Pendulum CNT-90, CNT-90XL, CNT-91, CNT-91R User Manual

Download

Timer/Counter/Analyzer

CNT-90, CNT-91

Frequency Calibrator/Analyzer

CNT-91R

Microwave Counter/Analyzer

CNT-90XL

User's Manual

4031 600

90001

2017 -

20th

Edition

Instruments

Table of

Contents

GENERAL INFORMATION ....................1-VIII

About this Manual ............................1-VIII

Warranty ..........................................1-VIII

Declaration of Conformity...................1-VIII

1 Preparation for Use

Preface.............................................. 1-2

Introduction ...............................................1-2

Powerful and Versatile Functions ........ 1-2

No Mistakes........................................1-3

Design Innovations.................................... 1-3

State of the Art Technology Gives

Durable Use........................................ 1-3

High Resolution................................... 1-3

Remote Control ......................................... 1-4

Fast GPIB Bus....................................1-4

Safety................................................ 1-5

Introduction ...............................................1-5

Safety Precautions .................................... 1-5

Caution and Warning Statements........1-6

Symbols..............................................1-6

If in Doubt about Safety....................... 1-6

Disposal of Hazardous Materials......... 1-6

Unpacking ........................................ 1-7

Check List ................................................. 1-7

Identification.............................................. 1-7

Installation................................................. 1-8

Supply Voltage.................................... 1-8

Battery Supply..................................... 1-8

Grounding ............................................1-8

Orientation and Cooling ...................... 1-8

Fold-Down Support .............................1-9

Rackmount Adapter ............................ 1-9

2 Using the Controls

Basic Controls ........................................... 2-2

Secondary Controls................................... 2-4

Connectors & Indicators...................... 2-4

Rear Panel.......................................... 2-5

Rear Panel (CNT-91R/71B).................2-6

Description of Keys ...................................2-7

Power ................................................. 2-7

Select Function...................................2-7

Autoset/Preset .................................... 2-8

Move Cursor ....................................... 2-8

Display Contrast.................................. 2-8

Enter................................................... 2-8

Save & Exit ......................................... 2-8

Don't Save & Exit................................ 2-8

Presentation Modes............................ 2-9

Entering Numeric Values.................. 2-10

Hard Menu Keys............................... 2-10

Default Settings ....................................... 2-19

3 Input Signal Conditioning

Input Amplifier............................................ 3-2

Impedance ......................................... 3-2

Attenuation......................................... 3-2

Coupling............................................. 3-3

Filter ................................................... 3-3

Man/Auto............................................ 3-4

Trig..................................................... 3-5

How to Reduce or Ignore Noise and

Interference ........................................... 3-6

Trigger Hysteresis .............................. 3-6

How to use Trigger Level Setting........ 3-7

4 Measuring Functions

Introduction to This Chapter........... 4-2

Selecting Function .................................... 4-2

Frequency Measurements .............. 4-3

FREQ A, B................................................ 4-3

FREQ C.................................................... 4-4

CNT-90/91(R)..................................... 4-4

CNT-90XL .......................................... 4-4

RATIO A/B, B/A, C/A, C/B ........................ 4-4

BURST A, B, C......................................... 4-4

1-8

Triggering........................................... 4-4

Burst Measurements using Manual

Presetting........................................... 4-5

Frequency Modulated Signals................... 4-6

Carrier Wave Frequency f0................. 4-6

max

.................................................... 4-7

min

..................................................... 4-7

Δf

p-p

.................................................... 4-8

Errors in f

max

, f

min

, and Δf

p-p

.................. 4-8

AM Signals ............................................... 4-8

Carrier Wave Frequency..................... 4-8

Modulating Frequency ........................ 4-9

Theory of Measurement............................ 4-9

Reciprocal Counting ........................... 4-9

Sample-Hold..................................... 4-10

Time-Out .......................................... 4-10

Measuring Speed ............................. 4-10

PERIOD.................................................. 4-13

Single A, B & Avg. A, B, C ................ 4-13

III

Environmental Considerations.............1-6

Single A, B Back-to-Back .................. 4-14

Frequency A, B Back-to-Back ........... 4-14

Time Measurements ...................... 4-15

Introduction ............................................. 4-15

Triggering.......................................... 4-15

Time Interval ........................................... 4-16

Time Interval A to B........................... 4-16

Time Interval B to A........................... 4-16

Time Interval A to A, B to B............... 4-16

CNT-91(R): Time Interval Error (TIE)....... 4-16

Rise/Fall Time A/B .................................. 4-16

Pulse Width A/B ...................................... 4-17

Duty Factor A/B....................................... 4-17

Measurement Errors................................ 4-18

Hysteresis.........................................4-18

Overdrive and Pulse Rounding.......... 4-18

Auto Trigger......................................4-19

Phase .............................................. 4-20

What is Phase?....................................... 4-20

Resolution............................................... 4-20

Possible Errors........................................ 4-20

Inaccuracies...................................... 4-21

Totalize [CNT-91(R) only] .............. 4-24

Totalize in General .................................. 4-24

TOT A MAN ............................................ 4-24

TOT B MAN ............................................ 4-24

TOT A+B MAN........................................ 4-24

TOT A–B MAN........................................ 4-25

Applications ...................................... 4-25

TOT A/B MAN......................................... 4-25

Totalize & Arming.................................... 4-25

Examples.......................................... 4-25

Voltage............................................ 4-27

MAX

, V

MIN

, VPP......................................... 4-27

RMS

........................................................ 4-28

5 Measurement Control

About This Chapter ...................................5-2

Measurement Time .............................5-2

Gate Indicator ..................................... 5-2

Single Measurements ......................... 5-2

Hold/Run & Restart .............................5-2

Arming ................................................ 5-2

Start Arming........................................ 5-3

Stop Arming........................................ 5-3

Controlling Measurement Timing... 5-4

The Measurement Process .......................5-4

Resolution as Function of Measurement

Time.................................................... 5-4

Measurement Time and Rates ............5-5

What is Arming?.................................. 5-5

Arming Setup Time ................................... 5-9

Arming Examples...................................... 5-9

Introduction to Arming Examples ........ 5-9

#1 Measuring the First Burst Pulse..... 5-9

#2 Measuring the Second Burst

Pulse #1 and #4 ................................ 5-12

#4 Profiling ........................................ 5-13

6 Process

Introduction............................................... 6-2

Averaging........................................... 6-2

Mathematics ............................................. 6-2

Example: ........................................... 6-2

Statistics................................................... 6-3

Allan Deviation vs. Standard Deviation6-3

Selecting Sampling Parameters.......... 6-3

Measuring Speed ............................... 6-4

Determining Long or Short Time

Instability ............................................ 6-4

Statistics and Mathematics ............... 6-5

Confidence Limits............................... 6-5

Jitter Measurements ........................... 6-5

Limits........................................................ 6-6

Limit Behavior..................................... 6-6

Limit Mode.......................................... 6-6

Limits and Graphics.................................. 6-7

7 Performance Check

General Information.................................. 7-2

Preparations ............................................. 7-2

Test Equipment ........................................ 7-2

Front Panel Controls................................. 7-3

Internal Self-Tests .............................. 7-3

Keyboard Test.................................... 7-3

Short Form Specification Test ................... 7-5

Sensitivity and Frequency Range ....... 7-5

Voltage............................................... 7-6

Trigger Indicators vs. Trigger Levels... 7-7

Input Controls ..................................... 7-8

Reference Oscillators (not CNT-91R) . 7-8 Rubidium Oscillator (CNT-91R and

CNT-91R/71B) ................................... 7-8

Resolution Test .................................. 7-9

Rear Inputs/Outputs.................................. 7-9

10 MHz OUT (all models) 0.1, 1, 5 & 10

MHz OUT(CNT-91R/71B

EXT REF FREQ INPUT...................... 7-9

EXT ARM INPUT.............................. 7-10

Measuring Functions ............................... 7-10

Check of HOLD OFF Function ................. 7-10

RF Inputs................................................. 7-12

Checking Input C............................... 7-12

Power Measurement

(CNT-90XL only)................................ 7-13

Battery Supply ......................................... 7-13

#3 Measuring the Time Between Burst

Pulse.................................................. 5-11

[CNT-90XL option 28 only].......................4-29

Pulsed Signals

Option 23/90 for CNT-90 & CNT-90XL

only................................................... 7-13

8 Specifications

CNT-90 .............................................. 8-2

Introduction ...............................................8-3

Measurement Functions............................ 8-3

Frequency A, B, C............................... 8-3

Frequency Burst A, B, C...................... 8-3

Period A, B, C Average....................... 8-3

Period A, B Single ............................... 8-4

Ratio A/B, B/A, C/A, C/B ..................... 8-4

Time Interval A to B, B to A, A to A, B to B

........................................................... 8-4

Pulse Width A, B ................................. 8-4

Rise and Fall Time A, B....................... 8-4

Phase A Rel. B, B Rel. A..................... 8-5

Duty Factor A, B.................................. 8-5

max

, V

min

, V

p-p

A, B.............................. 8-5

Timestamping A, B, C ......................... 8-6

Auto Set / Manual Set ......................... 8-6

Input and Output Specifications................. 8-6

Inputs A and B .................................... 8-6

Input C (Option 10).............................. 8-7

Input C (Option 13).............................. 8-7

Input C (Options 14 & 14B) ................. 8-8

Rear Panel Inputs & Outputs............... 8-8

Auxiliary Functions .................................... 8-8

Trigger Hold-Off .................................. 8-8

External Start/Stop Arming.................. 8-8

Statistics ............................................. 8-8

Mathematics ....................................... 8-9

Other Functions .................................. 8-9

Display................................................8-9

GPIB Interface .................................... 8-9

USB Interface.................................... 8-10

TimeView™ ...................................... 8-10

Battery Unit ............................................. 8-10

Option 23/90 ..................................... 8-10

Measurement Uncertainties..................... 8-11

Random Uncertainties (1) ............... 8-11

Systematic Uncertainties................... 8-11

Total Uncertainty (2) .......................8-11

Time Interval, Pulse Width, Rise/Fall Time

......................................................... 8-11

Frequency & Period .......................... 8-11

Frequency Ratio f1/f2......................... 8-12

Phase ............................................... 8-12

Duty Factor ....................................... 8-12

Calibration............................................... 8-13

Definition of Terms ............................ 8-13

General Specifications ............................ 8-13

Environmental Data........................... 8-13

Power Requirements......................... 8-13

Dimensions & Weight........................ 8-14

Ordering Information ............................... 8-14

Timebase Options................................... 8-15

Explanations..................................... 8-15

CNT-90XL ....................................... 8-16

Introduction............................................. 8-17

Measurement Functions ......................... 8-17

Frequency A, B, C ............................ 8-17

Frequency Burst A, B ....................... 8-17

Period A, B, C Average..................... 8-17

Period A, B Single ............................ 8-17

Ratio A/B, B/A, C/A, C/B................... 8-17

Time Interval A to B, B to A, A to A, B to B

.......................................................... 8-17

Pulse Width A, B............................... 8-18

Rise and Fall Time A, B.................... 8-18

Phase A Rel. B, B Rel. A .................. 8-18

Duty Factor A, B ............................... 8-18

max

, V

min

, V

p-p

A, B ........................... 8-18

Power C ........................................... 8-18

Timestamping A, B ........................... 8-19

(option 28 only).................................. 8-19

Auto Set / Manual Set....................... 8-19

Input and Output Specifications .............. 8-19

Inputs A and B.................................. 8-19

Input C.............................................. 8-20

Rear Panel Inputs & Outputs ............ 8-20

Auxiliary Functions.................................. 8-20

Trigger Hold-Off................................ 8-20

External Start/Stop Arming ............... 8-20

Statistics........................................... 8-20

Mathematics ..................................... 8-20

Other Functions................................ 8-20

Display ............................................. 8-21

GPIB Interface.................................. 8-21

USB Interface ................................... 8-21

TimeView™...................................... 8-21

Battery Unit............................................. 8-21

Option 23/90..................................... 8-21

Measurement Uncertainties .................... 8-22

Random Uncertainties (1σ)............... 8-22

Systematic Uncertainties .................. 8-22

Total Uncertainty (2σ)....................... 8-22

Time Interval, Pulse Width, Rise/Fall Time

.......................................................... 8-22

Frequency & Period A, B .................. 8-22

Frequency & Period C ...................... 8-22

Frequency Ratio fA/fB or fB/fA ............. 8-23

Frequency Ratio fC/fA or fC/fB ............. 8-23

Phase............................................... 8-23

Duty Factor....................................... 8-23

Calibration .............................................. 8-23

Definition of Terms ........................... 8-23

General Specifications............................ 8-24

Environmental Data .......................... 8-24

Power Requirements ........................ 8-24

Dimensions & Weight ....................... 8-24

Ordering Information............................... 8-24

Timebase Options................................... 8-25

Pulsed RF parameters input C

Explanations ..................................... 8-25

CNT-91(R) ....................................... 8-30

Introduction ............................................. 8-31

Measurement Functions.......................... 8-31

Frequency A, B, C............................. 8-31

Frequency Burst A, B, C.................... 8-31

Period A, B, C Average..................... 8-31

Period A, B Single ............................. 8-32

Period A, B Back-to-Back.................. 8-32

Ratio A/B, B/A, C/A, C/B ................... 8-32

Time Interval A to B, B to A, A to A, B to B

......................................................... 8-32

Pulse Width A, B ............................... 8-32

Rise and Fall Time A, B..................... 8-32

Time Interval Error (TIE) A, B............ 8-33

Phase A Rel. B, B Rel. A................... 8-33

Duty Factor A, B................................ 8-33

max

, V

min

, V

p-p

A, B............................ 8-33

Totalize A, B, A+B, A-B, A/B ............. 8-34

Timestamping A, B............................ 8-34

Auto Set / Manual Set ....................... 8-34

Input and Output Specifications............... 8-35

Inputs A and B .................................. 8-35

Input C (Option 10)............................ 8-35

Input C (Option 13)............................ 8-36

Input C (Options 14 & 14B) ............... 8-36

Rear Panel Inputs & Outputs............. 8-36

Auxiliary Functions .................................. 8-37

Trigger Hold-Off ................................ 8-37

External Start/Stop Arming................ 8-37

Statistics ........................................... 8-37

Mathematics ..................................... 8-38

Other Functions ................................ 8-38

Display..............................................8-38

GPIB Interface .................................. 8-38

USB Interface.................................... 8-39

TimeView™ ...................................... 8-39

Measurement Uncertainties..................... 8-40

Random Uncertainties (1) ............... 8-40

Systematic Uncertainties................... 8-40

Total Uncertainty (2) .......................8-40

Time Interval, Pulse Width, Rise/Fall Time

......................................................... 8-40

Frequency & Period .......................... 8-41

Frequency Ratio f1/f2......................... 8-41

Phase ............................................... 8-41

Duty Factor ....................................... 8-41

Calibration............................................... 8-42

Definition of Terms ............................ 8-42

General Specifications ............................ 8-42

Environmental Data........................... 8-42

Power Requirements......................... 8-43

Dimensions & Weight........................ 8-43

Ordering Information ............................... 8-43

Timebase Options CNT-91...................... 8-44

Explanations ..................................... 8-44

Timebase Specifications CNT-91R ......... 8-45

Explanations..................................... 8-45

CNT-91R/71B.................................... 8-46

Introduction............................................. 8-47

Measurement Functions ......................... 8-47

Frequency A, B, C ............................ 8-47

Frequency Burst A, B, C................... 8-47

Period A, B, C Average..................... 8-47

Period A, B Single ............................ 8-48

Period A, B Back-to-Back ................. 8-48

Ratio A/B, B/A, C/A, C/B................... 8-48

Time Interval A to B, B to A, A to A, B to B

......................................................... 8-48

Pulse Width A, B............................... 8-48

Rise and Fall Time A, B .................... 8-48

Time Interval Error (TIE) A, B ........... 8-49

Phase A Rel. B, B Rel. A .................. 8-49

Duty Factor A, B ............................... 8-49

max

, V

min

, V

p-p

A, B ........................... 8-49

Totalize A, B, A+B, A-B, A/B............. 8-50

Timestamping A, B ........................... 8-50

Auto Set / Manual Set....................... 8-50

Input and Output Specifications .............. 8-51

Inputs A and B.................................. 8-51

Input C.............................................. 8-51

Rear Panel Inputs & Outputs ............ 8-52

Auxiliary Functions.................................. 8-52

Trigger Hold-Off................................ 8-52

External Start/Stop Arming ............... 8-52

Statistics........................................... 8-52

Mathematics ..................................... 8-53

Other Functions................................ 8-53

Display ............................................. 8-53

GPIB Interface.................................. 8-53

USB Interface ................................... 8-54

TimeView™...................................... 8-54

Measurement Uncertainties .................... 8-55

Random Uncertainties (1)............... 8-55

Systematic Uncertainties .................. 8-55

Total Uncertainty (2)....................... 8-55

Time Interval, Pulse Width, Rise/Fall Time

......................................................... 8-55

Frequency & Period.......................... 8-56

Frequency Ratio f1/f2......................... 8-56

Phase............................................... 8-56

Duty Factor....................................... 8-56

Calibration .............................................. 8-57

Definition of Terms ........................... 8-57

General Specifications............................ 8-57

Environmental Data .......................... 8-57

Power Requirements ........................ 8-58

Dimensions & Weight ....................... 8-58

Timebase Specifications CNT-91R/71B..8-59

Explanations..................................... 8-59

9 Index

......................

10-2

Service

Sales and Service

Office

Appendix

New Look...............................................

11-2

VII

GENERAL INFORMATION

About this Manual

This manual contains directions for use that apply to the Timer/Counter/Analyzers CNT-90 and CNT-91 as well as the Frequency Calibrator/Analyzer CNT-91R and CNT-91R/71B and the Microwave Counter/Analyzer CNT-90XL.

In order to simplify the references, these instruments are further referred to throughout this manual as the '9X', whenever the information applies to all types. Differences are clearly marked.

Examples:

•

CNT-90 means CNT-90 and CNT-90/XL

•

CNT-90/91 means CNT-90 and CNT-91

•

CNT-91(R) means CNT-91, CNT-91R and CNT-91R/71B

Chapter 8; Specifications is divided into four separate sections to increase legibility. Much of

the contents is common, so redundant data is the price in this case.

Warranty

The Warranty Statement is part of the folder Important Information that is included with the shipment.

Declaration of Conformity

The complete text with formal statements concerning product identification, manufacturer and standards used for type testing is available on request.

VIII

Chapter 1

Preparation for Use

Preface

Introduction

Congratulations on your choice of instrument. It will serve you well and stay ahead of most competition for many years to come.

Your instrument is designed to bring you a new dimension to bench-top and system counting. It gives significantly increased performance compared to traditional Timer/Counters. The '9X' offers the following advantages:

—12 digits of frequency resolution per sec-

ond and 50 or 100 ps resolution, as a result of high-resolution interpolating reciprocal counting.

—Optional oven-controlled timebase oscilla-

tors, except th

e CNT-91R & CNT-91R/71B, which have a fixed ultrastabl

e rubidium oscillator.

—CNT-90, CNT-91(R): A variety of RF prescaler options with upper frequency limits ranging from 3 GHz t

o 20

GHz.

—CNT-91R/71B: 5 Reference frequency outputs covering 100 kHz, 1 MHz, 5 MHz and 10 MHz.

—CNT-90XL: A number of microwave inputs with upper frequency limits ranging from 27 GHz to 60 GHz.

—CNT-90(XL): Optional built-in Li-Ion battery supply realizes instant high-prescision measurements in the field and true UPS operation.

—Integrated high performance GPIB inter-

face using SCPI commands.

—A fast USB interface that replaces the tra-

ditional but slower RS-232 serial interface.

—Timestamping; the counter records the rel-

ative position in time of measurements with high resolution and accuracy.

—A high measurement rate of up to 250

k readings/s to internal memory.

Powerful and Versatile Functions

A unique performance feature in your new instrument is the comprehensive arming possibilities, which allow you to characterize virtually any type of complex signal concerning frequency and time.

For instance, you can insert a delay between the external arming condition and the actual arming of the counter. Read more about Arming in Chapter 5, "Measurement Control".

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-2

Preface

In addition to the traditional measurement

functions of a timer/counter, these

Design Innovations

instruments have a multitude of other functions such as phase, duty factor,

rise/fall-time and peak voltage. The counter

can perform all measurement functions on both main inputs (A & B). Most measurement functions can be armed, either via one of the main inputs or via a separate arming channel (E).

By using the built-in mathematics and statistics functions, the instrument can process the measurement results on your benchtop, without the need for a controller. Math functions include inversion, scaling and offset. Statistics functions include Max, Min and Mean as well as Standard and Allan Deviation on sample sizes up to 2*109.

No Mistakes

You will soon find that your instrument is more or less self-explanatory with an intuitive user interface. A menu tree with few levels makes the timer/counter easy to operate. The large backlit graphic LCD is the center of information and can show you several signal parameters at the same time as well as setting status and operator messages.

Statistics based on measurement samples can easily be presented as histograms or trend plots in addition to standard numerical measurement results like m

ax, min, mean and

standard deviation.

The AUTO function triggers automatically on any input waveform. A bus-learn mode simplifies GPIB programming. With bus-learn mode, manual counter settings can be transferred to the control

ler for later reprogramming. There is no need to learn code and syntax for each individual counter setting if you are an occasional GPIB bus user.

State of the Art Technology Gives Durable Use

These counters are designed for quality and durability. The design is highly integrated. The digital counting circuitry consists of just one custom-developed FPGA and a 32-bit microcontroller. The high integration and low component count reduces power consumption and results in an MTBF of 30,000 hours. Modern surface-mount technology ensures high production quality. A rugged mechanical construction, including a metal cabinet that withstands mechanical shocks and protects against EMI, is also a valuable feature.

High Resolution

The use of reciprocal interpolating counting in this new counter results in excellent relative resolution: 12 digits/s for all frequencies.

The measurement is synchronized with the input cycles instead of the timebase. Simultaneously with the normal "digital" counting, the counter makes analog measurements of the time between the start/stop trigger events and the next following clock pulse. This is done in four identical circuits by charging an integrating capacitor with a constant current, starting at the trigger event. Charging is stopped at the leading edge of the first following clock pulse.

The stored charge in the integrating capacitor represents the time difference between the start trigger event and the leading edge of the first following clock pulse. A similar charge integration is made for the stop trigger event.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-3

Preface

When the "digital" part of the measurement

is ready, the stored charges in the capacitors are measured by means of Analog/Digital Converters.

The counter's microprocessor calculates the result after completing all measurements, i.e. the digital time measurement and the analog interpolation measurements.

The result is that the basic "digital resolution" of + 1 clock pulse (10 ns) is reduced to 100 ps for the CNT-90 and 50 ps for the CNT-91(R).

Since the measurement is synchronized with the input signal

, the resolution for frequency measurements is very high and independent of frequency.

The counters have 14 display digits to ensure that the display itself does not restrict the resolution.

Remote Control

This instrument is programmable via two interfaces, GPIB and USB.

The GPIB interface offers full general functionality and compliance with the latest standards in use, the IEEE 488.2

1987 for HW

and the SCPI 1999 for SW.

In addition to this 'native' mode of operation there is also a second mode that emulates the Agilent 53131/132 command set for easy exc

hange of instrume

nts in operational ATE

systems.

The USB interface is mainly intended for the

lab environment in conjunction with the optional TimeView™ analysis software. The communication protocol is a proprietary version of SCPI.

Fast GPIB Bus

These counters are not only extremely powerful and versatile bench-top instruments, they also feature extraordinary bus properties.

The bus transfer rate is up to 4000 triggered measurements/s in CNT-91(R). Array measurements to the internal memory can reach 250 k measurements/s.

This very high measurement rate makes new measurements possible. For example, you can perform jitter analysis on several tens of thousands of pulse width measurements and capture them in less than a second.

An extensive Programmer's Handbook helps you understand SCPI and counter programming.

The counter is easy to use in GPIB environments. A built-in bus-learn mode enables you to make all counter settings manually and transfer them to the controller. The response can later be used to reprogram the counter to the same settings. This eliminates the need for the occasional user to learn all individual programming codes.

Complete (manually set) counter settings can also be stored in 20 internal memory locations and can easily be recalled on a later occasion. Ten of them can be user protected.

USER MANUAL ● CNT 9x Series ● Rev. 20 December 2017

1-4

Unpacking

Safety

Introduction

Even though we know that you are eager to get going, we urge you to take a few minutes to read through this part of the introductory chapter carefully before plugging the line connector into the wall outlet.

This instrument has been designed and tested for Measurement Category I, Pollution Degree 2, in accordance with EN/IEC 61010-1:2001 and CAN/CSA-C22.2 No. 61010-1-04 (including approval). It has been supplied in a safe condition.

Study this manual thoroughly to acquire adequate knowledge of the instrument,

especially the section on Safety Precautions hereafter and the section on Installation on page 1-7.

Safety Precautions

All equipment that can be connected to line power is a potential danger to life. Handling restrictions imposed on such equipment should be observed.

To ensure the correct and safe operation of the instrument, it is essential that you follow generally accepted safety procedures in addition to the safety precautions specified in this manual.

The instrument is designed to be used by trained personnel only. Removing the cover for repair, maintenance, and adjustment of the instrument must be done by qualified personnel who are aware of the hazards involved.

The warranty

commitments are rendered void if unauthorized access to the interior of the instrument has taken place during the given warranty period.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-5

Unpacking

Caution and Warning

Statements

CAUTION: Shows where incorrect

procedures can cause damage to, or destruction of equipment or other property.

WARNING: Shows a potential danger that

requires correct procedures or practices to prevent personal injury.

Symbols

Shows where the protective ground

terminal is connected inside the instrument. Never remove or loosen this screw.

This symbol is used for identifying

the functional ground of an I/O signal. It is

always connected to the instrument chassis.

Indicates that the operator should

consult the manual.

One such symbol is printed on the instrument, below the A and B inputs. It points out that the damage level for the input voltage decreases from 350 V

p to 12Vrms when you

switch the input impedance from 1 M to 50.

If in Doubt about Safety

Whenever you suspect that it is unsafe to use the instrument, you must make it inoperative by doing the following:

—Disconnect the line cord

—Clearly mark the instrument to prevent its

further operation

Fig. 1-1

Do not overlook the safety instructions!

— Inform your Spectracom representative.

For example, the instrument is likely to be unsafe if it is visibly damaged.

Disposal of Hazardous

Materials

■ CNT-90 & CNT-90XL only

If your instrument was ordered with a built-in battery supply (Option 23/90), it contains 12 Li-Ion cells arranged as a fixed battery pack with internal protection circuitry.

Even though this type of cell does not cause environmental damage in the same way as NiCd, for instance, you should dispose of a worn-out battery pack at an authorized recycling station or return it to Pendulum.

Note: Individual cells cannot be replaced.

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1-6

Unpacking

Environmental Considerations

This section provides information about the environmental impact of the product.

Product End-of-Life Handling

Observe the following guidelines when recycling an instrument or component:

Equipment recycling

Production of this equipment required the extraction and use of natural resources. The equipment may contain substances that could be harmful to the environment or human health if impropely handled at the product's end of life. To avoid release of such substances into the environment and to reduce the use of natural resources, we encourage you to recycle this product in an appropriate system that will ensure that most of the materials are reused or recycled appropriately.

This symbol indicates that this product complies with European Union requirements according to Directives 2012/19/EU and 2006/66/EC on waste electrical and electronic equipment (WEEE) and batteries.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-6

Unpacking

Check that the shipment is complete and that no damage has occurred during transportation. If the contents are incomplete or damaged, file a claim with the carrier immediately. Also notify your local Pendulum sales or service organization in case repair or replacement may be required.

Check List

The shipment should contain the following: — Counter/Timer/Analyzer CNT-90/91 or

Frequency Calibrator/Analyzer CNT-91R or CNT-91R/71B or Microwave Counter/Analyzer CNT-90XL

—

Line cord

—

N-to-BNC Adapter (only CNT-90/91(R) with prescaler options having a type N connector)

—

Printed version of the Getting Started

Manual

—

Brochure with Important Information

—

Certificate of Calibration

—

Options you ordered should be installed.

See Identification below.

—CDincluding

the

following

documentation in PDF:

•

Getting Started Manual

•

User's Manual

•

Programmer's Handbook

•

Service Manual (CNT-91R/71B only)

Identification

The type plate on the rear panel shows type number and serial number. See illustrations on page 2-5 and 2-6. Installed options are listed under the menu User Options - About, where you can also find information on firmware version and calibration date. See page 2-15.

The CNT-91R/71B version is identified by a unique identification marking, or UID. This permanent tag contains a barcode and allows customers to track easily their inventory and property.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-7

Unpacking

Installation

Supply Voltage

■ Setting

The Counter may be connected to any AC supply with a voltage rating of 90 to 265 Vrms , 45 to 440 Hz. The counter automatically adjusts itself to the input line voltage.

■ Fuse

The secondary supply voltages are electronically protected against overload or short circuit. The primary line voltage side is protected by a fuse located on the power supply unit. The fuse rating covers the full voltage range. Consequently there is no need for the user to replace the fuse under any operating conditions, nor is it accessible from the outside.

CAUTION: If this fuse is blown, it is likely

that the power supply is badly damaged. Do not replace the fuse. Send the counter to the local Service Center.

Removing the cover for repair, maintenance

and adjustment must be done by qualified and trained personnel only, who are fully aware of the hazards involved.

The warranty commitments are rendered void if unauthorized access to the interior of the instrument has taken place during the given warranty period.

Battery Supply

■ CNT-90 & CNT-90XL only

It is possible to run the counter from an optional battery supply, Option 23/90.

You must charge the battery before use or

storage. The counter charges the battery automatically when connected to line power or an external DC source, whether the instrument is in standby or turned on. See the specifications for charging time in different modes of operation.

Grounding

Grounding faults in the line voltage supply will make any instrument connected to it dangerous. Before connecting any unit to the power line, you must make sure that the protective ground functions correctly. Only then can a unit be connected to the power line and only by using a three-wire line cord. No other method of grounding is permitted. Extension cords must always have a protective ground conductor.

CAUTION: If a unit is moved from a cold to

a warm environment, condensation may cause a shock hazard. Ensure, therefore, that the grounding requirements are strictly met.

WARNING: Never interrupt the grounding

cord. Any interruption of the protective ground connection inside or outside the instrument or disconnection of the protective ground terminal is likely to make the instrument dangerous.

Orientation and Cooling

The counter can be operated in any position desired. Make sure that the air flow through the ventilation slots at the top, and side panels is not obstructed. Leave 5 centimeters (2 inches) of space around the counter.

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1-8

Unpacking

Fold-Down Support

Capacitors inside the instrument can hold

their charge even if the instrument has

For bench-top use, a fold-down support is

been separated from all voltage sources.

available for use underneath the counter. This support can also be used as a handle to carry the instrument.

Fig. 1-2 Fold-down support for comfortable Fig. 1-4

Fitting the rack mount brackets on the

bench-top use.

counter.

Rackmount Adapter

Fig. 1-3

Dimensions for rackmounting hardware.

If you have ordered a 19-inch rack-mount kit

for your instrument, it has to be assembled after delivery of the instrument. The rackmount kit consists of the following:

— 2 brackets, (short, left; long, right)

— 4 screws, M5 x 8

— 4 screws, M6 x 8

WARNING: Do not perform any internal

service or adjustment of this instrument unless you are qualified to do so

Before you remove the cover, disconnect

mains cord and wait for one minute.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-9

Unpacking

■ Assembling the Rackmount Kit

— Make sure the power cord is

disconnected from the instrument.

— Turn the instrument upside down.

See Fig. 1-5.

Fig. 1-5

Remove the screws and push the counter

out of the cover.

—

Undo the two screws (A) and remove

them from the cover.

—

Remove the rear feet by undoing the two screws (B).

—

Remove the four decorative plugs (C) that cover the screw holes on the right and left side of the front panel.

—

Grip the front panel and gently push at the rear.

—

Pull the instrument out of the cover.

—

Remove the four feet from the cover.

Use a screwdriver as shown in the following

illustration or a pair of pliers to remove the springs holding each foot, then push out the feet.

Fig. 1-6

Removing feet from the cover.

—

Push the instrument back into the cover.

See Fig. 1-5.

—

Mount the two rear feet with the screws (B) to the rear panel.

—

Put the two screws (A) back.

—

Fasten the brackets at the left and right side with the screws included as illustrated in Fig 1-6

—

Fasten the instrument in the rack via screws in the four rack-mounting holes

The long bracket has an opening so that cables

for Input A, B, and C can be routed inside the rack.

■ Reversing the Rackmount Kit

The instrument may also be mounted to the right in the rack. To do so, swap the position of the two brackets.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

1-10

Chapter 2

Using the Controls

Basic Controls

A more elaborate description of the front and

survey, the purpose of which is to make you

rear panels including the user interface with familiar with the layout of the instrument.

its menu system follows after this See also the appendix.

introductory

INPUT B

Opens the menu from which you can adjust all settings for Input B like Coupling, Impedance and Attenuation.

INPUT A

Opens the menu from which you can adjust all settings for Input A like Coupling, Impedance and Attenuation.

SETTINGS

Select measurement parameters such as measurement time, number of measurements, and so on.

STANDBY LED

The LED lights up when the

counter is in STANDBY mode, indicating that power is still applied to an internal optional OCXO, if one has been installed, or to the rubidium oscillator in the CNT-91R.

STANDBY/ON

Toggling secondary power switch. Pressing this button in standby mode turns the counter ON and restores the settings as they were at power-down.

MATH/LIMIT

Menu for selecting one of a set of formulas for modifying the measurement result. Three constants can be entered from the keyboard.

Numerical limits can also be entered for status reporting and recording.

USER OPT.

Controls the following items:

1. Settings memory

2. Calibration

3. Interface

4. Self-test

5. Blank digits

6. About

2-2

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

The cursor position,

inversion on the

moved in four

ENTER

Confirms menu selections without leaving the menu level.

selections and moves up one level in the menu tree.

CURSOR CONTROL

marked by text

display, can be

directions.

AUTO SET

Adjusts input trigger

voltages automatically to the optimum levels for the chosen measurement function.

Double-click for

default settings.

presentation mode with one main parameter and a number of auxiliary parameters.

presentation modes. Switching

is done by toggling the key.

STAT/PLOT

Enters one of three

statistics

between the modes

VALUE

Enters the normal

numerical

RESTART

Initiates one new measurement if HOLD is active.

MEAS FUNC

Menu tree for selecting measurement function. You can use the seven softkeys below the display for confirmation.

EXIT/OK

Confirms menu

HOLD/RUN

Toggles between

HOLD (one-shot) mode and RUN (continuous) mode. Freezes the result after completion of a measurement if HOLD is active.

CANCEL

Moves up one menu level without confirming selections made. Exits REMOTE mode if not

LOCAL

LOCKOUT.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-3

Using the Controls

Secondary Controls

Connectors & Indicators

GRAPHIC DISPLAY

SOFTKEYS

320 x 97 pixels LCD with backlight for showing The function of these seven keys is menu demeasurement results in numerical as well

as pendent. Actual function is indicated on the LCD. graphical format. The display is also the center of the Depressing a softkey is often a faster alternative to dynamic user interface, comprising menu trees, moving the cursor to the desired position and then indicators and information boxes. pressing OK.

TRIGGER IN-

GATE INDI-

MAIN INPUTS

DICATORS

CATOR

The two identical DC

Blinking LED dicates correct

in-

A pending measurement causes

coupled channels A& B

are used for all types of

measurements, either

triggering.

the LED to light up.

one at a time or both

together.

NUMERIC INPUT KEYS

Sometimes you may want to enter numeric values like the constants

and limits asked for when you are utilizing the postprocessing features in MATH/LIMIT mode. These twelve keys are to be used for this purpose.

RF/MICROWAVE

INPUT

CNT-90/91(R): A number of optional RF prescalers are

available.

CNT-90XL: One of

number of

microwave

converters is

mounted.

Conn. type dep.

frequency

spec.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-4

CNT-90

MER/

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Using the Controls

Rear Panel

Type Plate

Indicates instrument type and serial number.

Pulse Output

[CNT-91(R) only]

User definable to serve as output for built-in pulse generator, gate indicator or alarm.

Optional Main Input Connectors (not

with Option 23/90)

The front panel inputs can be moved to the rear panel by means of an optional cable kit. Note that the input capacitance will be higher.

Fan

A temp. sensor controls the speed of the fan. Normal bench-top use means low speed, whereas rack-mounting and/or options may result in higher speed.

Protective Ground

Terminal

This is where the protective ground wire is connected inside the instrument. Never tamper with this screw!

Line Power Inle

AC 90-265 VRMS, 45-440 Hz no range switching needed.

External Arming Input

See page 5-7.

Reference Output 10

MHz derived from the internal or,

if present,

the external reference.

GPIB Connector

Address set via User Options Menu.

External Reference Input

Can be automatically selected if a signal is present and approved as timebase source, see Chapter 9.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

USB Connector

Universal Serial Bus (USB) for data communication with PC.

Ext. DC Connector

Part of Option 23/90 for CNT-90(XL). Range: 12-18 V Note the polarity.

2-5

Using the Controls

Rear Panel

Additional output frequencies

(CNT-91R/71B)

Type Plate

Indicates instrument

type and serial

number.

Connectors

These connectors provide additional output frequencies which are, from left to right, 100kHz,

1MHz, 5MHz and 10MHz.

Protective Ground

Terminal

This is where the protective ground wire is connected inside the instrument. Never tamper with this screw!

Pulse Output

Fan

User definable to serve

A temp. sensor controls the speed

as output for built-in

of the fan. Normal bench-top use

pulse generator, gate indicator or alarm.

means low speed, whereas rack-mounting and/or options may

Line Power Inlet

AC 90-265 VRMS, 45-440

result in higher speed.

Hz, no range switching needed.

Reference Output

External Arming Input

USB Connector

10 MHz derived from the internal or, if present, the

See page 5-7.

Universal Serial Bus (USB) for data communication with external reference.

PC.

External Reference

GPIB Connector

Input

Address set via User Options

Can be automatically

Menu.

selected if a signal is present and approved as timebase source, see Chapter 9.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-6

Using the Controls

The charging level indicator shows:

Description of Keys

• the relative charging level in percent

Power

The ON/OFF key is a toggling secondary power switch. Part of the instrument is always ON as long as power is applied, and this standby condition is indicated by a red LED above the key. This indicator is consequently not lit while the instrument is in operation.

■ CNT-91R and CNT-91R/71B only

While the rubidium oscillator is warming up, an open padlock symbol labeled RB is flashing at the top right corner of the display, indicating that the control loop is not locked. Normal time to lock is about 5 min. Do not start measuring until the unlock symbol disappears.

New Message Box

Information exchange between the rubidium oscillator and the CPU takes place over a serial bus. Any malfunction in the UART-con-trolled communication link will be reported in a pop-up message box on the display.

■ CNT-90(XL) w. Option 23/90

The User Interface Screens have two indicators near the upper right corner of the display. One is a power supply status indicator and

the other is a battery charging level indicator.

The status indicator shows:

•

a fixed battery symbol when the internal battery is the active power source

•

a charging battery symbol when the internal battery is being charged

•

a power plug symbol when the mains is the active power source

•

a power plug symbol on top of a battery symbol when the instrument has been prepared for UPS operation and charging is not going on

Select Function

This hard key is marked MEAS FUNC. When you depress it, one of the menus below will open.

Fig. 2-1

CNT-90: Select measurement function.

Fig. 2-2

CNT-90XL: Select measurement

function.

Fig. 2-3

CNT-91(R): Select measurement

function.

The current selection is indicated by text in-

version that is also indicating the cursor position. Select the measurement function you want by depressing the corresponding softkey right below the display.

Alternatively you can move the cursor to the wanted position with the

RIGHT/LEFT arrow

keys. Confirm by pressing

ENTER.

A new menu will appear where the contents depend on the function. If you for instance

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-7

Using the Controls

have selected Frequency, you can then select

between Frequency, Frequency Ratio and Frequency Burst. Finally you have to decide which input channel(s) to use.

Autoset/Preset

By depressing this key once after selecting the wanted measurement function and input channel, you will most probably get a measurement result. The

AUTOSET system

ensures that the trigger levels are set optimally for each combination of measurement function and input signal amplitude, provided relatively normal signal

waveforms are applied. If Manual Trigger has

been selected before pressing the AUTOSET key, the system will make the necessary adjustments once (Auto Once) and then return to its inactive condition.

AUTOSET performs the following functions:

•

Set automatic trigger levels

•

Switch attenuators to 1x

•

Turn on the display

•

Set Auto Trig Low Freq to

■

100 Hz, if fin>100Hz, or to

■

in,

if 10<fin<100 Hz, or to

■

10 Hz, if fin <10Hz

A higher value means faster settling time.

By depressing this key twice within two seconds, you will enter the

Preset mode, and a

more extensive automatic setting will take place. In addition to the functions above, the following functions will be performed:

• Set Meas Time to 200 ms

• Switch off Hold-Off

• Set HOLD/RUN to RUN

• Switch off MATH/LIM

• Switch off Analog and Digital Filters

• Set Timebase Ref to Auto

• Switch off Arming

■ Default Settings

An even more comprehensive preset function can be performed by recalling the factory default settings. See page 2-16.

Move Cursor

There are four arrow keys for moving the cursor, normally marked by text inversion, around the menu trees in two dimensions.

Display Contrast

When no cursor is visible (no active menu selected), the

UP/DOWN arrows are used for

adjusting the LCD display contrast ratio.

Enter

The key marked ENTER enables you to confirm a choice without leaving your menu position.

Save & Exit

This hard key is marked EXIT/OK. You will confirm your selection by depressing it, and at the same time you will leave the current menu level for the next higher level.

Don't Save & Exit

This hard key is marked CANCEL. By de- pressing it you will enter the preceding menu level without confirming any selections made at the current level.

If the instrument is in

REMOTE mode, this key

is used for returning to

LOCAL mode, unless

LOCAL LOCKOUT has been programmed.

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2-8

Using the Controls

Presentation Modes

■ STAT/PLOT

■ VALUE

Fig. 2-4

Main and aux. parameters.

Value mode gives single line numerical pre-

sentation of individual results, where the main parameter is displayed in large characters with full resolution together with a number of auxiliary parameters in small characters with limited resolution.

Fig. 2-5

Limits presentation.

If Limit Behavior is set to Alarm and Limit

Mode is set to Range you can visualize the de-

viation of your measurements in relation to the set limits. The numerical readout is now combined with a traditional analog pointer-type instrument, wh

ere the current value is represented by a "smiley". The limits are presented as numerical values below the main parameter, and their positions are marked with vertical bars labelled LL (lower limit) and UL (upper limit) on the autoscaled graph.

If one of the limits has been exceeded, the limit indicator at the to

p of the display will be flashing. In case the current measurement is out of the visible graph area, it is indicated by means of a left or a right arrowhead.

If you want to treat a number of measurements with statistical methods, this is the key to operate. There are three display modes available by toggling the key:

•

Numerical

•

Histogram

•

Trend Plot

Numerical

Fig. 2-6

Statistics presented numerically.

In this mode the statistical information is dis-

played as numerical data containing the following elements:

•

Mean: mean value

•

Max: maximum value

•

Min: minimum value

•

P-P: peak-to-peak deviation

•

Adev: Allan deviation

•

Std: Standard deviation

Histogram

Fig. 2-7

Statistics presented as a histogram.

The bins in the histogram are always

autoscaled based on the measured data. Limits, if enabled, and center of graph are shown as vertical dotted lines. Data outside the limits are not used for autoscaling but are replaced by an

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-9

Using the Controls

arrow indicating the direction where

example is the Trig Lvl setting as part of the

non-displayed values have been recorded.

INPUT A (B) settings.

Trend Plot

Fig. 2-8

Running trend plot.

This mode is used for observing periodic fluc-

tuations or possible trends. Each plot terminates (if

HOLD is activated) or restarts (if RUN is

activated) after the set number of samples. The trend plot is always autoscaled based on the measured data, starting with 0 at restart. Limits are shown as horizontal lines if enabled.

■ Remote

When the instrument is controlled from the GPIB bus or the USB bus, the operating mode changes to

Remote, indicated by the label

REM on the display. All front panel keys ex-

cept

CANCEL are then disabled. See also page

2-8 for more information on this key.

Entering Numeric Values

Sometimes you may want to enter constants and limits in a value input menu, for instance one of those that you can reach when you press the

MATH/LIMIT key.

You may also want to select a value that is not in the list of fixed values available by pressing the

UP/DOWN arrow keys. One example is

Meas Time under SETTINGS.

A similar situation arises when the desired

value is too far away to reach conveniently by

incrementing or decrementing the original

value with the UP/DOWN arrow keys. One

Whenever it is possible to enter numeric val-

ues, the keys marked with 0-9;. (decimal

point) and ± (stands for Change Sign)take on

their alternative numeric meaning.

It is often convenient to enter values using the scientific format. For that purpose, the rightmost softkey is marked

EE (stands for

Enter Exponent), making it easy to switch between the manti

ssa and the exponent.

Press EXIT/OK to store the new value or

CANCEL to keep the old one.

Hard Menu Keys

These keys are mainly used for opening fixed menus from which further selections can be made by means of the softkeys or the cursor/select keys.

■ Input A (B)

Fig. 2-9

Input settings menu.

By depressing this key, the bottom part of the

display will show the settings for Input A (B).

The active settings are in bold characters and can be changed by depressing the corresponding softkey below the display. You can also move the cursor, indicated by text inversion, to the desired position with the

RIGHT/LEFT

arrow keys and then change the active setting with the

ENTER key.

The selections that can be made using this menu are:

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-10

Using the Controls

•

Trigger Slope: positive or negative, indicated by corresponding symbols

•

Coupling: AC or DC

•

Impedance: 50  or 1 M

•

Attenuation: 1x or 10x

•

Trigger:1Manual or Auto

•

Trigger Level:2numerical input via front panel

keyboard. If Auto Trigger is active, you can change the default trigger level manually as a percentage of the amplitude.

•

Filter:3On or Off

Notes:

Always Auto when measuring

risetime or falltime

The absolute level can either be

adjusted using the up/down arrow keys or by pressing ENTER to reach the numerical input menu.

Pressing the corresponding softkey or ENTER opens the Filter Settings menu. See Fig. 2-10. You can select a fixed 100 kHz analog filter or an adjustable digital filter. The equivalent cutoff frequency is set via the value input menu that opens if you select Digital LP Frequency from the menu.

Fig. 2-10

Selecting analog or digital filter.

■ Input B

The settings under Input B are equal to those under Input A.

■ Settings

This key accesses a host of menus that affect the measurement. The figure above is valid

after changing the default measuring time to 10

ms.

Fig. 2-11

Meas Time

The main settings menu.

Fig. 2-12

Submenu for entering measuring time.

This value input menu is active if you select a

frequency function. Longer measuring time means fewer measurements per second and gives higher resolution.

Burst

Fig. 2-13

Entering burst parameters.

This settings menu is active if the selected

measurement function is BURST - a special case of FREQUENCY - and facilitates measurements on pu

lse-modulated signals. Both the carrier frequency and the modulating frequency - the pulse repetition frequency (PRF) can be measured, often without the support of an external arming signal.

2-11

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

Arm

start. A typical use is to clean up signals gen-

erated by bouncing relay contacts.

Fig. 2-14

CNT-90 & CNT-90XL: Setting arming

conditions.

Fig. 2-15

CNT-91(R): Setting arming conditions.

Arming is the general term used for the means

to control the actual start/stop of a

measurement. The normal free-running mode is

inhibited and triggering takes place when

certain pretrigger conditions are fulfilled.

The signal or signals used for initiating the arming can be applied to three channels (A, B, E), and the start channel can be different from the stop channel. All conditions can be set via this menu.

NOTE: Stop Delay can only be used for realizing the

function Timed Totalize in the CNT-91(R).

Trigger Hold-Off

Fig. 2-16

The trigger hold-off submenu.

A value input menu is opened where you can set the delay during which the stop trigger conditions are ignored after the measurement

2-12

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

Statistics

Fig. 2-17

Entering statistics parameters.

In this menu you can do the following:

•

Set the number of samples used for calculation of various statistical measures.

•

Set the number of bins in the histogram view.

•

Pacing

The delay between measurements, called pacing, can be set to ON or OFF, and the time can be set within the range 2 s - 500 s.

Timebase Reference

Fig. 2-18

Selecting timebase reference source.

Here you can decide if the counter is to use an

Internal or an External timebase. A third al-

ternative is

Auto. Then the external timebase

will be selected if a valid signal is present at the reference input. The EXT REF indicator at the upper right corner of the display shows that the instrument is using an external timebase reference.

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

2-13

Using the Controls

Miscellaneous

Fig. 2-19

CNT-90: The 'Misc' submenu.

Fig. 2-22 CNT-90XL: The 'Input C Acquisition'

submenu.

Fig. 2-20

CNT-90XL: The 'Misc' submenu.

Fig. 2-21

CNT-91(R): The 'Misc' submenu.

The options in this menu are:

•

Smart Measure with submenus:

■ Smart Time Interval (valid only if the

selected measurement function is Time Interval) The counter decides by means

timestamping which measurement channel precedes the other.

■ Smart Frequency (valid only If the selected measurement function is Frequency or Period Average) By means of continuous timestamping and regression analysis, the resolution is increased for measuring times between 0.2 s and

100 s.

• Input C Acquisition (CNT-90XL only)

Auto

means

that the whole specified frequency range

is scanned for valid input signals.

Manual means that a narrow band around the manually entered center frequency is monitored for valid input signals. This mode is compulsory when measuring burst signals but is also recommended for FM signals, when the app

roximate fr

equency is known. An additional feature is that the measurement results are presented much faster, as the acquisition process is skipped.

NOTE: Signal frequencies outside the manual

capture range may cause erroneous results. In order to draw the operator's attention

to this eventuality, the sign "M.ACQ" is visible in the upper right corner of the display.

•

Auto Trig Low Freq In a value input menu you can set the lower frequency limit for automatic triggering and voltage measurements within the range 1 Hz 100 kHz. A higher limit means faster settling time and consequently faster measurements.

•

Timeout From this submenu you can activate/deactivate the timeout function and set the maximum time the instrument will wait for a pending measurement to finish before outputting a zero result. The range is 10 ms to 1000 s.

•

Interpolatator Calibration By switching off the interpolator calibration, you can increase the measurement speed at the expense of accuracy.

•

TIE (CNT-91 only) From a submenu you can either let the counter choose the reference frequency automatically (Auto) or enter it manually.

2-14

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

■ Math/Limit

Fig. 2-23

Selecting Math or Limits parameters.

You enter a menu where you can choose be-

tween inputting data for the Mathematics or the Limits postprocessing unit.

Fig. 2-24

The Math submenu.

The Math branch is used for modifying the

measurement result mathematically before presentation on the display. Thus you can make the counter show directly what you want without tedious recalculations, e.g. revolutions/min instead of Hz.

The

Limits branch is used for setting numerical

limits and selecting the way the instrument will report the measurement results in relation to them.

Let us explore the

Math submenu by pressing

the corresponding softkey below the display.

The display tells you that the Math function is

not active, so press the Math Off key once to open the formula selection menu.

Select one of the five different formulas, where K, L and M are constants that the user can set to any value. X stands for the current non-modified measurement result.

Fig. 2-26

Selecting formula constants.

Each of the softkeys below the constant labels

opens a value input menu like the one below.

Fig. 2-27

Entering numeric values for constants.

Use the numeric input keys to enter the man-

tissa and the exponent, and use the EE key to toggle between the input fields. The key marked X0 is used for entering the display reading as the value of the constant.

The

Limit submenu is treated in a similar way,

and its features are explored beginning on page 6-6.

Fig. 2-25

Selecting Math formula for

postprocessing.

2-15

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

■ User Options

Fig. 2-28

CNT-90: The User Options menu.

Fig. 2-29

CNT-90XL & CNT-90 with Option 23/90:

The User Options menu.

Fig 2-30

CNT-91(R): The User Options menu.

From this menu you can reach a number of

submenus that do not directly affect the measurement. You can choose between a number of modes by pressing the corresponding softkey.

Save/Recall Menu

Twenty complete front panel setups can be

stored in non-volatile memory. Access to the first ten memory positions is prohibited when

Setup Protect is ON. Switching OFF Setup Protect releases all ten memory positions si-

multaneou

sly.

The different setups can be individually labeled to make it easier for the operator to remember the application.

Fig. 2-32 The memory management menu

after pressing Setup.

The following can be done:

• Save current

setup

Fig. 2-33

Selecting memory position for saving

a measurement setup.

Browse through the available memory positions by using the

RIGHT/LEFT arrow keys. For

faster browsing, press the key

Next to skip to

the next memory bank. Press the softkey below the number (1-20) where you want to save the setting.

Fig. 2-31

The menu appearance after pressing

Save/Recall.

2-16

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

• Recall setup

Fig. 2-34 Selecting memory position for recalling

a measurement setup.

Select the memory position from which you want to retrieve the contents in the same way as under Save current setup above. You can also choose Default to restore the preprogrammed factory settings. See the table on page 2-19 for a complete list of these settings.

•

Modify labels Select a memory position to which you want to assign a label. See the descriptions under Save/Recall setup above. Now you can enter alphanumeric characters from the front panel. See the figure below.

The seven softkeys below the display are

used

for entering letters and digits in the same way as you write SMS messages on a cell phone.

•

Setup protection Toggle the softkey to switch between the

ON/OFF modes. When ON is active,

the memory positions 1-10 are all protected against accidental overwriting.

Fig. 2-35

Entering alphanumeric characters.

Dataset Menu

Fig. 2-36

The memory management menu after

pressing Dataset.

This feature is available in statistics mode

only, and if HOLD has been pressed prior to initiating a measurement with

RESTART. Up

to 8 different datasets can be saved in FLASH memory, each containing up to 32000 samples. If the pending measurement has more than 32000 samples, only the last 32000 will be saved. A default label will be assigned to the dataset. It can be changed in a similar way as the setup labels. See Modify labels above.

•

Save Select a memory position, accept or change the

name, and press

OK.

•

Recall Select a memory position and press

OK.

•

Total Reset The safety screen below will appear. Pressing

OK will restore all factory settings and erase all

user information.

Fig. 2-37

The Total Reset safety screen.

Calibrate Menu

This menu entry is accessible only for calibration purposes and is password-protected.

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2-17

Using the Controls

Interface Menu

Press Test Mode to open the menu with avail-

able choices.

Fig. 2-38 Selecting active bus interface. Bus Type

Select the active bus interface. The alternatives are GPIB and USB. If you select GPIB, you are also supposed to select the GPIB Mode and the GPIB Address. See the next two paragraphs.

GPIB Mode

There are two command systems to choose from.

•

Native

The SCPI command set used in this mode fully exploits all the features of this instrument series.

•

Compatible

The SCPI command set used in this mode is adapted to be compatible with Agilent 53131/132/181.

GPIB Address

Value input menu for setting the GPIB address.

Test

A general self-test is always performed every time you power-up the instrument, but you can order a specific test from this menu at any time.

Fig. 2-39

Self-test menu.

Fig. 2-40

Selecting a specific test.

Select one of them and press Start Test to run it.

Digits Blank

Jittery measurement results can be made easier for an operator to read by masking one or more of the LSDs on the display.

Place the cursor at the submenu Digits Blank and increment/decrement the number by means of the UP/DOWN arrow keys, or press the soft key beneath the submenu and enter the desired number between 0 and 13 from the keyboard. The blanked digits will be represented by dashes on the display. The default value for the number of blanked digits is 0.

Misc (CNT-90XL & CNT-90 with Option 23/90)

The CNT-90XL without Option 23/90 has a single submenu called Units. By pressing this softkey you get to the submenu Power. Press Power and then select dBm or W as the unit of measurement, when either of the functions Frequency C or Power C is selected from the

MEAS FUNC menu.

The CNT-90 with Option 23/90 has a single submenu called Use Battery in Standby. By toggling this softkey you can decide if the internal OCXO will remain powered or not when you turn off the instrument in battery operation mode.

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2-18

Using the Controls

The CNT-90XL with Option 23/90 has a

combination of the two submenus mentioned

above. See the figure below.

Fig. 2-41

The 'Misc' submenu for

CNT-90XL with battery option.

Output [CNT-91(R) only]

The rear panel pulse output can be used for three different purposes:

•

pulse generator

•

gate indicator

•

alarm

Press the softkey Output to open the submenu

below.

Fig. 2-42 Selecting output mode and pulse parameters.

Off is the default mode and inhibits all activity

on the output connector.

The pulse generator parameters Period and Width can be entered by first pressing the corresponding softkeys, then setting the numerical values as usual. By placing the cursor over the parameter, you can also set the values directly in 1-2-5 steps with the

UP/DOWN arrow keys.

Press Output Mode to enter the mode selection menu below:

• Gate Open indicates to external equipment when a measurement is in progress.

•

Pulse Generator activates a continuous pulse train having the parameters entered in the previous menu.

•

Alarm can be set to be active low or active high. The MATH/LIM menu is used for setting up the behavior and the numerical limits that trigger the alarm.

Fig 2-43

Output Mode selection menu.

The amplitude is fixed at TTL levels into

50irrespective of the output mode.

About

Here you can find information on:

•

model

•

serial number

•

instrument firmware version

•

timebase option & calibration date

■ The CNT-91R reports "Rubidium" in this

field.

•

RF input option

■ The CNT-90XL reports the upper

frequency limit.

■ Hold/Run

This key serves the purpose of manual arming. A pending measurement will be finished and the result will remain on the display until a new measurement is triggered by pressing the

RESTART key.

■ Restart

Often this key is operated in conjunction with the

HOLD/RUN key (see above), but it can also

be used in free-running mode, especially when long measuring times are being used, e.g. to initiate a new measurement after a change in the input signal.

RESTART will not affect any

front panel settings.

2-19

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Using the Controls

Default Settings

See page 2-16 to see how the following preprogrammed settings are recalled by a few keystrokes.

PARAMETER

VALUE/SETTING

Input A & B

Trigger Level

AUTO

Trigger Slope

POS

Impedance

1 M 

Attenuator

Coupling

Filter

OFF

Arming

Start

OFF

Start Slope

POS

Start Arm Delay

Stop

OFF

Stop Slope

POS

Hold-Off

Hold-Off State

OFF

Hold-Off Time

200 s

Time-Out

Time-Out State

OFF

Time-Out Time

100 ms

Statistics

OFF

No. of Samples

100

No. of Bins

Pacing State

OFF

Pacing Time

20 ms

Mathematics

OFF

PARAMETER

VALUE/SETTING

Math Constants

K=1, L=0, M=1

Limits

Limit State

OFF

Limit Mode

RANGE

Lower Limit

Upper Limit

Burst

Sync Delay

400 s

Start Delay

Meas. Time

200 s

Freq. Limit

400 MHz

Miscellaneous

Function

FREQA

Smart Frequency

AUTO

Smart Time Interval

OFF

Meas. Time

200 ms

Auto Trig Low Freq

100 Hz

Timebase Reference

AUTO

Blank Digits

Interpolator calibration

Output (CNT-91(R))

OFF

2-20

USER MANUAL

● CNT 9x Series ● Rev.20 December 2017

This page is intentionally left blank.

Chapter 3

Input Signal

Conditioning

Input Signaling Conditioning

Input Amplifier

The input amplifiers are used for adapting the widely varying signals in the ambient world to the measuring logic of the timer/counter.

These amplifiers have many controls, and it is essential to understand how these controls work together and affect the signal.

The block diagram below shows the order in which the different controls are connected. It is not a complete technical diagram but in- tended to help understanding the controls.

The menus from which you can adjust the settings for th

e two main measurement

channels are reached by pressing

INPUT A

respectively INPUT B. See Figure 3-2. The active choices are shown in boldface on the bottom line.

Fig. 3-2

Input settings menu.

Impedance

The input impedance can be set to 1 M or 50 

by toggling the corresponding softkey.

CAUTION: Switching the impedance to 50



when the input voltage is above 12 VRMS may cause permanent damage to the input circuitry.

Attenuation

The input signal's amplitude can be attenuated by 1 or 10 by toggling the softkey marked

1x/10x.

Use attenuation whenever the input signal ex-

ceeds the dynamic input voltage range ±5 V or

Fig. 3-1

Block diagram of the signal conditioning.

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3-2

Input Signaling Conditioning

else when attenuation can reduce the influence of

noise and interference. See the section dealing with these matters at the end of this chapter.

Coupling

Switch between AC coupling and DC coupling by toggling the softkey

AC/DC.

Fig. 3-3

AC coupling a symmetrical signal.

Use the AC coupling feature to eliminate un-

wanted DC signal components. Always use AC coupling when the AC signal is superimposed on a DC voltage that is higher than the trigger level setting range. However, we recommend AC coupling in many other measurement situations as well.

When you measure symmetrical signals, such as sine and square/triangle waves, AC cou- pling filters out all DC components. This means that a 0 V trigger level is always cen- tered around the middle of the signal wher

e triggering is most stable.

Fig. 3-4

Missing trigger events due to AC coupling

of signal with varying duty cycle.

Signals with changing duty cycle or with a

very low or high duty cycle do require DC coupling. Fig. 3-4 shows how pulses can be missed, while Fig. 3-5shows that triggering

does not occur at all because the signal ampli-

tude and the hysteresis band are not centered.

NOTE: For explanation of the hysteresis band, see

page 4-3.

Fig. 3-5

No triggering due to AC coupling of signal

with low duty cycle.

Filter

If you cannot obtain a stable reading, the signal-to-noise ratio (often designated S/N or SNR) might be too low, probably less than 6 to 10 dB. Then you should use a filter. Certain conditions call for special solutions like highpass, bandpass or notch filters, but usually the unwanted noise signals have higher frequency than the signal you are interested in. In that case you can utilize the built-in lowpass filters. There are both analog and digital filters, and they can also work together.

Fig. 3-6

The menu choices after selecting FILTER.

■ Analog Lowpass Filter

The counter has analog LP filters of RC type, one in each of the channels A and B, with a cutoff frequency of approximately 100 kHz, and a signal rejection of 20 dB at 1 MHz.

Accurate frequency measurements of noisy LF signals (up to 200 kHz) can be made when the noise components have significantly higher frequencies than the fundamental signal.

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3-3

Input Signaling Conditioning

■ Digital Lowpass Filter

The digital LP filter utilizes the Hold-Off function described below.

With trigger Hold-Off it is possible to insert a deadtime in the input trigger circuit. This means that the input of the counter ignores all hysteresis band crossings by the input signal during a preset time after the first trigger event.

When you set the Hold-Off time to approx. 75% of the cycle time of the signal, erroneous triggering is inhibited around the point where the input signal returns through the hysteresis band. When the signal reaches the trigger point of the next cycle, the set Hold-Off time has elapsed and a new and correct trigger will be initiated.

Instead of letting you calculate a suitable HoldOff time, t

he counter will do the job for you by converting the filter cutoff frequency you enter via the value input menu below to an equivalent Hold-Off time.

Fig. 3-7 Value input menu for setting the cutoff

frequency of the digital filter.

You should be aware of a few limitations to be

able to use the digital filter feature effectively and unambiguously. First you must have a rough idea of the frequency to be measured. A cutoff frequency that is too low might give a perfectly stable reading that is too low. In such a case, triggering occurs only on every 2nd, 3rd or 4th cycle. A cutoff frequency that is too

high (>2 times the input frequency) also leads

to a stable reading. Here one noise pulse is counted for each half-cycle.

Use an oscilloscope for verification if you are in doubt about the frequency and waveform of your input signal.

The cutoff frequency setting range is very wide: 1 Hz - 50 MHz

Fig. 3-8

Digital LP filter operates in the measuring

logic, not in the input amplifier.

Man/Auto

Toggle between manual and automatic triggering with this softkey. When

Auto is active the

counter automatically measures the peak-to-peak levels of the input signal and sets the trigger level to 50% of that value. The attenuation is also set automatically.

At rise/fall time measurements the trigger levels are automatically set to 10% and 90% of the peak values.

When

Manual is active the trigger level is set

in the value input menu designated

Trig. See

below. The current value can be read on the display before entering th

e menu.

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3-4

Input Signaling Conditioning

■ Speed

The Auto-function measures amplitude and calculates trigger level rapidly, but if you aim at higher measurement speed without having to sacrifice the benefits of automatic triggering, then use the

Auto Trig Low Freq func-

tion to set the lower frequency limit for voltage measuremen

If you know that the signal you are interested in always has a frequency higher than a certain value f

lo w , then you can enter this value

from a value input menu. The range for flo w is

1 Hz to 100 kHz, and the default value is 100

Hz. The higher value, the faster measurement speed due to more rapid trigger level voltage detection.

Even faster measurement speed can be reached by setting the trigger levels manually. See

Trig below.

Follow the instructions here to change the low-frequency limit:

Press SETTINGS  Misc  Auto

Trig Low Freq.

Use the UP/DOWN arrow keys or the numeric input keys to change the low frequency limit to be used during the trigger level calculation, (default 100 Hz).

Confirm your choice and leave the SET-

TINGS

menu by pressing EXIT/OK three

times.

Trig

Value input menu for entering the trigger level manually.

Use the

UP/DOWN arrow keys or the numeric

input keys to set the trigger level.

A blinking underscore indicates the cursor po-

sition where the next digit will appear. The

LEFT arrow key is used for correction, i.e.

deleting the position preceding the current cursor position.

Fig. 3-9

Value input menu for setting the trigger

level.

NOTE:

is probably easier to make small ad-

justments around a fixed value by using the

arrow

keys

for

incrementation

decrementation. Keep the keys depressed for faster response

NOTE:

Switching over from AUTO to MAN Trigger Level is automatic if you enter a trigger level manually.

■ Auto Once

Converting "Auto" to "Fixed"

The trigger levels used by the auto trigger can be frozen and turned into fixed trigger levels simply by toggling the

MAN/AUTO key. The

current calculated trigger level that is visible on the display under

Trig will be the new fixed

manual level. Subsequent measurements will be considerably faster since the signal levels are no longer monitored by the instrument. You should not use this method if the signal levels are unstable.

NOTE: You can use auto trigger on one input and fixed trigger levels on the other.

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3-5

Input Signaling Conditioning

How to Reduce or

Ignore Noise and Interference

Sensitive counter input circuits are of course also sensitive to noise. By matching the signal amplitude to the counter's input sensitivity, you reduce the risk of erroneous counts from noise and interference. These could otherwise ruin a measurement.

Fig. 3-10

Narrow hysteresis gives erroneous

triggering on noisy signals.

Fig. 3-11

Wide trigger hysteresis gives correct

triggering.

To ensure reliable measuring results, the coun-

ter has the following functions to reduce or eliminate the effect of noise:

10x input attenuator

Continuously variable trigger level

Continuously variable hysteresis for some functions

Analog low-pass noise suppression filter

Digital low-pass filter (Trigger Hold-Off)

To make reliable measurements possible on

very noisy signals, you may use several of the above features simultaneously.

Optimizing the input amplitude and the trigger level, using the attenuator and the trigger control, is independent of input frequency and useful over the entire frequency range. LP filters, on the other hand, function selectively over a limited frequency range.

Trigger Hysteresis

The signal needs to cross the 20 mV input hysteresis band before triggering occurs. This hysteresis prevents the input from self-oscillating and reduces its sensitivity to noise. Other names for trigger hysteresis are "trigger sensitivity" and "noise immunity". They explain the various characteristics of the hysteresis.

Fig. 3-12

Erroneous counts when noise passes

hysteresis window.

Fig. 3-10 and Fig. 3-12 show how spurious

signals can cause the input signal to cross the trigger or hysteresis window more than once perinputcycleandgiveerroneouscounts.

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3-6

Input Signaling Conditioning

Fig. 3-13

Trigger uncertainty due to noise.

Fig. 3-13 shows that less noise still affects the

trigger point by advancing or delaying it, but it does not cause erroneous counts. This trigger uncertainty is of particular importance when measuring low frequency signals, since the signal slew rate (in V/s) is low for LF signals. To reduce the trigger uncertainty, it is desirable to cross the hysteresis band as fast as possible.

Fig. 3-14

Low amplitude delays the trigger point

Fig. 3-14 shows that a high amplitude signal

passes the hysteresis faster than a low amplitude signal. For low frequency measurements where the trigger uncertainty is of importance, do not attenuate the signal too much, and set the sensitivity of the counter high.

In practice however, trigger errors caused by

erroneous counts (Fig. 3-10 and Fig. 3-12) are much more important and require just the opposite measures to be take

To avoid erroneous counting caused by spurious signals, you need to avoid excessive input signal amplitudes. This is particularly valid when measuring on high impedance circuitry and when using 1 M input impedance. Under these conditions, the cables easily pick up noise.

External attenuation and the internal 10x attenuator reduce the signal amplitude, including the noise, while the internal sensitivit

y control in the counter reduces the counter's sensitivity, including sensitivity to noise. Reduce excessive signal amplitudes with the 10x attenuator, or with an external coaxial attenuator, or a 10:1 probe.

How to use Trigger Level

Setting

For most frequency measurements, the optimal triggering is obtained by positioning the mean trigger level at mid amplitude, using either a narrow or a wide hysteresis band, depending on the signal characteristics.

Fig. 3-15

Timing error due to slew rate.

When measuring LF sine wave signals with little noise, you may want to measure with a high sensitivity (narrow hysteresis band) to reduce the trigger uncertainty. Triggering at or

3-7

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Input Signaling Conditioning

close to the middle of the signal leads to the smallest trigger (timing) error since the signal slope is steepest at the sine wave center, see Fig. 3-15.

When you have to avoid erroneous counts

due to noisy signals, see Fig. 3-12, expanding

the hysteresis window gives the best result if

you still center the window around the middle

of the input signal. The input signal

excursions beyond the hysteresis band should

be equally large.

■ Auto Trigger

For normal frequency measurements, i.e.

without arming, the Auto Trigger function

changes to Auto (Wide) Hysteresis, thus wid-

ening the hysteresis window to lie between

70 % and. 30 % of the peak-to-peak ampli-

tude. This is done with a successive approxi-

mation method, by which the signal's MIN.

and MAX. levels are identified, i.e., the

levels where triggering just stops. After this

MIN./MAX. probing, the counter sets the

trigger levels to the calculated values. The

default relative trigger levels are indicated by

70 % on Input A and 30 % on Input B. These

values can be manually adjusted between 50

% and 100 % on Input A and between 0 %

and 50 % on Input B. The signal, however, is

only applied to one channel.

Before each frequency measurement the counter repeats this signal probing to identify new MIN/MAX values. A prerequisite to enable AUTO triggering is therefore that the input signal is repetitive, i.e., >100 Hz (default). Another condition is that the signal amplitude does not change significantly after the measurement has started.

NOTE: AUTO trigger limits the maximum measuring

rate when an automatic test system makes many measurements per second. Here you can increase the measuring rate by switching off this probin

g if the signal

amplitude is constant. One single command

and the AUTO trigger function determines the trigger level once and enters it as a fixed trigger level.

■ Manual Trigger

Switching to Man Trig also means Narrow Hysteresis at the last Auto Level. Pressing

AUTOSET once starts a single automatic

trigger level calculation (Auto Once). This calculated valu

e, 50 % of the peak-to-peak amplitude, will be the new fixed trigger level, from which you can make manual adjustments if need be.

■ Harmonic Distortion

As rule of thumb, stable readings are free from noise or interference.

However, stable readings are not necessarily correct; harmonic distortion can cause erroneous yet stable readings.

Sine wave signals with much harmonic distortion, see Fig. 3-17, can be measured correctly by shifting the trigger point to a suitable level or by using continuously variable sensitivity, see Fig. 3-16. You can also use Trigger Hold-Off, in case the measurement result is not in line with your expectations.

Fig. 3-16

Variable sensitivity.

Fig. 3-17

Harmonic distortion.

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3-8

Chapter 4

Measuring Functions

Introduction to

This Chapter

This chapter describes the different

measuring functions of the counter. They have been grouped as follows:

Frequency measurements

— Frequency

— Period

— Ratio

— Burst frequency and PRF.

— FM

— AM

Time measurements

— Time interval.

— Pulse width.

— Duty factor.

— Rise/Fall time.

Phase measurements

Voltage measurements

— VMAX, VMIN.

— VPP.

Selecting Function

See also the front panel layout on page 2-3 to find the keys mentioned in this section together with short descriptions.

Press MEAS FUNC to open the main menu for

selecting measuring function. The two basic methods to select a specific function and its subsequent parameters are described on page 2-7.

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4-2

Measuring Functions

Frequency Measurements

FREQ A, B

The counter measures frequency between 0 Hz and 400 MHz on Input A and Input B. The Auto Trig function operates from 1 Hz (default

100 Hz) to 300 MHz. The Manual Trig covers the whole frequency range Frequencies above 100 Hz and up to 300 MHz are best measured using the Default Setup. See page 2-16. Then Freq A will be selected automatically. Other important automatic settings are AC Coupling, Auto

Trig and Meas Time 200 ms. See below for

an explanation. You are now ready to start

sing the most common function with a fair chance to get a result without further adjustments

Summary of Settings for Good

Frequency Measurements

—

AC Coupling, because possible DC offset is normally undesirable.

—

Auto Trig means Auto Hysteresis in this case, (comparable to AGC) because superimposed noise exceeding the normal narrow hysteresis window will be suppressed.

—

Meas Time 200 ms to get a reasonable tradeoff between measurement speed and

resolution.

Some of the settings made above by recalling the Default Setup can also be made by

activating the AUTOSET key. Pressing it once means:

—

Auto Trig. Note that this setting will be

made once only if Man Trig has been selected earlier.

Pressing AUTOSET twice within two seconds also adds the following setting:

— Meas Time 200 ms.

Fig. 4-1 Frequency is measured as the

inverse of the time between one trigger point and the next;

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4-3

Measuring Functions

FREQ C

BURST A, B, C

CNT-90/91(R)

With an optional prescaler the counter can measure up to 3, 8, 15 or 20 GHz on Input C. These RF inputs are fully automatic and no setup is required.

CNT-90XL

The four versions cover the frequency ranges 27, 40, 46 and 60 GHz by means of an automatic down-conversion technique described on page 4-12. Faster (manual) acquisition is an alternative if the measured frequency is fairly known. Then its nominal value can be entered via the keyboard as a fixed starting point for the acquisition process.

An additional feature is the possibility to measure signal power with high resolution.

RATIOA/B, B/A, C/A,

C/B

To find the ratio between two input frequencies, the counter counts the cycles on two channels simultaneously and divides the result on the primary channel by the result on the secondary channel.

Ratio can be measured between Input A and Input B, where either channel can be the primary or the secondary channel. Ratio can also be measured between Input C and Input A or between Input C and Input B. Here Input C is the primary channel.

Note that the resolution calculations are very different as compared to frequency measurements. See page 8-55 for details.

A burst signal as in Fig. 4-2 has a carrier wave (CW) frequency and a modulation frequency, also called the pulse repetition frequency (PRF), that switches the CW signal on and off.

Both the CW frequency, the PRF, and the number of cycles in a burst are measured without external arming signals and with or without selectable start arming delay. See Chapter 5 "Measurement Control" for a fundamental discussion of arming and arming delay.

The general frequency limitations for the re-

spective measuring channel also apply to

burst measurements. The minimum number of

cycles in a burst on Input A or Input B is 3

below 160 MHz and 6 between 160 MHz and

400 MHz (using Manual Trigger). Burst mea-

surements on Input C involve prescaling, so

the minimum number of cycles will be 3 x

prescaling factor. The 3 GHz option, for ex-

ample, has a prescaling factor of 16 and re-

quires at least 48 cycles in each burst.

The minimum burst duration is 40 ns below

and 80 ns above 160 MHz.

Triggering

Bursts with a PRF above 50 Hz can be measured with auto triggering on.

The out-of-sync error described under heading "Possible errors" on page 4-6 may occur more frequently when using A

uto Trigge

When PRF is below 50 Hz and when the gap between the bursts is very small, use manual triggering.

Always try using AUTOSET first. Then the

Auto Trigger and the Auto Sync functions in combination will give satisfactory results

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4-4

Measuring Functions

without further tweaking in most cases. Some-

times switching from AUTO to MANual trig- gering in the

INPUT A/B menus is enough to

get stable readings. The continually calculated trigger levels will then be fixed.

Input C has always automatic triggering and

AUTOSET only affects the burst synchroni-

zation.

Fig. 4-2

Burst signal.

Burst Measurements using

Manual Presetting

You can measure the frequency on Input A and Input B to 400 MHz and on Input C with limited specifications to the upper frequency limit of the prescaler with the internally synchronized BURST function as follows:

—

Select Freq Burst under the Freq menu

—

Select A, B, or C as measurement input.

—

Press SETTINGS and Burst. Select a Meas

Time that is shorter than the burst duration

minus two CW cycles.

If you do not know the approximate burst pa-

rameters of your signal, always start with a short measurement time and increase it gradually until the readout gets unstable.

—

Press Sync Delay and enter a value longer

than the burst duration and shorter than the inverse of the PRF. See Fig. 4-3.

—

Press Start Delay and enter a value longer than the transient part of the burst pulse.

—

Select Frequency Limit (160/400 MHz) if Input A or Input B is to be used.

Fig. 4-3

Set the sync delay so that it expires

in the gap between the bursts.

Use the low limit if possible to minimize the

number of cycles necessary to make a measurement.

— Press EXIT/OK to measure.

All relevant burst parameters can be read on the display simultaneously.

Fig. 4-4

Three time values must be set to

measure the correct part of a burst

■ Selecting Measurement Time

The measurement time must be shorter than the duration of the burst. If the measurement continues during part of the burst gap, no matter how small a period of time, then the measurement is ruined. Ch

oosing a measurement time that is too short is better since it only reduces the resolution. Making burst frequency measurements on short bursts means using short measurement times, giving a poorer resolution than normally achieved with the counter.

4-5

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Measuring Functions

■ How Does the Sync Delay Work?

The sync delay works as an internal start arming delay: it prevents the start of a new measurement until the set sync delay has expired. See Fig. 4-5.

After the set measurement time has started, the counter synchronizes the start of the measurement with the second trigger event in the burst. This means that the measurement does not start erroneously during the Burst Off du-ration or inside the burst.

Fig. 4-5

Measuring the frequency of the

carrier wave signal in a burst.

■ Possible Errors

Before the measurement has been synchronized with the burst signal, the first measurement(s) could start accidentally during the presence of a burst. If this would happen and if the remaining burst duration is shorter than the set measurement time, the readout of the first measurement will be wrong. However, after this first measurement, a properly set start-arming sync delay time will synchronize the next measurements.

In manually operated applications, this is not a problem. In automated test systems where the result of a single measurement sample must be reliable, at

least two measurements must be made, the first to synchronize the measurement and the second from which the measurement result can be read out.

Frequency

Modulated Signals

A frequency modulated signal is a carrier wave signal (CW frequency = f

) that changes

in frequency to values higher and lower than the frequency f0. It is the modulation signal that changes the frequency of the carrier wave.

The counter can measure:

f0 = Carrier frequency.

fmax = Maximum frequency.

fmin = Minimum frequency.

Δf = Frequency swing = fmax -f0.

Carrier Wave Frequency f

To determine the carrier wave frequency measure f

mean which is a close approximation

of f

Press STAT/PLOT to get an overview of all the statistical parameters.

Select the measurement time so that the counter measures an integral number of

modulation periods. This way the positive

frequency deviations will compensate the negative deviations during the measurement.

Example: If the modulation frequency is 50 Hz, the measurement time 200 ms will make the counter measure 10 complete modulation cycles.

If the modulation is non-continuous, like a voice signal, it is not possible to fully compensate positive deviations with

negative deviations.

Here, part of a

modulation

swing

may

remain

uncompensated for,

and lead to a

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Measuring Functions

measuring result that is too high or too low.

Fig. 4-6

Frequency modulation

In the worst case, exactly half a modulation

cycle would be uncompensated for, giving a maximum uncertainty of:

Δfmax

f0 - fmean =

tmeasuring x f

modulation

For very accurate measurements of the carrier

wave frequency f0, measure on the unmodulated signal if it is accessible.

■ Modulation Frequencies above 1 kHz

— Turn off SINGLE.

— Set a long measurement time that is an

even multiple of the inverse of the modulation frequency.

You

will obtain a good approximation when you select a long measurement time, for instance 10 s, and when the modu

lation fre-

quency is high, above 1000 Hz.

■ Low Modulation Frequencies

Press SETTINGS



STAT and make the No. of

samples

parameter as large as possible considering the maximum allowed measurement time. Press

STAT/PLOT and let the

counter calculate the mean value of the samples.

You will usually get

good results with 0.1 s

measurement time per sample and more than 30 samples (n 30). You can try out the opti- mal combination of sample size and measurement time for specific cases. It depends on the actual f0 and Δfmax.

Here the sampling frequency of the measurement (1/measurement time) is asynchronous with the modulation frequency. This leads to individual measurement results which are randomly higher and lower than f0. The statisti- cally averaged value of the frequency fmean approaches f0 when the number of averaged samples is sufficiently large.

When the counter measures instantaneous frequency values (when you select a very short measurement time), the RMS measurement uncertainty of the measured value of f0 is:

where n is the number of averaged samples of

max

—

Press SETTINGS



STAT and set No. of

samples to 1000 or more.

—

Press Meas Time and select a low value.

—

Press STAT/PLOT and watch fmax.

min

—

Press SETTINGS  STAT and set No. of

samples to 1000 or more.

—

Press Meas Time and select a low value.

—

Press STAT/PLOT and watch fmin.

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Measuring Functions

Δf

p-p

— Press SETTINGS



STAT and set No.of

samples to 1000 or more.

— Press

Meas Time and select a low value.

Press

STAT/PLOT and watch Δf

p-p

Δf

p - p

= f

max

- f

min

= 2 x Δf.

Errors in fmax, f min, and Δfp-p

A measurement time corresponding to 1/10cycle, or 36° of the modulation signal, leads to an error of approx. 1.5%.

Select the measurement time:

—

Fig. 4-7

Error when determining f

max

—

To be confident that the captured maximal fre-

quency really is fmax, you must select a sufficiently large number of samples, for instance n



1000.

AM Signals

The counter can usually measure both the carrier wave frequency and modulation frequency of AM signals. These measurements are much

like the burst measurements described earlier

in this manual.

Carrier Wave Frequency

The carrier wave (CW) is only continuously present in a narrow amplitude band in the middle of the signal if the modulation depth is high. If the sensitivity of the counter is too low, cycles will be lost, and the measurement ruined.

Fig. 4-8 Effects of different sensitivity when

measuring the CW Frequency of an AM signal.

To measure the CW frequency:

Enter the INPUT A menu.

Select a measurement time that gives you the resolution you want.

Turn on

Manual trigger.

Press

Trig level and enter 0 V trigger level

(press the numeric key 0 and EXIT/OK).

—

Select AC coupling.

—

Select 1x attenuation to get a narrow hysteresis band.

—

If the counter triggers on noise, widen the hysteresis band with the 'variable hysteresis' function, i.e. enter a trigger level >0 V but <V

P-Pmin. See Fig. 4-8.

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Modulating Frequency

The easiest way to measure the modulating frequency is after demodulation, for instance by means of a so-called RF-detector probe (also known as a demodulator probe, e.g. Pomona type 5815) used with AC-coupling of the input channel. If no suitable demodulator is available, use the

Freq Burst function to

measure the modulation frequency in the same way as when measuring Burst PRF.

Fig. 4-9

Measuring the modulating fre-

quency.

—

Press MEAS FUNC and select Freq

Burst A.

—

Press SETTINGS



BurstMeas Time

and enter a measurement time that is approximately 25 %

of the modulating

period.

—

Press Sync Delay and enter a value that

is approximately 75 %

of the

modulating period. See Fig. 4-3.

—

Press INPUT A and turn on Manual trigger.

—

Press Trig and enter a trigger level that makes the counter trigger according to Fig. 4-9.

Even though the main frequency reading may

now be unstable, the PRF value on the display will represent the modulating frequency.

Theory of

Measurement

Reciprocal Counting

Simple frequency counters count the number of input cycles during a preset gate time, for instance one second. This leads to a  1 input cycle count error that, at least for low-frequency measurements, is a major contribution to uncertainty.

However, the counters described here use a high-resolution, reciprocal counting technique, synchronizing the measurement start with the input signal. In this way an exact number of integral input cycles will be counted, thereby omitting the1

input cycle

error

Fig. 4-10

Synchronization of a

measurement.

After the start of the set measurement time,

the counter synchronizes the beginning of the actual gate time with the first trigger event (t

1) of the input signal. See also Fig. 4-10.

In the same way, the counter synchronizes the stop of the actual gate time with the input signal, after the set measurement time has elapsed. The multi-register counting technique allows you to simultaneously

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Measuring Functions

quency according to Mr. Hertz

measure the actual gate time (tg) and the

number of cycles (n) that occurred during this gate time.

Thereafter, the counter calculates the fre-

's definition:

The '9X' measures the gate time, tg, with a

resolution of 100 ps, independent of the measured frequency. Consequently the use of prescalers does not influence the quantization error. Therefore, the relative quantization error is: 100 ps/tg.

For a 1-second measurement time, this value is:

Except for very low frequencies, tg and the set

measurement time are nearly identical.

Sample-Hold

If the input signal disappears during the measurement, the counter will behave like a voltmeter with a sampl

e-and-hold feature and will

freeze the result of the previous measurement.

Time-Out

Mainly for GPIB use, you can manually select a fixed time-out in the menu reached by pressing

SETTINGSMiscTimeout. The range

of the fixed time-out is 10 ms to 1000 s, and the default setting is

Off.

Select a time that is longer than the cycle time of the lowest frequency you are going to measure; multiply the time by the prescaling factor of the input channel and enter that time as time-out.

When no triggering has occurred during the

time-out, the counter will show NO SIGNAL.

Measuring Speed

The set measurement time determines the measuring speed for those functions that utilize averaging -

Frequency and Period Avg.

For continuous signals,

Speed 

readings/s when Auto

trigger is on and can be increased to:

Speed 

readings/s

when Manual trigger is on, or via GPIB:

Speed 

readings/s

■ Average and Single Cycle

Measurements

To reduce the actual gate time or measuring aperture, the counters have very short meas

urement times a

nd a mode called

Single for

period measurements. The latter means that the counter measures during only one cycle of the input signal. In applications where the counter uses an input channel with a prescaler, the

Single measurement will last as

many cycles as the division factor. If you want to measure with a very short aperture, use an input with a low division factor.

Averaging is th

e normal mode for frequency and period measurements when you want to reach maximum resolution. There is always a tradeoff between time and precision,

however, so decide how many digits you

need and use as short a measurement time as possible to arrive at your objective.

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■ CNT-90/91(R): Prescaling May

Influence Measurement Time

Prescalers do affect the minimum measurement time, inasmuch as short bursts have to contain a minimum number of carrier wave periods. This number depends on the prescaling factor.

Fig. 4-11

Divide-by-16 Prescaler.

Fig. 4-11 shows the effect of the 3 GHz

prescaler. For 16 input cycles, the prescaler gives one square wave output cycle. When the counter uses a prescaler, it counts the number of prescaled output cycles, here f/16. The display shows the correct input frequency since the microcomputer compensates for the effect of the division factor d as follows:

Prescalers do not reduce resolution in recipro-

cal counters. The relative quantization error is

still:

See Table 4-1 to find the prescaling factors

used in different operating modes.

■ LF Signals

Signals below 100 Hz should be measured with manual triggering, unless the default setting (100 Hz) is changed. See page 2-13. The low limit can be set to 1 Hz, but the measurement process will be slowed down considerably if auto triggering is used in conjunction with very low frequencies.

Function

Prescaling

Factor

FREQ A/B (400 MHz)

BURST A/B (<160MHz)

BURST A/B (>160MHz

PERIOD A/B AVG (400 MHz)

PERIOD A/B SGL (400 MHz)

FREQ C (3 GHz)

FREQ C (8 GHz)

256

FREQ C (15 & 20 GHz)

128

All other functions

Table 4-1

Prescaling factors.

When measuring pulses with a low repetition

rate, for example a 0.1 Hz pulse with a non-prescaled function like PERIOD SGL, the measurement will require at least the duration of one cycle, that is 10 seconds, and at worst nearly 20 seconds. The worst case is when a trigger event took place just before the beginning of a measure

ment time (Fig. 4-12). Measuring the frequency of the same signal will take twice as long, since this function involves prescaling by a factor two.

Fig. 4-12

Measurement Time.

Even if you have chosen a short measurement

time, this measurement will require between 20 and 40 seconds (for this example).

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Measuring Functions

■

CNT-90/91(R): RF Signals

■ CNT-90XL: Microwave Conversion

As mentioned before, a prescaler in the

C-in-put divides the input frequency before it is counted by the normal digital counting logic. The division factor is called prescaler factor and can have different values depending on the prescaler type. The 3 GHz prescaler is designed for a prescaling factor of 16. This means that an input C frequency of, e.g., 1.024 GHz is transformed to 64 MHz.

Prescalers are designed for optimum perform

ance when measu

ring stable continuous RF. Most prescalers are inherently unstable and would self-oscillate without an input signal. To prevent a prescaler from oscillating, a "go-detector" is incorporated. See Fig. 4-13.

The go-detector continuously measures the level of the input signal and simply blocks the prescaler output when no signal, or a signal that is too weak, is present.

Fig. 4-13

Go-detector in the prescaler.

The presence of a burst signal to be measured

makes certain demands upon the signal itself. Regardless of the basic counter's ability to measure during very short measurement times, the burst duration must meet the following minimum conditions:

Burst

min

> (presc. factor) x (inp. cycle time) x 3

or at least 80 ns

Normally the real minimum limit is set by other factors, like the speed of the GO-detector. This speed depends on the specific input option used.

Measuring frequencies up to 20 GHz is possible with the top-performance prescaler option

14B. The general principles of prescalers are

described in the preceding paragraph.

The different versions of the CNT-90XL are intended for applications with upper frequency limits between 27 GHz and 60 GHz.

Here another technique is utilized, down conversion by means of mixing the unknown signal with known LO frequencies until there is a signal present within the passband of the IF amplifier, in this case 10 - 200 MHz.

A simplified block diagram can be seen in Fig. 4-14.

Fig. 4-14

Microwave acquisition in the

CNT-90XL.

The basic LO frequency range is 430 -

550

MHz and is divided into a number of discrete frequencies fetched from a look-up table. The LO output is fed to a comb generator that creates a harmonic spectrum covering the whole specified microwave range.

The automatic process of calculating the input frequency consists of the following steps:

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Measuring Functions

—

Preacquisition

The purpose of this process is to find out if there is a measurable signal present at the input, and if so, fix the LO frequency that gives rise to an IF signal above a certain threshold lev

el. This is done by sequentially stepping the LO from the highest value in the look-up table to the lowest value and applying the resulting comb generator spectrum to the mixer. The process is stopped when the signal detector outputs a status signal to the processor.

—

Acquisition

We don't know yet which harmonic generates the IF signal. First we measure the IF with the counter. Then we decrease the LO frequency by 1 MHz and measure the IF once more. If, for instance, the difference between the two values is 5 MHz, then we know that the fifth harmonic is the origin. By examining the sign of the difference, we can decide if the original IF should be added to or subtracted from the calculated harmonic in order to arrive at

the final value.

—

Final RF calculation

Now we know the LO frequency, the multiplication factor 'n' and the sign. What remains to be done is to count the IF during a measurement time corresponding to the desired resolution, and then the result is used for calculating the final value to be presented on the display as: f

x = n x f

± IF

There are a number of conditions that can

complicate the acquisition process. All of them are handled by measures taken by the instrument firmware. Two examples:

— One of the step frequencies produces an

IF but not its shifted value. Action: go to the next table value.

—

Frequency modulation causes an unstable

'n' value calculation. Action: increase the measuring time.

Power measurement

Another feature in this instrument is the ability to measure power with high resolution and moderate accuracy over the entire frequency range, achieved by storing individual frequency dependent power correction factors in a memory located inside the conversion unit. This memory is also used for storing other inf

ormation about t

he converter like identification data.

PERIOD

Single A, B & Avg. A, B, C

From a measuring point of view, the period function is identical to the frequency function. This is because the period o f a cyclic signal has the reciprocal value of the frequency (1/f).

In practice there are two minor differences.

1. The counter calculates FREQUENCY

(always AVG) as:

numberofcycles

f =

actual gate time

while it calculates PERIOD AVG as:

actualgatetime

p =

number of cycles

2. In the PERIOD SINGLE mode, the counter

uses no prescaler.

All other functions and features as described earlier under "Frequency" apply to Period measurements as well.

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Single A, B Back-to-Back

■ CNT-91(R) only

This function benefits from the basic time-stamping calculation method utilized by this series of counters to obtain consecutive measurement results without dead time.

Every positive or negative zero crossing (depending on the selected slope) up to the maximum frequency (125 kHz with interpolator calibration ON or 250 kHz with interpolator calibration OFF) is time-stamped. For every new time stamp the previous value is subtracted from the current value and displayed.

In VALUES mode the display is updated every new period if the period time exceeds 200 ms. For shorter times every second, third etc. result is displayed due to the limited updating rate.

In STATISTICS mode the graphs and statisti-cal data contain all periods up to the maxi-mum input frequency (see above). For higher frequencies the average period time during the 4 or 8 µs observation time is displayed. So, for higher frequencies the actual function is rather Period Average Back-

to-Back.

The main purpose of this function is to make continuous measurements of relatively long period times without losing single periods due to result processing. A typical example is the 1-pps timebase output from GPS receivers.

Frequency A, B Back-to-Back

■ CNT-91(R) only

This is the inverse function of Period Back-to-Back. In

STATISTICS mode mea-

surement time is used for pacing the time

stamps. The pacing parameter is not used in this case.

Thus a series of consecutive frequency aver-

age measurements without dead time can be made in order to fulfil the requirements for correct calculation of Allan variance or deviation. These statististical measures are, for instance, widely used by oscillator manufacturers to describe short-term stability.

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Time Measurements

Introduction

Measuring the time between a start and a stop condition on two separate channels is the basis for all time interval measurements. In addition to the fundamental function

Time Interval A to

the counters also offer other channel

combinations and derived functions like

Pulse

Width

and Rise/Fall Time.

Fig. 4-15

Time is measured between the

trigger point and the reset point. Accurate measurements are possible only if the hysteresis band is narrow.

Triggering

The set trigger level and trigger slope define the start and stop triggering.

If Auto is on, the counter sets the trigger level to 50% of the signal amplitude, which is ideal for most time measurements.

■

Summary of Conditions for Reliable

Time Measurements:

—

Auto Once, that is freezing the levels de-

termined by

Auto Trig, is normally the

best choice when making time measurements. Choose

Man Trig and press

AUTOSET once.

—

DC coupling.

—

1x Attenuation. Selected automatically if AUTOSET was used before to set the

trigger levels.

—

High signal level.

—

Steep signal edges.

Even though the input amplifiers have high

sensitivity, the hysteresis band has a finite value that would introduce a small timing error for signals with different rise and fall times, for instance asymmetrical pulse signals like the one in Fig. 4-15. This timing error is taken care of by using hysteresis compensation

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Measuring Functions

that virtually moves the trigger points by half

the hysteresis band.

Time Interval

All time interval functions can be found under the function menu

Time.

The toggling SLOPE keys (marked with a

positive

or negative

edge symbol)

under the menus INPUT A/B decide which edge

of the signal will start resp. stop the measurement.

Time Interval A to B

The counter measures the time between a start condition on input A and a stop condition on input B.

Time Interval B to A

The counter measures the time between a start condition on input B and a stop condition on input A.

Time Interval A to A, B to B

When the same (common) signal source supplies both start and stop trigger events, connect the signal to either input A or input B.

These functions can be used for measuring rise and fall times between arbitrary trigger levels.

CNT-91(R): Time Interval Error (TIE)

This function can be found under the function menu Time and is only applicable to clock signals, not data signals.

TIE measurement uses continuous time-stamping to observe slow phase shifts (wander) in nominally stable signals during extended periods of time. Monitoring distributed PLL clocks in synchronous data transmission systems is a typical application.

The frequency of the signal to be checked can be either manually or automatically set. Auto detects the frequency from the first two samples. The value is rounded to four digits, e.g.

2.048 MHz and is output on the bus when a query is sent. It is also displayed as an auxiliary parameter in VALUE mode.

TIE is measured as the time interval between the input signal and the internal or external timebase clock. These signals are not phase-locked, so irrespective of the real time interval value at the start of a measurement, the result at t = 0 is mathematically nulled. Thus the graphic representation in STATIS-TICS mode starts at the origin of coordinates.

Rise/Fall Time A/B

These functions can be found under the function menu Time.

Rise and fall time can be measured on both input A and input B.

By convention, rise/fall time measurements are made with the trigger levels set to 10 % (start) and 90 % (stop) of the maximum pulse amplitude, see Figure 4-16.

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Measuring Functions

The counter measures the time from when the signal passes 10 % of its amplitude to when it passes 90 % of its amplitude. The trigger levels are calculated and set automatically.

Auxiliary parameters shown simultaneously are Slew Rate (V/s), Vmax and Vmin

Fig. 4-16

Trigger levels for rise/fall mea-

surements.

For ECL circuits, the reference levels are 20

% (start) and 80 % (stop). In this case you can

use either of two methods:

1. Select the general Time Interval function described above and set the trigger levels manually after calculating them from the absolute peak values. Then you can benefit from the auxiliary parameters Vmax and Vmin. For measurements made on input A, use the following settings:

Rise Time:

Trig Level A = V

min +0.2(Vmax - Vmin)

Trig Level B = Vmin +0.8(Vmax - Vmin)

Fall Time:

Trig Level A = Vmin +0.8(Vmax -

Vmin)

Trig Level B = Vmin +0.2(V

max

- V

min

)

2. Select one of the dedicated Rise/Fall Time functions, and exploit the possibility to manually adjust the re

lative trigger levels (in %) when Auto Trigger is active. Both input channel menus are used for entering the levels, but only one channel is the active signal input.

See the paragraph on Auto Trigger (page 4-19)

to find out how overshoot or ringing may 71Bfect your measurement.

Pulse Width A/B

The function menu designation is Pulse. Either input A or input B can be used for measuring, and both po

sitive and negative pulse

width can be selected.

—

Positive pulse width means the time be-

tween a rising edge and the next falling edge.

—

Negative pulse width means the time between a falling edge and the next rising edge.

The selected trigger slope is the start trigger

slope. The counter automatically selects the inverse polarity as stop slope.

Duty Factor A/B

The function menu designation is Pulse. Either input A or input B can be used for measuring, and both positive and negative duty factor can be selected. See the preceding paragraph for a definition of positive and negative in this context.

Duty factor (or duty cycle) is the ratio between

pulse width and period time. The counter determines this ratio by first making a pulse width measurement, then a period measurement, and calculates the duty factor as:

Pulsewidth

Duty factor =

Period

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The total measurement time will be doubled compared to a single measurement, because "Duty" requires 2 measurement steps.

Measurement Errors

Hysteresis

The trigger hysteresis, among other things, causes measuring errors, see Figure 4-17. Actual triggering does not occur when the input signal crosses the trigger level at 50 percent of the amplitude, but when the input signal has crossed the entire hysteresis band.

Fig. 4-17

Trigger hysteresis

The hysteresis band is about 20 mV with at-

tenuation 1x, and 200 mV with attenuation

10x.

To keep this hysteresis trigger error low, the

attenuator setting should be 1x when possible. Use the

10x position only when input signals

have excessively large amplitudes, or when you need to set trigger levels higher than 5 V.

Overdrive and Pulse

Rounding

Additional timing errors may be caused by triggering with insufficient overdrive, see Figure 4-18. When triggering occurs too close to the maximum voltage of a pulse, two phenomena may influence your measurement uncertainty: overdrive and rounding.

Insufficient overdrive causes

Trigger Error.

Overdrive: When the input signal crosses the

hysteresis band with only a marginal overdrive, triggering may take some 100 ps longer than usual. The specified worst case 500 ps systematic trigger error includes this error, but you can avoid it by having adequate overdrive.

Rounding: Very fast pulses may suffer from

pulse rounding, overshoot, or other aberrations. Pulse rounding can cause significant trigger errors, particularly when measuring on

fast circuitry.

Fig. 4-18

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Measuring Functions

Auto Trigger

Auto Trigger is a great help especially when you measure on unknown signals. However, overshoot and ringing may cause

Auto to

choose slightly wrong MIN and MAX signal levels. This does not affect measurements like frequency, but transition time measurements may be affected.

Therefore, when working with known signals such as logic circuitry, set the trigger levels manually.

Always use manual trigger levels if the signal repetition rate is > 300 MHz or drops below 100 H

z (default), or below the low frequency limit set by entering a value between 1 Hz and 50 kHz in the menu

Auto Trig Low Freq. You

can reach it by pressing

SETTINGSMisc.

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Phase

What is Phase?

channel B are enough to calculate the result,

including sign.

Phase is the time difference between two signals of the same frequency, expressed as an angle.

Resolution

Fig. 4-19

Phase delay.

The traditional method to measure phase de-

lay with a timer/counter is a two-step process consisting of two consecutive measurements, first a period measurement and immediately after that a time interval measurement. The phase delay is then mathematically calculated as:

360°x (Time Interval A- B )

Period

or in other words:

Phase A -B = 360°x Time Delay x FREQ

A somewhat more elaborate method is used in these counters. It allows the necessary meas

urements to be pe

rformed in one pass by using time-stamping. Two consecutive time-stamps from trigger events on channel A and two corresponding time-stamps from

Fig. 4-20

Traditional phase definition.

The frequency range for phase is up to 160

MHz and the resolution depends on the frequency. For frequencies below 100 kHz the resolution is 0.001° and for frequencies above 10 MHz it is 1°. It can be further improved by averaging through the built-in statistics functions.

Possible Errors

Phase can be measured on input signal frequencies up to 160 MHz. However, at these very high frequencies the phase resolution is reduced to:

100ps x 360° x FREQ

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Measuring Functions

Inaccuracies

The inaccuracy of Phase A-B measurements depends on several external parameters:

—

Input signal frequency

—

Peak amplitude and slew rate for input signals A and B

—

Input signal S/N-ratio

Some internal parameters are also important:

— Internal time delay between channel A

and B signal paths

— Variations in the hysteresis window be-

tween channel A and B

Let us look deeper into the restrictions and possibilities of using phase measurements.

Inaccuracy: The measurement errors are of two kinds:

— Random errors

— Systematic errors

The random errors consist of resolution (quantization) and noise trigger error.

Systematic errors consist of "inter-channel delay difference" and "trigger level timing" errors. Systematic errors are constant for a given set of input signals, and in general, you can compensate for them in the controller (GPIB-systems) or locally via the

MATH/LIM

menu (manual operation) after making calibration measurements. See Methods of Com- pensation on page 4-23.

■ Random Errors

The phase quantization error algorithm is:

100 ps x FREQ x 360°

For example, the quantization error for a 1 MHz input signal is thus:

100 ps x 1 x 10

x 360°  0.04°

The trigger noise error consists of start and

stop trigger errors that should be added. For sinusoidal input signals each error is:

Let's use the example above and add some

noise so that the S/N ratio will be 40 dB. This corresponds to an amplitude ratio of 100 times (and power ratio of 10000 times). Then the trigger noise will contribute to the random error with:

The sum of random errors should not be

added linearly, but in an "RMS way", because of their random nature. Let's do so for our examples above.

Random error =

The total random errors are thus:

(single-shot)

What about random errors caused by internal

amplifier noise? Internal noise contribution is normally negligible. The phase error caused by noise on the signal, whether internal or external, is:

For an input signal of 250 mV

rms

and the typi-

cal internal noise figure of 250 V

rms gives us

a S/N-ratio of a minimum of 60 dB (1000 times). This gives us a worst case error of

0.06°. Increasing the input signal to 1.5 V

rms

decreases the error to 0.01°.

Another way to decrease random errors is to use the statistics features of the instrument

4-21

USER MANUAL

● CNT 9x Series ● Rev.20 December 2017

Measuring Functions

and calculate the mean value from a number

of samples.

■ Systematic Errors in Phase

Measurements

Systematic errors consist o f 3 elements:

— Inter-channel

propagation

delay

difference.

— Trigger level timing errors (start and

stop), due to trigger level uncertainty.

The inter-channel propagation delay differ-

ence is typically 500 ps at identical trigger

conditions in both input channels. Therefore, the corresponding Phase difference is:

<0.5 ns x360°x FREQ

See the following table.

160 MHz

28.8°

100 MHz

18.0°

10MHz

1.8°

1MHz

0.18°

100 kHz

0.018°

10 kHz and below

0.002°

Table 4-2

Phase difference caused by

inter-channel propagation delay difference

Trigger level timing error

The "trigger level timing error" is depending on two factors:

— The actual trigger point is not exactly

zero, due to trigger level DAC uncertainty and comparator offset error.

— The two signals have different slew rates

at the zero-crossing.

Every counter has input hysteresis. This is necessary to prevent noise to cause erroneous input triggering. The width of the hysteresis band determines the maximum sensitivity of the counter. It is approximately 30 mV, so when you set a trigger level of 0 V, the actual trigger point would normally be +15 mV and

the recovery point -15 mV. This kind of tim-

ing error is cancelled out by using hysteresis compensation.

Hysteresis compensation means that the microcomputer c

an offset the trigger level so that actual triggering (after offset) equals the set trigger level (before offset). This general hysteresis compensation is active in phase as well as in time interval and rise/fall time measurements. There is a certain residual uncertainty of a few mV and there is also a certain temperature drift of the trigger point.

The nominal trigger point is 0 V with a

n un-

certainty of ± 10 mV.

A sine wave expressed as V(t) = VPx sin (2π

ft), has a slew rate

of VPx 2πf close to

the zero-crosssing. That gives us the

systematic time error when crossing 10 mV,

instead of crossing 0 mV.

And the corresponding phase error in

degrees is:

which can be reduced to:

This error can occur on both inputs, so the

worst case systematic error is thus:

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4-22

Measuring Functions

Vpeak (A)

Vpeak (B)

Worst case systematic error

150 mV

4°+4°=8°

1.5 V

150 mV

0.4°+4° = 4.4°

1.5 V

1.5V

0.4°+0.4°=0.8°

Table 4-3

Systematic trigger level timing

error (examples).

Methods of Compensation

The calculations above show the typical uncertainties in the constituents that make up the total systematic error. For a given set of input signals you can compensate for this error more or less completely by making calibration measurements. Depending on the acceptable residual error, you can use one of the methods described below. The first one is very simple but does not take the inter-channel propagation delay difference into account. The second one includes all systematic errors, if it is carried out meticulously, but it is often not practicable.

Common

settings for the two inputs are:

Slope: Coupling: Impedance:

Pos or Neg AC

1 Mor 50depending on source and frequency

Trigger: Trigger Level: Filter:

Man

0 V Off

Method 1:

Connect the test signals to Input A

and Input

B. Select the function Phase A rel A to find the initial error. Use the MATH/LIM menu to enter this value as the constant L in the formula K*X+L by pressing X0 and change sign. Now the current measurement result (X0) will be subtracted from the future phase measurements made by selecting Phase A rel B.A considerable part of the systematic phase errors will thus be cancelled out. Note that this

calibration has to be repeated if the

frequency or the amplitude changes.

Method 2:

Connect one of the signals to be measured to both Input A and Input B via a 50  power splitter or a BNC T-piece, depending on the source impedance. Make sure the cable lengths between power splitter / T-piece and instrument inputs are equal. Select the function

Phase A rel B and read the result. Enter

this value as a correction factor in the same way as described above for Method 1.

In order to minimize the errors you should also maintain

the signal amplitudes at the inputs, so that the deviation between calibration and measurement is kept as small as possible.

The same restrictions as for Method 1 regarding frequency and amplitude apply to this method, i.e. you should recalibrate whenever one of these signal parameters changes.

Residual Systematic Error:

By mathematically (on the bench or in the controller) applying corrections according to one of the methods mentioned above, the systematic error will be reduced, but not fully eliminated. The residual time delay error will most probably be negligible, but a trigger level erro

r will always remain to a certain extent, especially if the temperature conditions are not constant.

USER MANUAL ● CNT 9x

Series ● Rev.20 December 2017

4-23

Measuring Functions

Totalize

[CNT-91(R) only]

Totalize in General

The Totalize functions add up the number of trigger events on the two counter inputs A and B. Several combinations of them are

theoretically possible. Five have been realized

and made available by entering the Totalize menu.

In addition to controlling the gate manually by toggling

HOLD/RUN you can also open

and close the gate by using the arming facilities under

SETTINGS. The different

functions are described below.

The display is updated continually while the gate is open. Events are accumulated during consecutive open periods until

RESTART is

pressed.

The manual Totalize functions can not be used in conjunction with the Statistics functions and parameters like block and pacing. Nor does Auto Trigger work the normal way. An Auto Once is performed instead before the start of a measurement in order to calculate suitable trigger levels once.

TOT A MAN

This mode enables you to totalize (count) the number of trigger events on Channel A. Aux- iliary calculated parameters are

A-B

and A/B. Start/Stop is manually controlled by toggling the key

HOLD/RUN, and the

counting regis- ters are reset by pressing

RESTART.

TOT B MAN

This mode enables you to totalize (count) the number of trigger events on Channel B. Aux- iliary calculated parameters are A-B and A/B. Start/Stop is manually controlled by toggling the key

HOLD/RUN, and the

counting regis- ters are reset by pressing

RESTART.

TOT A+B MAN

This mode enables you to calculate the sum of trigger events on Channel A and Channel B. Auxiliary parameters are

A and B.

Start/Stop is manually controlled by toggling the key HOLD/RUN, and the counting registers are reset by pressing

RESTART.

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4-24

Measuring Functions

TOT A-B MAN

This mode enables you to calculate the difference between trigger events on Channel A and Channel B. Auxiliary parameters are A and B. Start/Stop is manually controlled by toggling the key HOLD/RUN, and the counting registers are reset by pressing

RESTART.

Applications

The TOT A-B MAN makes it possible, for instance, to make differential flow measurements in control systems.

Example: the number of cars in a parking lot equals the number of cars passing the entrance (A) gate, minus the ones passing the exit (B) gate.

TOT A/B MAN

This mode enables you to calculate the ratio of trigger events on Channel A and Channel B. Auxiliary parameters are A and B. Start/Stop is manually controlled by toggling the key HOLD/RUN, and the counting registers are reset by pressing RESTART

Totalize & Arming

By using Arming together with Totalize you can open and close the gate with an external signal applied to one of the channels A, B or

E. In this way you can realize a host of func-

tions like TOT A START/STOP by B, TOT

A-B Gated by E and TOT B Timed by A,

simply by selecting channel, slope and delay time for Start/Stop.

Unlike the manual Totalize functions, the

armed variants resemble the other measurement functions inasmuch as they allow block and pacing control. Consequently all the Sta- tistics functions are available. A new result is displayed after each stop condition.

Examples

The comprehensive Arming features can be found under

SETTINGS  Arm.

In order to set up the Totalize functions above, do the following:

■ TOT A START/STOP by B

—Select Totalize from the MEAS FUNC

menu and then A.

— Connect the signal to be measured to

Input A.

— Set the trigger level for Input A

manually to a suitable value.

—

Connect the control signal to Input B.

—

Set the trigger level for Input B manually to a suitable value.

— Go to the Arming menu (SETTINGS 

Arm)

and set the seven parameters:

— Arm on Sample/Block

Decide if each event or each block of events

(STATISTICS mode) shoul

d be

armed.

— Start Channel

Select B.

— Start Slope

Select POS (marked by a rising edge symbol).

— Start Delay

Decide if you need to insert a delay (10 ns - 2 s) between the control signal and the actual opening of the gate.

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USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Measuring Functions

—

Stop Delay

Decide if you need to insert a delay (10 ns - 2 s) during which the gate will not respond to the control signal on the Stop Channel. The main application is to prevent relay contact bounces from closing the gate prematurely.

—

Stop Channel

Select B.

—

Stop Slope

Select POS (marked by a rising edge

symbol).

■ TOT A-B Gated by E

—Select Totalize from the MEAS FUNC

menu and then A-B.

— Connect the signals to be measured to In-

puts A and B.

— Set the trigger levels for Inputs A and B

manually to suitable values.

—

Connect the control signal (TTL levels) to

Input E.

— Go to the Arming menu (SETTINGS 

Arm) and set the seven parameters:

— Arm on Sample/Block

Decide if each event or each block of events (STATISTICS mode) should be armed.

—

Start Channel

Select

—

Start Slope

Select POS (marked by a rising edge

symbol).

—

Start Delay

Decide if you need to insert a delay (10 ns - 2 s) between the control signal and the actual opening of the gate.

— Stop Delay

Decide if you need to insert a delay (10

ns - 2 s) during which the gate will not respond to the control signal on the Stop Channel. The main application is to prevent relay contact bounces from closing the gate prematurely.

— Stop Channel

Select E.

—

Stop Slope

Select NEG (marked by a falling edge symbol).

■ TOT B Timed by A

—

Select Totalize from the MEAS FUNC

menu and then B.

—

Connect the signal to be measured to Input B.

—

Set the trigger level for Input B manually to a suitable value.

—

Connect the control signal to Input A.

—

Set the trigger level for Input A manually to a suitable value.

—

Go to the Arming menu (SETTINGS



Arm)

and set the six parameters:

— Arm on Sample/Block

Decide if each event or each block of events

(STATISTICS mode) should be armed.

— Start Channel

Select

— Start Slope

Select POS (marked by a rising edge symbol).

— Start Delay

Decide if you need to insert a delay (10 ns - 2 s) between the control signal and the actual opening of the gate.

— Stop Delay

Set the measurement time between 10 ns and 2 s.

— Stop Channel

Select Time.

With this function you can synchronize the start of an accurate gate time to an external event.

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4-26

Measuring Functions

Voltage

MAX

, Vmin, V

Press MEAS FUNC  Volt. The counter can measure the input voltage levels V

MAX , VMIN

and VPP on DC-input voltages and on repetitive signals between 1 Hz and 400 MHz.

Fig. 4-21

The default low frequency limit is 20 Hz but can be changed via the SETTINGS



The voltage is determined by making a series of trigger leve settings and sensing when the counter triggers.

Miscellaneous menu between 1 Hz and 50

kHz. A higher low-frequency limit means faster measurements.

The voltage capacity is -50 V to +50 V in two automatically selected ranges.

For LF signals the measurement has "voltmeter performance", i.e. an accuracy of about 1 % of the reading.

You can select any one of the parameters to be the main parameter that is displayed in large digits and with full resolution, while the others are displayed simultaneously at the bottom of the display in smaller characters.

4-27

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Measuring Functions

RMS

When the waveform (e.g. sinusoidal, triangular, square) of the input signal is known, its crest factor, defined as the quotient (QCF) of the peak (V

p) and RMS (Vrms) values, can be

used to set the constant K in the mathematical function K*X+L. The display will then show the actual Vrms value of the input signal, assuming that Vpp is the main parameter.

EXAMPLE: A sine wave has a crest factor of

1.414 ( ), so the constant in the formula above will be 0.354.

Press MATH/LIM and after that Math→ Math(Off) →K*X+L Press K= and enter

0.354 via the NUMERIC ENTRY keys.

Check that the L constant is set to its default setting 0. Confirm your choices with the softkeys below the display. If the input is AC coupled and V

selected, the display will now show the RMS value of any sine wave input.

If the sine wave is superimposed on a DC

voltage, the RMS value is found as: 0.354*Vpp + VDC. If VDC is not known it can be found as:

To display the rms value of a sine wave super-

imposed on a DC voltage, follow the example above, but set L = VDC.

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4-28

√2

Pulsed

Signals

[CNT-90 XL option 28 only]

NTRO DUCTI ON

The option 28, named Pulsed RF option, allows the CNT-90X L counter to measure pulsed RF signal characteristics. A pulsed signal can be defined by some parameters, as described below.

Frequency in pulse (FP): also known as carrier frequency or frequency in burst is the signal frequency within the pulse.

Pulse Width (PW): it is the pulse duration.

Pulse Repetition Interval (PRI): it is the pulse period of repetition.

Pulse Repetition Frequency (PRF): it is the reciprocal of the PRI.

Pulse Power level (Pon) is the signal level and is showed in dBm.

ETTING S

To be able to measure pulsed signal main characteristics, the CNT-90XL option 28 has a special set of measurement settings.

Start Dela y: internal signals may need some time to be properly

established. Start delay is useful to skip initial signal transie nt response to make sure that measurement starts when signal is stable.

Measurement time is the time during which the measurement is

actually done. NOTE that measurement time must be shorter than the pulse width with some margin.

Pulse Sensitivity setting ; High/Medium/Low, corresponds to the

trigger level selected for the internal pulse envelope detector signal. This setting depends on the RF signal power level.

RECOMME NDE D

ETTING S

In most cases, best values to use for start delay and

measurement

time are respectively 10% and 80% of pulse width.

NOTE: Measurement time should end min. 40 ns before end of pulse! Example: For a 100ns pulse, the recommended start delay is 10 ns and Meas. Time is 50ns

Sensitivity

Pon<

-8dBm

HIGH

sensitivity

-8dBm < Pon<

-3dBm

MEDIUM

sensitivity

Pon>

-3dBm

LOW

sensiti vity

settings:

NEED ED

ETTING S

Depending on selected measurement, some parameters need to be properly defined to get a correct result. The following table summarizes needed settings for each measurement function.

Manual Acq & Center freq.

Start Delay

Measurement Time

Sensitivity

Frequency in Pulse X X X X

Pulse Width X X

Pulse repetition interval X X

Power in pulse X X X

4-29

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

ELECTINGPULSED RF

ME ASUREMEN TS

If Option 28 is installed in CNT-90XL, the Pulsed RF measurements are found by pressing the MEAS key and selecting Pulsed RF:

The various Pulsed RF parameters are thereafter displayed:

Frequency in pulse

Repetition (PRI and PRF)

Width (Pulse Width and Duty Factor)

Power On (the power in Pulse during On time)

REQUENCY IN PULSE

Select Frequency measurement via the menu

MEAS

→

Pulsed RF

→

Freq in Pulse

→

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

(this function is also available via the

- MEAS

→

Repetition

→

PRI →C menu).

Set corresponding masureent settings in the following menus:

SETTINGS → Pulsed RF

4-29

SETTINGS  P ulsed RF  Start Delay

SETTINGS  P ulsed RF  Meas Time

SETTINGSPulsed RFSensitivity

And finally select Manual acquisition with a center frequency that is close to expected frequency (within 500 MHz)

- SETTINGS

→

Misc → Input C Acq → Center frequency

PRI

Select Pulse Repetition Interval measurement via the

- MEAS →Pulsed RF →Repetition →PRI →C.

Set corresponding measurement settings in the following menus:

(this function is also available via the

- MEAS

→

Frequency in Pulse →Freq in Burst →C menu).

Set corresponding masureent settings in the following menus:

SETTINGS → Pulsed RF

And finally select Manual acquisition with a center frequency that is

- SETTINGS

→

Misc

→

Input C Acq

→

Center frequency

- SETTINGS → Pulsed RF → Sensitivity

PRF

Select Pulse Repetition Frequency measurement via th e

- MEAS  Pulsed RF  Repetition  PRF  C.

Set corresponding measurement settings in the following menus:

SETTINGSPulsed RFSensitivity.

- SETTINGS  Misc  Input C Acq  Center frequency

ULSE WIDTH POS

Select Positive Pulse Width measurement via the

- MEAS →Pulsed RF →Width →Pos →C.

Set corresponding measurement settings in the following menus:

SETTINGS →Pulsed RF →Sensitivity.

SETTINGS →Misc →Input C Acq →Center frequency

ULSE WIDTH NEG

Select Negative Pulse Width measurement via the

- MEAS

→

Pulsed RF

→

Width

→

Neg

→

This measure corresponds to pulse off width, or the time between pulses.

Set corresponding measurement settings in the following menus:

- SETTINGS  Pulsed RF  Sensitivity.

close to expected frequency (within 500 MHz)

SETTINGSMiscInput C AcqCenter frequency

DUTY

ACTO R POS

Select Positive duty factor measurement via the

- MEAS  Pulsed RF  Width  Duty Factor Pos  C.

Set corresponding measurement settings in the following menu s:

- SETTINGS  Pulsed RF  Sensitivity.

- SETTINGS



MiscInput C AcqCenter frequency

Positive Duty Factor corresponds to the pulse width divided by the Pulse Repetition Interval.

DUTY

ACTO R NEG

Select Negative duty factor measurement via the

- MEAS  Pulsed RF  Width  Duty Factor Neg  C.

Set corresponding measurement settings in the following menu s:

- SETTINGS  Pulsed RF  Sensitivity.

- SETTINGS



MiscInput C AcqCenter frequency

The Negative Duty Factor corresponds to the negative pulse width divided by the Pulse Repetition Interval.

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USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

OWER ON

CNT-90XL option 28 is also able to measure the power within pulses by selecting the function in the appropriate menu:

Set corresponding measurement settings in the following menus:

- SETTINGS



Pulsed RFStart Delay.

- SETTINGS  P ulsed RF  Measurement Time.

SETTINGS  Misc  Input C Acq  Center frequency

- MEAS → Pulsed RF → Power ON → C

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

4-29

Chapter 5

Measurement

Control

Measurement Control

The exceptions are Frequency and Period

About This Chapter

Average.

This chapter explains how you can control the start and stop of measurements and what you can obtain by doing so. The chapter starts by explaining the keys and the functions behind them, then gives some theory, and ends with actual measurement examples.

Measurement Time

This parameter is only applicable to the functions

Frequency and Period Average.

Increasing the measurement time gives more digits, i.e. higher resolution, but fewer meas

urements per second. T

he default value is

200 ms but can be changed via

SETTINGS



Meas Time

between 20 ns and 1000 s.

The default value gives 11 digits on the display and 4 to 5 measurem

ents each second.

Varying the measurement time is a

hardware-based averaging method in contrast

to the software-based mean value function that can be found in the STAT/PLOT menu.

The measurement time changes in 1/2/5 steps

if you use the arrow keys for stepping. By using the numeric entry keys you can set any value within the specified range with a resolution of 10 ns.

To quickly select the lowest measurement time, enter 0. The counter will select 20 ns automatically.

Gate Indicator

The GATE LED is on when the counter is busy counting input cycles.

Single Measurements

SINGLE is implicitly the normal measure-

ment mode, which means that the counter shows the results from a single input cycle.

Single or Average is not relevant for V

max

Vmin or Vpp measurements.

Hold/Run & Restart

Pressing HOLD completes the current measurement and freezes the result on the display.

Pressing

RESTART initiates a new

measurement.

If you are performing a statistics measurement and press HOLD, the pending sample will be finished. Then the measurement will stop, and you can, for instance, watch the graphic representation of the samples taken so far.

Pressing RESTART starts a new measurement from sample 1, and the measurement will stop when the preset number of samples has been taken.

Arming

Arming gives you the opportunity to start and stop a measurement when an external qualifier event occurs.

Start and stop of the arming function can independently be set to positive slope, negative slope, or it can be turned off. A delay between 10 ns and 2 s can be applied to the start arming channel to facilitate certain measurements. The resolution is 10 ns.

Input E on the rear panel is the normal arming input, but also input A and input B can be used. The frequency range for input E is 80 MHz, whereas it is 160 MHz for the other inputs.

All the versatile arming functions can be reached under

SETTINGS  Arm.

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5-2

Measurement Control

Arming is somewhat complicated yet gives

the flexibility to perform a measurement on a specific portion of a complex signal, like a frequency measurement on the color burst contained in a composite video signal.

Other examples of arming can be found later in this chapter, starting on page 5-9.

Start

Arming

Start arming acts like an External Trigger on an oscilloscope. It allows the start of the ac- tual measurement to be synchronized to an external trigger event.

In a complex signal, you may want to select a certain part to perform measurements on. For this purpose, there is an arming delay function, which delays the actual start of measurement with respect to the arming pulse, similar to a "delayed timebase" in an oscilloscope.

You can choose to

delay start arming by a

preset time.

Start arming can be used for all functions except Frequency Burst, Ratio and Volt. If you use start arming to arm an average measurement, it only con

trols the start of the first

sample.

Stop Arming

Stop arming prevents the stop of a measurement until the counter detects a level shift on the arming input. Combining Start and Stop Arming results in an "external gate" function which determines the duration of the measurement.

Stop arming can be used for all functions except

Frequency Burst, Ratio, Volt and

Rise/Fall Time.

Stop Delay can only be used for realizing the function Timed Totalize in the CNT-91(R).

5-3

USER MANUAL ● CNT 9x Series ● Rev.20 December 2017

Measurement Control

Controlling

Measurement

Timing

The Measurement

Process

Basic Free-running Measurements

Since these counters use the reciprocal counting technique, they a

lways synchronize the start and stop of the actual measuring period to the input signal trigger events. A new mea- surement automatically starts when the previ- ous measurement is finished (unless

HOLD is on). This is ideal for

continuous wave signals.

The start of a measurement takes place when the following conditions have been met (in order):

—

The counter has fully processed the previ-

ous measurement.

—

All preparations for a new measurement are made.

—

The input signal triggers the counter's measuring input.

The measurement ends when the input signal

meets the stop trigger conditions. That happens directly after the following events:

—

The set measurement time has expired (ap-

plies to Frequency and Period Average measurements only).

—

The input signal fulfils the stop trigger conditions, normally when it passes the trigger window the second time.

Resolution as Function of

Measurement Time

The quantization error and the number of digits on the display mainly define the resolution of the counter, that is the least-significant digit

displayed.

As explained on page 4-10 under Reciprocal Counting, the calculated frequency f is:

while the relative rms quantization error Eq =

+100ps/tg.

The counter truncates irrelevant digits so that the rms quantization resolution cannot change the LSD (least-significant digit) more than  5 units. This occurs when the displayed value is 99999999, and the quantization error is worst

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5-4

Measurement Control

case. The best case is when the displayed

value is 10000000. Then the quantization resolution corresponds to  0.5 LSD units.

± 1 unit in 99999999 (=1E8) means 10 times more relative resolution than ± 1 unit in 10000000 (=1E7), despite the same number of digits.

A gradual increase of the measurement time reduces the instability in the LSD caused by the quantization uncertainty. At a specific measurement time setting, the counter is justified to display one more digit. That one additional digit suddenly gives ten times more display resolution, but not a ten times less quantization uncertainty. Consequently, a measurement time that gives just one more display digit shows more visual uncertainty in the last digit.

For a stable LSD readout, the maximum measurement time selected should be one that still gives the required number of digits. Such optimization of the measurement time enables the total resolution to be equal to the quantization resolution.

Measurement Time and Rates

The set measurement time decides the length of a measurement if Frequency or Period Average is selected.

This is important to know when you want to make fast measurements, for example when you are using the statistics features, or when you are collecting data over the GPIB bus.

The so-called "dead time", that is the time between the stop of one measurement and the start of the next one in the course of a block measurement, can be below 2 µs.

A block is a collection of consecutive measurements, the results of which are stored in local memory for statistics or plotting purposes (STAT/PLOT menu) or for later transfer to a controller over one of the data communication links (GPIB, USB or ETHERNET).

Additional controls over start and stop of measurements

Free-running measurements may be easy to understand, but measurements can get more complex.

Besides input signal triggering, the start of a measurement is further controlled by the following elements:

— Manual RESTART, if HOLD is selected.

— GPIB triggering (<GET> or *TRG), if bus

triggering is selected.

—

External arming signal, if Start Arming is selected.

—

Expired start arming delay, if Arming

Delay is selected.

In addition to expired measurement time and stop signal triggering, the stop of measurement is further controlled by:

— External arming signal triggering, if Stop

Arming is selected.

GPIB triggering is described in the Programmer's Handbook.

Now let's look deeper into the concept of

arming.

What is Arming?

Arming is a pretrigger condition ("qualifier") that must be fulfilled before the counter allows a measurement to start.

5-5

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Measurement Control

Arming can also be used to qualify the stop of

— A selected part of a complex waveform

a measurement. This is called "stop arming" as opposed to the more common "start arming".

When you use arming, you disable the normal free-run mode, i.e. individual measurements must be preceded by a valid start arming signal transition.

If you use start arming and stop arming tog

ether you get an exte

rnally controlled mea-

surement time, a s

o-called "External Gate".

■ Manual Arming

The counters have a manual start arming function called

HOLD. Here you manually arm the

start of each individual measurement by pressing the

RESTART key.

Use this manual arming mode to measure single-shot pheno

mena, which are either triggered manually or occur at long intervals. Another reason for using this manual arming could simply be to allow sufficient time to write down individual results.

■ When Do I Use Start Arming?

Fig. 5-1

A synchronization signal starts the measurement when start arming is used.

Start arming is useful for measurements of fre-

quency in signals, such as the following:

—

Single-shot events or non-cyclic signals.

—

Pulse signals where pulse width or pulse positions can vary.

—

Signals with frequency variations versus

signal.

Signal sources that generate complex wave forms like pulsed RF, pulse bursts, TV line signals, or sweep signals, usually also produce a sync signal that coincides with the start of a sweep, length of an RF burst, or the start of a TV line. These sync signals can be used to arm the counter. See Fig. 5-1.

■

When Do I Use Stop Arming?

You normally use stop arming together with

start arming. That means that the external gating signal controls both the start and the stop of the measurement. Such a gating signal can be used to force the counter to measure the frequency of a pulsed RF signal. Here the position of the external gate must be inside a burst. See Fig. 5-2.

Fig. 5-2

Start and stop arming together is used for burst signal gating.

Note that burst measurements with access to an external sync signal are performed in the normal Frequency mode, whereas burst measurements without an external sync signal are performed in the self-synchronizing

Frequency Burst mode.

In time interval measurements, you can use the stop arming signal as a sort of "external trigger Hold Off signal." Here you block stop triggering during the external period. See Fig. 5-3.

time ("profiling").

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Measurement Control

Fig. 5-3

Using arming as an external Hold

Off.

■

The Arming Input

—

Input E is the normal arming input. It is

suitable for arming (sync) signals that have TTL levels. The trigger level is fixed at

1.4V and cannot be changed. The trigger slope can be set to positive or negative. The Input E connector can be found on the rear panel of the instrument.

—

Input A or Input B can also be used as

arming input for all single channel measurements and d

ual channel measurements where the arming signal is one of the meas

uring signa

ls. This input is more suitable if your arming signal does not have TTL levels. All input controls such as AC/DC, Trigger Level, 50/ 1 Metc. can be used to condition the arming signal.

Using the measuring signal as arming

signal

When performing time or frequency measurements on complex signals having a unique trigger point, input B arming can be used to make the measuring signal itself "auto-arm" the counter, e.g. to measure the frequency of a signal after it has reached a specified voltage limit (= set trigger level), see Fig. 5-4.

— Connect the signal to input A.

— Press

INPUT A and adjust the settings to

suit the interesting part of the signal.

— Press INPUT B and adjust the settings so

that the unique trigger point can be detected. Normally

DC coupling and Manual

trigger level should be preferred.

— Activate start arming with or without delay

on input B via the

SETTINGS menu.

The signal on input A will be internally connected to input B, so no ext

ernal signal tap is

necessary.

■ When Do I Use Arming With Delay?

You can delay the start arming point with respect to the arming signal. Use this function when the external arming signal does not coincide with the part of the signal that you are interested in.

The time delay range is 20 ns to 2 s with a setting resolution of 10 ns.

■ Getting The Whole Picture

The flowchart in Fig. 5-5 illustrates how arming a trigger hold off enables precise control of

the start and stop of the actual measurement when you operate the counter from the front panel. If you control the counter via the GPIB or USB, read more about bus arming and triggering under the heading "How to use the trigger system" in the Programmer's Handbook.

Fig. 5-4

Auto-arming using the trigger level

on B as qualifier.

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Measurement Control

yes

PRESS

HOLD?

RESTART

DISPLAY

START

ARMING

yes

WAIT FOR

EXT SIGNAL

DELAY?

yes

WAIT FOR INPUT

SIGNAL TO TRIGGER

WAIT PRESET TIME

START OF MEASUREMENT

TRIGGER HOLD-OFF?

yes

WAIT PRESET TIME

STOP

ARMNG?

yes

WAIT FOR

EXT. SIGNAL

END OF PRESET MEASURING TIME

WAIT FOR INPUT

SIGNAL TO TRIGGER

STOP MEASUREMENT

PROCESS RESULT & DISPLAY

Fig. 5-5 Measurement control flow diagram.

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Measurement Control

Arming Setup Time

Arming Examples

The arming logic needs a setup time of about 5 nanoseconds before the counter is really armed; see Fig. 5-6.

Fig. 5-6

Time from active external control

edge until measurement is armed:

When arming delay is selected, the setup time

is different; see Fig. 5-7. It illustrates the effect of the 100 ns delay resolution.

Fig. 5-7

Time from expired time delay until measurement is armed:. -60 to +40 ns.

Fig. 5-7 shows that a start trigger signal may

be detected although it appears 60 nanoseconds before the programmed time delay has expired. The start trigger signal must come 40 nanoseconds after the programmed time delay has expired to guarantee correct start of the measurement.

Introduction to Arming Examples

The following arming examples are available:

#1 Measuring the first pulse in a burst #2 Measuring the second pulse in a burst

#3 Measuring the time between pulse #1 and #4

in a burst

#4 Profiling

Examples 1 and 2 measure the pulse width of a selected positive pulse in a burst. You can, however, also measure the period, rise time, or duty factor by changing FUNCTION, and you can measure on a negative pulse by changing trigge

r slope.

If you do not know the basic parameters of the signal to be measured, we recommend to use an oscilloscope for monitoring. Then you can estimate roughly how to set trigger slope, arming slope and arming delay.

#1 Measuring the First Burst

Pulse

Fig. 5-8

Synchronizing the measurement so that the pulse width of the first pulse is measured.

In the first example we will measure the width of pulse #1 in a repetitive pulse burst. In this example, a synchronization signal (SYNC) with TTL levels is also available. See Fig. 5-8.

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Measurement Control

However, the quick and simple method described first does not employ arming at all but rather draws on the fact that a counter of this type tends to self-synchronize its internal processes to the input signal.

Our task is to synchronize the start of the measurement (start trigger) to the leading edge of the first pulse. Depending on the signal timing, this can be easy, difficult, or very difficult.

■ A. Auto Synchronization Without

Arming

If we are lucky, we can manage without using the arming function at all. Often, the counter can automatically synchronize the measurement start to the triggering of the first pulse. The conditions for success are that the PRF is not too high, preferably below 50 Hz and certainly not abo

ve 150 Hz. The duration of a pulse burst (between first and last pulse) should be substantially less than the distance to the next burst, and the number of pulses in the burst should be more tha

n 100 to avoid

occasional miscounts.

Do the following steps to perform auto syn-

chronization without arming:

—

Connect the burst signal to input A.

—

Adjust the manual sensitivity and trigger level until the burst signal triggers the counter correctly.

—

Use the MEAS/FUNC key to select Pulse

Width A.

—

Use Pacing Time to select a value that

approaches the time between the bursts.

Absolute synchronization will not be guaranteed in this way, but there is a high probability that auto-synchronization will work anyway. However, occasional

erroneous values will be displayed. To

achieve guaranteed synchronization, use the

Start Arming function.

■ B. Synchronization Using Start Arming

The SYNC signal can be directly used to arm the measurement. This requires that the leading edge of the SYNC signal occurs more than 5 nanoseconds before the leading edge of the first pulse in the burst. See Fig. 5-9.

Fig. 5-9

Synchronization using start arming.

Do the following steps to perform synchroni-

zation using start arming:

—

Connect SYNC to input E.

—

Connect the burst signal to input A.

—

Adjust the trigger level to match the burst signal under study.

—

Press SETTINGS  Arm

—

Select Start Arm Delay = 0 and Start Chan

—

Use MEAS/FUNC to select Pulse Width A.

If there is no (or too little) time difference be-

tween the arming signal and the first pulse in the pulse burst, arming must be combined with a delay. See example C.

■ C. Synchronization Using Start Arming

With Time Delay

If the pulse bursts have a stable repetition frequency, you synchronize the measurement using Start Arming with Time Delay. Here you use the SYNC pulse belonging to a preceding burst to synchronize the start of measurement.

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Measurement Control

Set the time delay to a time longer than the

duration of a pulse burst and shorter than the repetition time of the pulse bursts. See Fig. 5-10.

Fig. 5-10

Synchronization using start arming

with time delay.

Use the same test setup as in the preceding ex-

ample but enter a suitable Start Arm Delay.

#2 Measuring the Second Burst

Pulse

The next task is to measure the width of the second pulse in the pulse train from example 1. How can we now synchronize the measurement start to the start of the second pulse? In this case auto-synchronization, without the use of the arming function, cannot work. Auto-synchronization can be used only to synchronize on the fir

st trigger event in a burst.

Depending on the SYNC signal's position relative to the burst, and the duration of the SYNC signal, the measurement can be performed with or wit

hout using arming delay.

If the trailing edge of the SYNC signal occurs

after the leading edge of the first pulse but before the second pu

lse in the pulse burst, then normal start arming without delay can be used. Select triggering on positive slope on input A and negative slope on input E. The slope for the active arming channel is set in the

SETTINGS  Arm Start Slope menu. This

example is shown in the following figure:

If the SYNC-pulse timing is not so suitable as

in the above measurement example, then

Fig. 5-11 If the trailing edge of the sync signal

appears before the second pulse use arming without delay.

arming must be used combined with a time

delay; see the following figure:

Fig. 5-12

Use arming with delay if the

trailing edge of the sync signal appears too late to be useful.

Use the same test setup as in the preceding ex-

ample but enter a suitable Start Arm Delay.

The set delay time must be set to expire in the gap between pulse #1 and #2.

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Measurement Control

#3 Measuring the Time Between

Burst Pulse #1 and #4

In the previous examples, the synchronization task has been to identify the start of a measurement and to perform a single-shot time interval measurem

ent. Now, we will complicate the picture even more. In our next example we will not only arm the start, but also the stop of a measurement. We will measure the time bet

ween the first and t

he fourth pulse in the

pulse burst. We still have the SYNC signal

available, see Fig. 5-13.

Fig. 5-13

Measuring a time interval inside a

burst.

The measurement function is not Pulse Width Abut

Time Interval A to A where the settings

for input B are used for controlling the stop conditions. The desired start and stop trigger points are marked in the preceding illustration. Our task is now to arm both the start and the stop of this measurement. The start arming is already described in example

#1, i.e., synchronize measurement start to the

leading edge of the first pulse. The challenge

is to synchronize the stop of the measurement,

i.e., to arm the stop. If we do nothing, the time

interval measured will be the time between the first and the second pulse. We must thus delay the stop. This can be done in different ways.

■ A. Using Trigger Hold Off to Delay the

Stop a Certain Time

Trigger Hold Off is used to inhibit stop triggering during a preset time. The Hold Off

Fig. 5-14

period starts synchronously with the start trigger event. The Hold Off time should be set to expire somewhere between pulse number 3 and 4, see Fig. 5-14.

If Hold Off expires between pulses

three and four, the correct time interval is measured.

Use the same test setup as in the preceding

examples. Then proceed as follows:

—

Use the MEAS/FUNC key to select

Time Interval A to A.

—

Press INPUT B and choose positive slope and a suitable trigger level.

—

Press SETTINGS Trigger Hold Off (On) and enter a suitable Hold Off time.

—

Make sure the start arming conditions from example #1 are maintained, i.e. no arming delay.

—

Measure.

■ B. Using Stop Arming (i.e., External Hold

Off) to Delay the Stop

So far in our examples, the sync signal has been

used exclusively as a start arming signal; i.e.,

we have been concerned only about the leading

edge of the sync signal, and not its duration. However, the sync signal can also be used as an External Trigger Hold Off when you select stop arming on the trailing edge of the sync signal.

If the duration of the sync pulses can be

externally varied, we can select a duration that

expires in the gap between pulse #3 and #4.

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Measurement Control

Fig. 5-15

Using both start and stop arming to select the part of the burst that is of interest.

Use the same test setup as in the preceding ex-

ample. Then proceed as follows:

— Press

SETTINGS  Arm and select Stop

Chan E

and negative Stop Slope.

— Measure.

#4 Profiling

Profiling means measuring frequency versus

time. Examples are measuring warm-up drift

in signal sources over hours, measuring the

linearity of a frequency sweep during seconds,

VCO switching characteristics during milli-

seconds, or the frequency changes inside a

"chirp radar" pulse during microseconds.

These counters can handle many profiling

measurement situations with some limitations.

Profiling can theoretically be done manually,

i.e., by reading individual measurement re-

sults and plotting in a graph. However, to

avoid getting bored long before reaching your

800th or so measurement result, you must use

some computing power and a bus interface. In

profiling applications, the counter acts as a

fast, high-resolution sampling front end, stor-

ing results in its internal memory. These re-

sults are later transferred to the controller for

analysis and graphical presentation. The

TimeView™ software package greatly simpli-

fies profiling.

You must distinguish between two different

types of measurements called free-running and

repetitive sampling.

■ Free-Running Measurements

Free-running measurements are performed over a longer period, e.g., to measure the sta-bility over 24 hours of oscillators, to measure initial drift of a generator during a 30-minute warm-up time, or to measure short-term sta-bility during 1 or 10s. In these cases, measure-ments are performed at user-selected intervals in the range 2 µs to 1000 s. There are several different ways of performing the measurements at regular intervals.

Measurements using the statistics features for setting the "pacing time"

By setting the pacing time to 10 s for example, measurements are automatically made at 10 s intervals until the set number of samples has been taken. The range is 2 - 2*109 .Use

HOLD/RUN and RESTART if you want to stop

after one full cycle. You can watch the trend or spread on the graphic display while the measurement is proceeding.

Using a controller as a "pacer"

As an alternative, the timer in the controller can be used for pacing the individual measure-ments. This allows for synchronization with external events, for instance a change of DUT when checking a series of components.

Using external arming signals

External arming signals can also be used for "pacing." For example with an arming signal consisting of 10 Hz pulses, individual measurements are armed at 100 ms intervals.

Letting the counter run free

When the counter is free-running, the shortest delay between measurements is approximately 4 µs (internal calibration OFF) or 8 µs (internal calibration ON) plus set measurement time. For example, with a measurement time of 0.1 ms,

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Measurement Control

the time between each sample is ap-

proximately 104-108s.

■ Repetitive Sampling Profiling

The measurement setup just described will not work when the profiling demands less than 4

s intervals between samples.

How to do a VCO step response profiling with 100 samples during a time of 10 ms.

This measurement scenario requires a repeti-

tive input step signal, and you have to repeat

your measurement 100 times, taking one new sample per cycle. And every new sample should be delayed 100s with respect to the previous one.

The easiest way to do this is by means of a controller, e.g. a PC, although it is possible but tedious to manually set and perform all 100 measurements.

The following are required to setup a meas

urement:

—

A repetitive input signal (e.g., frequency

output of VCO).

—

An external SYNC signal (e.g., step voltag

e input to VCO).

—

Use of arming delayed by a preset time (e.g., 100, 200, 300s

See Fig. 5-16 and Fig. 5-17.

When all 100 measurements have been made, the results can be used to plot frequency versus time. Note that the a

bsolute accuracy of the time scale is dependent on the input signal itself. Although the measurements are armed

at 100 s  100 ns intervals, the actual start of

measurement is always synchronized to the first input signal trigger event after arming.

The TimeView™ software package will do this measurement quickly and easily.

Fig. 5-16

Setup for transient profiling of a

VCO.

Fig. 5-17

Results from a transient profiling

measurement.

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5-13

Chapter 6

Process

Introduction

Three different ways to process a measuring result are available: Averaging, Mathematics and Statistics. They can be used separately or all together.

In addition to postprocessing you can also monitor the measurement results in real time by setting limits and deciding how to react when they are crossed.

Averaging

Hardware averaging by means of counting clock pulses during several full input signal cycles is only used for the measurement functions Frequency and Period Average. The parameter to be set by the operator in this case is Meas Time under SETTINGS, and the range is 20 ns to 1000 s. Longer measuring times mean higher resolution.

The other functions employ single cycle measuring, and the method to get average results is to utilize the statistics features described later.

Mathematics

The counter can use five mathematical expressions to process the measurement

result before it is displayed:

1. K*X+L

2. K/X+L

3. (K*X+L)/M

4. (K/X+L)/M

5. X/M-1

Press MATH/LIM  Math to enter the first

mathematics submenu. See page 2-14 how to

enter the constants K, L and M and how to

select the formula that best suits your need.

The default values of K, L and M are chosen so that the measurement result is not affected directly after activating

Math. Recalling the

default setting will restore these values as well.

Example:

If you want to observe the deviation from a certain initial frequency instead of the absolute frequenc

y itself, you can do like this:

—

Recall the default settings by pressing

USER OPT  Save/Recall  Recall Setup  Default.

—

Connect the signal to be measured to

input A.

—

Press AUTO SET to let the counter find

the optimum trigger conditions on its

own.

—

Press MATH/LIM Math  L

—

If the current display value is suitable for your purpose, then press X

. It will

then be transferred to the constant L. You can repeat pressing

until you are satisfied. The constant will be updated with the latest measurement result.

—

Instead of using X0you can enter any numerical value from the front panel. Let's assume that 10 MHz is your reference frequency. The mantissa is marked by text inversion for immediate editing. Press

1  0  ±  EE  6.

—

Confirm by pressing EXIT/OK. Now the

constant L is updated and displayed as

-10E6.

—

Press Math and choose the expression

K*X+L by pressing the softkey below it.

—

Now the display will show the deviation from the value you have just entered.

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Process

By changing the constant K you can scale the result instead.

Use the expression X/M-1 if you want the result to be displayed as a relative deviation.

Statistics

Statistics can be applied to all measuring functions and can also be applied to the result from Mathematics.

The available statistics functions are as follows:

X MAX: Displays the maximum value within a

mpled population of N x

-values.

X MIN: Displays the minimum value within a

sampled population of N xi-values.

XP-P: Displays the peak-to-peak deviation

within a sampled population of N

values.

MEAN: Displays the arithmetic mean value (x

of a sampled population of N x

values and is calculated as:

ST DEV: Displays the standard deviation (s) o

a sampled population of N xi-values and is calculated as:

It is defined as the square root of the variance.

A DEV: Displays the Allan deviation (σ) of a

sampled population of N xi-values and is calculated as:

It is defined as the square root of the Allan variance.

The number N in the expressions above can

assume any value between 2 and 2*109.

Allan Deviation vs. Standard

Deviation

The Allan Deviation is a statistic used for characterizing short-term instability (e.g. caused by jitter and flutter) by means of samples (measure

ments) taken at short intervals. The fundamental idea is to eliminate the influence of long-term drift due to aging, temperature or wander. This is done by making consecutive comparisons of adjacent samples.

The Standard Deviation, which is probably a more familiar statistic, considers the effects of all types of deviation, as all samples in the population are compared with the total mean value.

As you can

see, both the Allan Deviation and the Standard Deviation are expressed in the same units as the main parameter, e.g. Hz or s.

Selecting Sampling

Parameters

—

Press SETTINGS  Stat..

—

Press No. of samples and enter a new value by means of the numerical keys or the

UP/DOWN arrow keys, if you want to

change the default value of 100.

—

Proceed in the same way for No. of bins

if you want to present the measurement results graphically in a histogram.

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Process

Note that the six statistic measures are calculated and displayed simultaneously only in the non-graphic presentation mode under STAT/PLOT.

Use the same key for toggling between the three modes Numerical -

Histogram - Trend.

—

Press Pacing time and enter a new value

if you want to change the default value 20 ms. The range is 2s - 500 s. The pacing parameter sets the sampling interval.

—

Activate the set pacing time by pressing

Pacing Off. The status is changed to Pacing On. Status Pacing Off means that

the set number of samples will be taken with minimum delay.

—

Press HOLD/RUN to stop the measuring

process.

—

Press RESTART to initiate one data cap-

ture

—

Toggle STAT/PLOT to view the measurement result as it is displayed in the different presentation modes.

Note that you can watch the intermediate results update the display continually until the complete data capture is ready.

This is particularly valuable if the collection of data is lengthy.

Measuring Speed

When using statistics, you must take care that the measurements do not take too long time to perform. Statistics based on 1000 samples does not give a complete measurement result until all 1000 measurements have been made, although it is true that intermediate results are displayed in the course of the data capture. Thus it can take quite some time if the setting of the counter is not optimal.

—

Here are a few tips to speed up the

process:

—

Do not use a longer measuring time than necessary for the required resolution.

—

Remember to use a short pacing time, if your application does not require data collection over a long period of time.

NOTE:

If AUTO trigger is ON, the counter makes an "AUTO Once" and uses the found trigger level as fixed settings until all samples are captured. Thus the use of auto trigger does not reduce capture speed.

Determining Long or Short

Time Instability

When making statistical measurements, you must select measuring time in accordance with what you want to obtain: Jitter or very short time (cycle to cycle) variations require that the samples be taken as Single measurements.

Fig. 6-1

Jitter and drift.

If average is used (Freq or Period Average only), the samples used for the statistical calculations ar

e already averaged, unless the set measuring time is less than the period time of the input signal (up to 160 MHz). Above this frequency prescaling by two is introduced anyhow, and as a consequence a certain amount of averaging. This can be a great ad-

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Process

vantage when you measure medium or long time instabilities. Here averaging works as a smoothing function, eliminating the effect of jitter.

The signal in Fig. 6-1 contains a slower variation as well as jitter. When measuring jitter you should use a limited number of samples, so that the slow variation does not become noticeable or alternatively use the dedicated statistic measure for this kind of

measurement, the Allan deviation.

To measure the slower variation you calculate Max, Min or Mean on a long series of averaged samples. Here averaging eliminates the jitter in each sample and the long measuring time and large number of samples means that the measurement can record very slow variations. The maximum pacing time is 500 s, the maximum measuring time for each sample is

1000 s, and the maximum number of samples

is 2*109, which in effect means that a single

data capture could theoretically span up to

3*1012s, or more than 95000 years.

Statistics and Mathematics

The counter allows you to perform mathematical operations on the measured value before it is presented to the display or to the bus. See Page 6-2 to get an overview of the four available equations.

Any systematic measurement uncertainty can be measured for a particular measurement setup, and the needed correction constants can be entered into these equations. Statistics will then be applied to the corrected measured value.

Confidence Limits

The standard deviation can be used to calculate the confidence limits of a measurement.

Confidence limits =ks

Where:

x = standard deviation

k = 1 for a confidence level of 68.3% (1-

limits)

k = 2 for a confidence level of 95.5% (2

- limits)

k = 3 for a confidence level of 99.7% (3



- limits)

■ Example

A measurement of a time interval of 100 s is

used to illustrate how the confidence limits

are calculated from the measurement result.

Use the statistics to determine the mean value and standard deviation of the time interval. Take sufficient samples to get a stable

reading. Assume further that the start and stop

trigger transitions are fast and do not

contribute to the measurement uncertainties.

The counter displays:

MEAN value = 100.020 s and a STD DEV

= 50 ns, then the 95.5% confidence limits =

±2sx = ±2 * 50 ns = ±100 ns.

The 3- limit will then be ±3 * 50 ns = ±150 ns

Jitter Measurements

Statistics provides an easy method of determining the short term timing instability, (jitter) of pulse parameters. The jitter is usually specified with its rms value, which is equal to the standard deviation based on single meas

urements. T

he counter can then directly

measure and display the rms jitter.

Otherwise, the standard deviation of mean values can be measured. The rms value is a good measure to quantify the jitter, but it gives no information about the distribution of the measurement values.

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Process

To improve a design, it might be necessary to

analyze the distribution. Such measurements as well as trend analysis can be performed by means of the built in graphic capability - toggle the

STAT/PLOT key to see the two graphic

presentation modes.

Even higher versatility can be exploited with a controller and the optional TimeView™ Frequency and Time Analyzing Software Package.

Limits

The Limits Mode makes the counter an efficient alarm condition monitor with high flexibility as to the report possibilities.

Press MATH/LIM



Limits to enter the first

Limits Menu. See below.

Fig. 6-2

The Limit Menu, level 1

You can set two levels by entering the

submenus named Lower Limit resp. Upper Limit. Any numerical value can be entered using scientific notation. The active keys are the digits 0-9, the decimal point, the change sign (±) and the softkey designated EE for toggling between the mantissa and the exponent.

Typos are erased by pressing the left arrow key. Confirm by pressing ENTER.

Limit Behavior

Press Limit Behavior to set how the counter will react on limit crossings. The following choices exist:

• Off

No action taken. LIM indicator is OFF. In all other behavior modes, the LIM indicator is

and

non-flashing, unless the limits set in

the Limit Mode menu have been crossed.

• Capture

The measurements are compared with the limits set under Lower Limit and Upper

Limit, and the LIM symbol will be flashing

when the active Limit Mode has set the

LIM flag.

Only samples meeting the test criterion will be part of the population in statistics presentations.

• Alarm

The measurements are compared with the limits set under Lower Limit and Upper

Limit, and the LIM symbol will be flashing

when the active Limit Mode has set the

LIM flag.

All samples, i.e. also those outside the limits, will be part of the population in statistics presentations.

• Alarm_stop

The measurements are compared with the limits set under Lower Limit and Upper

Limit, and the LIM symbol will be flashing

when the active Limit Mode has set the

LIM flag.

The measurement process will stop, and the value that caused the limit detector to trigge can be read on the display.

Only samples taken before the alarm condition will be part of the population in statistics presentations.

The alarm conditions can also be detected via the SRQ function on the GPIB. See the Programmer's Handbook.

Limit Mode

The Limit Mode offers three choices:

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Pendulum CNT-90, CNT-90XL, CNT-91, CNT-91R User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual