Stanford Research Systems SR620 Service manual

MODEL SR620

Universal Time Interval Counter

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: [phone] • Fax: [phone]

email: [e-mail] • www.thinkSRS.com

Copyright © 1989 by SRS, Inc.

All Rights Reserved.

Revision 2.7 (2/2006)

SR620 Universal Time Interval Counter

Table of Contents

Condensed Information

Safety and Use v
SRS Symbols vi
Specifications vii
Abridged Command List xi

Operation

Quick Start Instructions 1
Instrument Overview 2

Front Panel Operation 3

Choosing the Measurement 3
Setting the Mode 3
Setting the Source 3
Setting the Arming Mode 3
Setting the Number of Samples 3
Starting Measurements 3
Arming Mode Summary 4

Choosing the Output Display 5
Setting the Front Panel Display 5
Graphics Outputs 6
Graph Types 6
Scaling Graphs 6
Graphics Cursor 6
Graphics Zoom 6
Hardcopy Output 7
Chart Outputs 7

Setting the Inputs 9
Setting Trigger Levels 9
Setting Trigger Slopes 9
Input Termination 9
UHF Prescalers 9
Input Coupling 10
Reference Output 10
Timebase Input and Output 10
DVM Inputs 10

Sample Arming 11

TIME Mode 11

Time Interval Arming 11
+TIME 11
+TIME, EXT 12
+TIME, EXT with HOLDOFF 12
+/-TIME 12
+/-TIME, EXT 12

Width Arming 13
WIDTH:+TIME 13
WIDTH:+TIME, EXT 13
WIDTH:+TIME, EXT with HLDF 13

Rise and Fall Time Arming 13
RISE/FALL: +TIME 13
RISE/FALL: EXT 13

Frequency Arming 14
Frequency Ratio 14

Period Arming 14
Period Ratio 14

Phase Arming 14

Count Arming 15
Count Ratio 15

Delayed Arming Modes 15

Configuration Menus 17

CONTROL Menu 17
GPIB Parameters 17
RS232 Parameters 17

CALIBRATION Menu 17
AutoCal 18
Clock Source 18

OUTPUT Menu 18
Graphics Enable 18
Printer/Plotter Control 18
Jitter Type Selection 19
Gate Scale Multiplier 19
Trigger Knob Range 19

SCAN Menu 19
Enabling Scans 19
Setting D/A outputs 20
Scanning the D/A's 20
External Trigger Delays 20
Scan Examples 20

Specification Guide 23

Definition of Terms 23
Least Significant Digit 23
Resolution 23
Error 23
Differential Non-linearity 23

Table of Contents i

SR620 Universal Time Interval Counter

ii Table of Contents

Timebase Specifications 24
Short-Term Stability 24
Long-Term Stability 24
External Timebases 25

Trigger Input Specifications 25

Measurement Accuracy 26

Programming

Programming the SR620 29

Communications 29
GPIB Communication 29
RS-232 Communication 29
Data Window 29

Command Syntax 29

Detailed Command List 30
Trigger Commands 30
Measurement Control 31
Data Transmission 32
Binary Dump 33
Scan Control 34
Graphics Control 35
Front Panel Control 36
Interface Control 37
Status Reporting 39
Calibration Commands 39
Serial Poll Status Byte 40
Event Status Byte 41
TIC Status Byte 41
Error Status Byte 41

Programming Examples 43
BASIC and RS-232 44
FORTRAN/National Instruments 45
BASIC/GPIB/Binary Dump 46
FORTRAN/GPIB/Binary Dump 48
C/GPIB/Binary Dump 52

Test and Calibration

Troubleshooting Tips 57

Troubleshooting 57
Power-up Error Messages 57
CAL Error Messages 57
Common Operational Problems 57
Error Messages 57
Error Indicators 58
Wrong Value 58
Excessive Jitter 58

Scope Display Problems 58
Printer and Plotter Problems 58
GPIB Interface Problems 58
RS-232 Interface Problems 59

Performance Tests 61

Necessary Equipment 61

Functional Tests 61
Front Panel Test 61
Self-Test 61
Trigger Input Tests 61
Counter Channel Tests 62
Rear Panel Tests 62

Performance Tests 63
Timebase Frequency 63
Accuracy 63
Time Interval 63
Trigger Sensitivity 63
Trigger Accuracy 64
D/A Output Accuracy 64
DVM Input Accuracy 64

Test Scorecard 67

Calibration 69

Overview 69

Calibration Bytes 69

Simple Calibration 70

Complete Calibration 70
Trigger Input Calibration 70
Clock Oscillator Calibration 71
Insertion Delay Calibration 71
D/A Output Calibration 72
DVM Input Calibration 73

SR620 Circuitry

Circuit Description 75
Processor System 75
GPIB Interface 75
Printer Interface 75
RS-232 Interface 76
Scope Display 76
Counter Input Ports 76
Display Control Ports 76
Front-End Status Bits 77
ADC and DAC Control Bits 77
REF OUT 78
Delay and Gate Generator 78

SR620 Universal Time Interval Counter

Timebase 79
Front-End Inputs 79
Trigger Multiplexers 80
Frequency Gates 80
Event Gating 81
Counting Channels 81
Fast Time Interval Logic 82
Time Interval Arming 82
Time Integrators 83
Analog to Digital Converter 83
Autolevel Circuits 83
Digital to Analog Converter 84
Unregulated Power Supplies 84
Power Supply Regulators 84
Power Supply Bypass 85
Front Panel Display PCB 85

Component Parts List 87

Schematic Circuit Diagrams Sheet No.

Front/Rear Panel Summary
Counter Visual Index
Microprocessor System 1/16
GPIB/RS232/Printer Interfaces 2/16
Scope Graphics Controller 3/16
I/O Ports and LED Drivers 4/16
Analog and ECL I/O 5/16
Slow Counters and REF OUT 6/16
Fast Clocks and Timebase 7/16
Front-End Input Comparators 8/16
ECL Mpx/Freq Gating/Counters 9/16
Fast Time Interval Logic 10/16
Time Interpolators 11/16
A/D, D/A and Threshold pots 12/16
Unregulated Power Supplies 13/16
Power Supply Regulators 14/16
Spares and Decoupling 15/16
Front Panel Display PCB 16/16
Component Placement on PCB

Table of Contents iii

SR620 Universal Time Interval Counter

iv Table of Contents

SR620 Universal Time Interval Counter

Safety and Preparation for Use v

Safety and Preparation for Use

WARNING: Dangerous voltages, capable of causing death, are present in this instrument.
Use extreme caution whenever the instrument covers are removed.

This instrument may be damaged if operated
with the LINE VOLTAGE SELECTOR set for the
wrong ac line voltage or if the wrong fuse is
installed.

LINE VOLTAGE SELECTION

The SR620 operates from a 100V, 120V, 220V, or
240V nominal ac power source having a line
frequency of 50 or 60 Hz. Before connecting the
power cord to a power source, verify that the LINE
VOLTAGE SELECTOR card, located in the rear
panel fuse holder, is set so that the correct ac
input voltage value is visible.

Conversion to other ac input voltages requires a
change in the fuse holder voltage card position
and fuse value. Disconnect the power cord, open
the fuse holder cover door and rotate the fuse-pull
lever to remove the fuse. Remove the small
printed circuit board and select the operating
voltage by orienting the printed circuit board to
position the desired voltage to be visible when
pushed firmly into its slot. Rotate the fuse-pull
lever back into its normal position and insert the
correct fuse into the fuse holder.

LINE FUSE

Verify that the correct line fuse is installed before
connecting the line cord. For 100V/120V, use a 1
Amp fuse and for 220V/240V, use a 1/2 Amp fuse.

LINE CORD

The SR620 has a detachable, three-wire power
cord for connection to the power source and to a
protective ground. The exposed metal parts of the
instrument are connected to the outlet ground to
protect against electrical shock. Always use an
outlet which has a properly connected protective
ground.

CONNECTION TO OTHER INSTRUMENTS

All front panel BNC shields are connected to the
chassis ground and to the power outlet ground via
the power cord. Do not apply any voltage to either
the shields or to the outputs. The outputs are not
protected against connection to any potential other
than ground.

SR620 Universal Time Interval Counter

vi Safety and Preparation for Use

SR620 Universal Time Interval Counter

Specifications vii

Specifications

Functions Time Interval, Pulse Width, Rise and Fall Times, Frequency, Period, Phase,

and Event Counting.

Measurement statistics (mean, min, max, standard deviation or Allan

variance) and graphics are available in all modes of operation.

Time Interval Time, Width, Rise and Fall Times

Range -1000 to +1000 s in +/- TIME mode; -1 ns to +1000 s in all other modes
Trigger Rate 0 to 100 MHz
Least Significant Digit 4 ps single sample, 1 ps with averaging
Resolution (((25 ps typ [50 ps max] )2+ (0.2 ppb x Interval)2) / N)1/2 rms
(((25 ps typ [50 ps max])2 + (0.05 ppb x Interval)2) / N)1/2 rms (Opt 01)
Error < ±(500ps typ [1 ns max] + Timebase Error x Interval +trigger error)
< ±(50ps typ[100ps max] + Timebase Error x Interval) (REL)
Arming Modes +TIME Stop is armed by Start
+TIME EXT Ext arms Start
+TIME EXT HOFF Leading EXT edge arms Start, trailing EXT edge arms

Stop.
±TIME Armed by Start/Stop pair
±TIME CMPL Armed by Stop/Start pair
±TIME EXT Armed by EXT input edge

EXT arming may be internally delayed or scanned with respect to the EXT

input in variable steps. The step size may be set in a 1,2,5 sequence from 1
µs to 10 ms. The maximum delay is 50,000 steps.

Display 16 digit fixed point with 1 ps LSD
Sample Rate For a sample size of N, the total sample time is N x(800 µs + measured time

interval) + Calculation time.

The calculation time occurs only after N measurements are completed and

varies from zero (N=1, no graphics, binary responses) to 5 ms (N=1, no
graphics) to 10 ms (display mean or std dev) to 60 ms (display histogram).

Frequency

Range 0.001 Hz to 300 MHz via comparator inputs.
40 MHz to 1.3 GHz via internal UHF prescalers.

RATIO A/B range: 10
Error < ± ((100ps typ [350 ps max])/Gate + Timebase Error ) x Frequency
Gates External, 1 period, 1ms to 500s in 1,2,5 sequence. Gates may be externally

triggered with no delay. Gates may be delayed relative to an EXTernal
trigger. The delay from trigger may be set from 1 to 50,000 gate widths.

Display 16 digit fixed point with LSD = Freq x 4ps/Gate. 1 µHz max. resolution (1

nHz with x1000 for frequencies ≤ 1 MHz)

Period

Range 0 to 1000 seconds. RATIO A/B range: 10
Error < ±((100 ps typ [350ps max])/Gate + Timebase Error) x Period
Gates Same as frequency
Display 16 digit fixed point with LSD = 1 ps (1 fs with x1000 for periods ≤ 1 s)

Phase Phase = 360 x (Tb - Ta) / Period A

Range -180 to +180 degrees (0 to 100 MHz frequency)
Resolution (25ps x frequency x 360 + 0.001) degree

-9

to 10

3

-9

to 103

SR620 Universal Time Interval Counter

viii Specifications

Gate 0.01 seconds (1 period min.) for period measurement and 1 sample for time

interval measurement. Period may also be measured using externally
triggered internal gates as in frequency mode.

Error < ±(1ns x Frequency x 360 + 0.001) degree

Events

Range 10
Count Rate 0 to 300 MHz
Gates Same as frequency
Display 12 digits

Timebase
Standard

Frequency 10.000 MHz 10.000 MHz
Type TCVCXO Ovenized VCXO

Aging 1x10
Allan Variance (1s)(typ.) 3x10

Stability 0-50° C 1 ppm 0.002 ppm
Settability 0.01 ppm 0.001 ppm
External User may supply 5 or 10 MHz timebase. 1 Volt nominal.

Inputs A, B, and EXTernal

Threshold -5.00 to +5.00 VDC with 10 mV resolution
Accuracy 15mV + 0.5% of setting
Sensitivity see graph next page

Autolevel (A&B) Threshold set between peak input excursions. (f>10 Hz, duty cycle >10
Slope Rising or falling edge
Impedance (1 M Ohm + 30 pf) or 50 Ohms. 50 Ohm termination has SWR < 2.5:1 from

Coupling AC or DC. Ext is always DC coupled.
Input Noise 350mV rms typical
Bandwidth 300 MHz BW provides 1.2 ns risetime
Prescaler (A&B) see graph next page
Protection 100 V. 50 Ohm terminator is released if input exceeds ±5 Vpeak.

REF Output Calibration and Trigger source

Frequency 1.00 KHz (Accuracy same as timebase)
Rise/Fall 2 ns
Amplitude TTL: 0 to 4 VDC (2 VDC into 50 Ohms)
ECL: -1.8 to -0.8 VDC into 50 Ohms

DVM Inputs Two differential rear panel DVM inputs

Full Scale ±1.999 or ±19.99 VDC
Type Sample & hold with successive approximation converter
Impedance 1 M Ohm
Accuracy 0.3% of full scale
Speed Formatted response in approximately 5 ms.

D/A Outputs Two rear panel outputs which may be ramped

Full Scale ±10.00 VDC
Resolution 5 mV
Impedance < 1 Ohm

12

. RATIO A/B range: 10

Option/01

-6

/yr 5x10

-10

5x10

0 -1.3GHz

-10

-12

/day

-9

to 103

-6

)

SR620 Universal Time Interval Counter

Specifications ix

Default Voltage proportional to Mean & Deviation
Accuracy 0.3% of full scale
Graphics Live scope displays and hardcopy

Scope Two rear panel outputs to drive x-y scope
Displays Histograms and strip charts of mean & deviation
X-axis -5 to +5 V for 10 division deflection
Y-axis -4 to +4 V for 8 division deflection
Resolution 250 (H) x 200 (V) pixels
Hardcopy Via Centronics port to Epson graphics compatible dot matrix printers.
RS-232, IEEE-488 to HP-GL compatible Digital Plotters.

Interfaces

RS-232C 300 to 19.2 KBaud. All instrument functions may be controlled.
PC compatible serial cable.
GPIB IEEE-488 compatible interface. All instrument functions may be controlled.
Speed Approximately 150 ASCII formatted responses per second.
1400 binary responses per second.

General

Operating 0 to 50° C
Power 100, 120, 220 or 240 VAC +5% -10%. 50/60 Hz. 70 Watts.
Dimensions 14" x 14" x 3.5". Rack mounting hardware included.
Weight 11 lbs

SR620 Universal Time Interval Counter

x Specifications

SR620 Universal Time Interval Counter

Abridged Command List xi

Abridged Command List

Syntax

Variable i is an integer that selects an input channel for the command:

i
0 External Gate
1 A input
2 B input

Variables i,j,k,l,m, and n are all integers.Variable x is a real number in integer,real, or exponential notation.

Commands which may be queried have a ? in parentheses (?) after the mnemonic. The ( ) are not sent.
Commands that may only be queried have a '?' after the mnemonic. Commands which may not be queried
have no '?'. Optional parameters are enclosed by {}.

Trigger Control Commands

LEVL(?) i,x Set channel i threshold to x volts. Only allowed in remote operation.
MTRG j Same as pushing MAN trigger button. In external gate or holdoff arming

RLVL(?) j Sets the reference output level to 0 = ecl,1=ttl.
TCPL(?) i{,j} Sets the input ac/dc coupling of chans A and B to 0 = dc, or 1 = ac.
TERM(?) i{,j} Sets the 50 ohm terminator of input, also prescaler n=0= 50 ohm, n=1= 1

TMOD(?) i{,j} Sets autolevel on/off. 0 = auto off, 1 = auto on.
TSLP(?) i{,j} Set the trigger slope to 0 = positive, 1 = negative.

Measurement Control Commands

*TRG Device trigger, same as pushing start button
ARMM(?) j Sets the arming mode
0 +- time
1 + time
2 1 period
3 0.01 s gate
4 0.1 s gate
5 1.0 s gate
6 ext trig +- time
7 ext trig + time
8 ext gate/trig holdoff
9 ext 1period
10 ext 0.01 s gate
11 ext 0.1 s gate
12 ext 1.0s gate

AUTM(?) j Sets/resets autostart of measurements. 0 = off,1= on
COMP Complements parity in +-time arming
DREL(?) j Sets/clears the display REL. 0 = clear, 1 = set, 2 = clear REL and display, 3=

GATE(?) x Sets the value of the frequency, period, of count gate to x. x may bebetween

JTTR(?) j Sets variance type to 0 = std dev, 1= Allan variance.

Channel

modes n=0= start gate, n=1= stop gate. Otherwise n is ignored.

meg, n= 2 = prescale. Prescalers can only be used in freq and per modes.

set rel to position of cursor.

1ms and 500s in a 1,2,5 sequence. If x < 0 a externally triggered gate of
length x is set.

SR620 Universal Time Interval Counter

xii Abridged Command List

MODE(?) j Sets the instrument mode to 0 = time,1 = width, 2=tr/tf, 3 = freq, 4 = period, 5

= phase, 6 = count.
SIZE(?) j Sets the number of samples.
SRCE(?) j Sets the measurement source to 0 = A, 1= B, 2= REF, 3 = ratio.
STRT Same as pushing start button.
STOP Same as pushing stop button.

Data Commands

MEAS? j Startts a measurement and returns the result when it is complete. If j=0 the

mean is returned, j=1 returns the jitter, j=2 returns the max, and j=3 the min.
XALL? Returns mean,rel,jitter,max,min of a measurement.
XAVG? Returns the mean of measurement.
XJIT? Returns the jitter of a measurement.
XMAX? Returns the max of a measurement.
XMIN? Returns the min of a measurement.
XREL(?) x Sets the value of the display rel to x.
XHST? j Returns section j of the histogram display as 4 byte binary integers.j=0 to 9.
HSPT? j Return the value of the point n on the histogram. j=1 to j = 250. Returns

9E20 if graph is blank.
SCAV? j Returns the value of point n (1 - 250) of the mean stripchart or the value of

scan point j. returns 9E20 if the stripchart is blank or the scan has not

reachedpoint j.
SCJT? j Returns the value of point n (1 - 250) of the jitter stripchart or the value of

scan point j. returns 9E20 if the stripchart is blank or the scan has not

reached point j.
BDMP j Binary dumps j points. Sample size = 1.

Scan Commands

ANMD(?) j Sets the DAC output mode of the mean and jitter DAC's to 0 = chart/chart, 1=

DAC/chart, 2= chart/DAC,3 = DAC/DAC.
DBEG(?) j Sets the delay scan start position to 1-50000 step sizes after the external

trigger
DSEN(?) j Enables the internal delay scan . 0 = delay off, 1=delay hold, 2 = delay scan.

DSTP(?) x Sets the delay scan step size to 1,2,5 x 10

maximum step size.
HOLD(?) x Sets the hold time between scan points from .01s to 1000 s in .01 s steps
SCAN Clears and restarts a scan.
SCEN(?) j Enables scanning. 0 = disabled,1= single scan mode 2 = repeat scan mode
SCLR Clears the scan.
SLOC? Returns the number of the last completed scan point. Returns 0 if no points

are complete.
SCPT(?) j Sets number of scan points to 2,5,10,25,50,125,250.
VBEG(?) j,x Sets the DAC output scan starting voltage. If scans are disabled or the step

size is zero the output is immediately set to the new voltage. Otherwise, the

voltage is updated at the start of the next scan.
VOUT? j Reads the current output voltage of DAC channel j.
VSTP(?) j,x Sets the DAC scan step size to x volts.

Graphics Control Commands

AUTP(?) j Sets the autoprint mode to 0 = off or 1 = on.
AUTS Autoscales the displayed graph.
CURS (?) j Sets the cursor position to j.
DGPH(?) j Sets the displayed graph to 0 = histogram,1= mean,2 = jitter
GCLR Clears the displayed graph.

-2,-3,-4,-5,-6

. 1 x 10

–2

is the

SR620 Universal Time Interval Counter

Abridged Command List xiii

GENA(?) j Turns graphs 0 = off, 1 = on.
GSCL(?) j,x Sets the graph scales. j = 0 = histogram verticalscale,j=1= histo

horizontal,j=2=histo bins,j=3= mean graph scale,j = 4= jitter graph scale. x =

units per division or # bins.
PDEV(?) j Sets output device to 0 = printer, 1= plotter.
PLAD(?) j Sets the plotter GPIB address.
PLPT(?) j Sets the plotter port to 0= RS232 , 1= GPIB.
PLOT Initiates a print/plot
PCLR Clears plots/prints

Front Panel Control

DISP(?) j Sets the display source. 0=MEAN,1=REL,2=JITTER,3=MAX
4 =MIN, 5 = TRIG, 6 = DVM's
EXPD(?) j Set the x1000 expand status in freq and per mode 0 = off, 1 = on
KEYS(?) j Simulates a keypress, or reads the most recently pressed key

Rear Panel Control

CLCK(?) j Sets the clock source. 0 = internal, 1= external.
CLKF(?) j Sets the external clock frequency. 0=10MHz,1= 5MHz.
PORT(?) j Sets/reads the binary I/O port when enabled.
PRTM(?) j Sets the printer port mode. 0 = print, 1 = input,2 =output.
RNGE(?) i{,j} Sets the full scale voltage of DVM input i.
VOLT? j Reads DVM channel 0 or 1.

Interface Control Commands

*RST Clears instrument to default settings.
*IDN? Returns the device identification .
*OPC(?) Operation complete common synchronization command. Either sets a status

bit or returns a value when all operations (scans , prints, measurements) are

complete.
*WAI Wait synchronization command. Holds off further command execution until all

in progress operations (scans, prints,measurements) are complete.
ENDT {j,k,l,m} Sets the RS232 end transmission terminator
LOCL j Sets the RS232 local/remote function. 0 = local, 1=remote, 2 = local lockout
WAIT(?) j Sets the RS232 intercharacter time delay.

Status Reporting Commands

*CLS Clears all status registers.
*ESE(?) j Sets/reads the standard status byte enable register.
*ESR? {j} Reads the standard status register, or just bit j of register.
*PSC(?) j Sets the power on status clear bit. This allows SRQ'son power up if desired.
*SRE(?) j Sets/reads the serial poll enable register.
*STB? {j} Reads the serial poll register, or just bit n of register.
EREN(?) j Sets/reads the error status enable register.
ERRS? {j} Reads the error status register, or just bit n of register.
STAT? {j} Reads the TIC status register, or just bit n of register.
TENA(?) j Sets/reads the TIC status enable register.
STUP? Returns the complete setup in one string.

Calibration Control (NOTE: these commands are not needed during normal operation)

$TAC? j Reads the value of the time-to-amplitude converters.
$PHK? j Sets the printer handshake lines.

SR620 Universal Time Interval Counter

xiv Abridged Command List

$POT? j Reads the value of trigger pot j
*CAL? Starts autocal and returns status when done.
*TST? Starts self-test and returns status when done.
BYTE(?) j,k Reads/sets linearization byte values.
WORD(?) j,k Reads/sets cal words.

SR620 Universal Time Interval Counter

Quick Start

Use this procedure as a quick orientation to
the instrument's features and capabilities. If
you encounter problems, read the detailed
discussions on operation or see the
troubleshooting section.

1) Make sure that the correct line voltage has

been selected on the rear panel power entry
module.

2) With the unit's power switch on "STBY", hold

the "CLR" key in the "DISPLAY" section down
and turn the unit "ON". This will return all of
the instrument settings to their default state.
The message, "SELF TEST PASS" should
briefly appear.

Note: The fan will not run until the unit warms

up. The red LED labeled "CLOCK" in the
CONFIG section may stay on for a few
minutes. The red LED's labeled "START" and
"STOP" may blink if no inputs are applied.

3) Press the MODE down key once to select the

WIDTH mode. Press the source key twice to
select REF ( the 1 kHz REFerence ) as the
signal source. After 1/2 hour warmup, the
display should read 500 us +- 1 ns. If the
displayed value is outside this range see
instructions for running the AUTOCAL
procedure.

4) Press the SAMPLE SIZE up-key five times to

select a sample size of 500. This will slow the
display update rate to 2 Hz, and provide a
more consistent MEAN value.

5) Press the SET key in the DISPLAY section to

set the REL offset to the current value of the

Quick Start Instructions 1

MEAN. The REL LED will turn on and the
display will show a mean value within a few ps
of zero. An 'r' appears on the display to
indicate a relative value.

6) Press the DISPLAY down-key to show each of
the following:

MEAN 0+-100 ps (The REL is set)
REL 500 us +-100 ps
JITTER 5-20 ps
MAX Erratic, but usually <100 ps
MIN Erratic, but usually <100 ps
TRIG +-5 V per the three level knobs
DVM Within 5 mV of zero.

7) Configure an oscilloscope for the X-Y display
mode with the horizontal and vertical inputs
set for dc coupling ( not 50 Ohms ) and 1 V/div
sensitivity. If the scope has scale factor
displays, turn them "off", and set the 20 MHz
bandwidth limit. Center the beam in the middle
of the screen and attach the inputs to the X-Y
display outputs on the rear panel of the
SR620.

8) Press the AUTO key in the SCOPE AND
CHART section to scale the display. If the
DISPLAY REL is set, the HISTOgram of the
pulse widths will be displayed with about 20
ps/div horizontal resolution.

9) In the SCOPE AND CHART section, press the
display button to change from HIST to MEAN.
Press the AUTO button to scale the display.
The scope will now show a 'strip chart' of the
mean values for each set of measurements.
Now select JITTER and press AUTO to
display a 'strip-chart' of standard deviations for
the scope display.

SR620 Universal Time Interval Counter

2 Instrument Overview

Introduction

The SR620 Time Interval Counter can do a variety
of time interval and frequency measurements. The
instrument's high single-shot timing resolution,
low jitter, and reciprocal counting architecture
allow rapid, high resolution measurements.

Modes of Operation

Time Intervals between the A and B inputs may

be measured with 4 ps LSD, 25 ps rms resolution,
100 ps relative accuracy, and 1ns absolute
accuracy. Time intervals from -1ns to 1000 s or +­1000 s may be measured.

Pulse Widths of either input may be measured.
The start and stop thresholds are set separately.
The resolution, jitter, and accuracy are the same
as for Time interval measurements.

Rise and Fall Times of either input may be
measured. The start and stop thresholds may be
set with 10 mV resolution. The 350 MHz
bandwidth of the inputs allows measurements of
rise and fall times down to 1 ns.

Frequencies from 0.001 Hz to 1.3 GHz may be
measured. The SR620 will provide 11 digits of
resolution when a one second gate is used.
Frequencies above 300 MHz may be measured on
either input using the UHF prescalers. NanoHertz
resolution is available in the x1000 display mode.

Periods may be measured with femtosecond
resolution. Period measurements are done the
same way as frequency measurements, except
the reciprocal of frequency is reported to the
display.

The Phase between the signals on the A and B
input may be measured with 0.001° resolution.
The phase shift between signals from 0.001 Hz to
100MHz may be measured.

The Count mode is used to count input
transistions during a gate. Count rates up to 300
MHz will be tallied.

Arming and Gating

Each of these modes are supported with powerful
arming and gating modes. Time arming modes
include: +Time, +Time External, +Time External
with Stop Holdoff, +-Time, and +-Time External.

Frequency gating modes include fixed gates of
one period, 0.01 s, 0.1 s, 1 s, Externally triggered
fixed gates, External gates, Externally triggered
adjustable gates from 1 us to 10 ms. Gates which
are externally triggered may be delayed and
scanned by 1 to 50,000 gate widths to allow
transient frequency measurements.

Statistics

The SR620 will compute and display statistics for
sample sizes of one to one million . The mean,
standard deviation or Allan variance, minimum
and maximum deviations may be displayed.
Statistics are available for all modes of operation.
Displayed values may be offset by the REL for
measurements relative to a previous mean value.

Scope Displays and Hardcopy

The SR620 can display histograms and strip
charts on any xy scope. Histograms show the
distribution of values within a group of
measurements. Strip charts of the previous 250
mean values or deviations show data trends.
Hardcopy of scope displays may be made to
printers or plotters via rear panel Centronics,
RS232 or GPIB ports.

Reference Output

The front panel REF OUT provides a precision
1KHZ square wave at TTL or ECL levels. This
source may be used for calibration, and is a
convienent trigger for many types of
measurements.

DVM's and DAC Outputs

Two rear panel DVM inputs allow dc voltage
measurements with 0.3% accuracy on 2 or 20 V
full scale ranges. The inputs may be displayed on
the front panel or read via the RS232 or GPIB
interfaces.

Two DAC outputs default to output voltages
proportional to the mean and jitter readings to
drive analog strip chart recorders. The DAC output
voltages may also be set or scanned from the front
panel of through one of the computer interfaces.

Computer Interfaces

Both GPIB and RS232 interfaces allow complete
control of the instrument. A fast binary dump
mode allows up to 1500 measurements per
second to be taken and transfered to a computer.

SR620 Universal Time Interval Counter

Front Panel Operation 3

FRONT PANEL OPERATION

The SR620 Universal Time Interval Counter can perform an extremely wide variety of time interval and
frequency measurements. The SR620 is designed so that the values of virtually all of the important
measurement parameters are visible at a glance on the front panel. Setting the SR620 to perform a particular
measurement can be separated into three steps: choosing the measurement, choosing the output display,
and setting the inputs. The SR620 is different than most counters in that a "measurement" consists of from 1
to 1,000,000 "samples" and the SR620 reports statistical information on these samples. The SR620 can
report the mean , jitter, maximum , and minimum values found in a measurement.

CHOOSING THE MEASUREMENT

SETTING THE MODE

Pressing the MODE up and down arrow keys sets
the measurement type. The SR620 can measure
time intervals, pulse widths, pulse rise and fall
times, frequency, period, phase, and can count
events. The ARMING section has detailed
explanations of the modes.

SETTING THE SOURCE

The source key selects the signal source for a
particular measurement. In time interval mode the
source specifies which input will "start" the time
interval. Normally the start source is channel A,
but channel B, and the 1kHz REF output may also
be selected. The other input then "stops" the time
interval. In pulse width, frequency, period, and
counts the source may be A, B, or the 1kHz REF.
Additionally, in frequency, period, and count
modes if both the A and B LEDs are on the RATIO
A/B is measured. In rise/fall time mode only A and
B may be the source, while in phase mode the
phase of B relative to A is always measured.

SETTING THE ARMING MODE

"Arming" controls the conditions under which a
sample is started and stopped. The arming mode
is chosen using the ARMING up and down arrow
keys. The SR620 has a large selection of arming

modes. The various modes relevant to each
particular type of measurement are treated in
detail in the ARMING section of this manual. A
summary of the arming modes is given on the next
page.

SELECTING THE NUMBER OF SAMPLES

The sample size up and down arrow keys select
between 1 and 1,000,000 samples for the
measurement.

STARTING AND STOPPING MEASUREMENTS

The "START" and "RESET" keys start and stop
measurements. Press the "START" key to start a
single measurement. The "BUSY" LED will remain
on while the measurement is in progress. When
the measurement is finished the SR620 will
display the results and stop. Press and hold the
START key to turn on the "AUTO" LED and
"automeasure". When automeasure is on the
SR620 will automatically start a new measurement
when the present one is complete. Press the
"RESET" key to terminate the present
measurement. Press and hold the "RESET" key
to turn the "AUTO" LED and automeasure off.

SR620 Universal Time Interval Counter

4 Front Panel Operation

SUMMARY OF ARMING MODES

mode arming mode LED indication

time +TIME +TIME LED on
+TIME EXTERNAL +TIME and EXT LEDs on *
+TIME EXTERNAL STOP HOLDOFF +TIME, EXT, and HLDF LEDs on *

±TIME ± TIME LED on
±TIME COMPLEMENT ±TIME and CMPL LEDs on
±TIME EXTERNAL ±TIME and EXT LED on *

width,rise/fall time +TIME +TIME LED on
+TIME EXTERNAL +TIME and EXT LED on *
+TIME EXTERNAL STOP HOLDOFF + TIME, EXT, and HLDF LEDs on *

frequency,period 1 PERIOD 1 PERIOD LED on

0.01s gate 0.01s LED on **

0.1s gate 0.1s LED on **

1.0s gate 1.0s LED on **
ext gate EXT LED on *
ext 1 PERIOD EXT and 1 PERIOD LEDs on
ext 0.01s gate EXT and 0.01s LEDs on **
ext 0.1s gate EXT and 0.1s LEDs on **
ext 1.0s gate EXT and 1.0s LEDs on **

phase +TIME +TIME LED on
EXT EXT LED on *

count 0.01s gate 0.01s LED on **

0.1s gate 0.1s LED on **

1.0s gate 1.0s LED on **
ext gate EXT LED on *
ext 0.01s gate EXT and 0.01s LEDs on **
ext 0.1s gate EXT and 0.1s LEDs on **
ext 1.0s gate EXT and 1.0s LEDs on **

NOTES:

* arming delay or scanning delay/gate may be used in this mode (see CONFIGURATION MENU and

ARMING sections). The EXT LED will flash if scanning is enabled.

** the gate time LED will flash if gate width multiplier is not equal to 1 (see CONFIGURATION MENU

and ARMING sections).

SR620 Universal Time Interval Counter

Front Panel Operation 5

CHOOSING THE OUTPUT DISPLAY

The SR620 can display statistical information
about the measurement of N samples. The
SR620 computes and reports the mean, standard
deviation or root Allan variance, minimum, and
maximum values seen during the measurement.
The equation for the statistical functions are given
by:

SETTING THE FRONT PANEL DISPLAY

The DISPLAY up and down arrow keys control
what is displayed on the 16 digit LED display. All
of the statistical data is always calculated and may
be viewed by scrolling through the displays. The
settings are:

MEAN: displays the mean value of the
measurement. If the display is prefaced by "r"
then a relative reading is displayed and the value
displayed is the mean minus the REL.

REL: displays the value of the REL. The REL is
set by pressing the "SET" button and is cleared by

pressing the "CLR" button. Normally the REL is
set to the value of the mean when the "SET"
button is pressed. However, the REL may be set
to an arbitrary value using the ZOOM feature
described in the GRAPHICS ZOOM section below.
Pressing the "CLR" button with the REL off clears
the display.

JITTER: displays either the standard deviation
(display prefaced by "d") or Allan variance (
display prefaced by "A"). The statistic that is used
is set in the CONFIGURATION MENU.

MAX: displays the maximum sample found in this
measurement. If the display is prefaced by "r"
then a relative reading is displayed and the value
displayed is the max minus the REL.

MIN: displays the minimum sample found in this
measurement. If the display is prefaced by "r"
then a relative reading is displayed and the value
displayed is the min minus the REL.

TRIG: displays the trigger level set by the EXT, A,
and B trigger knobs (The full scale range of the
knobs may be set in the CONFIGURATION
MENU.).

DVM: displays the voltage values at the rear panel
DVM inputs.

Pressing both the up and down arrow keys
together toggles the x1000 expand on and off in
frequency and period mode. In frequency mode
the x1000 expand moves the decimal point 3
places to the left and displays frequencies with
nHz resolution. The maximum frequency
measurable in this mode is 1 MHz. In period
mode the x1000 expand moves the decimal point
3 places to the left and displays periods with fs

SR620 Universal Time Interval Counter

6 Front Panel Operation

resolution. The maximum period measurable in
this mode is 1 s.

In both period and frequency modes the statistical
data is displayed with the number of significant
digits allowed by the SR620's resolution. Using
longer gate times increases the resolution.

GRAPHICS OUTPUTS

In addition to the 16 digit LED display the SR620's
scope and chart outputs may be used to give the
user alternative methods of viewing the data.

SCOPE OUTPUTS

The SR620 may be attached to an oscilloscope
operating in x-y mode to provide a graphical
presentation of the output data. The oscilloscope
should be set in x-y mode with sensitivities set to
1V/div and the SR620's x and y rear panel
outputs attached.

GRAPH TYPES

The SR620 can then display either a histogram of
number of samples vs. measured parameter for
the samples within a measurement, a stripchart of
mean values for successive measurements, or a
stripchart of jitter values for successive
measurements. In histogram mode a new graph
will be displayed after each measurement of N
samples is complete, showing the distribution of
samples in that measurement. In the stripchart
modes a new point will be added to the graph after
each measurement indicating the mean and jitter
values for that measurement. The display desired
is chosen by pressing the select button below the
indicator LEDs. The data for all three graphs are
saved so that all of the graphs may be viewed by
cycling through the three choices. The data, the
scale values, and the cursor position are all
displayed on the scope screen. In the stripchart
modes up to 250 points will be displayed. When
the display fills up new data will start to overwrite
the old starting from the left. The graph may be
cleared by pressing the CLR button below the
PRINT button.

SCALING GRAPHS

In histogram mode the vertical scale, horizontal
scale, and number of bins may be adjusted. In the
stripchart modes the vertical scales may be
adjusted. The easiest way to scale the graphs is
to press the "AUTO" button to autoscale the graph.
Autoscale will automatically adjust the scales so

that the data fits on the screen. In histogram
mode the actual scaling of the graph does not
occur until the next measurement is complete.
The scale may also be adjusted manually.
Pressing the up and down arrow keys with the
normal display on the 16 digit LEDs will adjust the
vertical scale on the displayed graph. In the
histogram mode , incrementing the vertical scale
past the largest value (200,000 / div) will change
the vertical scale to a log scale. To view the
scales on the LED display press the "DISP"
button. First, the cursor position (discussed
below) will be displayed. Pressing the button
again will display the vertical graph scales in the
appropriate units. In the histogram mode pressing
the button again will display the horizontal scale
and then the number of bins. The scales may be
adjusted using the up and down arrow keys.
When the REL is set the graphs are centered
about the REL and, thus, very fine detail may be
observed on a large number.

GRAPHICS CURSOR

The displayed graphs have a moveable cursor that
allows one to read the values of individual points
on the screen. The cursor is represented on
screen by a dotted line. The cursor is moved by
pressing the "DISP" button to display the cursor
position and using the up and down arrow keys to
move it about. The cursor x position is displayed
on the LED display ( in the correct units of s, Hz,
etc.) for the histogram display and measurement
number for the stripcharts. Both the x and y
positions are displayed on the scope screen.

GRAPHICS ZOOM

The SR620 has a feature which allows one to
zoom in on any feature in a displayed histogram,
this feature also allows the REL to be set to any
value desired. First, press the DISP button to
display the cursor position. Then, move the cursor
until it is at the desired position ( or the cursor
value is the desired REL value if setting the REL).
Push the "SET" button. This will set the REL to
the cursor position. Now, adjust the graph
horizontal scale to get the magnification desired.
If setting the REL value, note that the amount that
the cursor value changes for each press of the
arrow keys is determined by the horizontal
histogram scale and that the scale may have to be
adjusted to get the REL value desired.

SR620 Universal Time Interval Counter

Front Panel Operation 7

HARDCOPY OUTPUT

The displayed graph may be copied to either an
Epson compatible graphics printer or a HP-GL
compatible plotter by pressing the "PRINT" key.
The SR620 will continue to take data while the
hardcopy is being generated. If a second print/plot
request is made (by pressing the button or a
programming command) before the current copy is
finished the SR620 will stop taking data until the
current copy is done. This is to avoid corrupting
the second copy as the SR620 only has a 1 deep
graphics output queue.The output device (printer
or plotter) is chosen in the CONFIGURATION
menu (see that section for detail). When using a
printer the SR620 may be put into autoprint mode
by pressing and holding the PRINT key until the
"AUTO" LED turns on. In autoprint mode the
SR620 will automatically print every histogram or a
new stripchart each time the stripchart fills up.
There is no autoprint when using a plotter because
the paper needs to be changed. In autoprint mode
the speed of the printer may determine the
measurement rate if new graphs are generated

faster than the printer can print them. A print or
plot may be aborted by pressing the "CLR" button
under the PRINT button. Pressing and holding the
CLR button will turn off autoprint. If the message
"print error" or "plot error" appears while printing or
plotting please refer to the TROUBLESHOOTING
section.

CHART OUTPUTS

In addition to the scope outputs the SR620 also
has two rear panel analog outputs designed to go
to analog chart recorders. One output puts out a
voltage proportional to the mean of the
measurement while the other output puts out a
voltage proportional the jitter of the measurement.
The output range is 0-8V corresponding to the 8
vertical scope divisions. The scale is the same as
the scope scales. In the cases where zero is at
the center of the scope screen (the REL is set, for
example) zero will correspond to 4 volts output.
The chart outputs may also be configured as
general purpose D/A outputs (see
CONFIGURATION MENU section).

Sample Histogram. The graph scales are 20ps/div in the horizontal direction and 10 /div
in the vertical. The REL is 500.000023 us and is at the center of the graph. This
measurement has a mean value 6ps greater than the REL and a standard deviation of
9ps. The cursor (dotted line) is at the REL and there are 46 events in that bin.

SR620 Universal Time Interval Counter

8 Front Panel Operation

Sample mean value graph. Each point corresponds to the mean value from one
measurement. The vertical scale is 10 Hz/div. The measurements are relative to a REL
of 10.022176 kHz. The REL is at the center of the graph. The cursor (dotted line) shows
that measurement number 67 was 1.635 kHz below the REL.

Sample jitter stripchart. This graph shows the jitter associated with each measurement
above. The vertical scale is 10 Hz/div. The cursor is at measurement number 165 and
the jitter value there is 32.6 Hz.

SR620 Universal Time Interval Counter

Front Panel Operation 9

SETTING THE INPUTS

SETTING TRIGGER LEVELS

The trigger levels are set by rotating the trigger
level adjust knobs. These knobs may have a full
scale range of ±5.00V, ±2.50V, or ±1.25V. The
full scale range is set in the CONFIGURATION
menu (see that section of the manual for details).
In all cases the trigger level resolution is 10mV
and the actual level may be displayed on the front
panel. The trigger inputs have about 40mV of
hysteresis and the trigger levels are corrected for
this hysteresis so that the inputs will trigger at the
selected voltage independent of the selected
trigger slope.

The LED's above the trigger knobs will flash when
the input comparator triggers. The A and B inputs
may also be set to autolevel by rotating the knobs
completely counter-clockwise. The "AUTO" LED
under the knob will come on and the trigger
threshold will automatically be set to the midpoint
of the signal. Autolevel will work for a signal faster
than about 10Hz and a duty cycle greater than
about 0.0001%. The autolevel circuit will not
change the trigger level until the input stops
triggering for more than 1/2 second. Then it will
try to reset the trigger level to a new value. The
red LEDs behind the words "START" and "STOP"
will flash each time the autolevel circuit tries to
adjust the trigger threshold.

NOTE: in width and rise/fall time modes, which
use only one input, the A trigger knob sets the
start trigger voltage and the B trigger knob sets the
stop trigger voltage.

SETTING TRIGGER SLOPES

The A and B inputs can be set to trigger on either
a rising or falling edge by pressing the "SLOPE"
buttons. The EXT input can be set to rising edge
or falling edge by pressing the "LOGIC" button. If
the EXT input is being used to supply a gate pulse
the SR620 will use the time above threshold as
the gate if POSitive logic is selected and the time
below threshold as the gate if NEGative logic is
selected.

INPUT TERMINATION

The EXT, A, and B input may all be terminated in
either 1MOhm or 50 Ohms by pressing the
"INPUT" or "TERM" buttons. If the inputs are
terminated in 50 ohms and the input signal
exceeds ±6V peak the 50 Ohm terminator will
automatically be removed to prevent damage to
the terminator. When this overload condition
occurs the 50 ohm LED will flash.

UHF PRESCALERS

In frequency and period modes the input signal
may be fed to the SR620's UHF prescalers to
measure signals between 40MHz and 1.3GHz. To
engage the prescalers press the "INPUT" button
for the desired channel repeatedly until the UHF
LED comes on. The sensitivity of the prescalers
may be adjusted by adjusting the channel A and B
trigger knobs. Setting the knobs for 0V or to
autolevel will set the sensitivity to maximum.
Setting the knobs to 5V will reduce the sensitivity

SR620 Universal Time Interval Counter

10 Front Panel Operation

to about 200mV rms. Both positive and negative
settings of the knob have the same effect. The
sensitivity adjustment is useful because at
maximum sensitivity the prescalers will self­oscillate with no signal input. With an input,
however, this is not a problem, but by reducing the
sensitivity slightly these oscillations will disappear.

INPUT COUPLING

The A and B inputs may be either AC or DC
coupled by pressing the AC/DC button. The
coupling is independent of the input termination
impedance. The EXT input is always DC coupled.

REFERENCE OUTPUT

The front panel REF output puts out a 1kHz 50%
duty cycle square wave synchronized to the
SR620's internal 10MHz clock. This output may
provide 4V into high impedance or 2V into 50
Ohms, or ECL levels into 50 Ohms. An example
of the use of this output is to set the mode to
measure the time from REF to B. If a cable is then
connected from the REF output to B the cable
delay may be directly measured.

TIMEBASE INPUT AND OUTPUT

A rear panel BNC outputs the SR620's 10MHz
clock. This output supplies approximately 1V pk­pk into a 50 ohm load. Another BNC allows the
input of a 5 or 10 MHz external timebase. This
input presents a 1 kOhm load to the signal. The
SR620 can then phase-lock its internal timebase
to this external source. See the
CONFIGURATION MENU chapter for detail on
using an external timebase.

DVM INPUTS

The SR620 has two rear panel DVM inputs.
These 1 MOhm differential inputs allow the
SR620 to measure DC voltages on either a 2V or
20V full scale range. The SR620 can either
autorange the inputs (default) or they may be set
to a fixed scale. See the CONFIGURATION
MENU chapter for detail on setting the DVM
scales.

SR620 Universal Time Interval Counter

SAMPLE ARMING

The SR620 Time Interval counter has a wide
variety of arming modes that allow the user great
flexibility in controlling the desired measurement.
The various measurement modes and their
respective arming modes are discussed in detail
below.

NOTE: references to delayed or scanning gates
are discussed at the end of this section

TIME MODE

In this mode, the unit measures the time interval
between a Start and a Stop pulse. The time
interval is a positive number if the Start occurs
before the Stop and negative if the Stop occurs
before the Start. The SOURCE LED's indicate the
source of the START pulse.

Source LED
A A B

B B A

REF REF B

(The knobs which are directly above each input
set the voltage threshold for that input when in the
TIME mode.)

Usually, A will be selected as the Start source and
the time interval from A to B will be measured. The
internal 1kHz REF output may be selected in
cases where the front panel REF out will be used
to trigger an event, and the "event" will provide a
Stop pulse to the B input. A simple example of this
would be the measurement of cable lengths. The
length of a cable may be measured by selecting
REF as the Start source, and connecting the cable
between the REF output and the B input.

Start Source Stop Source

Sample Arming 11

When REF is used as the Start source, the rising
edge of REF is used when a positive slope is
selected for the A input, and the falling edge is
used when a negative slope is set. In this case,
the threshold knob above the A input has no effect
on operation.

TIME INTERVAL ARMING

There are several arming modes for Time interval
measurements. The arming mode controls when
the instrument will be ready to take a sample. Start
and Stop pulses are ignored when the instrument
is not armed. The ARM LED will turn on when the
unit is armed. The SR620 remains busy for about
800us after receiving a Start/Stop pair and may
not be rearmed during this time.

+TIME MEASUREMENTS (-1ns to 1000s)

In +TIME arming the Start input is armed as soon
as the instrument is ready and Stop is armed
when a Start is received. In this mode only a
positive or zero time interval may be measured.

In +TIME EXTERNAL mode the Start input is
enabled by the GATE and Stop is armed by Start.
A delay from an EXT input may be set or scanned
in the CONFIG menu, otherwise the EXT input will
be the trigger.

Time Arming Mode Function

+TIME Start arms automatically, Stop is armed by Start
+TIME EXTERNAL Start is armed by EXT input, Stop armed by Start
+TIME EXTERNAL STOP HOLDOFF Start is armed by leading edge of EXT input and the Stop is

armed by the trailing edge.

±TIME Starts and Stops are armed by Start/Stop pair
±TIME COMPLEMENT Starts and Stops are armed by Stop/Start pair
±TIME EXTERNAL Starts and Stops are armed by EXT input

SR620 Universal Time Interval Counter

12 Sample Arming

TIME EXTERNAL with HOLDOFF is similar

except that Stops are not enabled until the trailing
edge of the EXT input. A particular Stop pulse
may be selected to end the time interval by using
the EXT input to inhibit, or holdoff, the Stop input.
An example of this might be measuring the time
from the index mark on a hard disk drive to a
particular data bit. By adjusting the holdoff time
one could measure the time to any data bit instead
of just the first one.

The trigger may be delayed or scanned from an
EXT trigger input by setting SCAN parameters in
the CONFIG menu. A blinking EXT LED indicates
that the EXT is used as a trigger for the delayed
gate.

and Stop are both 1 KHz square waves, with the
Stop edge following the Start edge by 1 uS, then
the unit will report a Time interval of either +1 us or

-999 us. In the ±TIME and ±TIME COMPLEMENT
modes the start and stop inputs are armed by
parity, that is, the reception of either a Start/Stop
pair or a Stop/Start pair of pulses. By changing
between these two modes one may choose to
measure either the time from Stop to the next Start
or Start to the next Stop. Since the parity of the
input signals is randomly determined at power-up
there is no way to specify which arming mode will
correspond to which measurement. But by
changing between these modes both
measurements may always be obtained.

Alternately, one may arm a ±Time measurement
with a signal applied to the Ext input in which case
the measurement that is made (Start to Stop or
Stop to Start) is determined only by the
relationship of the Ext input to the Start and Stop
signals.

±TIME MEASUREMENTS (-1000s<t< 1000s)

In all of the ± TIME modes Starts and Stops are
armed simultaneously and so either a positive or
negative time interval may be measured. There is,
unfortunately, some ambiguity to this method of
arming. For periodic inputs there is no way for the
instrument to know if the desired time interval
should be measured from the Start to the previous
Stop or to the next Stop. For example, if the Start

SR620 Universal Time Interval Counter

The EXT input requires about 10 ns setup prior to
the Start or Stop inputs.

Time intervals as a function of delay from an EXT
trigger may be measured by using the internal
delay generator. The delay generator is triggered
by the EXT input. The trigger delay may be set or
scanned via the CONFIG Menu.

Sample Arming 13

Time interval samples may also be armed by
pressing the MAN key when in the EXT arming
mode.

WIDTH

Pulse widths may be measured in the WIDTH
mode. The pulse source may be either the A input,
the B input, or the internal 1 kHz REF source.

The Start threshold and slope are set by the
controls just above the A input. The Stop threshold
and slope are set by the controls just above the B
input. If the Start slope is positive, the Stop slope
will be negative, and the time from a rising edge to
a falling edge will be measured. These controls
are not used when measuring the width of the
internal 1 KHz REF.

As pulse widths are always positive times, only the
+TIME arming modes are available. The three
arming modes for WIDTH measurements are
shown here. The trigger source may be either the
EXT input or the EXT triggered internally
delayed/scanned gate.

RISE and FALL TIMES

The transition time for an input may be measured
in this mode. Either the A or B input may be
selected as the source to be measured. The
selected source is used as the input to both
comparators. The threshold knob above the A
input is used to specify the Start voltage threshold,
and the knob above the B input is used to specify
the Stop voltage threshold. The rise time of the
input is reported if positive slope is selected, and
the fall time is reported if negative slope is
selected. Either slope key changes both
comparators' slope LED's.

For example, to measure the 20-80% rise time of
a one volt input, the A threshold would be set to

0.20 VDC, the B threshold would be set to 0.80

VDC, and the slope would be set to positive to
measure the rising edge. The 80-20% transition
time of the falling edge could be measured by
setting the slope to negative and adjusting the
trigger thresholds. Reported times are not
corrected for the finite bandwidth of the input of
the instrument. The inputs have a bandwidth of
about 300 MHz, and so the 10-90% transition time
of an infinitely fast input would be reported as 1.2
ns. When measuring 10-90% transition times, the
actual transition time may be found by:

T actual =

As transition times are always positive numbers,
only +TIME and +TIME_EXT arming modes are
allowed.

sqrt (T

measured2 - 1.2 ns2 )

SR620 Universal Time Interval Counter

14 Sample Arming

FREQUENCY

The frequency of either the A or B inputs may be
measured in this mode of operation. The SR620 is
a "reciprocal" frequency counter. That is, it
measures frequency by measuring the time
interval for an integer number of cycles of the
input. The frequency is then equal to (number of
cycles)/(Measured Time). Since there is no
fraction-of-a-cycle error ( as would be seen if the
unit merely counted cycles in a fixed gate ) a
frequency measurement can be made to 11 digits
of resolution in one second. For frequencies
between 0 and 200MHz the SR620's front end
comparators may be directly used. For
frequencies between 200MHz and 1.3GHz the
Ultra High Frequency (UHF) prescalers must be
used.

The number of cycles used in the sample is
determined by the GATE . The instrument will
always measure at least 1 period of the input.
Gates from 1ms to 500s, or EXTernal gates may
be selected from the front panel. The actual gate
time for the internal gates is the gate time set on
the front panel multiplied by the "gate scale" set in
the configuration menu. Thus if one desired a 20s
gate one would set the gate scale to 200 and the
gate time to 0.1s (0.1s x 200 = 20s). If the "gate
scale" is not set to the default of 1 the gate time
led on the front panel will blink. Due to internal
synchronization circuitry, the frequency
measurement starts on the 2nd input edge after
the gate opens and ends on the second input
edge after the gate closes. Thus, a frequency
measurement always requires at least 2 complete
cycles of the input waveform.

The choice of GATE mode is determined mainly
by the desired resolution and measurement
speed. A longer gate results in a higher resolution
measurement with 11 digits obtainable in a one
second sample. The SR620 always adjusts the
number of displayed digits to reflect the
appropriate resolution depending on gate time.
The accuracy of frequency measurements is

determined by the accuracy of time interval
measurements. For very short gates, the
accuracy is determined by the 200ps time interval
accuracy, while for long gates the accuracy is
limited by the accuracy of the timebase.

EXTERNAL gates may be applied to the EXT
input and may range from 5ns to 1000s, although
the SR620 always measures for at least 1 input
period regardless of the gate time. The setup time
for an external gate is about 10 ns. Additionally,
the EXT input may be used to trigger any of the
internal gates (in this mode both the "ext" and the
gate time LED will be on). This is useful , for
example, to synchronize a 0.1s gate to an external
event. Additionally, gates of 1 us to 10 ms may
be scanned using the SR620's scanning facility.
These gates must be externally triggered. The
gate may either be fixed in time relative to the EXT
input, or may be automatically scanned at the end
of a measurement of N samples. If scanned, the
step size is equal to the gate width and the initial
delay may be set between 1 and 50000 gates
times. This scanning ability allows one to make
measurements of frequency as a function of time
from some event, such as the time response of a
VCO to a step change in control voltage.

The RATIO of the frequencies of two inputs will be
displayed if both the A & B source LED are on. It
should be noted that the A and B frequencies are
not measured simultaneously but are measured
on alternate measurement cycles.

PERIOD

Period measurements are done virtually the same
way as frequency measurements, however the
reciprocal of the frequency is reported instead of
the frequency. Gating modes are identical to
those used in the frequency mode.

When both the A and B source lights are on, the
ratio of the periods of signals applied to the A and
B inputs may be displayed. Period ratios may span
from 10-9 to 10+3.

PHASE

The phase between the A and B inputs is
measured in this mode of operation. Two
measurements are actually being done: the period
of the A input and the time interval between the A
and B inputs. For example, suppose the A and B
inputs are both 1 KHz square waves (1000us
period) with the rising edge of B coming 250us
after the rising edge of A. The unit would measure

SR620 Universal Time Interval Counter