Stanford Research Systems SR785 User Manual

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Operating Manual and Programming Reference

Model SR785 Dynamic Signal Analyzer

1290-D Reamwood Avenue

Sunnyvale, CA 94089 U.S.A.

Phone: (408) 744-9040 • Fax: (408) 744-9049

Email: info@thinkSRS.com • www.thinkSRS.com

Stanford Research Systems, Inc.

Revision 1.3 (September, 2004)

Certification

Stanford Research Systems certifies that this product met its published specifications at the time of shipment. Stanford Research Systems further certifies that its calibration measurements are traceable to the United States National Institute of Standards and Technology (NIST).

Warranty

This Stanford Research Systems product is warranted against defects in materials and workmanship for a period of one (1) year from the date of shipment.

Service

For warranty service or repair, this product must be returned to a Stanford Research Systems authorized service facility. Contact Stanford Research Systems or an authorized representative before returning this product for repair.

Information in this document is subject to change without notice.

Stanford Research Systems, Inc. 1290-D Reamwood Avenue Sunnyvale, California 94089

Printed in U.S.A.

SR785 Dynamic Signal Analyzer

Safety and Preparation For Use

WARNING!

Dangerous voltages, capable of causing injury or death, are present in this instrument. Use extreme caution whenever the instrument cover is removed. Do not remove the cover while the unit is plugged into a live outlet.

Caution

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 SR785 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 so that the desired voltage is 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

Line Cord

Service

Fan

Verify that the correct line fuse is installed before connecting the line cord. For 100V/120V, use a 1.5 Amp fuse. For 220V/240V, use a 3/4 Amp fuse.

The SR785 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.

Do not attempt to service or adjust this instrument unless another person, capable of providing first aid or resuscitation, is present.

Do not install substitute parts or perform any unauthorized modifications to this instrument. Contact the factory for instructions on how to return the instrument for authorized service and adjustment.

The fans in the SR785 are required to maintain proper operation. Do not block the vents in the chassis or the unit may not operate properly.

SR785 Dynamic Signal Analyzer

iii

Contents

Safety and Preparation For Use i Contents iii Table of Figures vii Features ix Specifications xi

Chapter 1 Getting Started

General Installation 1-3 Front Panel Quick Start 1-4 Things To Watch Out For 1-6 Analyzing a Sine Wave 1-9 Measuring a Frequency Response Function 1-13 Linking (Advanced Operation) 1-17 Triggering and the Time Record 1-21 Octave Analysis 1-25 Capture 1-29 Waterfall Display 1-37 Swept Sine Measurement 1-45 Saving and Recalling 1-51 User Math Functions 1-57 Limit Testing 1-61 Exceedance Statistics 1-65

Chapter 2 Analyzer Basics

Measurement Groups 2-5 What is an FFT? 2-7 FFT Frequency Spans 2-9 FFT Time Record 2-11 FFT Windowing 2-13 FFT Measurements 2-17 Views 2-22 FFT Averaging 2-25 Real Time Bandwidth and Overlap 2-28 Waterfall Display 2-31 Capture Buffer 2-34 The Source 2-37 Correlation Analysis 2-39 Octave Analysis 2-42 Swept Sine Measurements 2-48 Order Analysis 2-57 Time/Histogram Measurements 2-60 Trace Storage 2-62 User Math Functions 2-63 Signal Inputs 2-67 Input Connections 2-70

SR785 Dynamic Signal Analyzer

iv Contents

Intrinsic Noise Sources 2-72 External Noise Sources 2-73 Curve Fitting and Synthesis 2-77

Chapter 3 Operation

Overview 3-3 Front Panel Connectors 3-6 Rear Panel Connectors 3-8 Screen Display 3-11 Status Indicators 3-18 Keypad 3-24 Normal and Alternate Keys 3-24 Menu Keys 3-25 Entry Keys 3-26 Control Keys 3-29 Function Keys 3-35 Macros 3-39

Chapter 4 Menus

Frequency Menus 4-7

FFT Frequency Menu 4-7 Octave Frequency Menu 4-12 Swept Sine Frequency Menu 4-15 Order Frequency Menu 4-19 Time/Histogram Frequency Menu 4-23

Display Setup Menu 4-25

Display Options Menu 4-46

Marker Menu 4-50

Normal Marker Menu 4-54 Harmonic Marker Menu 4-58 Sideband Marker Menu 4-61 Band Marker Menu 4-64

Frequency/Damping Marker Menu 4-66

Source Menus 4-67

Sine Source Menu 4-71 Chirp Source Menu 4-73 Noise Source Menu 4-75 Arbitrary Source Menu 4-78 Swept Sine Source Menu 4-83

Input Menu 4-87

Input Configuration Menu 4-89 Transducer Parameter Menu 4-92 Tachometer Input Menu 4-95 Playback Input Menu 4-97

Trigger Menu 4-100

Average Menus 4-107

SR785 Dynamic Signal Analyzer

Contents v

FFT, Correlation, and Order Average Menu 4-107 Octave Average Menu 4-115 Swept Sine Average Menu 4-119

User Math Menu 4-122

Window Menu 4-131

Waterfall Menu 4-135

Capture Menu 4-143

Analysis Menu 4-147

Data Table Analysis Menu 4-150 Limit Testing Analysis Menu 4-152 Marker Statistics Analysis Menu 4-156 Exceedance Statistics Analysis Menu 4-159 Curve Fit Menu 4-161

Disk Menu 4-167

Nodal Degree-of-Freedom Menu 4-171 Recall Settings Menu 4-174 Disk Buffers Menu 4-177 Disk Upkeep Menu 4-182

Output Menu 4-184

System Menu 4-191

System Remote Menu 4-194 System Preferences Menu 4-196 System Date/Time Menu 4-199 System Diagnostics Menu 4-200 Edit Macro Menu 4-203

Chapter 5 Programming

Index of Commands 5-2 Alphabetical List of Commands 5-12 Introduction 5-21 Command Syntax 5-25 Frequency Commands 5-28 Display Setup Commands 5-37 Display Options Commands 5-43 Marker Commands 5-45 Source Commands 5-53 Input Commands 5-60 Trigger Commands 5-66 Average Commands 5-68 User Math Commands 5-74 Window Commands 5-78 Waterfall Commands 5-80 Capture Commands 5-83 Memory Allocation Commands 5-84 Data Table Commands 5-85 Limit Test Commands 5-87

SR785 Dynamic Signal Analyzer

vi Contents

Marker Statistics Commands 5-89 Exceedance Statistics Commands 5-90 Disk Commands 5-94 Output Commands 5-97 System Commands 5-100 Front Panel Commands 5-102 Data Transfer Commands 5-107 Interface Commands 5-117 Nodal Degree-of-Freedom Commands 5-118 Status Reporting Commands 5-120 Status Word Definitions 5-124 Example Program 5-129

Chapter 6 File Conversion

Why File Conversion? 6-2 SR785 File Types 6-3 Supported External File Types 6-4 Using the File Conversion Utility 6-5

SR785 Dynamic Signal Analyzer

vii

Table of Figures

Figure 2-1 Waterfall Display 2-31 Figure 2-2 Transfer Functions 2-49 Figure 2-3 Capacitive Coupling 2-61 Figure 2-4 Inductive Coupling 2-73 Figure 2-5 Resistive Coupling 2-74

Figure 3-1 Front Panel 3-3 Figure 3-2 Rear Panel 3-8 Figure 3-3 Dual Display Screen 3-11 Figure 3-4 Overlay Display Screen 3-12 Figure 3-5 Vertical Scale Bar 3-13 Figure 3-6 Horizontal Scale Bar 3-14 Figure 3-7 Marker Region 3-14 Figure 3-8 Marker Position Bar 3-15 Figure 3-9 Status Indicator Panel 3-18 Figure 3-10 Front Panel Keypad 3-24

SR785 Dynamic Signal Analyzer

viii

SR785 Dynamic Signal Analyzer

Features

Measurements FFT Group

FFT (Linear Spectrum) Time Record Windowed Time Time Capture Frequency Response Power Spectrum Cross Spectrum Coherence Orbit User Math Functions

Correlation Group

Cross Correlation Autocorrelation Time Record Windowed Time Time Capture User Math Functions

Octave Analysis Group

1/1, 1/3, 1/12 Octave Time Capture User Math Functions L

Impulse

Swept Sine Group

Spectrum Transfer Function Cross Spectrum Normalized Variance User Math Functions

Time Histogram Group

Histogram Probability Density Cumulative Density Unfiltered Time Record User Math Functions Time Capture

Views Log Magnitude Linear Magnitude Magnitude Squared

Real Part Imaginary Part Phase Unwrapped Phase Nichols Plot Nyquist Plot

Displays Single Dual Upper/Lower Front/Back Waterfall

with Skew Zoom and Pan

FFT Resolution 100, 200, 400 or 800 FFT lines

FFT Windows Hanning Blackman-Harris Flattop

Kaiser Uniform Force/Exponential User defined

Correlation Windows [-T/2..T/2] [-T/4..T/4] [0..T/2]

Averaging RMS Vector Peak Hold

Linear or Exponential Preview Time Records Equal Confidence

Analysis Harmonic Sideband Band

Peak Finding THD/THD+N Limit Testing Data Table Exceedance (L Waterfall Slice Curve Fitting Curve Synthesis

User Math +, -, *, / Conjugate Magnitude/Phase

Real/Imaginary Sqrt FFT/Inverse FFT

ω Log/Exp d/dx

j Group Delay A, B, C Wt

) Statistics

SR785 Dynamic Signal Analyzer

x Features

Source Outputs Sine Two Tone Swept Sine

Offset White/Pink Noise Burst Noise Chirp Burst Chirp Arbitrary

Trigger Arming Auto Arm Manual Arm RPM Arm

Time Arm

Trigger Sources Continuous Internal External

External/TTL Source Manual

Time Capture Capture time data for later analysis (FFT or Octave). Up to 2 Msamples

of data can be saved with standard memory, 8 Msamples with optional memory.

Storage 3.5”, 1.44 Mbytes, DOS formatted disk. Save data, setups and hardcopy.

Hard Copy Print to dot matrix or LaserJet/InkJet printers. Plot to HPGL or Postscript

plotters. Print/Plot on-line (serial, parallel or IEEE-488) or to disk file. GIF, EPS and PCX graphic formats available for disk output.

File Translation Native SR785 binary files can be converted to: ASCII, MATLAB

.MAT Files, Universal File Format, and HP SDF v3 Files.SDF and SR780 files can be converted to native SR785 binary format.

Interfaces RS232 serial, Centronics parallel and IEEE-488.

Help On screen help system provides Operating Manual and Programming

Reference on-line.

SR785 Dynamic Signal Analyzer

Specifications

Specifications apply after 30 minutes of warm-up and within 2

hours of last auto-offset. All specifications are with 400 line FFT resolution and anti-alias filters enabled unless stated otherwise.

Frequency

Range 102.4 kHz or 100 kHz (both displays have the same range). FFT Spans 195.3 mHz to 102.4 kHz or 191 mHz to 100 kHz. The 2 displays can have different spans and start frequencies. FFT Resolution 100, 200, 400 or 800 lines Real Time Bandwidth 102.4 kHz (highest FFT span with continuous data acquisition and averaging on both inputs). Accuracy 25 ppm from 20°to 40°C

FFT Dynamic Range

Dynamic Range -90 dBfs typical, -80 dBfs guaranteed (FFT and Octave), 145 dB (Swept Sine). Includes spurs, harmonic and intermodulation distortion and alias products. Excludes alias responses at extremes of span. Harmonic Distortion <-80 dB (Single tone in band). Intermodulation Distortion <-80 dB (Two tones in band, each <- 6.02 dBfs). Spurious <-80 dBfs Alias Responses <-80 dBfs (Single tone outside of span, < 0 dBfs, < 1 Mhz). Full Span FFT Noise Floor -100 dBfs typical (Input grounded, Input Range > -30 dBV, Hanning window, 64 RMS averages). Residual DC Response < -30 dBfs (FFT with Auto Cal On).

Amplitude Accuracy

Single Channel ± 0.2 dB (excluding windowing effects). Cross Channel ± 0.05 dB (dc to 102.4 kHz) (Transfer Function measurement, both inputs on the same Input Range, RMS averaged).

Phase Accuracy

Single Channel ± 3.0 deg relative to External TTL trigger. (-50 dBfs to 0 dBfs, freq < 10.24 kHz) (Center of frequency bin, DC coupled) For Uniform, Blackman-Harris, Hanning, Flattop and Kaiser windows, phase is relative to a cosine wave at the center of the time record. For Force and Exponential windows, phase is relative to a cosine wave at the beginning of the time record. Cross Channel ± 0.5 deg (dc to 51.2 kHz) ± 1.0 deg (dc to 102.4 kHz) (Transfer Function measurement, both inputs on the same Input Range, Vector averaged.)

SR785 Dynamic Signal Analyzer

xii Specifications

Signal Inputs

Number of Inputs 2 Full Scale Input Range -50 dBV (3.16 mVpk) to +34 dBV (50 Vpk) in 2 dB steps. Maximum Input Level 57 Vpk Input Configuration Single-ended (A) or True Differential (A-B). Input Impedance 1 M Shield to Chassis Floating Mode: 1 M Grounded Mode: 50 Shields are always grounded in differential input (A-B). Maximum Shield Voltage 4 Vpk AC Coupling -3 dB rolloff at 0.16 Hz. CMRR 90 dB at 1 kHz (Input Range < 0 dBV). 80 dB at 1 kHz (Input Range <10 dBV). 50 dB at 1 kHz (Input Range ICP Signal Conditioning Current Source: 4.8 mA Open Circuit Voltage +26 V A-weight Filter ANSI Standard S1.4-1983; 10 Hz to 25.6 kHz, Type 0 Tolerance. Crosstalk <-145 dB below signal, (Input to Input and Source to Inputs, 50 source impedance). Input Noise <10 nVrms/

Ω + 50 pF

Ω + 0.01 µF

Ω

≥10 dBV).

Ω receiving input

√Hz (< -160 dBVrms/√Hz) above 200 Hz.

Trigger Input

Modes Continuous, Internal, External, or External TTL. Internal Level adjustable to ±100% of input scale. Positive or Negative slope. Minimum Trigger Amplitude: 5% of input range External Level adjustable to ±5V in 40 mV steps. Positive or Negative slope. Input Impedance: 1 M

Max Input: ±5V

Ω

Minimum Trigger Amplitude: 100 mV External TTL Requires TTL level to trigger (low<0.7V, high>3.0V). Post-Trigger Measurement record is delayed up to 100,000 samples after

the trigger. Pre-Trigger Measurement record starts up to 8000 samples prior to the trigger.

Tachometer Input

Modes External, or External TTL. External Level adjustable to ±5V in 4 mV steps. or ±25V in 20 mV steps. Positive or Negative slope.

Time Capture

Mode Continuous real time data recording to memory. Maximum Rate 262,144 samples/sec for both inputs. Lower rates may be used for longer capture. Maximum Capture Length 2M samples standard,

SR785 Dynamic Signal Analyzer

Specifications xiii

8M samples optional.

Octave Analysis

Standards Conforms to ANSI S1.11-1986, Order 3, Type 1-D. Frequency Range Band centers: Single Channel 1/1 Octave 0.125 Hz - 32 kHz 1/3 Octaves 0.100 Hz - 40 kHz 1/12 Octaves 0.091 Hz - 12.34 kHz Two Channels 1/1 Octave 0.125 Hz - 16 kHz 1/3 Octaves 0.100 Hz - 20 kHz 1/12 Octaves 0.091 Hz - 6.17 kHz Accuracy < 0.2 dB (1 second stable average, single tone at band center). Dynamic Range 80 dB (1/3 Octave, 2 second stable average), per ANSI S1.11-1986. Sound Level Exponential time averaged broadband power (L), per ANSI S1.4-1983, Type 0. Broadband Impulse and Peak power, per IEC 651-1979, Type 0. Sum of octave bands total power.

Source Output

Amplitude Range 0.1 mVpk to 5 Vpk Amplitude Resolution 0.1 mVpk DC Offset <10.0 mV (typical) Output Impedance < 5

Ω; ±100 mA peak output current.

Sine

Amplitude Accuracy ±1% of setting, 0 Hz to 102.4 kHz

0.1 Vpk to 5.0 Vpk, high impedance load. Offset 0 V to Harmonics, Sub-Harmonics 0.1 Vpk to 5 Vpk, 0 V offset, and Spurious Signals <-80 dBc (fundamental < 30 kHz), <-74 dBc (fundamental > 30 kHz).

±5 V, max output ±5 V (ac+dc).

Two Tone

Amplitude Accuracy ±1% of setting, 0 Hz to 102.4 kHz

0.1 Vpk to 5 Vpk, high impedance load. Offset 0 V to Harmonics, Sub-Harmonics 0.1 Vpk to 2.5 Vpk, 0 V offset, and Spurious Signals < -80 dBc (fundamental < 30 kHz), <-74 dBc (fundamental > 30 kHz).

±5 V, max output ±5 V (ac+dc).

White Noise

Time Record Continuous or Burst Bandwidth DC to 102.4 kHz or limited to analysis span. Flatness <0.25 dB pk-pk (typical), <1.0 dB pk-pk (max), (5000 rms averages).

SR785 Dynamic Signal Analyzer

xiv Specifications

Pink Noise

Time Record Continuous or Burst Bandwidth DC to 102.4 kHz Flatness <2.0 dB pk-pk, 20 Hz - 20 kHz, (measured using averaged 1/3 Octave Analysis).

Chirp

Time Record Continuous or Burst Output Sine sweep across the FFT span. Flatness ±0.25 dB pk-pk, Amplitude = 1.0 Vpk. Crest Factor 1.94 (5.77 dB)

Swept Sine

Auto Functions Source Level, Input Range and Frequency Resolution. Dynamic Range 145 dB Offset 0 V to

±5 V, max output ±5 V (ac+dc).

Arbitrary

Amplitude Range ± 5V Record Length 2M samples (playback from Arbitrary Waveform memory or a Capture buffer), 4M and 8M samples optional. Variable output sample rate.

General

Monitor Monochrome CRT, 800H by 600V resolution. Interfaces IEEE-488, RS232 and Printer interfaces standard. All instrument functions can be controlled through the

IEEE-488 and RS232 interfaces. A PC (XT) keyboard input is

provided for additional flexibility. Hardcopy Print to dot matrix and HP LaserJet/InkJet compatible printers.

Plot to HPGL or Postscript plotters. Print/Plot to RS232 or

IEEE-488 interfaces or to disk file. Additional file formats

include GIF, PCX and EPS. Disk 3.5 inch DOS compatible format, 1.44 Mbytes capacity.

Storage of data, setups and hardcopy. Preamp Power Power connector for SRS preamplifiers. Power 70 Watts, 100/120/220/240 VAC, 50/60 Hz. Dimensions 17"W x 8"H x 22"D Weight 56 lb. Warranty One year parts and labor on materials and workmanship.

SR785 Dynamic Signal Analyzer

Specifications xv

SR785 Dynamic Signal Analyzer

1-1

Chapter 1

Getting Started

These example measurements are designed to acquaint the first time user with the SR785 Dynamic Signal Analyzer. They provide a foundation for understanding how to use the SR785. For a more complete overview of the instrument and its capabilities, refer to the ‘Analyzer Basics’ and ‘Operation’ sections of this manual.

Many of the examples use the test filter enclosed with this manual. The filter is a simple twin-tee 1 kHz passive notch filter. This filter provides an interesting frequency response for these measurements.

In This Chapter

General Installation 1-3

Caution 1-3 Line Voltage Selection 1-3 Line Fuse 1-3 Line Cord 1-3 Power Switch 1-3 Screen Brightness 1-3 Fan 1-3

Front Panel Quick Start 1-4

[Hardkeys] 1-4 <Softkeys> 1-4 [Alt] Keys 1-4 Knob 1-4 Help 1-5

Things To Watch Out For 1-6

Start 1-6 Live Display 1-6 Narrow Span 1-6 Low Detection Frequency 1-6 Averaging 1-6 Triggering 1-6 Scaling and Ranging 1-7 Local 1-7 Reset 1-7

Analyzing a Sine Wave 1-9

Measuring a Frequency Response

Function 1-13

Linking (Advanced Operation) 1-17

Triggering and the Time Record 1-21

Octave Analysis 1-25

Capture 1-29

Waterfall Display 1-37

Swept Sine Measurement 1-45

Saving and Recalling 1-51

User Math Functions 1-57

Limit Testing 1-61

Exceedance Statistics 1-65

SR785 Dynamic Signal Analyzer

1-2 Getting Started

SR785 Dynamic Signal Analyzer

Getting Started 1-3

General Installation

Caution

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

Line Fuse

Verify that the correct line fuse is installed before connecting the line cord. For 100V/120V, use a 1.5 Amp fuse. For 220V/240V, use a 3/4 Amp fuse.

Line Cord

Power Switch

The power switch is on the rear panel. Turn the unit on by depressing the upper half of the power switch. The green power LED on the front panel indicates that the unit is powered.

Screen Brightness

If the screen is too dark or too bright, adjust the brightness using the Brighter and Dimmer buttons below the softkeys (below right of the display). Do not set the brightness higher than necessary.

Fan

The fans in the SR785 are required to maintain proper operation. Do not block the vents in the chassis or the unit may not operate properly.

SR785 Dynamic Signal Analyzer

1-4 Getting Started

Front Panel Quick Start

There are two types of front panel keys which are referenced in this section. Hardkeys are those keys with labels printed on them. Their function is determined by the label and does not change. Hardkeys are referenced by brackets like this - [Hardkey]. Softkeys are the ten gray keys along the right edge of the screen. Their function is labeled by a menu box displayed on the screen next to the key. Softkey functions change depending upon the menu and instrument configuration. Softkeys are referenced as the <Softkey>.

[Hardkeys]

The keypad consists of four groups of hardkeys (keys with printed labels).

The ENTRY keys are used to enter numeric parameters which have been highlighted by a softkey. The MENU keys select a menu of softkeys. Pressing a menu key will change the menu boxes which are displayed next to the softkeys. Each menu presents a group of similar or related parameters and functions. The CONTROL keys start and stop data acquisition, toggle the active display and link parameters and functions. These keys are not in a menu since they are used frequently and within any menu. The FUNCTION keys perform common functions such as Auto Scale and Auto Range. These keys can be accessed at any time.

The SR785 has a menu driven user interface. The Menu keys each display a menu of softkeys. The softkeys are at the right of the video display and have different functions depending upon the displayed menu.

There are three types of softkeys - buttons, lists and numeric values. A button performs a function, such as <Full Span>. A list presents a list of choices or options in the entry field (at the top of the screen). Use the knob to make a selection and press [Enter]. <Measurement> is an example of a list. A numeric value presents the current value in the entry field and awaits numeric entry. Enter a new value with the entry keys and press [Enter]. <Start Freq> is an example of a numeric value.

[Alt] Keys

The [Alt] key is a special key which has no meaning by itself but instead modifies the meaning of another keypress or knob turn. Pressing the [Alt] key toggles the state of the ALT indicator at the top of the screen. Pressing a control key while the ALT indicator is lit selects the "alternate" function labeled underneath the key instead of the normal function. Turning the knob when ALT is lit affects the way the markers move in the waterfall, data tables, and other displays.

Knob

The knob normally moves the markers within the displays. If a parameter has been highlighted by its softkey, the knob adjusts the parameter. List parameters are most easily modified with the knob. Numeric parameters may also be adjusted with the knob.

Knob list selections are referenced in parenthesis like (Hanning).

SR785 Dynamic Signal Analyzer

Getting Started 1-5

Help

Enter the on screen help system by pressing [Help/Local]. Help on any hardkey or softkey is available simply by pressing the key. Press [1] for the Help Index. Press [0] to exit the help system and return to normal operation.

SR785 Dynamic Signal Analyzer

1-6 Getting Started

Things To Watch Out For

If the analyzer is on but doesn't seem to be taking data, there are a number of things to check.

Start

Press the [Start/Reset] key to start the measurements. Make sure the Run/Pause indicator at the top of the screen displays ‘RUN’ instead of ‘PAUSE’. Note that in many cases when settings are changed using the menus, the new settings will not take effect until [Start/Reset] is pressed.

Live Display

If the displays are showing recalled trace data, they are Off-Line and do not display the live measurement data. Set the Display to Live instead of Off-Line (in the [Display Options] menu).

Narrow Span

If the FFT span is very narrow, the time record is very long (up to 1000’s of seconds). Completely new data is available only every time record. Change the Time Record Increment in the [Average] menu) to display overlapped data more often.

Low Detection Frequency

Swept Sine measurements at very low frequencies (<< 1 Hz) take a very long time (at least 2 cycles and maybe longer). Do not set the sweep Start to a very low frequency to measure the DC response.

Octave measurements with a very low starting band take a long time to settle before the first measurement is valid. The settling time is related to the bandwidth of the lowest octave band. If the Lowest Band is less than 1 Hz, the settling time can be very long.

Averaging

Very long averaging times for any measurement may give the appearance that the display does not update. Check the FFT Number Of Averages, the Octave Integration Time or the Swept Sine Integration Time.

When Linear averaging is on, the measurement is paused after the average is completed (unless triggered or waterfall storage is on). Press [Start/Reset] to take another average.

Triggering

If the analyzer is waiting for a trigger, the Trig Wait indicator at the top of the screen is on.

If the measurement is not meant to be triggered, make sure the Trigger Mode is Auto Arm and the Trigger Source continuous.

If the measurement is meant to be triggered, make sure that the correct Trigger Source is selected and the Trigger Level is appropriate for the trigger signal.

SR785 Dynamic Signal Analyzer

Getting Started 1-7

Check that the Trigger Mode is set to Auto Arm. If the Trigger Mode is Manual Arm, then the analyzer will only trigger once and then wait for the next Manual Arm command.

Scaling and Ranging

Check that the inputs are not completely overloaded by using [Auto Range Ch1] and [Auto Range Ch2].

Scale the display to show the entire range of the data with [Auto Scale A] and [Auto Scale B].

Local

Make sure that the analyzer is not in the REMOTE state where the computer interfaces have setup the instrument and locked out the front panel. Press the [Local/Help] key to restore local control.

Reset

If the analyzer still seems to function improperly, turn the power off and turn it back on while holding down the [<-] (backspace) key. This will reset the analyzer into the default configuration. The analyzer should power on running and taking measurements.

SR785 Dynamic Signal Analyzer

1-8 Getting Started

SR785 Dynamic Signal Analyzer

Analyzing a Sine Wave 1-9

Analyzing a Sine Wave

This measurement investigates the spectrum of a 1.024 kHz sine wave. You will use the SR785 source to provide the sine signal (or you can use a function generator capable of providing a 1.024 kHz sine wave at a level of 100 mV to 1 V, such as the SRS DS345). The actual settings of the generator are not important since you will be using the SR785 to measure and analyze its output.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the Channel 1 A Input.

(Or connect a function generator's output to the

Ch1 A Input of the analyzer.)

3. Press [Source] Select the Source menu.

Press <Sine>

Press <Frequency 1>

Press [1] [.] [0] [2] [4], select (kHz) with the

knob, and press [Enter].

Press <On> Turn the source on. When the instrument is turned

(Or

turn on the generator, set the frequency to

1.024 kHz and the amplitude to approximately 1 Vrms.)

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Setup to analyze the source output.

(The input impedance of the analyzer is 1 M generator may require a terminator. Many generators have either a 50 impedance. Use the appropriate feedthrough termination if necessary. In general, not using a terminator means that the output amplitude will not agree with the generator setting and the distortion may be greater than normal.)

Choose Sine output.

Adjust the output Frequency.

Enter 1.024 kHz for the Frequency. Enter the value with the numeric entry keys. Select the units with the knob. Enter the new value with the [Enter] key.

on, the source is always off. (Setup the function generator for 1.024 kHz sine output.)

Ω or 600 Ω output

Ω. The

SR785 Dynamic Signal Analyzer

1-10 Analyzing a Sine Wave

4. Press [Auto Range Ch1]

5. Press [Freq]

Press <Span>

Use the knob to adjust the Span to 6.4 kHz and

press [Enter].

6. Press [Display Options]

Press <Format>

Select (Single) with the knob and press [Enter]. Select the desired option from the displayed list and

7. Press [Auto Scale A]

Press [Marker Max]

8. Use the knob to move the Marker around. Take a look at some of the harmonics.

9. Let’s look at the fundamental only.

Press [Span Down] twice to decrease the Span to

1.6 kHz. The Stop Frequency shown at the bottom right of the graph should read 1.6 kHz.

Press [Marker Max]

Let the analyzer automatically set the Input Range to agree with the signal (either from the Source or function generator). Note that the Ch1 Input Range readout at the top of the screen is displayed in inverse when Ch1 Auto Range is on.

Select the Frequency menu.

Adjust the FFT Span.

Set the Span to display the signal and its first few harmonics.

Select the Display Options menu.

Choose a new Display Format.

press [Enter]. Single Display Format shows a single large graph.

Automatically scale DisplayA (the active display) to show the entire range of the measurement.

This moves the Marker to the maximum data point in the active display (A). The Marker should now be on the 1.024 kHz signal. The Marker Position shown above the graph displays the frequency and amplitude of the signal.

The knob normally adjusts the Marker Position within the active display (DisplayA in this case). If a menu box is highlighted with a softkey, the knob adjusts the selected parameter shown in the entry field at the top of the screen.

You can also use the [Span Up] and [Span Down] keys to adjust the Span.

This isolates the 1.024 kHz fundamental frequency. You may notice that the spectrum takes a noticeable time to settle at this last span. This is because the time record is 250 ms long.

Move the Marker to the peak.

SR785 Dynamic Signal Analyzer

Analyzing a Sine Wave 1-11

Press [Marker Center]

10. Let's look at the signal distortion.

Press [Freq]

Press <Span>

Enter [1] [2] [.] [8], select (kHz) with the knob,

and press [Enter].

Press [Auto Scale A]

11. Let's measure some harmonics using the Marker Reference.

Press [Marker Max]

Press [Marker Ref]

Use the knob to move the Marker to the

harmonics.

Press [Marker Ref]

12. Let's have the analyzer measure the distortion.

Press [Marker]

Press <Mode>

Select (Harmonic) with the knob and press

[Enter].

This sets the span Center Frequency to the Marker Position (for the active display). The signal will be at the center of the span. Further adjustments to the span will keep the center frequency fixed.

Select the Frequency menu.

Adjust the Span.

You can also use the numeric keypad to enter the span.

Enter the 12.8 kHz span numerically. Note that the Center Frequency is no longer 1.024 kHz. This is because a 12.8 kHz span cannot be centered below

6.4 kHz without starting at a negative frequency.

Adjust the graph scale and reference to display the entire range of the data. This key can be used at any time.

Move the Marker to the fundamental peak.

Set the Marker Offset or Reference to the amplitude of the fundamental. The Marker Position above the graph now reads relative to this offset (∼0 dB). This is indicated by the ∆ in front of the Marker Position reading. A small flag shaped symbol is located at the screen location of the reference.

The Marker Position shows the distortion peaks relative to the fundamental.

Pressing [Marker Ref] again removes the Marker Offset and returns the Marker to absolute readings.

Select the Marker menu.

Adjust the Marker Mode.

Choose the Harmonic Marker for the active display.

SR785 Dynamic Signal Analyzer

1-12 Analyzing a Sine Wave

Press <# Harmonics>

Use the knob to adjust the Number Of

Harmonics to 10 and press [Enter].

Press [Marker Max]

This concludes this measurement example. You

Adjust the Number Of Harmonics for analysis.

Enter 10 harmonics.

Move the Marker to the peak (fundamental).

Notice that Harmonic Markers (little triangles) appear on top of all of the harmonic peaks. These indicate which data points are used in the harmonic calculations.

The harmonic calculations are displayed within the menu. THD (total harmonic distortion) is relative to the fundamental. Harmonic power is an absolute measurement of the harmonic power level.

should have a feeling for the basic operation of the menus, knob and numeric entry, marker movement and some function keys.

SR785 Dynamic Signal Analyzer

Measuring a Frequency Response Function 1-13

Measuring a Frequency Response Function

This example investigates the frequency response of the test filter (enclosed with this manual) using FFT measurements. You will use the SR785 source to provide a broad band chirp and both input channels to measure the input to and output from the device under test.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Use a BNC Tee to connect the Source Output to the filter input and the Ch1 A Input.

Connect the filter output to the Ch2 A Input.

3. Press [Source]

Press <Chirp>

Press <On> Turn the source On. Press [Window]

Press <Window>

Select (Uniform) with the knob and press

[Enter].

4. Press [Auto Range Ch1]

Press [Auto Range Ch2]

5. Press [Freq]

Press <Span>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Frequency Response is defined as Ch2 response divided by the Ch1 reference. Thus, Ch1 monitors the filter input (source output) and Ch2 measures the response of the device under test.

Select the Source menu.

Choose Chirp output. The output is an equal amplitude sine wave at each frequency bin of the FFT spectrum.

Select the Window menu.

Adjust the FFT Window function.

The Chirp source requires the use of the Uniform window since not all chirp frequency components are present at all points in the time record. The chirp is exactly periodic with the FFT time record and does not ‘leak’ with the uniform window.

Let the analyzer automatically set the Input Ranges to agree with the signals. Note that the Input Range readouts at the top of the screen are displayed in inverse when Auto Range is on.

Select the Frequency menu.

Adjust the FFT Span.

SR785 Dynamic Signal Analyzer

1-14 Measuring a Frequency Response Function

Use the knob to adjust the Span to 6.4 kHz and

press [Enter].

The top display (A) is measuring the filter input and

6. Press [Display Setup]

Press <Measurement>

Select (Freq. Resp.) with the knob and press

[Enter].

Press [Auto Scale A]

7. Press [Marker]

Press <Width>

Select (Normal) with the knob and press [Enter]. Change to Normal Width (1/2 division).

Press <Seeks>

Select (Min) with the knob and press [Enter].

Move the Marker Region with the knob to find

the notch frequency and depth. Or press [Marker Min].

8. Press [Display Options]

Press <X-Axis>

Press <Link> The <Link> key lets you make a choice for both

Set the Span to display the filter notch at 1 kHz.

should show a fairly flat spectrum. The bottom display (B) is measuring the filter output and should show a deep notch.

Both displays are measuring absolute signal levels.

Select the Display Setup menu.

Adjust the Measurement of the active display (A).

Choose Frequency Response for the Measurement in DisplayA (top).

Frequency Response is the ratio of the response (Ch2) to the input (Ch1) and is a unitless quantity.

Adjust the scale and reference for DisplayA to show the entire range of the data.

Select the Marker menu.

Adjust the Marker Width for DisplayA.

Adjust what the Marker Seeks within the Marker Region.

Seek the Minimum of the data within the Marker Region.

The Marker Region makes it easy to find narrow peaks and valleys in the graph. The notch should be around 1 kHz and about -60 dB deep.

Select the Display Options menu.

The graph might look better on a log x axis.

displays. We want both displays on a log xaxis.Note that the linkage indicator at the top of the screen changes from DispA to Link.

SR785 Dynamic Signal Analyzer

Measuring a Frequency Response Function 1-15

Select (Log) with the knob and press [Enter].

9. Let’s show phase response on DisplayB (bottom).

Press [Active Display]

Press [Display Setup]

Press <Measurement>

Select (<Freq. Resp.>) with the knob and press

[Enter].

Press <View>

Select (Phase) with the knob and press [Enter]. Choose Phase View to show the phase of the

Press [Auto Scale B]

10. Let’s link the Markers together.

Press [Active Display]

Press [Marker]

Press <Width>

Select (Spot) with the knob and press [Enter]. Change the Marker Width to Spot.

11. Press [Link] and use the knob to move the marker.

Press [Enter]

Log scale is a common way to display filter response functions. Note that both displays now have log x-axes.

The two displays have separate Measurements.

Make DisplayB the active display. The active display has its Marker Position Bar (above the graph) highlighted.

Select the Display Setup menu. The setup of DisplayB (the active display) is now shown in the menu.

Adjust the Measurement of DisplayB.

Choose Frequency Response also.

The measured data is a set of complex values which can be viewed in a number of different ways.

transfer function.

Scale DisplayB to show the entire phase transfer function.

Make DisplayA (top) the active display.

Select the Marker menu.

Adjust the Marker Width of DisplayA.

The [Link] key links the two display markers together. This allows simultaneous readout of Transfer Function Magnitude (top) and Phase (bottom).

Pressing any key removes the linkage between the markers.

SR785 Dynamic Signal Analyzer

1-16 Measuring a Frequency Response Function

To permanently link the Markers, go to the

Marker menu.

Press [Marker]

Press <Marker>

Select (Link) with the knob and press [Enter].

Move the Marker with the knob.

This concludes this measurement example. You

Select the Marker menu.

Adjust the Marker Type.

Linked Markers move together. Since we changed the DisplayA Marker to Linked, moving the DisplayA Marker moves the DisplayB Marker.

If DisplayB is active, moving its Marker does not move the DisplayA Marker. To do this, change the DisplayB Marker Type to Linked also.

should have a feeling for the basic operation of two channel measurements and the use of [Active Display].

SR785 Dynamic Signal Analyzer

Linking (Advanced Operation) 1-17

Linking (Advanced Operation)

This example investigates the test filter (enclosed with this manual) using FFT measurements. You will use the SR785 source to provide a broad band source and both displays to measure the output of the device under test. Display parameter linking and function linking will be explored in greater detail.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the filter input.

Connect the filter output to the Ch2 A Input.

3. Press [Source]

Press <Noise>

Press <On> Turn the source on. Press <Type>

Select (White) with the knob and press [Enter]. This source is White Noise which extends over the

4. Press [Auto Range Ch2]

5. Press [Display Setup]

Press <Measurement>

Select (FFT ch2) with the knob and press

[Enter].

6. Press [Input] Select the Input Menu

Press <Analyzer Config> Change the way the input channels are used.

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

In this example, only the filter output on Channel 2 is required.

Select the Source menu.

Choose Noise output. The output is random noise.

Adjust the Noise Type.

entire 0-102.4 kHz frequency range.

Let the analyzer automatically set the Input Range to agree with the signal. Note that the Input Range readouts at the top of the screen are displayed in inverse when Auto Range is on.

Select the Display Setup menu.

Change the Measurement of the active display (A).

Choose FFT spectrum of Ch2 for the measurement in DisplayA (top). Both displays are independently measuring the filter output spectrum.

SR785 Dynamic Signal Analyzer

1-18 Linking (Advanced Operation)

Select (Independent Chan.) In this example we'll want to set separate frequency

spans for the two displays. To do that the analyzer must be in the Independent Channels mode. In the default Dual Channel mode the span is the same for both channels, but dual channel measurements are allowed.

7. Press [Freq]

Press <Span>

Use the knob to adjust the Span to 12.8 kHz and

press [Enter].

Press <Span> again.

Press [Link]

Use the knob to adjust the Span to 3.2 kHz and

press [Enter].

Many parameters affect the displays separately.

8. Press [Average]

Press <Display Avg>

Select (RMS) with the knob and press [Enter]. Select RMS Averaging for both displays.

Select the Frequency menu. The menu shows the frequency parameters for the measurement in DisplayA (active display).

Highlight the Span. Note that the Link indicator at the top of the screen turns on. This indicates that the highlighted parameter (Span) is linked to both displays. Changing a linked parameter affects both displays at once.

Narrow the Span of both displays to show the filter notch at 1 kHz (noisy of course).

Highlight the Span again.

Pressing [Link] toggles parameter linking off. The Link indicator now shows ‘DispA’ indicating that this menu box adjusts the span for DisplayA only.

Change the Span of DisplayA to 3.2 kHz. The Span of DisplayB remains at 12.8 kHz. The SR785 allows the two displays to have differing Spans and Start frequencies in the Independent Channels mode.

Linking is a convenient way to adjust the two displays together and keep their settings the same.

The default settings link many measurement parameters, such as frequency and averaging, together as found in many other instruments.

Select the Average menu.

Select the averaging mode to display. The SR785 always computes all the averaging modes when <Compute Avgs> is set to Yes. The <Display Avg> softkey selects which averaging mode is currently displayed.

SR785 Dynamic Signal Analyzer

Linking (Advanced Operation) 1-19

Press <# Avgs>

Press [Link]

Press [2] [0] and press [Enter].

9. Press [Active Display]

Press [Window]

Press <Window>

Press [Link]

Select (Hanning) with the knob and press

[Enter].

So far we have unlinked measurement parameters.

10. Press [Display Setup]

Press <Units>

Press <Pk Units> Select between peak, rms and peak to peak units. Press [Link]

Select (rms) with the knob and press [Enter].

11. Press <Return>

Press <Y Max>

Change the Number Of Averages for DisplayA.

Unlink the Number Of Averages. ‘DispA’ is shown on the Link indicator.

Change the Number Of Averages for DisplayA to 20 (instead of 2). DisplayA will average for 10 times as many measurements as DisplayB and be quite a bit smoother.

Let’s change the Window for DisplayB. Make DisplayB the active display.

Select the Window menu.

The windows are linked by default.

Unlink the Window type. ‘DispB’ is shown as the Link indicator.

DisplayB is now using the Hanning window while DisplayA is still using the BMH window.

Frequency Span, Averaging and Window type affect the actual measurements within the displays. Most analyzers do not allow these measurement parameters to be unlinked.

Display parameters, such as Scaling, Views, Units and Marker functions, are usually unlinked. However, linking them can be a convenient way to adjust graph parameters together with a minimum of key presses.

Select the Display Setup menu.

Enter the Units

Link the Pk Units parameter (default is unlinked).

Both displays’ Units become dBVrms with a single parameter entry. The Pk Units remain linked until unlinked with the [Link] key.

Return to the Display Setup Menu.

Let’s change the Top Reference of the graphs.

SR785 Dynamic Signal Analyzer

1-20 Linking (Advanced Operation)

Press [Link]

Press [-] [1] [0] and press [Enter].

Press [Auto Scale A]

Press [Link] and then [Auto Scale A]

12. Press [Link] and use the knob to move the Marker. Both Markers move together when linked.

Press [Enter]

Press [Marker Min]

Press [Link] then [Marker Min]

This concludes this measurement example. You

Link the Y Max parameter (default is unlinked).

Change the Top Reference of both graphs to

-10 dBVrms with a single entry.

Change the scale of DisplayA to center the data.

Another simple way to adjust the scales of both graphs is using the Auto Scale keys.

Pressing [Link] [Auto Scale A] first auto scales DisplayA and then changes the scale of DisplayB to match. This is convenient when you are comparing the two displays.

The [Link] key temporarily links the two display Markers together.

Pressing any key removes the link between the Markers.

To permanently link the markers, go to the Marker menu and change <Marker> to Link.

[Marker Min] moves the Marker in the active display (B) to the graph minimum.

Pressing [Link] first moves both Markers to their graph minimums at the same time.

[Link] preceding a function key generally performs the function on both displays at once. [Link] [Auto Scale] matches the active display. [Span Up] and [Span Down] are always linked.

should have a feeling for linking and unlinking and the flexibility of unlinked measurements.

SR785 Dynamic Signal Analyzer

Triggering and the Time Record 1-21

Triggering and the Time Record

This example investigates the trigger and time record. You will need a function generator capable of providing a 100 0V.

Make sure you have read ‘The FFT Time Record’ in Chapter 2 before trying this exercise.

µs wide pulse at 256 Hz with an amplitude of 1V. The output should have a DC level of

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Turn on the generator and choose a pulsed output waveform. Set the frequency to 256 Hz, the pulse width to 100 1V. (These settings only need to be approximate.) Make sure that the DC level of the output is near 0V.

Connect the generator output to the Ch1 A input

of the analyzer.

3. Press [Input]

Press <Input Config> Select the input configuration submenu Press <Ch1 Input Range>

Press [1.5]. Select (Vpk) with the knob. Now

press [Enter]

4. Press [Active Display]

Press [Display Setup]

Press <Measurement>

µs and the amplitude to

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

The input impedance of the analyzer is 1 M generator may require a terminator. Many generators have either a 50 impedance. Use the appropriate feedthrough termination if necessary. In general, not using a terminator means that the output amplitude will not agree with the generator setting.

Select the Input master menu.

Choose an input range that doesn’t overload.

Set the input range to about 1.5 Vpk. The channel 1input range indicator at the top of the unit now should read 1.58 Vpk, the closest allowed input range to the value you entered. Adjust the pulse amplitude to that no overloads occur.

Let’s change the Measurement for DisplayB. Make DisplayB the active display.

Select the Display Setup menu.

Change the Measurement of the active display (B).

Ω or 600 Ω output

Ω. The

SR785 Dynamic Signal Analyzer

1-22 Triggering and the Time Record

Select (Time1) with the knob and press [Enter]. Choose Time Record of Ch1 for the measurement

in DisplayB (bottom). You should see the pulse on the bottom display.

5. Press [Trigger]

Press <Trigger Source>

Select (Ch 1) with the knob and press [Enter]. Select internal triggering from the Ch 1 input.

Press <Trigger Level>

Press [3] [0], select (%) with the knob, and

[Enter].

Press [Auto Scale B] DisplayB (bottom) should display the pulse

6. Press [Window]

Press <Window>

Select (Uniform) with the knob and press

[Enter].

Press [Auto Scale A]

7. Press <Window>

Select (Hanning) with the knob and press

[Enter].

Press [Display Setup]

Select the Trigger menu.

Change the Trigger Source.

Adjust the trigger level.

Set the trigger level as a percentage of full scale. Adjust the level for a stable time record in DisplayB.

waveform at the left edge. In this case, the display shows the signal pulse as a digital oscilloscope would.

Select the Window menu.

Because the pulse is much shorter than the time record, we need to use the Uniform (or Force) window. The other window functions taper to zero at the start and end of the time record. Always be aware of the effect windowing has on the time record and the FFT.

Select a new window type for both displays (window type is linked by default).

Notice how the spectrum in DisplayA is changed by the Uniform window.

The spectrum in DisplayA is the sinx/x envelope of a rectangular pulse. The zeroes in the spectrum occur at the harmonics of 1/pulse width (1/100 10 kHz.)

Choose a non-optimum window.

Choose the Hanning window. Notice how the spectrum in DisplayA goes away.

Select the Display Setup menu.

µs or

SR785 Dynamic Signal Analyzer

Triggering and the Time Record 1-23

Press <Measurement>

Select (WinTime1) with the knob and press

[Enter].

8. Press [Trigger]

Press <Delay1>

Press [-] [2], select (ms) with the knob, and

[Enter].

Press [Auto Scale B]

9. Press [Display Setup]

Change the Measurement of DisplayB to show the effect of the Hanning window on the time record.

The Hanning window is zero at the beginning of the time record and large in the center. This effectively zeroes the signal pulse at the start of the time record leaving nothing in the windowed time record. The FFT operates on this windowed time record and thus the spectrum shows no evidence of the signal pulse.

Select the Trigger menu again.

Change the Trigger Delay for the signal on Ch 1.

We can get the spectrum back by delaying the time record relative to the trigger so that the pulse is positioned in the center of the time record.

A negative delay means that the time record starts before the trigger event. In this case, the time record is about 4 ms long so a delay of -2ms will put the signal pulse in the center of the triggered time record.

The trigger delay is specified in time record bins at the current span.

Note that the windowed time record in DisplayB shows the signal at the center of the time record.

The amplitude of the windowed time record is not the same as the amplitude of the time record itself. This is because the window functions have gain and attenuation at different parts of the time record. The Hanning window is 2.0 at the center so the amplitude of the signal in the windowed time record is twice as large.

The Hanning, Flattop, BMH and Kaiser windows are not intended for use with narrow pulse signals. They are used for signals which last the entire time record and normalized as such.

The Uniform (and Force) windows have no gain and should be used with pulsed signals such as this.

Select the Display Setup menu.

SR785 Dynamic Signal Analyzer

1-24 Triggering and the Time Record

Press <Measurement>

Select (Time1) with the knob and press [Enter]. Time1 is the un-windowed time record.

Press [Window]

Press <Window>

Select (Uniform) with the knob and press

[Enter].

10. Press [Trigger]

Press <Trigger Source>

Select (Cont) with the knob and press [Enter]. The continuious trigger source triggers

Adjust the generator frequency to 255 Hz.

This concludes this measurement example. You

Change the Measurement of DisplayB back to Time Record.

Select the Window menu.

Change the window type for both displays.

Use the Uniform window (which is the correct window for this measurement).

Select the Trigger menu.

Change the Trigger Mode.

measurements as fast as the analyzer can go.

If the generator is set to 256 Hz pulse rate, the signal will drift slowly in the time record. This is because the SR785 time records are exactly 1/256 Hz (3.90625 ms) long (400 lines at full span) and the analyzer is running in real time (no missed data).

The drift in the time record is because the analyzer and the generator are using different time bases.

The time record is now unstable and the pulse moves through the entire time record.

The spectrum in DisplayA is mostly unaffected since the Uniform window allows the pulse to be anywhere in the time record.

Only when the pulse is not entirely within the time record is the spectrum disturbed.

should have a feeling for triggered time records and the effect of windowing on the resulting FFT.

SR785 Dynamic Signal Analyzer

Octave Analysis 1-25

Octave Analysis

This example investigates the test filter (enclosed with this manual) using Octave measurements. You will use the SR785 source to provide a broad band source and both displays to measure the output of the device under test.

Refer to ‘Octave Analysis’ in Chapter 2 for more about Octave Analysis measurements.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Use a BNC Tee to connect the Source Output to the filter input and the Ch1 A Input.

Connect the filter output to the Ch2 A Input.

3. Press [Display Setup]

Press <Measure Group>

Select (Octave) with the knob and press [Enter]. Choose the Octave group. Both displays are now

Press [Source], <On>. Turn the source on. Press [Link] [Auto Scale A]

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Ch1 measures the source (filter input) and Ch2 measures the filter output.

Select the Display Setup menu.

Change the Measurement Group.

making Octave Analysis measurements.

Auto Scale DisplayA and change the scale of DisplayB to match DisplayA.

The Octave measurement displays the output of logarithmically spaced bandpass filters. This is not an FFT based measurement. The last bin at the right is a Sound Level measurement and may be calculated independently from the octave bands.

Note that even though the source is a single frequency sine wave, the octave display shows a very broad peak. This is because the individual bandpass filters are very broad, 1/3 of an octave in this case.

SR785 Dynamic Signal Analyzer

1-26 Octave Analysis

4. Press [Source]

Press <Noise>

Press <Type>

Use the knob to select (Pink) and press [Enter]. Choose Pink noise. Pink noise rolls off at -3dB per

Press [Auto Range Ch1] and [Auto Range Ch2]. Adjust the input ranges to remove overloads.

Press [Auto Scale A] and [Auto Scale B].

5. Press [Average]

Press <Integration Time>

Press [1], select (s) with the knob, and press

[Enter].

6. Press [Freq]

Press <Octave Resolution>

Use the knob to select (Full) and press [Enter]. Choose Full octave bands.

Press <Octave Resolution>

Select the Source menu.

Choose Noise as the source type. Octave measurements are generally used to measure noise.

Change the type of noise.

octave. This maintains equal power per octave band and yields a flat octave spectrum.

Notice that the measurement needs to settle after the input range is changed. This is because the measurement is invalid until the input change has propagated through all of the octave band filters. This settling time is related to 1/bandwidth of each filter. Bands which are un-settled are graphed in half intensity. ‘Settle’ is displayed until all bands in the display are settled.

DisplayA (Ch1) shows the flat source spectrum and DisplayB (Ch2) shows the notch filter output.

Select the Average menu. Note that this menu is changed in Octave group.

The Integration Time is the averaging time constant. All Octave measurements are rms averaged.

Increase the Integration Time to smooth the fluctuations in the spectrum.

Select the Frequency menu.

Change the number of bands per octave.

Each band represents a full octave with very poor frequency resolution.

Change the number of bands per octave again.

SR785 Dynamic Signal Analyzer

Octave Analysis 1-27

Use the knob to select (Twelfth) and press

[Enter].

7. Press <Octave Resolution>

Use the knob to select (Third) and press [Enter]. Let’s return to 1/3 octaves.

Press <Highest Band>

Use the knob to select 20 kHz and press [Enter]. Set the highest band to 20 kHz.

Press <Octave Channels>

Use the knob to select (1 Channel) and press

[Enter].

Press <Highest Band>

Use the knob to select 40 kHz and press [Enter]. Set the highest band to 40 kHz.

8. Press [Display Setup]

Choose 1/12 octave bands.

Each band represents 1/12 of an octave with very good frequency resolution.

Note that the measurement requires a long settling time. Narrow bands increases the settling time (1/bandwidth).

Choosing narrow bands also increases the number of calculations required and decreases the maximum frequency which can be measured.

Change the number of bands per octave again.

Change the highest measured band.

Remember, changing the octave resolution has lowered the highest band in the measurement.

This is the highest allowed band for 2 channel, 1/3 octave analysis.

We can change the number of channels which are being measured.

Choose single channel octave analysis. This limits the insturment to analyzing only one input, but increases the measurement bandwidth.

Change the highest measured band.

Single channel analysis has twice the measurement bandwidth of two channel analysis. Thus, to increase the highest measured band, use 1 channel analysis.

This is the highest allowed band for 1 channel, 1/3 octave analysis.

Select the Display Setup menu.

SR785 Dynamic Signal Analyzer

1-28 Octave Analysis

Press <Measurement>

Use the knob to select (Oct ch2) and press

[Enter].

Press <Measurement>

Use the knob to select (Oct ch1) and press

[Enter].

9. Press [Input]

Press <Input Config>

Press <Ch1 A-Wt Filter>

Use the knob to select (On) and press [Enter]. Turn the A-Wt filter On. The Ch1 Awt indicator at

Press [Link] [Auto Scale A] The A-Weighted spectrum is a bandpass centered

This concludes this measurement example. You

Change the Measurement of both displays. Remember, in 1 channel analysis, both displays measure the same input.

Both displays now show the filter output on Ch2.

Change the Measurement again.

Show the source output on Ch1 on both displays.

Select the Input menu.

Show the Input Configuration submenu.

Choose the Ch1 Input A-Weighting filter.

the top of the screen is highlighted.

The hardware A-Wt filter conforms to the ANSI standard and is commonly used in sound measurements. This filter attenuates high and low frequencies according to how people hear and perceive sound.

around 2 kHz.

should have a feeling for Octave measurements and how they are setup.

SR785 Dynamic Signal Analyzer

Capture 1-29

Capture

This example investigates the Capture buffer using FFT measurements. You will use the SR785 to capture a signal and then analyze it from memory.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the Channel 1 A Input.

3. Press [Source]

Press <Sine>

Press <Frequency 1>

Press [1] [.] [0] [2] [4] select (kHz) with the

knob and press [Enter].

Press <On> Turn the source on.

4. Press [Capture]

Press <Allocate Memory>

Press <Waterfall Memory>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Setup to analyze the source output.

Select the Source menu.

Choose Sine output.

Adjust the output Frequency.

Enter 1.024 kHz for the Frequency. Enter the value with the numeric entry keys. Select the units with the knob. Enter the new value with the [Enter] key.

Select the Capture menu.

The data memory is allocated between Capture, Waterfall/Order Track, and Arbitrary Waveform storage.

Memory is allocated in blocks. Each block can store 2 kPoints. The total number of blocks available is displayed in the <Total Available> menu box. This number is for display only, it cannot be changed from the menu.

Larger memory options (up to 4000 blocks) are available.

To increase Capture memory, you must first decrease the other allocations so that the sum never exceeds the total available memory.

SR785 Dynamic Signal Analyzer

1-30 Capture

Press [0] and [Enter].

Press <Capture Memory>

Press [9] [0] [0] and [Enter].

Press <Confirm Allocation> and <Return>.

5. Press <Capture Channels>

Use the knob to select (Ch1) and press [Enter]. Choose Ch1 only. In this case, the entire capture

Press <Capture Length>

Press [1] [8] [0] [0] and [Enter].

6. Press [Start Capture]

Press [Active Display]

Press [Display Setup]

Press <Measurement>

Use the knob to select (Capture1) and press

[Enter].

Decrease the Waterfall allocation to the minimum allowed. Note that the analyzer displays an allocation of 4 blocks. The analyzer always maintains a minimum allocation for each function that uses memory..

Select the Capture allocation.

Increase it to 900 blocks. (1,843,200 points).

You must confirm the new allocation. Changing the memory allocation destroys previously stored data in the memory.

Select which inputs to capture.

buffer is available for Ch1. When both channels are captured, half of the buffer is available for each channel.

Increase the capture length.

All of the capture allocation (900 blocks) is available. Each block stores 2 kPoints for a total of 1800 kPoints.

Start the capture. The buffer will take 7.03 seconds to fill. Since the Capture Mode is 1-Shot, the capture stops once the buffer is full. During this time, Capture indicator is highlighted and the Capture Progress indicator shows how much of the buffer has been filled (up to 100%).

After capture is complete, the Capture indicator shows ‘Cap Data’ indicating that the Capture buffer contains data.

Make DisplayB (bottom) the active display.

Select the Display Setup menu.

Change the measurement of DisplayB.

Choose Capture1 to show the contents of the Ch1 Capture buffer.

SR785 Dynamic Signal Analyzer

Capture 1-31

Press <Zoom>

Press [1] [1] and [Enter].

7. Unplug the signal from the Ch1 input.

Press [Input]

Press <Input Source>

Use the knob to select (Playback) and press

[Enter].

Press <Playback Config> Enter the playback configuration submenu. Press <Playback Length>

Press [1] [8] [0] [0] and [Enter].

Press <Playback Mode>

Use the knob to select (Circular) and press

[Enter].

8. Use a BNC TEE to connect the source to both Ch1 and Ch2 A inputs.

There are far too many points in the buffer to graph each one. The graph shows the envelope of the data in this case.

Zoom in to show individual points. ‘Expand’ below the graph indicates that the graph has been graphically zoomed and does not show all of the data along the X axis.

Enter a zoom factor of 11 (211). The display now shows the signal sine wave clearly.

The signal should disappear from the spectrum in DisplayA.

Select the Input menu.

Change the Input Source to measure from the Capture buffer.

Choose Playback instead of the analog inputs.

The measurement now takes its input from the data stored in the Ch1 Capture buffer. The signal reappears in the spectrum in DisplayA.

Only single channel measurements using Ch1 are allowed in this case (there is no Ch2 data available).

You can choose to playback only a portion of the buffer if desired.

Choose the entire buffer by setting the Playback Length equal to the Capture Length.

Playback can be 1-Shot (once through the buffer and stop) or Circular (repeat when finished).

Choose circular playback. The % indicator at the top of the display shows the current progress through the playback buffer. uffer during playback.

DisplayB automatically pans to show the portion of the Capture buffer at the current playback position.

Reconnect the analog signal to both Ch1 and Ch2 inputs.

SR785 Dynamic Signal Analyzer

1-32 Capture

Press [Input]

Press <Input Source>

Use the knob to select (Analog) and press

[Enter].

Press [Display Setup]

Press <Measurement>

Use the knob to select (FFT ch2) and press

[Enter].

9. Press [Capture]

Press <Capture Channels>

Use the knob to select (Ch1+Ch2) and press

[Enter].

Press <Sampling Rate>

Use the knob to decrease the rate to 131.1 kHz

and press [Enter].

Press [Start Capture]

10. Disconnect the signal from both inputs.

Press [Input]

Press <Input Source>

Use the knob to select (Playback) and press

[Enter].

Select the Input menu.

Change the Input Source.

Choose Analog input again. The Capture parameters can not be modified while the measurement input is Playback.

Select the Display Setup menu.

Change the Measurement for DisplayB.

Measure the Ch2 input also.

Select the Capture menu.

Change which channels are captured.

Choose both channels.

The Capture Length is automatically halved to accommodate both channels in the allocated memory.

We can increase the capture time by decreasing the Sampling Rate. This decreases the bandwidth of the stored signal.

Choose 131.1 kHz as the Sampling Rate. The capture bandwidth is now 51.2 kHz (reduced from

102.4 kHz).

During playback from this buffer, the measurement bandwidth will not be allowed to exceed 51.2 kHz.

Capture both inputs for 7.03 seconds. Watch for the Capture Progress indicator to reach 100%.

The signal should disappear from the spectrum in both displays.

Select the Input menu.

Change the Input Source.

Choose Playback from Capture. Since there is captured data for both inputs, both displays start measuring from the capture.

SR785 Dynamic Signal Analyzer

Capture 1-33

Press [Auto Scale A] and [Auto Scale B].

Press [Start/Reset]

Press <Playback Config> Enter the Playback Configuration submenu. Press <Playback Speed>

Use the knob to select (Every Time Rec) and

press [Enter].

Press [Start/Reset]

Press [Trigger] Select the trigger menu. Press <Trigger Source> We'll use manual trigger to show how you can step

Use the knob to select (Manual) and press

[Enter]

Scale the displays to show the measurements.

Start playback from the beginning of the buffer. Since the default Playback Speed is Normal, the entire playback takes as long as the equivalent real time measurement. In this case, the capture represents 7.73 seconds of data so the playback takes 7.73 seconds as well.

At the current span (51.2 kHz), there are almost 1000 time records of captured data. Not all time records are displayed during Normal playback though all time records contribute to averaged measurements. The display is updated 8 times a second for 7.73 seconds for a total of 62 updates during this playback.

Normal playback is limited to the real time limitations of the equivalent real time analog input measurement.

Change the Playback Speed.

Choose Every Time Record. Since the data is stored in memory, we can choose to display every stored time record.

Start the playback at the beginning of the buffer.

In this case, all 1000 time records are measured AND displayed. The display still updates at 8 Hz so playback takes about 125 seconds to complete.

If the time records are overlapped, there may be more than 1000 measurements to display and playback will take even longer.

Every Time Record playback is not limited by real time considerations.

through each time record in the capture buffer. Each manual Trigger will step one time record into the capture buffer.

SR785 Dynamic Signal Analyzer

1-34 Capture

Press <Manual Trigger> several times Each time record is 1k points long, or represents

.11% of the 900k capture buffer. Note that the playback indicator increments by .11% each time <Manual Trigger> is pressed.

Press <Trigger Source>, select (Cont) with the

knob and press [Enter].

11. Press [Freq]

Press <Span>

Use the knob to select (6.4 kHz) and press

[Enter].

Press [Start/Reset]

12. Press [Input]

Press <Playback Config> Enter the playback configuration submenu. Press <Playback Length>

Press [8] [9] [6] and [Enter].

Press [Start/Reset]

Return to continuous playback.

Select the Frequency menu.

Change the measurement span. The span can not be increased above 51.2 kHz since the captured data is bandwidth limited to 51.2 kHz (because of our capture sampling rate).

Change the span to 6.4 kHz.

Capture playback allows the same captured data to be measured at different spans, windows, averaging, etc. This is useful if the signal is hard to reproduce or occurs infrequently.

At this span, the capture buffer only holds 112.48 time records and takes only 14 seconds to playback every time record.

Select the Input menu.

It is generally a good idea to playback an exact number of time records. This way, the last record doesn’t wrap around and use points from the start of the buffer.

At this span (with no overlap), each time record is

62.5 ms long. The capture sampling rate was 131.1 kHz so 2 kPoints of capture represents 15.625 ms of data. Thus each time record is 8 kPoints of capture long.

We want the Playback Length to be an integer multiple of 8 kPoints. 112 time records uses 896 kPoints and is close to the full capture length.

Change the Playback Length to an exact number of time records.

Start the playback again.

SR785 Dynamic Signal Analyzer

Capture 1-35

This concludes this example. Capture and Playback

is a way to record a signal and re-analyze it over and over.

SR785 Dynamic Signal Analyzer

1-36 Capture

SR785 Dynamic Signal Analyzer

Waterfall Display 1-37

Waterfall Display

This example demonstrates the use of waterfall displays. Waterfalls are available for FFT and Octave measurements for analog inputs as well as capture playback.

In this example, we will simulate a reverberation measurement measuring the SR785 source. To perform a real measurement, you would use the source to drive a power amplifier and a microphone to receive the signal.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the Channel 1 A Input.

3. Press [Display Setup]

Press <Measure Group>

Select (Octave) with the knob and press [Enter]. Choose the Octave group. Both displays are now

4. Press [Freq]

Press <Lowest Band>

Use the knob to select (50 Hz) and press [Enter]. Select 50 Hz as the lowest band. The lowest band

5. Press [Source]

Press <On> Turn the source on. Press <Noise>

Press <Type>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Setup to analyze the source output.

Select the Display Setup menu.

Change the Measurement Group.

making Octave Analysis measurements.

Select the Frequency menu.

Change the lowest band in the display.

places a limitation on the minimum integration time. Raising this lowest band allows shorter integration times.

Select the Source menu.

Choose Noise as the source type. Octave measurements are generally used to measure noise.

Change the type of noise.

SR785 Dynamic Signal Analyzer

1-38 Waterfall Display

Use the knob to select (Pink) and press [Enter]. Choose Pink noise. Pink noise rolls off at -3dB per

octave. This maintains equal power per octave band and yields a flat octave spectrum.

Press <Burst>

Press [9] [6] [Enter]

6. Press [Average]

Press <Averaging Type>

Use the knob to select (Linear Time) and press

[Enter].

Press <Integration Time>

Use the knob to select (8 ms) and press [Enter]. Choose the minimum time for the best resolution.

Press <Linear Avg Trig>

Change the Burst percentage.

Using a percentage less than 100% makes the noise source a triggered source. 100% burst outputs noise continuously with 100% duty cycle. Bursts less than 100% will output noise with less than 100% duty cycle and may be triggered.

In this case, the noise will be output for 96% of the source period (100 ms default) or 96 ms.

If the measurement is continuously or source triggered, then the output is noise for 96 ms out of every 100 ms.

If the measurement is externally or manually triggered, the output is 96 ms every trigger with the minimum trigger period equal to 100 ms (the source period).

Select the Average menu. Note that this menu is changed in Octave group.

Change the Averaging Type.

Use Linear Time for best time resolution. Exponential Time averaging takes about 5 integration times to fully respond to a transient. Linear Time averaging responds in a single integration time.

Change the Integration Time.

Change the Linear Average Trig. This determines how measurements behave when triggering is enabled (not Free Run).

SR785 Dynamic Signal Analyzer

Waterfall Display 1-39

Use the knob to select (Start) and press [Enter]. Choose Start. This means that a new measurement

is started as soon as the previous average is complete. In this case, a new measurement is made every 8 ms, regardless of triggering.

If we choose One Lin Avg., then a measurement is made only when triggered.

Press <Power Bin>

Use the knob to select (L) and press [Enter].

7. Press [Trigger]

Press <Trigger Source>

Use the knob to select (Manual) and press

[Enter].

Press <Trigd Source Mode>

Use the knob to select (One Shot) and press

[Enter].

8. Press [Start/Reset]

Press <Manual Trigger>

9. Press [Waterfall]

Change the Power Bin.

Choose L (Leq) as the sound level bin. Leq is a standard broad band sound measurement. When we start the next measurement the result will be displayed as the last bin in the display and labeled ‘L’.

Select the Trigger menu.

Change the Trigger Source.

Choose Manual trigger. We will start our measurement with a button press. We could also use an external trigger. Triggering on the signal itself requires the use of an external signal source.

Change the Triggered Source Mode. This determines whether the source triggers only once (on the first trigger after [Start/Reset]) or on every trigger.

Choose One Shot to trigger the noise burst only once at the start of the measurement.

[Start/Reset] starts the measurement. Since the measurement is triggered (not Free Run), nothing happens until the first trigger is received.

<Manual Trigger> supplies the first trigger. The source outputs a single noise burst (Trigd Source Mode=One Shot). The display starts a continuous stream of octave measurements, each linear averaged for 8 ms and each starting when the previous average is complete (Linear Avg Mode=Continuous).

Select the Waterfall menu.

SR785 Dynamic Signal Analyzer

1-40 Waterfall Display

Press <Display>

Use the knob to select (Waterfall) and press

[Enter].

Press <Storage>

Press (Save Options) and press [ENTER]

Use the knob to select (Active Meas Only) and press [ENTER]

Use the knob to select (One Shot) and press

[Enter].

Press <Total Count>

Press [5] [0] [Enter]

10. Press [Trigger]

Press [Start/Reset]

Press <Manual Trigger>

Change the Display.

Choose Waterfall display. This shows measurements scrolling down. Without waterfall storage, this is purely graphical, no data can be read from measurements other than the most recent.

Select waterfall Storage.

Choose One Shot to fill the waterfall buffer once and stop.

Change the number of measurements to store in the waterfall buffer.

The Total Count is linked to both displays by default. Entering 50 changes the total count for both displays to 50.

Select the Trigger menu again.

[Start/Reset] starts the measurement. Since the measurement is triggered (not Free Run), nothing happens until the first trigger is received.

50 averaged measurements are stored in the waterfall buffer starting with the trigger. The first 96 ms (12 measurements) are during the triggered noise burst. The remaining measurements are taken after the noise burst turns off and measure the decay response or reverberation.

The number of records stored in the waterfall is shown in the Vertical Scale Bar as ‘wf 50’.

SR785 Dynamic Signal Analyzer

Waterfall Display 1-41

11. Press [Alt] and turn the knob clockwise.

Continue until the display does not scroll any

further.

[Alt] knob moves the marker in the Z direction (time axis) in the waterfall display. This scrolls the display to show earlier measurement record.

When the keypad and knob are in the alternate mode, the alternate key functions (labeled below each key) are in effect.

The waterfall records are numbered starting with 0 (the most recent measurement) in the back. In this case, we stored 50 records so the earliest record is number 49.

The marker Z position is displayed next to the marker frequency in the Marker Position Bar above the graph. It should read ‘49’ when you have scrolled all the way to the beginning of the buffer.

Notice how the first few records show the spectrum growing at the start of the noise burst The low frequency bands grow more slowly than the high frequency bands because their filters have longer time constants..

Press [Alt] Pressing [Alt] removes the keypad and knob from

the alternate mode.

Turn the knob clockwise to move the marker

along the frequency axis of a single record.

Move the marker all the way to the right hand

edge of the display. The marker position should read ‘L:49’.

The normal knob function moves the marker along the X axis of a single record.

The last bin in the octave display is the total sound level L

(as selected by <Power Bin>).

12. Press [Waterfall]

Press <More>

Press <Slice to Trace>, select (Trace 1) with the

knob and press [Enter].

Select the Waterfall menu.

Show More of the Waterfall menu.

A waterfall slice is the time history of a single X axis location (data at the marker X position from all stored records). The slice data is stored in a trace.

Press [Active Display]

Make the bottom display active (DisplayB).

SR785 Dynamic Signal Analyzer

1-42 Waterfall Display

Press [Alt] [Help/Local], select (Trace 1) with

the knob and press [Enter].

Press [Auto Scale B]

Use the knob to move the marker around in the

waterfall slice (DisplayB).

13. Press [Active Display]

Press [Waterfall]

Press <View Count>

Press [5] [0] [Enter].

Press <Marker Z to>

Press [0] [Enter].

Press <Angle>

Use the knob to select (45°) and press [Enter].

Press <More>

Press <Paused Drawing>

Trace to Display is an alternate function. The alternate key functions are labeled below each key (in this case, the [Help/Local] key).

Trace to Display recalls trace data to the active display.

Auto Scale DisplayB to show the time history of Leq.

The points in a slice are numbered and displayed from 0 (oldest) to 49 (newest). Note that this differs from the waterfall display in which the newest record is numbered 0. This is because the slice is a time record with time advancing to the right and it is more natural to number it this way.

Note that the first 12 records (96 ms) show a large value for L

during the noise burst. If this was a

real reverberation measurement, the signal would not decay in a single 8 ms measurement but would last for a reverberation time.

Make the top display active (DisplayA).

Select the Waterfall menu.

Change the View Count. This is the number of records which are displayed.

Enter 50 to show the entire waterfall buffer.

Move the marker to a specific record number.

Enter record 0 (most recent at the back).

Change the skew angle of the display.

Choose 45° to skew the opposite way.

Show More of the Waterfall menu.

Change the waterfall direction.

SR785 Dynamic Signal Analyzer

Waterfall Display 1-43

Use the knob to select (Oldest at Top) and press

[Enter].

This concludes this example. There are many

While the measurement is running, the newest records are added at the top of the waterfall display. When the measurement is done or paused, the waterfall may be drawn with either the newest (Normal) or Oldest at Top.

Since this measurement is measuring a transient decay, it is better to draw the oldest record at the top and the subsequent (smaller amplitude) records in the front.

display parameters in the Waterfall menu which you should familiarize yourself with.

The transient response of any FFT or Octave measurement may be recorded in a waterfall buffer. Using a slice will give a time evolution of a single X axis bin.

SR785 Dynamic Signal Analyzer

1-44 Waterfall Display

SR785 Dynamic Signal Analyzer

Swept Sine Measurement 1-45

Swept Sine Measurement

This example investigates the test filter (enclosed with this manual) using Swept Sine measurements. You will use the SR785 source to provide a sweeping sine source and both inputs to measure the input to and output from the device under test.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Use a BNC Tee to connect the Source Output to the filter input and the Ch1 A Input.

Connect the filter output to the Ch2 A Input.

Press [Input], <Input Conifg>

Press <Ch1 Input Range>

Press [2], select (dBVpk) with the knob, and

[Enter].

3. Press [Display Setup]

Press <Measurements Group>

Select (Swept Sine) with the knob and press

[Enter].

4. Press [Freq]

Press <Start>

Press [9] [0] [0] select (Hz) with the knob and

press [Enter].

Press <Stop>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

In this instrument, transfer function is defined as Ch2 response over Ch1 reference. Thus, Ch1 monitors the filter input (source output) and Ch2 measures the response of the device under test.

Select the Input Configuration submenu.

Adjust the Ch1 input range.

Set the range to 2 dBV.

Select the Display Setup menu.

There are six Measurement Groups - FFT, Correlation, Octave, Order Analysis, Swept Sine, and Time/Histogram. The Measurement Group determines which Measurements are available to the displays.

Choose the Swept Sine group. The menus now configure swept sine measurements only.

Select the Frequency menu.

Adjust the sweep Start Frequency.

Enter 900 Hz.

Adjust the sweep Stop Frequency.

SR785 Dynamic Signal Analyzer

1-46 Swept Sine Measurement

Press [1] [.] [1] select (kHz) with the knob and

press [Enter].

Press <# of Points>

Press [1] [0] [2] [4] [Enter].

5. Press [Start/Reset]

Wait for the sweep to complete at least once.

Press [Auto Scale A] and [Auto Scale B]

6. Press [Display Setup]

Press <Measurement>

Select (Freq. Resp.) with the knob and press

[Enter].

Press [Display Options]

Press <Format>

Select (Single) with the knob and press [Enter]. Choose a Single Display with DisplayA (Frequency

Press [Auto Scale A]

7. Press [Marker]

Press <Width>

Select (Normal) with the knob and press [Enter]. Choose Normal Width (1/2 division).

Press <Seeks>

Enter 1.1 kHz.

The 900 - 1100 Hz sweep covers the filter region of interest.

Adjust the Number Of Points in the sweep.

Enter 1024 points.

Reset and start the sweep. The source is a sine wave whose frequency sweeps from 900 Hz to 1.1 kHz stopping at 1024 discrete frequencies. At each frequency point, the inputs are measured and displayed.

The small triangular marker moving across the bottom of the graphs indicates the position of the sweep in progress.

Scale the two displays. The top display is the filter input (source output) and should be fairly flat. The bottom display is the filter output and show the filter notch at 1 kHz.

Select the Display Setup menu.

Change the Measurement of DisplayA (active display).

Choose Frequency Response (filter output divided by filter input).

Select the Display Options menu.

Change the Display Format.

Resonse Function) active.

Scale the display to show the Frequency Response.

Select the Marker menu.

Change the Marker Region width.

Change the Marker Seeks function.

SR785 Dynamic Signal Analyzer

Swept Sine Measurement 1-47

Select (Min) with the knob and press [Enter].

Press [Marker Min]

8. Press [Average]

Press <Integration Time>

Press [4] [0] select (ms) with the knob and press

[Enter].

The greater of the Integration Cycles and

Press [Auto Range Ch1] and [Auto Range Ch2] Change both inputs to Auto Range (the Input Range

Seek the Minimum data within the Marker Region.

Move the Marker to the notch minimum. Read the notch depth and frequency in the marker Position display.

Select the Average menu.

Change the Integration Time.

Enter 40 ms. The Integration Time is set in increments of 3.9 ms so the entry is rounded to 39 ms.

The new estimated sweep time is displayed below the graph.

At each frequency point, the inputs measure the amount of signal at the source frequency. This is done by multiplying the input data by the source sine (and cosine) and averaging the results over an integration time. The actual integration time is always rounded up to an exact number of cycles of the source frequency. This rejects signals which are at different frequencies, such as noise and harmonics. Long integration times improve signal to noise while increasing the measurement time.

Integration Time (rounded to the next complete cycle) is used at each frequency. The sweep is at 1 kHz so each cycle is 1 ms. Setting the Integration Time to 40 ms increases the integration time.

So far, the Input Ranges, Source Level and Sweep Resolution have been constant over the sweep. Let’s change these to optimize both the measurement and the measurement time.

indicators at the top of the screen are highlighted).

SR785 Dynamic Signal Analyzer

1-48 Swept Sine Measurement

When Auto Range is on, the Input Range is

optimized at each frequency point in the sweep. If the signal is overloaded, the range moves up. If the signal is below half scale, the range moves down. This optimizes the input signal to noise at each point separately and can dramatically improve the S/N of measurements with a large dynamic range. Measurements in excess of 140 dB of dynamic range are possible with swept sine.

This notch filter is only -60 dB deep and does not

actually require Auto Range for a clean measurement. Note that the Ch2 Input Range changes as the sweep moves through the notch, following the filter output signal.

Auto Range increases the sweep time.

Press [Input], <Input Conifg>

Press <Channel>, select (Ch2) with the knob,

and press [Enter].

Press <Ch2 Coupling>

Select (AC) with the knob and press [Enter].

10. Press [Freq]

Press <Auto Resolution>

Select (On) with the knob and press [Enter].

If successive points differ by more than the Slower

Select the Input Configuration submenu.

Select Ch2 Input Configuration.

Change the Input Coupling for Ch2.

Choose AC coupling. This eliminates the DC offset from the source and allows Auto Range on Ch2 to cover the entire allowable input range.

Select the Frequency menu.

Change the Auto Resolution mode.

Choose Auto Resolution on.

If successive points differ by less than the Faster Threshold (on both Ch1 and Ch2), then the sweep starts to skip points. Each successive time this threshold is met, the number of points skipped is increased until the Max # of Skips is reached. This speeds up the sweep in regions where the response is slowly changing.

Threshold (on either input), then the sweep returns to the earlier point and continues with no skipping. This ‘fills’ in the region where the response is rapidly changing. The sweep continues from this point, speeding up when allowed and slowing down when required.

SR785 Dynamic Signal Analyzer

Swept Sine Measurement 1-49

Note that the sweep progress marker at the bottom

of the graph changes speed through the notch.

Auto Resolution greatly shortens the measurement

time while preserving the resolution where required.

11. Press [Source]

Press <Auto Level Ref>

Select (Channel 2) with the knob and press

[Enter].

Press <Ideal Ref>

Press [1] [0], select (mV) with the knob, and

press [Enter].

This is useful whenever the transfer function has

Press [Active Display]

Press [Auto Scale B]

12. Press [Active Display]

Select the Source menu.

Change the Auto Level Reference.

Choose Channel 2 as the Auto Level Reference.

Change the Ideal Reference level.

Enter 10 mV.

Auto Level will try to maintain the Ch2 signal level at the Ideal Reference level by changing the source level at each point of the sweep. The Max Source sets the largest source output allowed.

substantial gain as well as attenuation or if a test requires a constant level within the device under test (usually input or output). In this case, Auto Level is not really required but illustrates its use.

Change the active display to DisplayB (which is still measuring the spectrum of Ch2).

Scale the display. On both sides of the notch, you can see Auto Level keeping the Ch2 signal level at 10 mV (-40 dBV). The reference tolerance is 3 dB and is set by the Ref Limits. As the sweep moves into the notch, the source level reaches the Max Source level of 1 V and the Ch2 signal drops to

-60 dBV. The spectrum of Ch1 measures the actual source level at each point and the Frequency Response is still calculated correctly.

Switch back to DisplayA (Frequency Response).

SR785 Dynamic Signal Analyzer

1-50 Swept Sine Measurement

This concludes this measurement example. You

should have a basic understanding of Swept Sine measurements. The Input Range, Resolution and Source Level optimizations greatly extend the dynamic range of the measurement while minimizing the measurement times.

SR785 Dynamic Signal Analyzer

Saving and Recalling 1-51

Saving and Recalling

This example illustrates saving and recalling displays to reference displays, traces and disk files.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the Channel 1 A Input.

Press [Source], <On> Turn the source on. Press [Span Down] twice to change the span to

25.6 kHz.

Press [Auto Scale A]

3. Press [Display Ref]

Press [Input], <Input Conifg>

Press <Ch1 Input Range>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Setup to analyze the source output. The default source is a 10.24 kHz sine.

Narrow the span to display the signal better.

Scale the display to show the entire measurement range.

Copy the current measurement data into the reference graph for the active display (A).

The reference graph is stored data which is associated with each display. The reference graph is shown in half intensity.

We need to change the measurement data in order to see the reference graph since it is ‘underneath’ the current data.

Select the Input Configuration submenu.

Change the input range to raise the noise floor.

SR785 Dynamic Signal Analyzer

1-52 Saving and Recalling

Press [3] [0], select (dbVpk) with the knob, and

[Enter].

Select 30 dBV for the Ch1 Input Range.

Now you can see the reference graph below the current measurement’s noise floor. The reference graph allows visual comparison of live data with stored data. The marker can also be set to read the current data relative to the reference graph.

The reference graph can be loaded by copying the current live data or by copying a stored trace.

SR785 Dynamic Signal Analyzer

Saving and Recalling 1-53

4. Press [Display Setup]

Press <Ymid>

Press [-] [4] [0] and [Enter].

Press [Alt] [Start Capture]

Press [Display Ref]

Press [Auto Scale A]

5. Press [Alt] [Print Screen]

Use the knob to select Trace 3 and press [Enter]. Store the display in Trace 3.

Press [Span Up] twice to return to full span.

Press [Active Display]

Press [Alt] [Help/Local]

Select the Display Setup menu.

Change the top reference for the graph.

Set the middle reference to -400 dBV.

Note that the live data is graphed with the new vertical reference but the reference graph did not move. This allows the reference graph to be offset from the live data (so it is visible).

Snap Ref is an alternate function. The alternate key functions are labeled below each key (in this case, the [Start Capture] key).

Snap Ref redraws the reference graph with the current display scaling.

Pressing [Display Ref] again turns off the reference graph. The reference graph data is lost.

To save the reference graph, transfer the reference graph to a trace before turning it off. The trace may be saved to disk.

Scale the graph appropriately again.

Display to Trace is an alternate function. This function saves the current data in the active display to a trace.

There are 5 traces available for data storage. They can store the results of any measurement (other than capture buffer). They can be viewed as complex arrays of data which can be viewed like any other measurement data.

Change the live measurement.

Make DisplayB (bottom) active.

Trace to Display is an alternate function. This function recalls trace data to the active display.

SR785 Dynamic Signal Analyzer

1-54 Saving and Recalling

Press [Enter] to select Trace 3.

Press [Display Setup]

Press <Ymax>

Press [8] [0] and [Enter].

Press [Active Display]

Press [Display Options]

Press <Display>

Use the knob to select (Live) and press [Enter]. Choose Live to return the live measurement to

6. Put a blank 1.44MB, 3.5” disk into the disk drive.

Only Trace 3 contains data at this time. Only Trace 3 may be selected.

DisplayB now shows the data stored in Trace 3. The display is labeled ‘Off-Line’ (in its upper left corner) indicating that it is showing static data.

Select the Display Setup menu.

Note that the Measurement Group and Measurement menu boxes are shown in gray. This indicates that these parameters may not be changed for the active display. In this case, the active display contains stored data so the measurement parameters cannot be changed.

Other measurement parameters, such as averaging and window type, are also not allowed to be changed when the active display is Off-Line.

Change the graph scale of DisplayB.

The view and scale of DisplayB can be changed of course. These parameters simply change the way the stored data is graphed.

Move the graph down.

Make DisplayA (top) the active display. Note that the Measurement Group and Measurement menu boxes are not gray. This is because these parameters may be adjusted for the live measurement in DisplayA.

Make DisplayB (bottom) the active display again.

Select the Display Options menu.

Make DisplayB live again.

DisplayB.

Let’s save DisplayA to a disk file.

Use a blank disk if possible, otherwise any disk that you don’t mind formatting will do. Make sure the write protect tab is off.

SR785 Dynamic Signal Analyzer

Saving and Recalling 1-55

Press [Disk]

Press <Disk Upkeep>

Press <Format Floppy> and press [Enter] to

confirm.

Press <Return>

7. Press [Active Display]

Press <File Name>

Press [Alt]

Press [D] [A] [T] [A] [1] [Enter]

Press <Display to Disk>

8. Press [Active Display]

Press <File Name>

Turn the knob to display the file catalog.

Use the knob to select one of the disk files and

press [Enter].

Select the Disk menu.

Choose the Disk Upkeep menu.

Make sure that the disk does not contain any information that you want!

This function requires a confirmation. Go ahead and confirm. Formatting takes about a minute.

Go back to the main Disk menu.

Make DisplayA (top) the active display again. We are going to save DisplayA to disk.

We need a file name.

[Alt] lets you enter the letter characters printed below each key. The number and backspace keys function normally.

‘ALT’ is highlighted (at the top of the screen) when the Alternate keys are in use.

Enter a file name (any legal DOS file name up to 8 characters). Pressing [Enter] terminates the entry and removes the [Alt].

Save the measurement data in DisplayA to disk using the specified file name.

The extension .78D is appended automatically.

Save the measurement again. Notice that the <File Name> ahs been changed to DATA2. The SR785 will try to autoincrement any filename containing a number.

Make DisplayB (bottom) the active display.

To recall a file, first specify the file name. You can either enter the name or select from the file catalog.

Turning the knob while <File Name> is highlighted displays the file catalog of the current directory.

The knob selects a file and scrolls the display. Only the files with the appropriate extension (.78D) are shown. To show all files (*.*), press [Exp].

SR785 Dynamic Signal Analyzer

1-56 Saving and Recalling

Press <Disk to Display>

9. Press [Display Options]

Press <Display>

Use the knob to select (Live) and press [Enter]. Choose Live to return the live measurement to

This concludes this example. Remember, ‘Off-Line’

Recall the data in the file to the active display (DisplayB).

Once again, DisplayB is ‘Off-Line’ indicating that it is showing static data.

Select the Display Options menu.

Make DisplayB live again.

DisplayB.

displays are showing stored data, not live measurement results. Many measurement parameters can not be adjusted for an ‘Off-Line’ display.

SR785 Dynamic Signal Analyzer

User Math Functions 1-57

User Math Functions

This example measures the group delay of the test filter (enclosed with this manual) using User Math Functions. You will use the SR785 source to provide a broad band source and both displays to measure the output of the device under test.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Use a BNC Tee to connect the Source Output to the filter input and the Ch1 A Input.

Connect the filter output to the Ch2 A Input.

3. Press [Source]

Press <On> Turn on the source. Press <Chirp>

Press [Window]

Press <Window>

Select (Uniform) with the knob and press

[Enter].

4. Press [Auto Range Ch1]

Press [Auto Range Ch2]

5. Press [Freq]

Press <Span>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

In this instrument, transfer function is defined as Ch2 response over Ch1 reference. Thus, Ch1 monitors the filter input (source output) and Ch2 measures the response of the device under test.

Select the Source menu.

Choose Chirp output. The output is an equal amplitude sine wave at each frequency bin of the FFT spectrum.

Select the Window menu.

Adjust the FFT Window function.

Let the analyzer automatically set the Input Ranges to agree with the signals. Note that the Input Range readouts at the top of the screen are displayed in inverse when Auto Range is on.

Select the Frequency menu.

Adjust the FFT Span.

SR785 Dynamic Signal Analyzer

1-58 User Math Functions

Use the knob to adjust the Span to 6.4 kHz and

press [Enter].

The top display (A) is measuring the filter input and

6. Press [User Math]

Press <Function>

Use the knob to select (FFTUsrFn2) and press

[Enter].

Press <Edit Fn>

Press <Operands>

Use the knob to highlight FFT(2) and press

[Enter].

Use the knob to highlight / (divide) and press

[Enter].

Use the knob to highlight FFT(1) and press

[Enter].

Set the Span to display the filter notch at 1 kHz.

should show a fairly flat spectrum. The bottom display (B) is measuring the filter output and should show a deep notch.

Both displays are measuring absolute signal levels.

Select the User Math menu.

Choose one of the five user functions available in the FFT measurement group.

Select Function2 to edit.

Show the Edit Function menu.

The display shows the available operands for this function.

Use <Operands> and <Operations> to switch the display between operands and operators.

Use the knob to highlight the desired box and press [Enter] to insert the selection into the function string at the top of the screen.

Choose FFT(2) as the first operand. This is the FFT of Ch2 and is identical to the normal measurement. We are going to define a transfer function (FFT2/FFT1).

As soon as the operand is entered, the display switches to show operations.

Choose the divide operation next.

The display switches back to operands.

You can choose another operation instead by pressing <Operations>.

Choose FFT(1) as the denominator of the transfer function.

SR785 Dynamic Signal Analyzer

User Math Functions 1-59

Press <Function String>

Use the knob to move the insertion point to the

beginning of the function (highlight FFT2).

Press <Operations>

Use the knob to highlight GrpDly and press

[Enter].

Press <Enter Eq.>

7. Press [Display Setup]

Press <Measurement>

Use the knob to select (FFTUsrFn2) and press

[Enter].

Press <View>

Use the knob to select (Real Part) and press

[Enter].

Press [Auto Scale A]

This key moves the marker to the function string at the top of the screen. This allows you to delete terms and insert new ones.

Move to the start of the string by highlighting the first term (FFT2).

‘Ins’ above the function string indicates that new terms will be inserted before the highlighted term. ‘Rep’ indicates that the new term replaces the highlighted one. Use <Insert/Replace> to toggle between insert and replace.

Insert the Group Delay operator in front of the transfer function.

Closing parentheses are not required (if they are at the very end of the string).

Enter the equation.

Select the Display Setup menu.

Change the measurement of the active display (DisplayA).

Choose FFT User Function2.

Change the view. Group delay is the derivative of the phase with respect to

Choose Real Part. The units automatically switch to linear units.

User Functions are dimensionless quantities. You can choose dB (logarithmic) or units (linear).

Scale the display.

The group delay is the delay time caused by the filter at different frequencies. The data values are seconds of delay. Most points are in the neighborhood of 10 to 600

At the 1 kHz notch, the group delay has a singularity. Remember, the notch filter has a phase discontinuity at the notch frequency.

ω and is a real time.

µs.

SR785 Dynamic Signal Analyzer

1-60 User Math Functions

8. Press [Display Options]

Press <d/dx Window>

Press [.] [1] [Enter]

Press [2] [Enter]

This concludes this example. User Functions allow

Select the Display Options menu.

The group delay is the derivative of the phase. The derivative operation requires an aperture or window.

Set the window to 0.1% of the display length. This increases the resolution (makes things narrower) of the graph while decreasing the smoothing of noisy data.

Set the window to 2%. This decreases the resolution (makes thing wider) while increasing the smoothing of noisy data.

you to define your own measurements starting with the basic SR785 measurements. User Functions can also use stored trace data (for calibrations and normalizations) and user constants.

SR785 Dynamic Signal Analyzer

Limit Testing 1-61

Limit Testing

This example is intended to familiarize the user with limit testing. Limit Testing tests the measurement data against a set of defined Limit Segments. When measurement data exceeds a Limit Segment at any point, the test fails. Each display has its own set of Limit Segments.

A Limit Segment is defined as the line between the pair of points (X0,Y0) and (X1,Y1). The segment values between the endpoints are calculated for the displayed span. A segment may be defined as either an Upper or Lower limit. Measurement data which is greater than an Upper limit or less than a Lower limit cause the test to fail.

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the Channel 1 A Input.

Press [Source], <On> Turn the source on. Press [Auto Scale A]

3. Press [Analysis]

Press <Limit Test>

Press <Edit Limits>

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Setup to analyze the source output. The default source is a 10.24 kHz sine.

Scale the display to show all of the data.

Select the Analysis menu.

Choose Limit Test.

Show the Edit Limits menu. We can’t turn on any testing until limits are defined.

1-62 Limit Testing

Press <New Segment>

Press <X0>

Press [8] [0] [0] [0] [Enter]

Press <Y0>

Press [-] [1] [0] [Enter]

Press <X1>

Press [1] [2] [0] [0] [0] [Enter]

Press <Y1>

Press [-] [1] [0] [Enter]

Press <Return>

4. Press <Limit Testing>

Use the knob to select (On) and press [Enter]. Turn Limit Testing on. The limit test result is

Press <Edit Limits>

This function adds a new segment. The new segment has a default position and length.

The segment is defined by its endpoints, (X0,Y0) and (X1,Y1). These values are specified for the current view and units, in this case, Hz for the x values and dBVpk for the y values.

The segments are drawn in half intensity. The arrow markers at the end points point down for upper limits and point up for lower limits. The current segment (whose endpoints are shown and edited in the menu) has two additional arrows at the endpoints.

Select X0 first.

Enter a value of 8000 Hz.

Select Y0.

Enter a value of -10 dBVpk.

Select X1.

Enter a value of 12000 Hz.

Select Y1.

Enter a value of -10 dBVpk.

The segment should intersect the signal peak at

10.24 kHz.

Return to the Limit Test menu.

Select Limit Testing.

displayed to the left of the graph. In this case ‘Fail’ should be shown.

The limit that we drew is an upper limit. Since the data peak exceed this limit, the test fails.

Go back to the Edit Limits menu.

Limit Testing 1-63

Press <Shift All>

Press [7] [Enter]

5. Press <New Segment>

Press <X0>

Press [2] [0] [0] [0] [0] [Enter]

Press <Y0>

Press [-] [8] [0] [Enter]

Press <X1>

Press [9] [0] [0] [0] [0] [Enter]

Press <Y1>

Press [-] [8] [0] [Enter]

6. Press <Segment#>

Press [0] [Enter]

Press <Limit Type>

Use the knob to select (Lower) and press

[Enter].

Press <Return>

Select Shift All limit segments. This moves all of the segments together. In this case, there is only one segment.

Enter 7 to move the segment up by 7 dBVpk.

The new segment is above the signal peak and the limit test passes.

Add another segment.

Select X0 first.

Enter a value of 20000 Hz.

Select Y0.

Enter a value of -80 dBVpk.

Select X1.

Enter a value of 90000 Hz.

Select Y1.

Enter a value of -80 dBVpk.

The segment should be above the noise floor. The limit test should still pass.

Change the segment which we are editing. The current segment is identified in the order in which they are created.

Select segment 0 (the first one). Before editing a segment, make sure that you have chosen the correct one. The current segment is identified in the display by two additional arrows at the endpoints and its endpoint coordinates are displayed in the menu.

Select the Limit Type for segment 0.

Change the limit to a lower limit.

The limit test now fails since data falls below the segment.

Return to the Limit Testing menu.

1-64 Limit Testing

Press <Limit Beep>

Use the knob to select (On) and press [Enter]. On enables the audible alarm. This alarm alerts you

7. Press [Display Setup]

Press <Measurement>

Use the knob to select (Time1) and press

[Enter].

Press <Measurement>

Use the knob to select (FFT ch1) and press

[Enter].

8. Press [Analysis]

Press <Limit Test>

Press <Limit Segments>

Use the knob to select (Show) and press [Enter]. Show the segments. The two segments defined

This concludes this example. Limit testing is a

Select Limit Beep.

to limit test failures.

Select the Display Setup menu.

Change the Measurement.

The limit segments are defined for the current measurement, view and units. Changing any of these parameters turns limit testing off.

The Limit Testing and Beep are turned off since the limit segments we defined have no meaning for this measurement.

Change the Measurement to Time Record Ch1.

If we went back to the Limit Testing menu and tried to edit limits now, the previous limit segments would be lost. Each display only has a single set of limits and they are defined for a specific measurement, view and units.

Change the Measurement back.

Select FFT ch1 again.

Select the Analysis menu.

Go to the Limit Testing menu.

Select Limit Segments.

earlier are still available.

powerful tool for repetitive tests. In an automated test environment, limit segments are usually downloaded from a host computer. The SR785 performs the limit testing in real-time and the results are queried by the host computer.

Exceedance Statistics 1-65

Exceedance Statistics

This example is intended to familiarize the user with calculating exceedance centile statistics (Ln). Ln is calculated from measurements stored in the waterfall buffer.

is the amplitude at each bin which is exceeded by n% of the records in the waterfall. The SR785

simultaneously calculates L environmental noise levels.

at all frequencies in the spectrum. Ln is commonly used to characterize

1. Press [System]

Press <Preset>

Press [Enter] to confirm Preset.

2. Connect the Source Output to the Channel 1 A Input.

3. Press [Display Setup]

Press <Measure Group>

Select (Octave) with the knob and press [Enter]. Choose the Octave group. Both displays are now

4. Press [Source]

Press <On>

Press <Noise>

Press <Type>

Use the knob to select (Pink) and press [Enter]. Choose Pink noise. Pink noise rolls off at -3dB per

Display the System menu.

Preset returns the unit to its default settings.

Preset requires confirmation to prevent accidental reset. Wait until the self tests are completed.

Setup to analyze the source output.

Select the Display Setup menu.

Change the Measurement Group.

making Octave Analysis measurements.

Select the Source menu.

Turn the Source On

Choose Noise as the source type. Octave measurements are generally used to measure noise.

Change the type of noise.

octave. This maintains equal power per octave band and yields a flat octave spectrum.

1-66 Exceedance Statistics

5. Press [Average]

Press <Power Bin>

Use the knob to select (L) and press [Enter].

Press [Auto Scale A]

6. Press [Waterfall]

Press <Storage>

Use the knob to select (One Shot) and press

[Enter].

Press <Total Count>

Press [1] [0] [0] [Enter]

Press <Save Option> The SR785 has two waterfall storage options for

Use the knob to select (Active Meas Only) and

press [Enter]

Select the Average menu. Note that this menu is changed in Octave group.

We will leave the averaging at its default, 100 ms exponential time.

Change the Power Bin.

Choose L (Leq) as the sound level bin. Leq is a standard broad band sound measurement. The result is displayed as the last bin in the display and is labeled ‘L’.

Scale DisplayA to show the entire range of the data.

Select the Waterfall menu. Ln is calculated from measurements stored in the waterfall buffer.

Select waterfall Storage.

Choose One Shot to fill the waterfall buffer once and stop.

Change the number of measurements to store in the waterfall buffer.

Enter the Total Count for both displays (100).

The Storage Interval is 100 ms. A measurement snapshot is added to the waterfall buffer every Storage Interval so 100 measurements will take 10 s to complete.

saving measurements. (All) Means that all possible measurements for the currently selected measruement group will be stored in the waterfall buffer. This option saves more measurements but each trace takes more space in the waterfall buffer, so fewer traces will be available.

(Active Meas Only) means that only the currently selected measurement will be saved in the waterfall buffer. To calculate exceedence statistics, we must use the Active Measurement Only save option.

Exceedance Statistics 1-67

Press [Start/Reset] Start the measurement. This resets the waterfall

buffer. New measurements are added to the waterfall buffer every 100 ms. The number of records stored in the waterfall is shown in the Vertical Scale Bar and increments to 100.

Because the waterfall storage is One Shot, the waterfall buffer fills once (100 records). No more records are added after 10 seconds.

7. Press [Analysis]

Press <Exceedance Stats>

Press <Stop Index>

Press [9] [9] [Enter].

Press [Pause/Cont]

Press <Calculate Excd>, choose (Trace 1) with

the knob and press [Enter].

8. Press [Alt] [Link], choose (Trace 1) with the knob and press [Enter].

Select the Analysis menu.

Select the Exceedance Statistics menu.

The exceedance is calculated using the records in the waterfall buffer starting with the Start Index (0 is the most recent record) and continuing through the Stop Index (Total Count - 1).

Enter 99 to include records 0 through 99 (100 total).

Pause the measurement. Exceedance calculation requires that the active display be paused. This ensures that the waterfall buffer is static and no new records will be added during the calculation.

The exceedance results are stored in a data trace. The trace measurement is the same type as the waterfall measurements. In this case, the trace data for each bin is exceeded by only 1% (Exceedance Pct) of the records stored in the waterfall buffer.

Trace to Ref is an alternate function. The alternate key functions are labeled below each key (in this case, the [Link] key).

Trace to Ref copies the data in a trace to the reference graph of the active display. The reference graph is shown in half intensity.

In this case, the reference graph is the level of each octave bin which is exceeded only 1% of the time

Note that the exceedance is also calculated for the power bin (L

1-68 Exceedance Statistics

Press [Pause/Cont]

Continue the live measurement. Note that the live measurement data rarely exceeds the reference graph.

9. Press [Pause/Cont]

Press <Exceedance Pct>

Press [9] [9] [Enter].

Press <Calculate Excd>, choose (Trace 2) with

the knob and press [Enter].

Pause the live measurement again.

Change the Exceedance Percentage.

Enter 99%. This level is exceeded 99% of the time.

The exceedance results are stored in Trace 2.

Press [Alt] [Link], choose (Trace 2) with the

knob and press [Enter].

Trace to Ref is an alternate function. The alternate key functions are labeled below each key (in this case, the [Link] key).

Trace to Ref copies the data in a trace to the reference graph of the active display. The reference graph is shown in half intensity.

In this case, the reference graph is the level of each octave bin which is exceeded 99% of the time (L

This concludes this example. Exceedance is a

common measurement for environmental noise levels, such as airports or highways. Change the octave integration time and waterfall storage interval to optimize the measurement rate. Use a larger waterfall buffer (up to 2000 measurements per display) for long monitoring times.

2-1

Chapter 2

Analyzer Basics

In This Chapter

Measurement Groups 2-5

Analyzer Configuration 2-5

What is an FFT? 2-7

Why Look At A Signal's Spectrum? 2-7 The FFT Analyzer 2-7 Advantages And Limitations 2-7

FFT Frequency Spans 2-9

Full Span 2-9 Spans Less Than Full Span 2-9 Baseband Spans 2-9 Starting the Span Above DC 2-10

FFT Time Record 2-11

Baseband Time Records 2-11 Heterodyned Time Records 2-11 The Time Record Display 2-12 Why Use The Time Record? 2-12 Watch Out For Windowing! 2-12

FFT Windowing 2-13

In The Frequency Domain 2-13 Uniform 2-13 Hanning 2-14 Flattop 2-14 BMH 2-14 Kaiser 2-15 Force-Exponential 2-15 User Defined 2-15

FFT Measurements 2-17

Definitions 2-17 FFT Spectrum 2-17 Power Spectrum 2-17 Time Record 2-18 Windowed Time Record 2-18 Orbit 2-18 Cross Spectrum 2-19 Frequency response 2-19 Coherence 2-20 Capture Buffer 2-20 User Function 2-21

Views 2-22

Log Magnitude 2-22 Linear Magnitude 2-22 Magnitude Real Part 2-22 Imaginary Part 2-22 Phase 2-22 Unwrapped Phase 2-23 Nyquist Plot 2-24 Nichols Plot 2-24

FFT Averaging 2-25

RMS Averaging 2-25 Vector Averaging 2-25 Peak Hold Averaging 2-26 Linear Weighting 2-26 Waterfall Storage 2-26 Exponential Weighting 2-26

Real Time Bandwidth and Overlap 2-28

What is Real Time Bandwidth? 2-28 Averaging Speed 2-28 Overlap Processing 2-28 Time Record Increment 2-29 Settling 2-29 Vector Averaging 2-30 Triggering 2-30

Waterfall Display 2-31

What is a Waterfall? 2-31 Waterfall Storage 2-31 Waterfall Display 2-32

Capture Buffer 2-34

Input Sampling 2-34 Capture Fill 2-34 Capture Playback 2-35 Capture as the Arbitrary Source 2-36

The Source 2-37

Sine 2-37 Two Tone 2-37 Chirp 2-37

2-22

SR785 Network Signal Analyzer

2-2 Analyzer Basics

Noise 2-37 Arbitrary 2-37 Windowing 2-38 Source Trigger 2-38 External Trigger 2-38

Correlation Analysis 2-39

What is Correlation Analysis? 2-39 Correlation Measurements 2-39 Capture 2-40 User Function 2-41

Octave Analysis 2-42

What is Octave Analysis? 2-42 Band Center Frequencies 2-42 Full Octave Bands 2-42 1/3 Octave Bands 2-42 1/12 Octave Bands 2-42 Octave Measurements 2-43 Octave 2-43 Capture 2-43 User Function 2-44 Octave Averaging 2-44 Linear Time 2-44 Exponential Time 2-44 Equal Confidence 2-45 Peak Hold 2-45 Sound Level Measurement 2-45 Leq 2-45 Impulse 2-45 Peak 2-46 Total Power 2-46 Settling Time 2-47

Swept Sine Measurements 2-48

Why Use Swept Sine? 2-48 Swept Sine Measurement Setup 2-50 Swept Sine Measurements 2-50 Spectrum 2-50 Cross Spectrum 2-51 Frequency response 2-51 Normalized Variance 2-51 User Function 2-51 Averaging - Settling and Integration 2-51 Sweep Frequency and Auto Resolution 2-52 Input Auto Ranging 2-53 Source Auto Level and Ramping 2-54

Order Analysis 2-57

What is Order Analysis? 2-57 Order Spectra and Order Tracking 2-57 Order Measurement Setup 2-57 Order Measurements 2-58 Linear Spectrum 2-58 Power Spectrum 2-58 Time Record 2-58 Windowed Time Record 2-58

RPM Profile 2-58 Orbit 2-59 Track 2-59 Capture 2-59 User Function 2-59

Time/Histogram Measurements 2-60

Time and Histogram Measurements 2-60 Measurements in the Time/Histogram Group 2-60 Histogram 2-60 Time Record 2-60 Probability Density Function (PDF) 2-60 Cumulative Density Function (CDF) 2-61 Capture 2-61 User Function 2-61

Trace Storage 2-62

User Math Functions 2-63

What is a User Function? 2-63 Measurement Groups and Traces 2-63 Operands 2-63 X Axis 2-64 Operations 2-65 User Function Limits 2-66

Signal Inputs 2-67

Manual Range 2-67 Auto Range 2-67 Input Noise 2-67 Input Impedance 2-68 Anti-aliasing Filter 2-68 A-Weighting Filter 2-69 Input Transducer Units 2-69

Input Connections 2-70

Single-Ended Connection (A) 2-70 Differential Connection (A-B) 2-70 Common Mode Signals 2-70 AC vs DC Coupling 2-71

Intrinsic (Random) Noise Sources 2-72

Johnson Noise 2-72 Shot Noise 2-72 1/f Noise 2-72 Total Noise 2-72

External Noise Sources 2-73

Capacitive Coupling 2-73 Inductive Coupling 2-74 Resistive Coupling (Ground Loops) 2-75 Microphonics 2-75 Thermocouple Effects 2-76

Curve Fitting and Synthesis 2-77

Curve Tables 2-77 Polynomial 2-77 Pole-Zero 2-77

SR785 Dynamic Signal Analyzer

Analyzer Basics 2-3

Pole Residue 2-77 Frequency Scale 2-78 Delay 2-78 Trace 2-78 Curve Fitting 2-78 Weighting 2-78 Curve Synthesis 2-79

SR785 Dynamic Signal Analyzer

Measurement Groups 2-5

Measurement Groups

The SR785 is organized into six Measurement Groups - FFT, Correlation, Octave Analysis, Order Analysis, Time/Histogram, and Swept Sine. Choose the Measurement Group in the [Display Setup] menu.

The SR785 calculates all the measurements in a given group simultaneously, regardless of which measurements are actually being displayed. When a measurement is paused, or finished, you can view the results of all measurements in the group without having to take new data.

The Measurement Group determines how the input data is processed. In FFT group, the input data is gathered into time records which are then transformed into spectra. In Correlation group, these spectra are transformed back into the time domain to yield auto and cross correlations. In Time/Histogram group, the time records are processed to give a statistical description of the input signal. In Octave group, the data is passed through a parallel bank of filters and averaged. In Swept Sine group, the input data is processed to determine the spectral content at a single frequency by integrating over an exact number of source cycles. Finally, in the Order Analysis group the input data is processed together with information from a tachometer to give spectra corresponding to the revolutions of a rotating machine, rather than a fixed sampling frequency.

The data processing in each group is governed by the parameters chosen in the menus. For some menus, the choice of the Measurement Group determines which parameters are shown in the menu. The [Frequency], [Average] and [Source] menus have different sets of parameters in each group. For other menus, part or all of the menu is unavailable in certain groups. The [Window], [Capture] and [Waterfall] parameters do not apply to all groups.

Each group has its own set of available measurements. Only the FFT and Order group have time and frequency domain measurements. The Time Histogram and Correlation groups have only time domain measurements, while the Octave and Swept Sine groups have only frequency domain measurements. To select a measurement, choose the Measurement Group, then the Measurement, View and Units.

Analyzer Configuration

Some measurements made by the SR785 are intrinsically two channel measurements, for instance frequency response, or cross-correlation. Other measurements, such as Autocorrelation or FFT1, only involve a single input channel. The SR785 has a uniquely flexible architecture with regard to processing single and dual channel measurements which is set by the [Input]<Analyzer Coniig> softkey. If two channel measurements are required, this softkey should be set to the default Dual Channel setting. This is the traditional mode in which 2-channel analyzers are operated. All measurements, including two channel measurements are allowed, however, many insturement parameters, such as frequency span, and number of FFT lines must be the same for both channels. Obviously it wouldn't make much sense to take a frequency response measurement where the input and output had different frequency spans.

If two-channel measurements aren't required however, the SR785 allows selection of the Independent Channel configuration. In this mode, each display of the SR785 functions as

SR785 Dynamic Signal Analyzer

2-6 Analyzer Basics

a completely separate single channel analyzer with an independent set of operating parameters. For instance in the Independent Channel mode display A could be configured to show a broadband measurement of channel 1 with linear averaging while display B could show a narrowband detail of the same input with exponential averaging. The restriction, of course, is that in Independent Channel mode, no two-channel measurments can be computed.

SR785 Dynamic Signal Analyzer

Measurement Groups 2-7

What is an FFT?

An FFT analyzer takes a time varying input signal, as you would see on an oscilloscope trace, and computes its frequency spectrum. Fourier's basic theorem states that any waveform in the time domain can be represented by the weighted sum of pure sine waves of all frequencies. If the signal in the time domain is periodic, then its spectrum is probably dominated by a single frequency component. The spectrum analyzer represents the time domain signal by its component frequencies.

Why Look At A Signal's Spectrum?

For one thing, some measurements that are very hard in the time domain are very easy in the frequency domain. Take harmonic distortion. It is hard to quantify the distortion by looking at a good sine wave output from a function generator on an oscilloscope. When the same signal is displayed on a spectrum analyzer, the harmonic frequencies and amplitudes are displayed with amazing clarity. Another example is noise analysis. Looking at an amplifier's output noise on an oscilloscope basically measures just the total noise amplitude. On a spectrum analyzer, the noise as a function of frequency is displayed. It may be that the amplifier has a problem only over certain frequency ranges. In the time domain it would be very hard to tell.

Many of these types of measurements can be done using analog spectrum analyzers. In simple terms, an analog filter is used to isolate frequencies of interest. The filtered signal power is measured to determine the signal strength in certain frequency bands. By tuning the filters and repeating the measurements, a reasonable spectrum can be obtained.

The FFT Analyzer

An FFT spectrum analyzer works in an entirely different way. The input signal is digitized at a high sampling rate. Nyquist's theorem says that as long as the sampling rate is greater than twice the highest frequency component of the signal, then the sampled data will accurately represent the input signal (in the frequency domain). In the SR785, sampling occurs at 262 kHz. To make sure that Nyquist's theorem is satisfied, the input signal passes through an analog anti-aliasing filter that removes all frequency components above 102.4 kHz. The resulting digital time record is then mathematically transformed into a frequency spectrum using an algorithm known as the Fast Fourier Transform or FFT. The resulting spectrum shows the frequency components of the input signal.

The original digital time record comes from discrete samples taken at the sampling rate. The corresponding FFT yields a spectrum with discrete frequency samples or bins. In fact, the spectrum has half as many frequency bins as there are time points. (Remember Nyquist's theorem.) Suppose that you take 1024 samples at 262 kHz. It takes 3.9 ms to take this time record. The FFT of this record yields 512 frequency points, but over what frequency range? The highest frequency will be determined by the period of 2 time samples or 131 kHz. The lowest frequency is just the period of the entire record or 1/(3.9 ms) or 256 Hz. The output spectrum thus represents the frequency range from DC to 131 kHz with 512 points spaced every 256 Hz.

Advantages And Limitations

The advantage of this technique is its speed. The entire spectrum takes only 3.9 ms to measure. The limitation of this measurement is its resolution. Because the time record is

SR785 Dynamic Signal Analyzer

2-8 Analyzer Basics

only 3.9 ms long, the frequency resolution is only 256 Hz. Suppose the signal has a frequency component at 380 Hz. The FFT spectrum will detect this signal but place part of it in the 256 Hz bin and part in the 512 Hz bin. One way to measure this signal accurately is to take a time record that is 1/380 Hz or 3.846 ms long with 1024 evenly spaced samples. Then the signal would land all in one frequency bin. But this would require changing the sampling rate based upon the signal (which you haven't measured yet). This is not a practical solution. Instead, the way to measure the signal accurately is to lengthen the time record and change the span of the spectrum.

SR785 Dynamic Signal Analyzer

FFT Frequency Spans 2-9

FFT Frequency Spans

Full Span

Full span is the widest frequency span corresponding to the fastest available sampling rate. In the SR785, this is DC to 131 kHz using a sampling rate of 262 kHz. Because the signal passes through an anti-aliasing filter at the input, the entire frequency span is not useable. The filter has a flat response from DC to 102.4 kHz and then rolls off steeply from 102.4 kHz to 156 kHz. The range between 102.4 kHz and 131 kHz is therefore not useable and the actual displayed frequency span stops at 102.4 kHz. There is also a frequency bin labeled 0 Hz (or DC). For a time record of 1024 samples (3.9 ms), this bin actually covers the range from 0 Hz to 128 Hz (the lowest measurable frequency) and contains the signal components whose period is longer than the time record (not only DC). So our final displayed spectrum contains 401 frequency bins. The first covers 0 128 Hz, the second 128 - 384 Hz, and the 401st covers 102.272 - 102.528 kHz.

Spans Less Than Full Span

The duration of the time record determines the resolution of the spectrum. What happens if we want a resolution better than 256 Hz? We need to increase the duration of the time record. There are two ways to do this - take more points in each time record or lower the sampling rate. Taking more points is difficult since both the memory and processing requirements increase with the number of points. The longest time record the SR785 can process is 2048 points (800 point FFT).

Instead, we take the approach of lowering the sample rate and making the same number of samples cover a longer time. If we halve the sample rate, this doubles the time record duration and gives us better resolution. However, the sample rate also determines the frequency span. By halving the sample rate, we also halve the frequency span. At a constant number of points in the FFT, we must tradeoff better resolution with narrower frequency spans.

Changing the sample rate of the A-D converter is not practical since that requires changing the analog anti-aliasing filter cutoff frequency. Instead, the incoming data samples (at 262 kHz) are digitally filtered and down-sampled. The advantage is that the digital filter's cutoff frequency can be easily changed. For example, to decrease the sampling rate from 262 kHz to 131 kHz, the incoming data is low-pass filtered to remove any signals above 51.2 kHz. This filter rolls off steeply from 51.2 kHz to 65.6 kHz. Since output of this filter only contains frequencies up to 65.6 kHz, Nyquist only requires a sample rate of 131 kHz and only every other point is kept as part of the time record. The result is a time record of 1024 points sampled at 131 kHz to make up an 7.8 ms record. The FFT processor operates on a constant number of points and the resulting FFT will yield 400 bins from DC to 51.2 kHz. The resolution or linewidth is 128 Hz (1/7.8 ms).

This process of halving the sample rate and span can be repeated by using multiple stages of digital filtering. The SR785 can process a 400 bin spectra with a span of only

195.3 mHz and a time record of 2048 seconds if you have the patience. However, this filtering process only yields baseband measurements (frequency spans which start at DC).

Baseband Spans

Some points to remember are:

SR785 Dynamic Signal Analyzer

2-10 FFT Frequency Spans

1. The FFT resolution (number of frequency bins in the spectrum) determines the number of points required in the time record.

2. The duration of the time record determines the frequency resolution of the spectrum (spacing of frequency bins in the spectrum).

3. The sampling rate determines the frequency span of the spectrum (Nyquist’s theorem). The sampling rate is the number of points in the time record divided by the duration of the time record.

The SR785 allows FFT resolutions of 100, 200, 400 or 800 bins (not counting DC). Changing the resolution does not change the span, instead the time record length is changed.

The various FFT resolutions are summarized below.

FFT Frequency Resolution 100 bins Span/100 100/Span 200 bins Span/200 200/Span 400 bins Span/400 400/Span 800 bins Span/800 800/Span

Resolution Time Record

Starting the Span Above DC

Using digital filtering alone requires that every span start at DC. Frequency shifting is accomplished by heterodyning. Heterodyning is the process of multiplying the incoming signal by a sine (and cosine) wave. The resulting spectrum is shifted by the heterodyne frequency. If we incorporate heterodyning with our digital filtering, we can shift any frequency span so that it starts at DC. The resulting FFT yields a spectrum offset by the heterodyne frequency. When this spectrum is displayed, the frequencies of the X axis are the original frequencies of the signal.

Heterodyning allows the analyzer to compute zoomed spectra (spans that start at frequencies other than DC). The digital processor must filter and heterodyne the input in real time to provide the appropriate filtered and down-sampled time record at all spans and center frequencies.

SR785 Dynamic Signal Analyzer

FFT Time Record 2-11

FFT Time Record

The FFT operates on time records. A time record is simply a sequence of data samples. The duration of the time record is the FFT resolution/span. There are two types of time records, those corresponding to baseband spans (starting at DC) and those corresponding to zoomed spans (not starting at DC). Zoomed time records are heterodyned (frequency shifted) and do NOT contain the input signal at its original frequencies.

Baseband Time Records

Baseband time records are very simple to understand. They represent the input signal passed through low-pass filters. At full span, the signal has passed through the analog anti-aliasing filter. The sample rate is 262 kHz. To get the time records for narrower spans, the data is digitally filtered and down-sampled. At a given FFT resolution, each time the span is halved, the bandwidth of the time record is halved and the sampling rate is halved. The length of the time record (in seconds) doubles.

Heterodyned Time Records

Zoomed time records are more complicated. Heterodyning is a complex operation. The input points are multiplied by cos(

π times the span center frequency. The real and imaginary parts of each point are

is 2 orthogonal. You can think of the complex time record as two separate records, one real and one imaginary.

ω t) and sin(ωt) to yield a real and an imaginary part. ω

The input signal is frequency shifted or heterodyned. This moves signals at the span center to DC and frequencies below span center to negative frequencies. If the span center is at 51.2 kHz, the input range from 0 to 102.4 kHz is shifted to -51.2 kHz to +51.2 kHz. This data is then passed through a low-pass filter which cuts off at This results in a ±51.2 kHz (102.4 kHz) useable span centered at 51.2 kHz. The output data only requires a sampling rate of 131 kHz (instead of the original 262 kHz real input rate) so only every other point is saved. Thus, the original 102.4 kHz span is represented by a time record with half as many points and half the sampling rate and the same duration. How can this be?

The complex time record has half as many points as the baseband (real) time record with the same span and resolution. This is because the negative frequency part of the spectrum is kept in the heterodyned case. You can think of the real and imaginary parts of the complex time record as completely independent data streams, each at half of the original sample rate and each with half of the original span. Together, they represent the original span with the original number of samples and the original time record length.

Digital filtering and down-sampling is used to narrow the span of the heterodyned data. This ‘zooms’ in around the heterodyne frequency (span center). The first digital filter reduces the sample rate by 2 (to 131 kHz) but does not reduce the span.

The second digital filter cuts off at ±25.6 kHz and reduces the sample rate by 2 again. The number of points in the time record is NOT halved again (this only happens at the first filter due to the splitting of the real time record into two parts, real and imaginary). The new time record must have twice the original duration and thus, half of the original span. This results in a 51.2 kHz (±25.6 kHz) span centered at 51.2 kHz. The time record duration is twice the full span time record. The sample rate is 1/4 of the full span

±51.2 kHz.

SR785 Dynamic Signal Analyzer

2-12 FFT Time Record

baseband sample rate. In comparison, the baseband 0-51.2 kHz span has a sample rate 1/2 of the full span baseband sample rate. This is because the baseband time record is all real and the zoomed time record is complex.

Further filtering and down-sampling reduces the span even further. At each span, the zoomed time record is complex and has half as many points (half the sample rate) as the corresponding real baseband time record.

The Time Record Display

The baseband time record display resembles a digital oscilloscope display. Signals at frequencies above the span have been filtered out. The anti-aliasing filters (both analog and digital) have a steep but finite roll-off at their cutoff frequencies. Signals which are just above the cutoff frequency are outside of the FFT span (not in the displayed spectrum) but appear attenuated in the time record.

Baseband time records are entirely real, they have no imaginary part.

The zoomed time record display does not resemble the original data. The data has been frequency shifted. Signals at the center of the span appear at DC while frequencies at both edges of the span appear as high frequencies. The anti-aliasing filters have a steep but finite roll-off at their cutoff frequencies. Signals which are just outside of the span are not displayed in the FFT but appear frequency shifted and attenuated in the time record.

Zoomed time records are complex, they have both a real and an imaginary part. You can display the magnitude and phase as well as the real or imaginary part. The sampling rate is always half of the equivalent baseband span.

Why Use The Time Record?

The time record display can be useful in determining whether the time record is triggered properly. If the analyzer is triggered and the signal has a large component synchronous with the trigger, then the signal should appear stationary in the time record. If the signal triggers randomly, then the time display will jitter back and forth.

Remember, the time record has a resolution of 1/(sample rate). A triggered time record will always jitter by 1 sample. This jitter is removed in the computation of the phase of the spectrum relative to the trigger.

Watch Out For Windowing!

The SR785 can display both the time record and the windowed time record. Most window functions taper off to zero at the start and end of the time record. If a transient signal occurs at the start of the time record, the corresponding windowed time record and FFT may not show anything because the window function reduces the transient to zero.

SR785 Dynamic Signal Analyzer

FFT Windowing 2-13

FFT Windowing

A signal which is not exactly periodic within the time record does not fall on an exact frequency bin of the FFT spectrum (integer multiple of the FFT frequency resolution). Its energy is split across multiple adjacent frequency bins. This is true but it's actually worse than that. An FFT spectrum models the time domain as if a time record repeated itself forever. This means the end of the time record is followed by the start of the time record in a circular fashion. If the data is not continuous across the stop to start boundary, the FFT will actually compute the spectrum of the discontinuity and ‘leak’ energy into all frequencies in the spectrum.

Windows are functions defined over a time record which are periodic in a circular time record. They generally start and end smoothly at zero and are smooth functions in between. When the time record is windowed, its data samples are multiplied by the window function, time point by time point, and the resulting windowed time record is definitely periodic in the circular sense. Windowing eliminates the leakage in the spectrum from signals not exactly periodic with the time record.

In The Frequency Domain

In the frequency domain, a window acts like a filter. The amplitude of each frequency bin is determined by centering this filter on each bin and measuring how much of the signal falls within the filter.

If the filter is narrow, then only frequencies near the exact bin frequency will contribute to the bin. A narrow filter is called a selective window - it selects a small range of frequencies around each bin. However, since the filter is narrow, frequencies slightly off bin are attenuated and phase shifted. Selective windows are useful for resolving adjacent peaks or improving signal to noise. They should not be used for accurate amplitude measurements (except for signals at exact bin frequencies).

If the filter is wide, then frequencies farther from each exact bin will contribute to the bin amplitude making the signal peaks very wide. However, off bin frequencies are not attenuated. These windows should be used for accurate amplitude measurements rather than good frequency resolution.

Windowing allows the FFT to accurately measure signals at frequencies which are not exact frequency bins. The different types of windows trade off selectivity, amplitude accuracy, and noise floor.

The SR785 offers many types of window functions - Uniform (no windowing), Flattop, Hanning, Blackman-Harris (BMH), Kaiser, Force-Exponential, and User Defined windows.

Uniform

The Uniform window is actually no window at all. The entire time record is used with equal weighting. A signal will appear in a single frequency bin in the spectrum if its frequency is exactly equal to a frequency bin. (It is exactly periodic within the time record). If its frequency is between bins, it will leak into every bin of the spectrum. These two cases also have a great deal of amplitude variation between them (up to 4 dB).

SR785 Dynamic Signal Analyzer

2-14 FFT Windowing

In general, this window is only useful when looking at transients which do not last the entire time record. The Uniform window may also be used with signals which are exactly periodic in the time record such as a chirp or exact bin sine frequencies.

In the Order Analysis measurement group the uniform window can be used in situations where most of the harmonics of interest will be at integer multiples of the shaft rotation speed. In this case, these harmonics will fall on exact bin frequencies and will be most accurately reproduced with the uniform windows.

Hanning

The Hanning window is a commonly used window. However, it has an amplitude variation of about 1.5 dB for signals which are not at exact bin frequencies and provides only reasonable selectivity. Its side-lobes are very high and broad for off-bin frequencies. As a result, the Hanning window can limit the performance of the analyzer when looking at signals close together in frequency and very different in amplitude.

The Hanning window is most often used in noise measurements since it has the lowest noise floor.

The Hanning window function is

Flattop

The Flattop window has the best amplitude accuracy of any window. The amplitude variation is only about 0.02 dB for signals between exact frequency bins. However, the selectivity is worse. Unlike the other windows, the Flattop window has a very wide pass band and very steep rolloff on either side. Thus, signals appear wide but do not leak across the whole spectrum.

The Flattop window is the best window to use for accurate amplitude measurements.

The Flattop window function is

for i = 0 .. N-1 and N = number of time record points.

BMH

The BMH window combines good selectivity and reasonable accuracy (about 0.8 dB for signals between exact frequency bins). The BMH window has much lower side-lobes than the Hanning window and very little broadening of non-bin frequencies.

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SR785 Dynamic Signal Analyzer

FFT Windowing 2-15

for i = 0 .. N-1 and N = number of time record points.

Kaiser

The Kaiser window combines excellent selectivity and reasonable accuracy (about 0.8 dB for signals between exact frequency bins). The Kaiser window has the lowest side-lobes and least broadening for non-bin frequencies. This makes this window the best for selectivity.

The Kaiser window is the best window to use for measurements requiring a large dynamic range.

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Force-Exponential

Many impact measurements require a Force window for excitation channel and an Exponential window for the response channel. With the SR785, this is accomplished by selecting the Force/Exponential Window. The Force/Exponential window is actually two windows, either of which can be applied to either input channel.

The Force window is uniform over the beginning of the time record and sets the remainder of the time record to its average value. This method minimizes sepectral artifacts caused by zeroing the remainder of the time record. The force length is user specified. This window is used to isolate impulsive signals, such as impact excitations, from noise and other oscillations later in the time record.

The Exponential window attenuates the time record with a decaying exponential time constant. This window is often used in impact testing on the response channel to remove oscillations which last longer than the time record.

When the Force-Exponential window is selected and the analyzer is in the Dual Channel configuration the choice of which window is applied to which input channel is made by the <Channel 1 Window> and <Channel 2 Window> softkeys.When the analyzer is in the Independent Channels configuration, these softkeys govern which window is applied to the measurement in the two displays.

User Defined

The User window is any function that the user provides. The User window is copied from a stored trace using <Trace to Window>. The trace may contain display data or be loaded from disk or via the computer interface.

SR785 Dynamic Signal Analyzer

Stanford Research Systems SR785 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual