Stanford Research Systems SR785 User Manual

Operating Manual and
Programming Reference

Model SR785
Dynamic Signal Analyzer

1290-D Reamwood Avenue

Sunnyvale, CA 94089 U.S.A.

Phone: [phone] • Fax: [phone]

Email: [e-mail] • www.thinkSRS.com

Copyright © 1995, 1996, 1998

Stanford Research Systems, Inc.

All Rights Reserved

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.

Copyright © Stanford Research Systems, Inc., 1995, 1996, 1998. All rights reserved.

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

Printed in U.S.A.

SR785 Dynamic Signal Analyzer

i

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

ii

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

ix

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

EQ

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

N

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

xi

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

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

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

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.

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.

<Softkeys>

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