Stanford Research Systems SR770 User’s Manual

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User’s Manual

Model SR770

FFT Network Analyzer

1290-D Reamwood Avenue

Sunnyvale, California 94089

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

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

Revision 1.3 (09/2001)

TABLE OF CONTENTS

GENERAL INFORMATION

Safety and Preparation for Use 1-3 Specifications 1-5 Abridged Command List1-9

GETTING STARTED

Your First Measurement 2-1 Analyzing a Sine Wave 2-2 Second Measurement Example 2-7 Amplifier Noise Level 2-8 Using Triggers and the Time Record 2-11 Using the Disk Drive 2-15 Using Data Tables 2-21 Using Limit Tables 2-25 Using Trace Math 2-29 Using the Source 2-33

Sine 2-34 Two Tone 2-38 Noise 2-40 Chirp 2-43

Things to Watch Out For 2-49

ANALYZER BASICS

What is an FFT Spectrum Analyzer? 3-1 Frequency Spans 3-2 The Time Record 3-3 Measurement Basics 3-4 Display Type 3-5 Windowing 3-6 Averaging 3-7 Real Time Bandwidth and Overlap 3-8 Input Range 3-9 The Source 3-10

MARKER 4-9 ACTIVE TRACE 4-9 AUTO RANGE 4-9 AUTOSCALE 4-9 SPAN UP/DOWN 4-9 MARKER ENTRY 4-9 MARKER MODE 4-9 MARKER REF 4-9 MARKER CENTER 4-10 MARKER MAX/MIN 4-10 PRINT 4-10 HELP 4-10 LOCAL 4-10

Rear Panel 4-11

Power Entry Module 4-11 IEEE-488 Connector 4-11 RS232 Connector 4-11 Parallel Printer Connector 4-11 PC Keyboard Connector 4-11

MENUS

Frequency Menu 5-1 Measure Menu 5-3 Display Menu 5-15 Marker Mode Menu 5-17 Input Menu 5-19 Scale Menu 5-27 Analyze Menu 5-29 Average Menu 5-49 Source Menu 5-53 System Menu 5-61 Store/Recall Menu 5-77 Default Settings 5-85

OPERATION

Front Panel 4-1

Power On/Off 4-1 Reset 4-1 Video Display 4-1 Soft Keys 4-2 Keypad 4-2 Spin Knob 4-2 Disk Drive 4-2 BNC Connectors 4-2

Screen Display 4-3

Data Display 4-3 Single/Dual Trace Displays 4-3 Marker Display 4-5 Menu Display 4-5 Status Indicators 4-5

Keypad 4-7

Normal and Alternate Keys 4-7 Menu Keys 4-7 Entry Keys 4-8 START and PAUSE/CONT 4-8

PROGRAMMING

GPIB Communications 6-1 RS232 Communications 6-1 Status Indicators and Queues 6-1 Command Syntax 6-1 Interface Ready and Status 6-2

Detailed Command List 6-3

Frequency Commands 6-4 Measurement Commands 6-5 Display and Marker Commands 6-6 Scale Commands 6-8 Input Commands 6-9 Analysis Commands 6-10 Data Table Commands 6-11 Limit Table Commands 6-12 Averaging Commands 6-13 Source Commands 6-14 Print and Plot Commands 6-15 System Setup Commands 6-16 Store and Recall Commands 6-18 Trace Math Commands 6-19 Front Panel Control Commands 6-20

TABLE OF CONTENTS

Data Transfer Commands 6-21 Interface Commands 6-23 Status Reporting Commands 6-24

Status Byte Definitions 6-25

Serial Poll Status Byte 6-25 Serial Polls 6-25 Service Requests (SRQ) 6-26 Standard Event Status Byte 6-26 FFT Status Byte 6-27 Error Status Byte 6-27

Program Examples

Microsoft C, Nat'l Instruments GPIB 6-28 BASIC, Nat'l Instruments GPIB 6-31

TESTING

Introduction 7-1 Preset 7-1 Serial Number 7-1 Firmware Revision 7-1 General Installation 7-2 Necessary Equipment 7-3 If A Test Fails 7-3

Performance Tests

Self Tests 7-5 DC Offset 7-7 Common Mode Rejection 7-9 Amplitude Accuracy and Flatness 7-11 Amplitude Linearity 7-15 Anti-Alias Filter Attenuation 7-17 Frequency Accuracy 7-19 Phase Accuracy 7-21 Harmonic Distortion 7-23 Noise and Spurious Signals 7-25 Sine Source 7-27

Performance Test Record 7-29

Keyboard Interface 8-3 Spin Knob 8-4 Speaker 8-4 Clock/Calendar 8-4 Printer Interface 8-4 Video Graphics Interface 8-4 Disk Controller 8-4 GPIB Interface 8-4 RS232 Interface 8-4 Expansion Connector 8-4

Power Supply Board

Unregulated Power Supplies 8-5 Power Supply Regulators 8-5

DSP Logic Board

Overview 8-7 DSP Processors 8-7 Trigger 8-7 Timing Generator 8-8 I/O Interface 8-8 Source 8-8

Analog Input Board

Overview 8-9 Input Amplifier 8-9 Gain Stages and Attenuators 8-9 Anti-Alias Filter 8-9 A/D Converter 8-10 I/O Interface 8-10 Power 8-10

Parts Lists

CPU Board 8-11 Power Supply Board 8-15 DSP Logic Board 8-17 Analog Input Board 8-22 Chassis Assembly 8-29 Miscellaneous 8-32

CIRCUIT DESCRIPTION

Circuit Boards 8-1 Video Driver and CRT 8-1

CPU Board

Microprocessor System 8-3 Keypad Interface 8-3

Schematic Diagrams

CPU Board Power Supply Board DSP Logic Board Analog Input Board

SR770 FFT SPECTRUM 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 covers are removed. Do not remove the covers 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 SR770 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 Amp fuse and for 220V/240V, use a 1/2 Amp fuse.

LINE CORD

The SR770 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.

SERVICE

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.

iii

SR770 FFT SPECTRUM ANALYZER

SPECIFICATIONS

FREQUENCY

Measurement Range 476 µHz to 100 kHz, baseband and zoomed. Spans 191 mHz to 100 kHz in a binary sequence. Center Frequency Anywhere within the measurement range subject to span and range

limits. Accuracy 25 ppm from 20°C to 40°C. Resolution Span/400 Window Functions Blackman-Harris, Hanning, Flattop and Uniform. Real-time Bandwidth 100 kHz

SIGNAL INPUT

Number of Channels 1 Input Single-ended or true differential Input Impedance 1 MΩ, 15 pf Coupling AC or DC CMRR 90 dB at 1 kHz (Input Range < -6 dBV)

80 dB at 1 kHz (Input Range <14 dBV)

50 dB at 1 kHz (Input Range ≥ 14 dBV) Noise 5 nVrms/√Hz at 1 kHz typical, 10 nVrms/√Hz max.

(-166 dBVrms/√Hz typ., -160 dBVrms/√Hz max.)

AMPLITUDE

Full Scale Input Range -60 dBV (1.0 mVpk) to +34 dBV (50 Vpk) in 2 dB steps. Dynamic Range 90 dB typical Harmonic Distortion No greater than -90 dB from DC to 50 kHz. (Input Range ≤ 0 dBV)

No greater than -80 dB to 100 kHz. Spurious Input range ≥ -50 dBV:

No greater than -85 dB below full scale below 200 Hz.

No greater than -90 dB below full scale to 100 kHz. Input Sampling 16 bit A/D at 256 kHz Accuracy ± 0.2 dB ± 0.003% of full scale (excluding windowing effects). Averaging RMS, Vector and Peak Hold.

Linear and exponential averaging up to 64k scans.

TRIGGER INPUT

Modes Continuous, internal, external, or external TTL. Internal Level: Adjustable to ±100% of input scale.

Positive or Negative slope.

Minimum Trigger Amplitude: 10% of input range. External Level: ±5V in 40 mV steps. Positive or Negative slope.

Impedance: 10 kΩ

Minimum Trigger Amplitude: 100 mV. External TTL Requires TTL level to trigger (low<.7V, high>2V). Post-Trigger Measurement record is delayed by 1 to 65,000 samples (1/512 to 127

time records) after the trigger.

Delay resolution is 1 sample (1/512 of a record). Pre-Trigger Measurement record starts up to 51.953 ms prior to the trigger.

Delay resolution is 3.9062 µs. Phase Indeterminacy <2°

SR770 FFT SPECTRUM ANALYZER

DISPLAY FUNCTIONS

Display Real, imaginary, magnitude or phase spectrum. Measurements Spectrum, power spectral density, time record and 1/3 octave. Analysis Band, sideband, total harmonic distortion and trace math.

Trace Math Add, subtract, multiply, and divide with a constant, ω (2

trace. Log (base 10), square root, phase unwrap and d/dx functions.

Graphic Expand Display expands up to 50x about any point in the display.

MARKER FUNCTIONS

Harmonic Marker Displays up to 400 harmonics of the fundamental. Delta Marker Reads amplitude and frequency relative to defined reference. Next Peak/Harmonic Locates nearest peak or harmonic to the left or right. Data Tables Lists Y values of up to 200 user defined X points. Limit Tables Automatically detects data exceeding up to 100 user defined upper and

lower limit trace segments.

SOURCE OUTPUT

Amplitude Range 0.1 mVpk to 1.000 Vpk Amplitude Resolution 1 mVpk (Output>100 mVpk); 0.1 mVpk (Output ≤ 100.0 mVpk) DC Offset <10.0 mV (typical) Output Impedance < 5 Ω; ±50 mA peak output current.

SINE

Amplitude Accuracy ±1% (0.09 dB) of setting, 0 Hz to 100 kHz, 0.1 Vpk to 1.0 Vpk,

high impedance load. Frequency Resolution 15.26 mHz (1 kHz/65536) Harmonics, Sub-Harmonics, 0.1 Vpk to 1 Vpk 0 to 10 kHz <-80 dBc

10 kHz to 100 kHz <-70 dBc

Spurious Signals <-100 dBV (typical, line frequency related)

f), or another

TWO TONE

Amplitude Accuracy ±1% (0.09 dB) of setting, 0 Hz to 100 kHz, 0.1 Vpk to 0.5 Vpk,

high impedance load. Frequency Resolution 15.26 mHz (1 kHz/65536) Harmonics, Sub-Harmonics 0.1 Vpk to 0.5 Vpk 0 to 10 kHz <-80 dB below larger tone

10 kHz to 100 kHz <-70 dB below larger tone

Spurious Signals <-100 dBV (typical, line frequency related)

WHITE NOISE Output is 0 Hz to 100 kHz at all measurement spans.

Flatness <0.25 dB pk-pk (typical), <1.0 dB pk-pk (max)

(5000 rms averaged spectra, Source Cal on).

PINK NOISE Output is 0 Hz to 100 kHz at all measurement spans.

Flatness <4.0 dB pk-pk, 20 Hz - 20 kHz

(measured using 1/3 octave analysis, Source Cal on).

CHIRP Output is equal amplitude sine waves at each frequency bin of the

measurement span. Flatness Measured spectra (all spans, Source Cal on)

<0.05 dB pk-pk (typical), <0.2 dB pk-pk (max), Amplitude=1.0 Vpk. Phase Auto Phase function calibrates to current phase spectrum.

GENERAL

Monitor Monochrome CRT. 640H by 480V resolution.

Adjustable brightness and screen position. Interfaces IEEE-488, RS232 and Printer interfaces standard.

SR770 FFT SPECTRUM ANALYZER

All instrument functions can be controlled through the IEEE-488 and RS232 interfaces. A PC keyboard input is provided for additional flexibility.

Hardcopy Screen dumps and table and setting listings to dot matrix and HP

LaserJet compatible printers. Data plots to HP-GL compatible plotters (via RS232 or IEEE-488).

Disk 3.5 inch DOS compatible format, 720 kbyte capacity. Storage of data,

setups, data tables, and limit tables. Power 60 Watts, 100/120/220/240 VAC, 50/60 Hz. Dimensions 17"W x 6.25"H x 18.5"D Weight 36 lbs. Warranty One year parts and labor on materials and workmanship.

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SR770 FFT SPECTRUM ANALYZER

COMMAND LIST

VARIABLES g Trace0 (0), Trace1 (1), or Active Trace (-1)

FREQUENCY page description

SPAN (?) {i} 6-4 Set (Query) the Frequency Span to 100 kHz (19) through 191 mHz (0). STRF (?) {f} 6-4 Set (Query) the Start Frequency to f Hz. CTRF (?) {f} 6-4 Set (Query) the Center Frequency to f Hz. OTYP (?) {i} 6-4 Set (Query) the number of bands in Octave Analysis to 15 (0) or 30 (1). OSTR (?) {i} 6-4 Set (Query) the Starting Band in Octave Analysis to -2 ≤ i ≤ 35. WTNG (?) {i} 6-4 Set (Query) the Weighting in Octave Analysis to none (0) or A-weighting (1).

MEASUREMENT page description

MEAS (?) g {,i} 6-5 Set (Query) the Measurement Type to Spectrum (0), PSD (1), Time (2), or

DISP (?) g {,i} 6-5 Set (Query) the Display to LogMag (0),LinMag (1), Real (2), Imag (3), or

UNIT (?) g {,i} 6-5 Set (Query) the Units to Vpk or deg (0),Vrms or rads (1), dBV (2), or dBVrms

VOEU (?) g {,i} 6-5 Set (Query) the Units to Volts (0), or EU (1). EULB (?) g {,s} 6-5 Set (Query) the EU Label to string s. EUVT (?) g {,x} 6-5 Set (Query) the EU Value to x EU/Volt. WNDO (?) g {i} 6-5 Set (Query) the Window to Uniform (0), Flattop (1), Hanning (2), or BMH (3).

DISPLAY and MARKER page description

ACTG (?) {i} 6-6 Set (Query) the Active Trace to trace0 (0) or trace1 (1). FMTS (?) g {,i} 6-6 Set (Query) the Display Format to Single (0) or Dual (1) trace. GRID (?) g {,i} 6-6 Set (Query) the Grid mode to Off (0), 8 (1), or 10 (2) divisions. FILS (?) g {,i} 6-6 Set (Query) the Graph Style to Line (0) or Filled (1). MRKR (?) g {,i} 6-6 Set (Query) the Marker to Off (0), On (1) or Track (2). MRKW (?) g {,i} 6-6 Set (Query) the Marker Width to Norm (0), W ide (1), or Spot (2). MRKM (?) g {,i} 6-6 Set (Query) the Marker Seeks mode to Max (0), Min (1), or Mean (2). MRLK (?) {i} 6-6 Set (Query) the Linked Markers to Off (0) or On (1). MBIN g,i 6-6 Move the marker region to bin i. MRKX? 6-6 Query the Marker X position. MRKY? 6-6 Query the Marker Y position. MRPK 6-6 Move the Marker to the on screen max or min. Same as [MARKER MAX/MIN]

MRCN 6-6 Make the Marker X position the center of the span. Same as [MARKER

MRRF 6-6 Turns Marker Offset on and sets the offset equal to the marker position. MROF (?) {i} 6-6 Set (Query) the Marker Offset to Off (0) or On (1). MROX (?) {x} 6-7 Set (Query) the Marker Offset X value to x. MROY (?) {x} 6-7 Set (Query) the Marker Offset Y value to x. PKLF 6-7 Move the marker to the next peak to the left. PKRT 6-7 Move the marker to the next peak to the right. MSGS s 6-7 Display message s on the screen and sound an alarm.

i,j Integers f Frequency (real) x,y Real Numbers sString

Octave (3).

Phase (4).

(3).

key.

CENTER] key.

SCALE

TREF (?) g {,x} 6-8 Set (Query) the Top Reference to x. BREF (?) g {,x} 6-8 Set (Query) the Bottom Reference to x. YDIV (?) g {,x} 6-8 Set (Query) the Vertical Scale (Y/Div) to x. AUTS g 6-8 AutoScale graph g. Similar to the [AUTO SCALE] key. EXPD (?) g {,i} 6-8 Set (Query) the Horizontal Expand to no expand (5), 128, 64, 30, 15, or 8 bins

ELFT (?) g {,i} 6-8 Set (Query) the Left Bin when expanded to bin i. XAXS (?) g {,i} 6-8 Set (Query) the X Axis scaling to Linear (0) or Log (1).

page description

(4-0).

viii

SR770 FFT SPECTRUM ANALYZER

page description

INPUT

ISRC (?) {i} 6-9 Set (Query) the Input to A (0) or A-B (1). IGND (?) {i} 6-9 Set (Query) the Input Grounding to Float (0) or Ground (1). ICPL (?) {i} 6-9 Set (Query) the Input Coupling to AC (0) or DC (1). IRNG (?) {i} 6-9 Set (Query) the Input Range to i dBV full scale. -60 ≤ i ≤ 34 and i is even. ARNG (?) {i} 6-9 Set (Query) the Auto Range mode to Manual (0) or Auto (1). AOFF 6-9 Perform Auto Offset calibration. AOFM (?) {i} 6-9 Set (Query) the Auto Offset Mode to Off (0) or On (1). TMOD (?) {i} 6-9 Set (Query) the Trigger Mode to Cont (0), Int (1), Ext (2), or Ext TTL(3). TRLV (?) {x} 6-9 Set (Query) the Trigger Level to x percent. -100.0 ≤ x ≤ 99.22. TDLY (?) {i} 6-9 Set (Query) the Trigger Delay to i samples. -13300≤ i ≤ 65000. ARMM (?) {i} 6-9 Set (Query) the Arming Mode to Auto (0) or Manual (1). ARMS 6-9 Manually arm the trigger.

ANALYSIS

ANAM (?) g {,i} 6-10 Set (Query) the real time Analysis to None (0), Harmonic (1), Sideband (2), or

CALC? g,i 6-10 Query result i (0 or 1) of the latest real time analysis. FUND (?) g {,f} 6-10 Set (Query) the Harmonic Fundamental to frequency f Hz. NHRM (?) g {,i} 6-10 Set (Query) the Number of Harmonics to 0 ≤ i ≤ 400. NHLT 6-10 Move the Marker or Center Frequency to the next harmonic to the left. NHRT 6-10 Move the Marker or Center Frequency to the next harmonic to the right. SBCA (?) g {,f} 6-10 Set (Query) the Sideband Carrier to frequency f Hz. SBSE (?) g {,f} 6-10 Set (Query) the Sideband Separation to f Hz. NSBS (?) g {,i} 6-10 Set (Query) the Number of Sidebands to 0 ≤ i ≤ 200. BSTR (?) g {,f} 6-10 Set (Query) the Band Start to frequency f Hz. BCTR (?) g {,f} 6-11 Set (Query) the Band Center to frequency f Hz. BWTH (?) g {,f} 6-11 Set (Query) the Band Width to f Hz. TABL 6-11 Turn on Data Table display for the active trace. DTBL (?) g {,i}{,f} 6-11 Set (Query) Data Table line i to frequency f. DINX (?) {i} 6-11 Set (Query) Data Table index to i. DINS 6-11 Insert a new line in the data table. DIDT 6-11 Delete a line from the data table. DLTB 6-11 Delete the entire data table. LIMT 6-12 Turn on Limit Table display for the active trace. TSTS (?) {i} 6-12 Set (Query) the Limit Testing to Off (0) or On (1). PASF? 6-12 Query the results of the latest limit test. Pass=0 and Fail=1. LTBL (?) g {,i} {j,f1,f2,y1,y2} 6-12 Set (Query) Limit Table line i to Xbegin (f1), Xend (f2), Y1 and Y2. LINX (?) {i} 6-12 Set (Query) Limit Table index to i. LINS 6-12 Insert a new line in the limit table. LIDT 6-12 Delete a line from the limit table. LLTB 6-12 Delete the entire limit table. LARM (?) {i} 6-12 Set (Query) the Audio Limit Fail Alarm to Off (0) or On (1).

page description

Band (3).

AVERAGING page description

AVGO (?) {i} 6-13 Set (Query) Averaging to Off (0) or On (1). NAVG(?) {i} 6-13 Set (Query) the Number of Averages to 2 ≤ i ≤ 32000. AVGT (?) {i} 6-13 Set (Query) the Averaging Type to RMS (0), Vector (1), or Peak Hold (2). AVGM (?) {i} 6-13 Set (Query) the Averaging Mode to Linear (0) or Exponential (1). OVLP (?) {x} 6-13 Set (Query) the Overlap to x percent. 0 ≤ x ≤ 100.0.

SOURCE page description

STYP (?) {i} 6-14 Set (Query) Source Type to Off (0), SIne (1), 2-Tone (2), Noise (3) or Chirp

SLVL (?) i {,x} 6-14 Set (Query) the Level of sine, tone 1, tone 2, noise, chirp (i=0..4) to x mV. SFRQ (?) i {,f} 6-14 Set (Query) the Frequency of sine, tone 1, tone 2 (i=0..2) to f Hz.. NTYP (?) {i} 6-14 Set (Query) the Noise Type to white (0) or pink (1). SCAL (?) {i} 6-14 Set (Query) Source Cal off (0) or on (1) (noise and chirp only). APHS 6-14 Do Auto Phase (chirp source only).

PLOT AND PRINT

PLOT 6-14 Plot the entire graph (or graphs). PTRC 6-14 Plot the trace (or traces) only. PMRK 6-14 Plot the marker (or markers) only.

page description

(4).

SR770 FFT SPECTRUM ANALYZER

PTTL (?) {s} 6-14 Set (Query) the Plot Title to string s. PSTL (?) {s} 6-14 Set (Query) the Plot Subtitle to string s. PRSC 6-14 Print the screen. Same as the [PRINT] key. PSET 6-14 Print the analyzer settings. PLIM 6-14 Print the Limit Table of the active graph. PDAT 6-14 Print the Data Table of the active graph.

SETUP page description

OUTP (?) {i} 6-15 Set (Query) the Output Interface to RS232 (0) or GPIB (1). OVRM (?) {i} 6-15 Set (Query) the GPIB Overide Remote state to Off (0) or On (1). KCLK (?) {i} 6-15 Set (Query) the Key Click to Off (0) or On (1). ALRM (?) {i} 6-15 Set (Query) the Alarms to Off (0) or On (1). THRS (?) {i} 6-15 Set (Query) the Hours to 0≤ i ≤ 23. TMIN (?) {i} 6-15 Set (Query) the Minutes to 0 ≤ i ≤ 59. TSEC (?) {i} 6-15 Set (Query) the Seconds to 0 ≤ i ≤ 59. DMTH (?) {i} 6-15 Set (Query) the Month to 1 ≤ 1 ≤ 12. DDAY (?) {i} 6-15 Set (Query) the Day to 1 ≤ 1 ≤ 31. DYRS (?) {i} 6-15 Set (Query) the Year to 0 ≤ 1 ≤ 99. PLTM (?) {i} 6-15 Set (Query) the Plotter Mode to RS232 (0) or GPIB (1). PLTB (?) {i} 6-15 Set (Query) the Plotter Baud Rate to 300 (0), 1200 (1), 2400 (2), 4800 (3),

PLTA (?) {i} 6-15 Set (Query) the Plotter GPIB Address to 0 ≤ i ≤ 30. PLTS (?) {i} 6-15 Set (Query) the Plot Speed to Fast (0) or Slow (1). PNTR (?) {i} 6-15 Set (Query) the Trace Pen Number to 1 ≤ i ≤ 6. PNGD (?) {i} 6-16 Set (Query) the Grid Pen Number to 1 ≤ i ≤ 6. PNAP (?) {i} 6-16 Set (Query) the Alphanumeric Pen Number to 1 ≤ i ≤ 6. PNCR (?) {i} 6-16 Set (Query) the Cursor Pen Number to 1 ≤ i ≤ 6. PRNT (?) {i} 6-16 Set (Query) the Printer Type to Epson (0) or HP (1).

9600 (4).

STORE AND RECALL FILE page description

FNAM (?) {s} 6-17 Set (Query) the current File Name to string. SVTR 6-17 Save the Active Trace Data to the file specified by FNAM. SVST 6-17 Save the Settings to the file specified by FNAM. RCTR 6-17 Recall the Trace Data from the file specified by FNAM to the active graph. RCST 6-17 Recall the Settings from the file specified by FNAM.

MATH OPERATIONS

CSEL (?) {i} 6-18 Set (Query) the Operation to +, -, x, /, log, √ (0-5). COPR 6-18 Start the calculation. CARG (?) {i} 6-18 Set (Query) the Argument type to Constant (0), w (1), or Other Graph (2). CONS (?) {x} 6-18 Set (Query) the Constant Argument to x. CMRK 6-18 Set the Constant Argument to the Y value of the marker.

FRONT PANEL CONTROLS

STRT 6-19 Start data acquisition. Same as [START] key. STCO 6-19 Pause or Continue data acquisition. Same as [PAUSE CONT] key. PRSC 6-19 Print the screen. Same as [PRINT] key. ACTG (?) {i} 6-19 Set (Query) the Active Trace to trace0 (0) or trace1 (1). Similar to [ACTIVE

ARNG (?) {i} 6-19 Set (Query) the Auto Range mode to Manual (0) or Auto (1). Similar to [AUTO

AUTS 6-19 AutoScale the graph. Same as the [AUTO SCALE] key.

DATA TRANSFER page description SPEC? g {,i} 6-20 Query the Y value of bin 0 ≤ i ≤ 399. BVAL? g, i 6-20 Query the X value of bin 0 ≤ i ≤ 399.

SPEB? g 6-20 Binary dump the entire trace g. BDMP (?) g, {,i} 6-21 Set (Query) the auto binary dump mode for trace g.

page description

TRACE] key.

RANGE] key.

INTERFACE

*RST 6-22 Reset the unit to its default configurations. *IDN? 6-22 Read the SR770 device identification string.

page description

SR770 FFT SPECTRUM ANALYZER

LOCL(?) {i} 6-22 Set (Query) the Local/Remote state to LOCAL (0), REMOTE (1), or LOCAL

OVRM (?) {i} 6-22 Set (Query) the GPIB Overide Remote state to Off (0) or On (1).

STATUS

*CLS 6-23 Clear all status bytes. *ESE (?) {i} {,j} 6-23 Set (Query) the Standard Status Byte Enable Register to the decimal value i

*ESR? {i} 6-23 Query the Standard Status Byte. If i is included, only bit i is queried. *SRE (?) {i} {,j} 6-23 Set (Query) the Serial Poll Enable Register to the decimal value i (0-255). *STB? {i} 6-23 Query the Serial Poll Status Byte. If i is included, only bit i is queried. *PSC (?) {i} 6-23 Set (Query) the Power On Status Clear bit to Set (1) or Clear (0). ERRE (?) {i} {,j} 6-23 Set (Query) the Error Status Enable Register to the decimal value i (0-255). ERRS? {i} 6-23 Query the Error Status Byte. If i is included, only bit i is queried. FFTE (?) {i} {,j} 6-23 Set (Query) the FFT Status Enable Register to the decimal value i (0-255). FFTS? {i} 6-23 Query the FFT Status Byte. If i is included, only bit i is queried.

page description

LOCKOUT (2).

(0-255).

STATUS BYTE DEFINITIONS

SERIAL POLL STATUS BYTE (6-24)

bit name usage 0 SCN No measurements in progress 1 IFC No command execution in progress 2 ERR Unmasked bit in error status byte set 3 FFT Unmasked bit in FFT status byte set 4 MAV The interface output buffer is non-empty 5 ESB Unmasked bit in standard status byte

6 SRQ SRQ (service request) has occurred 7 Unused

STANDARD EVENT STATUS BYTE (6-25)

bit name 0 INP Set on input queue overflow 1 Limit Fail Set when a limit test fails 2 QRY Set on output queue overflow 3 Unused 4 EXE Set when command execution error

5 CMD Set when an illegal command is

6 URQ Set by any key press or knob rotation 7 PON Set by power-on

set

usage

occurs

received

FFT STATUS BYTE (6-25)

bit name 0 Triggered Set when a time record is triggered 1 Prn/Plt Set when a printout or plot is completed 2 NewData 0 Set when new data is available for trace 0 3 NewData 1 Set when new data is available for trace 1 4 Avg Set when a linear average is completed 5 AutoRng Set when auto range changes the range 6 High Voltage Set when high voltagedetected at input 7 Settle Set when settling is complete

ERROR STATUS BYTE (6-26)

bit name 0 Prn/Plt Err Set when an printing or plotting error

1 Math Error Set when an internal math error occurs 2 RAM Error Set when RAM Memory test finds an error 3 Disk Error Set when a disk error occurs 4 ROM Error Set when ROM Memory test finds an error 5 A/D Error Set when A/D test finds an error 6 DSP Error Set when DSP test finds an error 7 Overload Set when the signal input overloads

usage

occurs

SR770 FFT SPECTRUM ANALYZER

xii

YOUR FIRST MEASUREMENT

GETTING STARTED

This sample measurement is designed to acquaint the first time user with the SR770 Network Analyzer. Do not be concerned that your measurement does not exactly agree with this exercise. The focus of this measurement exercise is to learn how to use the instrument.

There are two types of front panel keys which will be 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]. The softkeys are the six gray keys along the right edge of the screen. Their function is labelled by a menu box displayed on the screen next to the key. Softkey functions change depending upon the situation. Softkeys will be referenced as the <Soft Key> or simply the Soft Key.

Hardkeys

The keypad consists of five groups of hardkeys. 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 groups together similar parameters and functions. The CONTROL keys start and stop actual data acquisition, select the marker and toggle the active trace the display. These keys are not in a menu since they are used frequently while displaying any menu. The SYSTEM keys output the screen to a printer and display help messages. These keys can also be accessed from any menu. The MARKER keys determine the marker mode and perform various marker functions. The marker functions can be accessed from any menu.

Softkeys

The SR770 has a menu driven user interface. The 6 softkeys to the right of the video display have different functions depending upon the information displayed in the menu boxes at the right of the video display. In general, the softkeys have two uses. The first is to toggle a feature on and off or to choose between settings. The second is to highlight a parameter which is then changed using the knob or numeric keypad. In both cases, the softkey affects the parameter which is displayed adjacent to it.

Knob

The knob is used to adjust parameters which have been highlighted using the softkeys. Most numeric entry fields may be adjusted with the knob. In addition, functions such as display zooming and scrolling use the knob as well. In these cases, the knob function is selected by the softkeys. The [MARKER] key, which can be pressed at any time, will set the knob function to scrolling the marker.

Example Measurement

This measurement is designed to investigate the spectrum of a 1 kHz sine wave. You will need a function generator capable of providing a 1 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 SR770 to measure and analyze its output. Choose a generator which has some distortion (at least -70 dBc) or use a square or triangle wave.

Specifically, you will measure the spectrum of the sine wave, measure its frequency, and measure its harmonic distortion.

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GETTING STARTED

ANAL YZING A SINE WAVE

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Turn on the generator, set the frequency to 1 kHz and the amplitude to approximately 1 Vrms.

Connect the generator's output to the A input of the analyzer.

3. Press [AUTO RANGE]

4. Press the <Span> softkey to highlight the span. Use the knob to adjust the span to

6.25 kHz.

You can also use the [SPAN UP] and [SPAN DOWN] keys to adjust the span.

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

The input impedance of the analyzer is 1 MΩ. The generator may require a terminator. Many generators have either a 50 Ω or 600 Ω output 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.

Since the signal amplitude may not be set accurately, let the analyzer automatically set its input range to agree with the actual generator signal. Note that the range readout at the bottom of the screen is displayed in inverse when the autoranging is on.

Set the span to display the 1 kHz signal and its first few harmonics.

You can also use the numeric keypad to enter the span. In this case, the span will be rounded to the next largest allowable span.

5. Press [MARKER MAX/MIN]

6. Use the knob to move the marker around. Take a look at some of the harmonics.

7. Let's measure the frequency exactly.

Decrease the span to 1.56 kHz using the <Span> key and knob, the [SPAN DOWN] key or by entering the span numerically.

This centers the marker region around the largest data point on the graph. The marker should now be on the 1 kHz signal. The marker readout above the graph displays the frequency and amplitude of the signal.

The [MARKER MAX/MIN] key can also be configured to search for the minimum point on the graph.

Pressing the [MARKER MAX/MIN] key also selects the knob to adjust the marker position. The Span Menu box becomes unhighlighted. A box is drawn around the marker readout to indicate that the knob will move the marker.

This isolates the 1 kHz fundamental frequency.

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GETTING STARTED

Press [MARKER MAX/MIN]

Press [MARKER CENTER]

8. Decrease the span to 97.5 Hz using the

<Span> key and knob, the [SPAN DOWN] key or by entering the span numerically.

9. Press [MARKER MAX/MIN]

10. Press [AUTO SCALE]

Move the marker to the peak at 1 kHz.

This sets the span center frequency to the marker frequency. The signal will be at the center of the span. Further adjustments to the span will keep the center frequency fixed.

You may notice that the spectrum takes a while to settle down at this last span. This is because the frequency resolution is 1/400 of the span or 244 mHz. This resolution requires at least

4.096 seconds of time data. Note that the Settling indicator at the lower left corner of the display will stay on while the data settles.

This centers the marker more accurately. The frequency of the signal can now be read with 244 mHz resolution.

This key adjusts the graph scale and top reference to display the entire range of the data. You can press this key at any time to optimize the graph display.

11. Press [ANALYZE]

Press <Harmonic>

12. Press <Next Harmonic Right>

Use the <Next Harmonic Right> and <Next Harmonic Left> keys to investigate the harmonics of the signal.

Display the Analysis menu.

Select Harmonic analysis. The menu displays the harmonic analysis menu. Notice that the fundamental frequency (first menu box) has been set to the frequency of the marker.

We used a narrow span to get an accurate reading of the fundamental signal frequency. We will use this measurement of the fundamental to accurately locate the harmonics.

The harmonic measurement readout at the upper left corner of the graph is under range because the span is not wide enough to include any harmonics.

This centers the span around the second harmonic (approx. 2 kHz). You are now making an accurate measurement of the 2nd harmonic content of the signal.

With this narrow span, the harmonics should be easily visible.

13. Press [FREQ]

Let's have the analyzer measure the distortion for us. First return to full span by displaying the

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GETTING STARTED

Press <Full Span>

Press [AUTO SCALE]

14. Press <Start Freq.>

Now adjust the span to 12.5 kHz using the <Span> key and knob, the [SPAN DOWN] key or by entering the span numerically.

15. Press [ANALYZE]

Press <Harmonic>

16. Press <# Harmonics>

Press [1] [1] <Enter>

frequency menu and choosing full span.

Return the graph to a scale where the fundamental is on screen.

This highlights the Start Frequency menu box. It also fixes the start frequency when the span is adjusted.

Reduce the span to resolve the first few harmonics of the signal.

Display the Analysis menu.

Choose Harmonic analysis. (It should still be on from before.) The fundamental frequency should still be accurately set.

Highlight the number of harmonics menu box.

Enter 11 for the number of harmonics.

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.

17. Now let's measure some harmonics using the reference marker.

Press <Return>

Press <None>

Press [MARKER MAX/MIN]

Press [MARKER REF]

Use the knob to measure the harmonic

The harmonic calculations are displayed in the upper left corner of the graph. The top reading is the harmonic level (absolute units) and the lower reading is the distortion (harmonic level divided by the fundamental level).

Return the menu display to the main Analysis menu.

Choose No analysis. This turns off the harmonic indicators and calculations.

This moves the marker to the fundamental peak.

This sets the marker reference or offset to the frequency and amplitude of the fundamental. The marker readout above the graph now reads relative to this offset. This is indicated by the ∆ in front of the marker readout. A small star shaped symbol is located at the screen location of the reference.

The [MARKER REF] key also allows the knob to

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GETTING STARTED

levels relative to the fundamental.

18. Press [MARKER REF]

move the marker.

The marker readout is now relative to the reference or fundamental level.

Pressing [MARKER REF] again removes the marker offset.

This concludes this measurement example. You should have a good feeling for the basic operation of the menus, knob and numeric entry, and marker movement and measurements.

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GETTING STARTED

SECOND MEASUREMENT EXAMPLE

This sample measurement is designed to further acquaint the user with the SR770 Network Analyzer. Do not be concerned that your measurement does not exactly agree with this exercise. The focus of this measurement exercise is to learn how to use the instrument.

The Measurement

This measurement is designed to investigate the noise of an audio amplifier. You will need an audio frequency amplifier such as the SRS SR560. We will use the SR770's source to provide the test signal.

Specifically, you will measure the output signal/noise ratio of the amplifier and its input noise level.

1-6

MEASURING AMPLIFIER NOISE

GETTING STARTED

1. Turn the analyzer on while holding down the

[<-] (backspace) key. Wait until the poweron tests are completed.

2. Turn on the amplifier and set its gain to at

least 20 dB.

Connect the amplifier output to the A input of the analyzer.

Press [SOURCE]

Press <Sine>

Press <Configure Source>

Press <Level>

Press [1] [0] [0] <mV>

Connect the SR770 Source output to the amplifier input.

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

The input impedance of the analyzer is 1 MΩ. The amplifier output may require a terminator. Many instruments have either a 50 Ω or 600 Ω output impedance. Use the appropriate feedthrough termination if necessary. In general, not using a terminator means that the output amplitude will not agree with the instrument setting and the distortion may be greater than normal.

Display the Source menu.

Turn on the Sine output.

Display the sine configuration menu.

Highlight the Sine Output Level. Leave the frequency at 1 kHz.

Enter 100 mV (pk).

This should be a small enough signal for the amplifier to handle. If not, simply lower the sine output level to a suitable level.

3. Press [AUTO RANGE]

Press [FREQ]

Press [SPAN DOWN] until the span is

6.25 kHz

Press [AUTO SCALE]

4. Press [MARKER MAX/MIN]

Press [MARKER REF]

Since the amplifier output amplitude may not be set accurately, let the analyzer automatically set its input range to the actual signal.

Display the Frequency menu.

Set the span to display the 1 kHz signal and its first few harmonics.

Set the graph scaling to display the entire range of the data.

Move the marker to the signal peak (1 kHz). The marker should read an amplitude equal to the source output level times the amplifier gain.

This turns on the marker offset and sets the reference marker to the current marker position. From now on, the marker will now read relative to the signal peak. A ∆ is displayed before the marker readout to indicate that the reading is relative. A small star symbol is located on the graph at the marker offset position.

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GETTING STARTED

Use the knob to move the marker to a region that is representative of the noise floor.

Press [MARKER REF] again

5. Press [MEAS]

Press <Measure Menu>

Press <PSD>

6. Press [AVERAGE]

Press <Average Mode>

The [MARKER REF] key automatically allows the knob to adjust the marker position. The marker is now providing a direct reading of the signal to noise ratio. Remember, this is the S/N for the source/amplifier combination.

The [MARKER REF] key toggles the marker offset on and off. We now want to turn the offset off.

Display the Measure menu.

Choose the Measurement type menu.

Select Power Spectral Density. The PSD approximates the amplitude of the signal within a 1 Hz bandwidth located at each frequency bin. This allows measurements taken with different linewidths (spans) to be compared.

To get a better measurement of noise, a little averaging can help.

Display the Average menu.

Select Exponential averaging.

Press <Number of Averages>

Press [2] [0] <Enter>

Press <Averaging>

7. Press [MARKER]

Use the knob to move the marker to a region representative of the noise floor.

Press [MEAS]

Press <Units Menu>

Press <Volts RMS>

Highlight the Number of Averages menu box.

Enter 20 averages.

Turn averaging on. Notice how the noise floor approaches a more stable value. W e are using RMS averaging to determine the actual noise floor. See the section on Averaging for a discussion of the different types of averaging.

The [MARKER] key allows the knob to move the marker.

The Marker reading should be in dBV/√Hz. This is the output noise amplitude at the marker frequency, normalized to a 1 Hz bandwidth. To generalize to other bandwidths, multiply by the square root of the bandwidth. This approximation only holds if the noise is Gaussian in nature.

Display the Measure menu.

Choose the Units menu.

Select Volts RMS as the display units.

The marker now reads in Volts RMS /√Hz. This is a typical way of specifying amplifier input noise

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GETTING STARTED

levels.

8. Disconnect the source output from the

amplifier. Leave the amplifier input terminated (with 50 Ohms).

Now we are measuring the amplifier's output noise with a shorted input. If you take the noise measurement and divide by the amplifier gain, then you will have the amplifier's input noise at the frequency of the marker reading.

An FFT is a convenient tool for measuring amplifier noise spectra since the noise at many frequencies can be determined in a single measurement.

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GETTING STARTED

USING TRIGGERS AND THE TIME RECORD

This sample measurement is designed to acquaint the user with the triggering capabilities of the SR770 Network Analyzer. Do not be concerned that your measurement does not exactly agree with this exercise. The focus of this measurement exercise is to learn how to use the instrument.

The Measurement

This measurement is designed to investigate the trigger and time record. You will need a function generator capable of providing a 100 µs wide pulse at 250 Hz with an amplitude of 1 V. The output should have a DC level of 0V.

Specifically, you will measure the output spectrum when the signal is triggered. In addition, the trigger delay will be used to delay the signal within the time record.

Make sure that you have read "The Time Record" in the Analyzer Basics section before trying this exercise.

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TRIGGERING THE ANALYZER

GETTING STARTED

1. Turn the analyzer on while holding down the

[<-] (backspace) key. Wait until the poweron tests are completed.

2. Turn on the generator and choose a pulsed

output waveform. Set the frequency to 250 Hz, the pulse width to 100 µs and the amplitude to approximately 1 V. Make sure that the DC level of the output is near 0V.

Connect the generator's output to the A input of the analyzer.

3. Press [INPUT]

Press <Coupling> to choose DC

Press <Input Range>

Press [4] <dBV>

4. Press [DISPLAY]

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section of this manual for a complete listing of the settings.

The input impedance of the analyzer is 1 MΩ. The generator may require a terminator. Many generators have either a 50 Ω or 600 Ω output impedance. Use the appropriate feedthrough termination if necessary. In general, not using a terminator only means that the output amplitude will not agree with the generator setting and the distortion may be greater than normal.

Let's choose DC coupling and an input range that doesn't overload.

Set the input range to 4 dBV. Adjust the pulse amplitude so that no overloads occur.

Show two traces.

Press <Format> to choose Up/Dn

5. Press [MEAS]

Press <Measure Menu>

Press <Time Record>

Press <Return>

Press <Display Menu>

Press <Linear Mag.>

6. Press [INPUT]

Press <Trigger Menu>

Press <Trigger> to select Internal

Press <Trigger Level>

Press [.] [5] <Volts>

Press [AUTO SCALE]

We will show the time record on the upper trace.

Go to the Measure menu to choose Time Record.

Let's show the time record on a linear scale.

Now set up the trigger.

Trigger on the signal itself.

The input is a 1 V pulse so set the trigger level to

0.5 V.

The upper trace should display the pulse waveform at the left edge. Auto scale will set the

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GETTING STARTED

display limits automatically. Remember that we are displaying the magnitude of the signal. Any negative portion of the signal will be folded back around zero and appear as a positive magnitude.

7. Press [MEAS]

Press <Window Menu>

Press <Uniform>

Press [ACTIVE TRACE]

Press [AUTO SCALE]

8. Press <Hanning>

9. Press [INPUT]

Press <Trigger Menu>

Because the pulse is much shorter than the time record, we need to use the Uniform 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 FFT of the time record.

There should now be a spectrum on the lower trace. Use [AUTO SCALE] to set the display.

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

Now choose the Hanning window. Notice how the spectrum goes away. 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.

Go back to the Trigger submenu.

Press <Trigger Delay>

Press [2] [5] [6] <Samples>

10. Press [4] [7] [5] <Samples>

Highlight the Trigger Delay menu box.

Enter 256 samples of delay. Because the pulse repetition rate is 250 Hz, the period between pulses is exactly equal to one time record. So setting the delay to half of a time record will place the pulse at the middle of the record.

Remember that the time record only displays the first 400 points (out of 512) so that the middle of the record is not the middle of the display trace.

The spectrum should reappear on the lower trace. This is because windowing preserves the central part of the time record.

Let's delay the signal some more. Now we've delayed the time record by almost a full period. The pulse is now near the end of the time record.

Notice how the spectrum is greatly attenuated. This is the effect of the window function attenuating the start of the timer record.

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GETTING STARTED

11. Press <Trigger> to select Continuous

12. Press [MEAS]

Press <Window>

Press <Uniform>

Now if we go to continuous triggering, the time record becomes unstable. The spectrum is also unstable because of the windowing. Some time records place the pulse at the middle, some at the ends.

If we set the window back to Uniform, we find that the spectrum does not vary with the position of the pulse within the time record.

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GETTING STARTED

USING THE DISK DRIVE

The disk drive on the SR770 may be used to store 3 types of files.

1. Data File

This includes the data in the active trace, the measurement and display type, the units and the graph scaling. In addition, the associated data and limit tables are stored in this file as well. Data files may be recalled into either trace0 or trace1.

2. ASCII Data File

This file saves the data in the active trace in ASCII format. These files may not be recalled to the display. This format is convenient when transferring data to a PC application.

3. Settings File

This files stores the analyzer settings. Recalling this file will change the analyzer setup to that stored in the file.

The disk drive uses double-sided, high density (DS/HD) 3.5" disks. The disk capacity is 1.44M. The SR770 uses the DOS format. A disk which was formatted on a PC or PS2 may also be used. Files written by the SR770 may be copied or read on a DOS computer.

The Measurement

This measurement is designed to familiarize the user with the disk drive. We will use the SR770 source to provide a test signal so that there is some data to save and recall.

Specifically, you will save and recall a data file and a settings file.

Data files can store data in either binary or ascii format. Binary format uses less disk space. Ascii format allows trace data to be read by other programs using a PC.

1-14

STORING AND RECALLING DATA

GETTING STARTED

1. Turn the analyzer on while holding down the

[<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A

input.

Press [SOURCE]

Press <Sine>

3. Press [AUTO RANGE]

4. Press [SPAN DOWN] until the span is

6.25 kHz

5. Press [AUTO SCALE]

6. Press [PAUSE CONT]

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Sine output. Leave the sine frequency and level at the default settings (1 kHz and 1 Vpk).

Let the analyzer automatically set its input range.

Set the span to display the 1 kHz signal.

Set the graph scaling to display the entire range of the data.

Stop data acquisition. The graph on the screen is the one we want to save. (You can actually save graphs while the analyzer is running.)

7. Put a blank double-sided, high density

(DS/HD)3.5" disk into the drive.

8. Press [STORE RECALL]

Press <Disk Utilities>

Press <Format Disk>

9. Press <Return>

Press <Save Data>

10. Press <File Name>

Press [ALT]

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

Let's format this disk.

Display the Store and Recall menu.

Choose Disk Utilities.

Make sure that the disk does not contain any information that you want. Formatting the disk takes about a minute.

Go back to the main Store and Recall menu.

Display the Save Data menu.

Now we need a file name.

[ALT] lets you enter the letter characters printed

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GETTING STARTED

below each key. The numbers and backspace function as normal.

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

11. Press <Save Data>

12. Press <Catalog>

13. Press <File Name>

Press [ALT]

Press [D] [A] [T] [A] [2] <Enter>

Press <Save Data>

14. Press <Return>

Press [START]

Remove the input signal cable.

Enter a file name such as DATA1 (or any legal DOS file name).

This saves the active trace data to disk using the file name specified above.

Display the disk catalog. This display lists all of the files on the disk.

Save the data again using a new file name. This way you can have multiple files in the disk catalog.

Go back to the main Store and Recall menu.

Resume data acquisition. The graph should be live again.

Now we have a spectrum which is different from the one we just saved. Recalling the data from disk will restore the graph to what it was.

15. Press <Recall Data>

Press <Catalog>

16. Press [MARKER]

17. Press <Recall Data>

18. Press [DISPLAY]

Display the Recall Data menu.

Display the disk catalog. The 2 files which you just saved should be listed.

Pressing the [MARKER] key allows the knob to adjust the marker. W hen the disk catalog is displayed, the marker highlights a file. Use the knob to choose a file to recall.

This recalls the data file from disk and displays it on the active graph. Data acquisition is stopped so that the graph is not updated. The file name is displayed below the graph.

The marker may be moved on the recalled graph to read specific data points. The graph scaling may also be changed.

Show the Display menu.

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GETTING STARTED

Press <Format>

Press [ACTIVE TRACE]

19. Press [START]

Choose the Up/Dn dual trace display format.

Make trace1 active (the lower graph). The active graph has a highlighted label at its upper right.

This restarts data acquisition, but only for the active trace (trace1). The recalled trace on trace0 is still displayed. To restart data acquisition on trace0, press [ACTIVE TRACE] to make trace0 (upper graph) active and then [START].

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GETTING STARTED

STORING AND RECALLING SETTINGS

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Press [SPAN DOWN] a number of times to change the span.

Press [INPUT]

Press <Coupling>

3. Press [STORE RECALL]

Press <Save Settings>

4. Press <File Name>

Press [ALT]

Press [T] [E] [S] [T] [1] <Enter>

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Change the analyzer setup so that we have a nondefault setup to save.

Show the Input menu.

Choose DC coupling.

Display the Store and Recall menu.

Choose the Save Settings menu.

Now we need a file name.

[ALT] lets you enter the letters printed below each key. The numbers and backspace function as normal.

Enter a file name such as TEST1 (or any legal DOS file name).

5. Press <Save Settings>

6. Press [SPAN UP] a number of times to change the span.

Press [INPUT]

Press <Coupling>

7. Press [STORE RECALL]

Press <Recall Settings>

Press <Catalog>

8. Press [MARKER]

Save the analyzer setup to disk using the file name specified above.

Change the analyzer setup again.

Show the Input menu.

Choose AC coupling.

Now let's recall the analyzer setup that we just saved.

Display the Store and Recall menu.

Choose the Recall Settings menu.

Display the disk catalog listing. Note that data files have the type DAT and setting files have the type SET.

Pressing the [MARKER] key allows the knob to adjust the marker. W hen the disk catalog is displayed, the marker highlights a file. Use the

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GETTING STARTED

knob to choose the file TEST1 to recall. (Or use the <File Name> key to enter the file name.)

9. Press <Recall Settings>

This recalls the settings from the file TEST1. The analyzer settings are changed to those stored in TEST1. The span and input coupling should be the same as those in effect when you created the file.

Note that the STOP-Invld indicator is flashing at the bottom of the screen. This means that the display data does not match the analyzer settings or the graph parameters. Remember, we just recalled the settings which paused the data acquisition before changing the settings. Pressing [START] will start data acqusition with the new settings.

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GETTING STARTED

USING DATA TABLES

A data table reports the Y values for user listed Xaxis values. For example, the entries could be a set of harmonic frequencies which need to be measured. The data table is a convenient way to measure the data values at various points without moving the marker around and manually recording the answers. To generate a printed report of the measurement, the data table may be printed using the Plot menu.

Each trace has its own data table though only the table associated with the active trace is on and displayed at any time.

Data tables are saved along with the trace data when data is saved to disk.

Data tables are not stored in non-volatile memory and are not retained when the power is turned off.

Remember that the values in the table do not have units associated with them. An X location of 10 kHz is stored as 10 k and a Y value of -20 dBV is reported as simply -20. The Y values come directly from the graph so it is important to use the proper display units to get consistent data table readings.

The Measurement

This measurement is designed to familiarize the user with the data tables. We will use the SR770 Source to provide a test signal so that there is some data to report.

Specifically, you will generate a data table to measure some harmonics as well as the noise floor.

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DATA TABLES

GETTING STARTED

1. Turn the analyzer on while holding down the

[<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A

input.

Press [SOURCE]

Press <Sine>

3. Press [AUTO RANGE]

4. Press [SPAN DOWN] until the span is

6.25 kHz

5. Press [AUTO SCALE]

6. Press [ANALYZE]

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Sine output. Leave the sine frequency and level at the default settings (1 kHz and 1 Vpk).

Let the analyzer automatically set its input range.

Set the span to display the 1 kHz signal.

Set the graph scaling to display the entire range of the data.

Display the Analysis menu.

Press <Data Table>

7. Press [MARKER MAX/MIN]

Press <X Value>

Press [MARKER ENTRY]

8. Press <Table Index>

Press [1] <Enter>

9. Press <X Value>

Select Data Table display. The display switches to dual trace format with the spectrum on top and the data table listed below.

This moves the marker to the peak of the spectrum. This should center the marker on the 1 kHz fundamental frequency.

Highlight the X Value menu box.

This copies the marker X position into the X Value menu box. The X value of data table line 0 is now equal to the 1 kHz signal frequency. The Y value of line 0 is updated each time the graph is updated.

This highlights the Table Index menu box. Let's add another line to the data table.

Entering an index or line number beyond the end of the table adds a new line to the end.

Highlight the X Value menu box.

Press [2] <kHz>

Enter the frequency of the 2nd harmonic into the

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GETTING STARTED

data table.

Line 1 now has the frequency of the 2nd harmonic. Note how the Y values update with the graph.

In the Analysis menu, many of the frequencies or X values may be entered by copying the X location of the marker (using [MARKER ENTRY]) or by entering the numerical value with the keypad.

10. Press <Table Index>

Press [2] <Enter>

11. Press <X Value>

Press [2] [.] [5] [4] <kHz>

12. Press <Insert Item>

Press <Delete Item>

13. Press [SYSTEM]

Press <Print>

Let's add another line to the table.

Highlight the X Value menu box.

Enter some frequency which is representative of the noise floor of the signal.

We decided that we wanted another harmonic in the table. This key inserts a new line before the highlighted line.

We could enter an X value for this new line now.

But we changed our mind. Let's delete this line.

Display the System menu.

Display the Printing submenu.

If we have a printer attached, then the <Print Data> function will print the data table, with updated Y values.

14. Press [DISPLAY]

Press <Format>

Show the Display menu.

Choose the Single trace display format. This removes the data table display and restores the screen to a single trace display.

1-22

USING LIMIT TABLES

GETTING STARTED

A limit table lists the X,Y coordinates of the line segments which define the trace test limits. When trace data exceeds these limit segments, then the test fails. The limit table is a convenient way to test devices against a specification defined over a range of frequencies. To generate a printed listing of a limit table, use the Print Limits function in the Plot menu.

Each trace has its own limit table though only the table associated with the active trace is on and displayed at any time.

Limit tables are saved along with the trace data when data is saved to disk.

Limit tables are not stored in non-volatile memory and are not retained when the power is turned off.

Remember that the values in the table do not have units associated with them. An X location of 10 kHz is stored as 10 k and a Y value of -20 dBV is simply -20. The limit test compares the data on the graph (in the display units) to the Y values in the table. It is important to use the correct units in the display to get consistent limit table tests.

The Measurement

This measurement is designed to familiarize the user with the limit tables. We will use the SR770 Source to provide a test signal so that there is some data to test.

Specifically, you will generate a limit table to test the signal level as well as the noise floor.

1-23

GETTING STARTED

LIMIT TABLES

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A input.

Press [SOURCE]

Press <Sine>

3. Press [AUTO RANGE]

4. Press [SPAN DOWN] until the span is

6.25 kHz

5. Press [AUTO SCALE]

6. Press [ANALYZE]

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Sine output. Leave the sine frequency and level at the default settings (1 kHz and 1 Vpk).

Let the analyzer automatically set its input range.

Set the span to display the 1 kHz signal and its first few harmonics.

Set the graph scaling to display the entire range of the data.

Display the Analysis menu.

Press <Limit Table>

7. Press [MARKER MAX/MIN]

8. Press <X Values>

Press [9] [0] [0] <Hz>

Press <X Values> again

Press [1] [.] [1] <kHz>

Select Limit Table display. The display switches to dual trace format with the spectrum on top and the limit table listed below.

This moves the marker to the peak of the spectrum and measures the fundamental frequency.

Let's define an upper limit for the 1 kHz peak.

Highlight the upper X Value menu field.

Enter a frequency below the signal frequency.

Highlight the lower X Value menu field.

Enter a frequency higher than the signal frequency.

As with data tables, it is also possible to copy the marker X location into the X value fields. But this time we want frequencies above and below the peak so we entered them numerically.

9. Press <Y Values>

Highlight the upper Y values menu field.

1-24

GETTING STARTED

Press [-] [5] <Enter>

Press <Y Values>

Press [-] [5] <Enter>

10. Press <More>

Press <Audio Alarm>

Press <Return>

Press <Y Values>

Enter a value somewhat less than the signal peak.

Highlight the lower Y values menu field.

Enter a value somewhat less than the signal peak.

Notice that small line segment is drawn on the display. This line starts at (Xbegin,Y1) and ends at (Xend, Y2) and represents a limit segment. If the data exceeds this limit (since it is an upper limit), then the FAIL indicator will flash at the bottom of the screen. The FAIL indicator should be flashing now.

Display the second limits menu.

Set the audio alarm on. Now whenever a trace is taken that exceeds the limit, an alarm sounds.

Set the audio alarm off. You're probably ready to turn off the alarms by now anyway.

Go back to the main limits menu.

Highlight the upper Y values menu field.

Press [2] <Enter>

Press <Y Values>

Press [2] <Enter>

11. Press <Table Index>

Press [1] <Enter>

Enter a value higher than the signal peak.

Highlight the lower Y values menu field.

Enter a value higher than the signal peak.

The limit segment is now entirely above the signal peak and the PASS indicator is on at the bottom of the screen. Remember, this segment is an upper limit.

Let's add another segment to this table.

Highlight the Table Index menu box.

Entering an index or line number beyond the end of the table adds a new line to the end.

Notice how the new segment is a continuation of the previous one. This makes building a continuous limit much simpler. The starting point of the new line equals the ending point of the previous one. The new segment's length along the X axis is the same as the previous segment's. The only thing you need to edit is the value of Y2 and your new segment is finished.

12. Press <X Values> until the upper field is

But let's go on to define a noise floor limit.

1-25

GETTING STARTED

highlighted.

Press [2] [.] [2] <kHz>

Press <X Values>

Press [2] [.] [8] <kHz>

13. Press <YValues> until the upper field is highlighted.

Press [-] [9] [0] <Enter>

Press <Y Values>

Press [-] [9] [0] <Enter>

14. Press <Limit Type>

15. Press <More>

Press <Testing>

Enter a segment which is between harmonics. In this case, between 2.2 and 2.8 kHz. This is representative of the noise floor.

Define an upper limit a little above the noise floor.

In this case, we define an upper noise limit of 90 dB.

There should now be a horizontal segment above the noise floor between 2.2 and 2.8 kHz. The limit test should still PASS.

This switches the noise limit from an upper limit to a lower limit. Since the data will now be below the lower limit, the test will FAIL.

Display the second limits menu.

Set limit testing to OFF. It is possible to display the limit table without testing taking place. This is helpful when a lot of the X values on the graph have defined limits. The testing can slow down the response of the analyzer noticeably. It is simpler to define the limits with testing off.

14. Press [DISPLAY]

Press <Format>

Show the Display menu.

Choose the Single trace display format. This removes the limit table display and restores the screen to a single trace display. No testing occurs when the limit table is not displayed.

1-26

USING TRACE MATH

GETTING STARTED

The Calculator submenu allows the user to perform arithmetic calculations with the trace data. Operations are performed on the entire trace, regardless of graphical expansion.

Calculations treat the data as intrinsic values, either Volts, Engineering Units or degrees. If a graph is showing dB, then multiplying by 10 will raise the graph by 20 dB and dividing by 10 will lower the graph by 20 dB.

Performing a calculation on the active trace will set the File Type to Calc to indicate that the trace is not Live. This is shown by the "File=Calc" message at the lower left of the graph. The analyzer continues to run but the calculated trace will not be updated. To return the trace to live mode, activate the trace and press the [START] key. The File Type will return to Live.

The Measurement

This measurement is designed to familiarize the user with the trace math capabilities. W e will use the SR770 Source to provide a test signal.

Specifically, you will ratio a spectrum with a reference spectrum.

1-27

GETTING STARTED

TRACE MATH

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A input.

Press [SOURCE]

Press <Sine>

Press <Configure Source>

Press <Level>

Press [-] [3] <dBV>

3. Press [AUTO RANGE]

4. Press [SPAN DOWN] until the span is

6.25 kHz

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Sine output.

Display the Sine frequency and level.

Select the Sine output level.

Enter -3 dBV for the Sine output level. Note that the Sine level can be entered in either mV (pk) or dBV (pk).

Let the analyzer automatically set its input range.

Set the span to display the 1 kHz signal and its first few harmonics.

5. Press [AUTO SCALE]

6. Press [MEAS]

Press <Calculator Menu>

7. Press <Do Calc>

8. Press [DISPLAY]

Press <Format>

Set the graph scaling to display the entire range of the data.

Display the Measure menu.

Select the Calculator menu.

This operation defaults to adding zero to the trace data. The default operation is +, the default argument is the constant zero. We're doing this so that the trace does not update. This is now the graph we will use as the reference data.

Reference data normally comes from a disk file. Recalling a stored file brings the data back to the active graph but does not update it. See "Using the Disk Drive" earlier in this section.

Bring up the Display menu.

Choose the Up/Dn dual trace format. The reference graph will be the upper trace (Trace0) and the live graph will be the lower trace (Trace1).

1-28

GETTING STARTED

Press <Marker Width> twice to choose Spot Marker.

Press [ACTIVE TRACE]

Press <Marker Width> twice to choose Spot Marker.

9. Press [MARKER MODE]

Press <Linked Markers>

Press [MAX/MIN]

10. Press [SOURCE]

Press <Configure Source>

Press <Level>

Press [-] [2] [0] <dBV>

Make the marker on the upper graph a spot marker.

Let's make the live graph the active trace.

Make the marker on the lower graph a spot marker.

Display the Marker Mode menu.

Link the two markers together. Now when the knob moves one marker, they both move together. Since they are both spot markers, the frequencies which they read on both graphs are identical.

Move the markers to the signal peak (1 kHz).

Bring up the Source menu.

Display the Sine frequency and level menu.

Select the Sine output level.

Enter -20 dBV.

11. Press [MEAS]

Press <Calculator Menu>

Press <Argument Type> twice to select Other Graph.

Press <Operation> three times to select '/' (divide).

12. Press <Do Calc>

Press [AUTO SCALE]

The reference amplitude may be read from the marker readout of the upper graph. The live amplitude may be read from the marker of the lower graph.

Now we have 2 traces which differ in amplitude. Let's take the ratio.

Go back to the Calculator menu.

We will divide the active graph (Trace1 Live) by the inactive graph (Trace0 reference).

Select the divide operation.

Do the calculation. Since the graphs are displayed in dBV, the ratio of the peaks should simply be the difference in their amplitudes expressed in dBV. Remember, the calculations work on the underlying data points (in Volts).

The active graph now displays the ratio of the two traces in dB.

The marker on the lower graph should read the difference between the two peak amplitudes (-

1-29

GETTING STARTED

17.0 dB). Clearly, only the frequencies which correspond to the signal and its harmonics have much meaning in this ratio. One noise floor divided by another noise floor is going to be pretty noisy.

A better way to read these harmonic ratios is using the data table. A data table can display the values of selected frequencies in easy to read form. See "Using Data Table" earlier in this section. The data table would be defined for Trace1.

Other operations which may be performed are +, , x, /, square root, log, phase unwrap and d/dx. The second argument may be a constant (for

scaling or offset), ω (2πf to differentiate or

integrate the spectrum), or the other graph (reference trace from disk).

1-30

USING THE SOURCE

GETTING STARTED

The SR770 has a built in signal source capable of providing a variety of test signals.

SINE

A low distortion sine wave for general purpose gain, distortion and signal/noise measurements. The sine source is synchronous with the FFT, i.e. sine waves can be generated at exact bin frequencies of the FFT. This can eliminate windowing effects in the measured amplitude and phase.

TWO TONE

Two low distortion sine waves can be generated simultaneously for intermodulation distortion tests (IMD). Each tone has independent frequency and amplitude settings.

NOISE

Broadband noise is useful for characterizing circuits, mechanical systems or even the audio response of an entire room. W hite noise provides equal amplitude per root Hz from 0 to 100 kHz. White noise is useful in electronic applications. Pink noise rolls off at 3 dB/oct providing equal amplitude per octave. Pink noise is preferred in audio applications.

The Measurement

This measurement is designed to familiarize the user with the source capabilities. We will use the SR770 Source to provide a test signal.

Specifically, you will measure the spectrum of each of the source types, taking advantage of the fact that the built in source is synchronous with the FFT.

CHIRP

The Chirp source provides an equal amplitude sine wave at each bin of the displayed spectrum. Since there are 400 bins in a spectrum, the chirp is the sum of 400 discrete sine waves. The phases of each sine wave are arranged so that they do not add in phase and the resulting output does not peak. This source is useful for measuring transfer functions quickly without having to make many discrete measurements using a single sine wave.

1-31

GETTING STARTED

USING THE SINE SOURCE

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A input.

Press [SOURCE]

Press <Sine>

Press <Configure Source>

Press <Level>

Press [-] [3] <dBV>

Press [MARKER MAX/MIN]

4. Press <Frequency>

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Sine output.

Display the Sine frequency and level.

Select the Sine output level.

Enter -3 dBV for the Sine output level. Note that the Sine level can be entered in either mV (pk) or dBV (pk).

Move the marker to the signal peak. The marker should be centered at the 1 kHz signal and display a level of -3 dBV.

Select the Sine frequency.

Use the knob to adjust the frequency to

50.00 kHz.

Press [MARKER MAX/MIN]

Press <Frequency>

Press [5] [0] [.] [0] [1] <kHz>

Use the knob to adjust the frequency to 10 kHz.

When the knob is used to adjust the sine frequency, the frequency resolution is equal to the linewidth of the displayed spectrum. In this case, since we are at full span, the linewidth is 250 Hz (100 kHz/400). This always puts the sine exactly on a frequency bin of the spectrum.

Move the marker to the signal peak.

Select the Sine frequency.

The keypad allows random frequencies to be entered. The fundamental frequency resolution of the sine source is 15.26 mHz. The entered frequency will be rounded to the nearest multiple of 15.26 mHz. In this case, 50.01 kHz is rounded to 50.0099945 kHz (only 5.5 mHz off).

When the knob is used again, the frequency resolution returns to the linewidth and the frequency immediately jumps to the nearest multiple of the linewidth.

Generally, the sine frequency should be an exact bin frequency, this eliminates windowing effects and allows for source triggering and vector averaging.

1-32

GETTING STARTED

5. Press [AVERAGE]

Press <Number of Averages>

Press [2] [0] <Enter>

Press <Average Mode> to select Exponential.

Press <Averaging> to turn on averaging.

6. Press [MARKER MAX/MIN]

Press [MARKER REF]

Display the Averaging menu.

Highlight the number of averages.

Change the number of averages to 20.

Select exponential averaging so the display is live.

Turn averaging on.

Notice how the noise floor has averaged to a stable value. RMS averaging averages the magnitude of the signal and reduces the fluctuations in the data. Note that harmonics may now be visible.

Move the marker to the signal peak at 10 kHz.

This turns on the marker offset and sets the reference marker to the current marker position. From now on, the marker will read relative to the signal peak. A ∆ is displayed before the marker readout to indicate that the reading is relative. A small star symbol is located on the graph at the marker offset position.

Use the knob to move the marker to the signal harmonics.

Press [MARKER REF]

7. Press [INPUT]

Press <Trigger Menu>

Press <Trigger> 4 times to select Source.

The ref marker gives a direct reading of the harmonic levels relative the fundamental.

Pressing [MARKER REF] again turns off the reference marker.

Display the Input menu.

Go to the Trigger menu.

Select Source Trigger. This is a special trigger mode. The spectra are taken with no overlap (at all spans) but without using the trigger circuits. When each time record finishes, the next one begins without delay. Any signal which is EXACTLY periodic over a time record will be the same in EVERY time record. For this type of signal, this has the same effect as triggering.

If the source sine frequency is an exact multiple of the linewidth, then the source will appear triggered. This only works with the SR770's own source since it is clocked with the same crystal timebase as the input time record. A separate sine generator will invariably drift relative to the SR770's timebase and the generator's sine frequency will drift away from an exact bin frequency.

1-33

GETTING STARTED

To see how this works, we need to vector average.

8. Press [AVERAGE]

Press <Average Type> to select Vector.

9. Press [SOURCE]

Press <Configure Source>

Press <Frequency>

Press [1] [0] [.] [1] <kHz>

Go back to the Average menu.

Vector averaging averages the complex FFT spectrum. This reduces the level of signals which are not phase coherent from time record to time record, such as noise.

Notice that the noise floor is actually reduced but the signal and its harmonics stay the same amplitude. This is because the source (and its harmonics) are at an exact bin frequency and are EXACTLY the same in each time record.

Even though the signal has a stable phase within each time record, the absolute phase of the signal is arbitrary. This is because the SR770 does not turn on and off the source synchronously with the FFT.

Go back to the Source menu.

Display the Sine frequency and level.

Select the frequency.

Set the frequency to 10.1 kHz. This is NOT an exact bin frequency. Note that the signal peak is dramatically reduced. This is the effect of vector averaging a non-coherent signal.

Use the knob to set the frequency back to 10 kHz.

10. Press [AVERAGE]

Press <Averaging> to turn averaging off.

Press [MEAS]

Press <Window Menu>

Press <Uniform>

The signal peak is restored to its correct value once the frequency is an exact bin frequency again.

Go back to the Average menu.

Turn averaging off.

Display the Measure menu.

Choose the Window menu. Windowing is used to turn a non-periodic time record into a periodic one. Read "Windowing" in the Analyzer Basics section for more information. In this case, the source sine is exactly periodic over a time record so windowing is not required.

The uniform window is no window at all. The signal peak is unchanged in amplitude but has no frequency width. This is because the signal is exactly on a frequency bin.

1-34

GETTING STARTED

Note that this only works when the noise level is very low. If the sine is noisy, then windowing will still be required to achieve a clean spectrum.

11. Press [SOURCE]

Press <Configure Source>

Press <Frequency>

Press [1] [0] [.] [1] <kHz>

Press [MEAS]

Press <Window Menu>

Press <BMH>

Go back to the Source menu.

Display the Sine frequency and level.

Select the frequency.

Set the frequency to 10.1 kHz. This is NOT an exact bin frequency. Note that the signal peak is dramatically affected in both amplitude and frequency width. This is because this frequency is not periodic in the time record. The end points of the time record are not equal and represent a large step discontinuity. The spectrum of this discontinuity is spread over the entire spectrum.

Display the Measure menu.

Choose the Window menu.

Choose a non-uniform window. The window function allows the non-periodic signal to be analyzed. Note that the signal peak frequency is reported as 10.00 kHz (even though the signal is at 10.1 kHz). This is because the FFT only results in 400 discrete frequency bins. The signal peak is also wider than with the uniform window. Windows decrease the selectivity of the spectrum by widening the signal peaks. The amplitude of the peak is also wrong. This is because window functions have amplitude variations for signals between bins.

All in all, the sine source should be used with an exact bin frequency whenever possible. This allows source triggering (no jitter trigger, regardless of signal to noise ratio) and vector averaging as well as eliminating window effects.

1-35

GETTING STARTED

USING THE TWO TONE SOURCE

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A input.

Press [SOURCE]

Press <2-Tone>

Press [AUTO RANGE]

Press [MARKER MAX/MIN]

3. Press <Configure Source>

Press <Frequency 1>

Use the knob to adjust the frequency to 10 kHz.

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Sine output.

Let the analyzer select the input range

The spectrum has two equal signal peaks at 1 kHz and 9 kHz. The two tone source is simply the sum of two sine waves. Each tone has its own amplitude and frequency which are adjusted in the same manner as the single sine.

Display the frequency and level of each tone.

Select the frequency of tone 1.

When the knob is used to adjust the tone frequency, the frequency resolution is equal to the linewidth of the displayed spectrum. In this case, since we are at full span, the linewidth is 250 Hz (100 kHz/400). This always puts the tone exactly on a frequency bin of the spectrum.

4. Press [MARKER MAX/MIN]

Press [MARKER CENTER]

Press [AVERAGE]

Press <Number of Averages>

Press [2] [0] <Enter>

The keypad allows random frequencies to be entered. The fundamental frequency resolution of the sine is 15.26 mHz. The entered frequency will be rounded to the nearest multiple of 15.26 mHz.

Generally, the tone frequency should be an exact bin frequency, this eliminates windowing effects and allows for source triggering and vector averaging.

Move the marker to the signal peak. The marker picks the larger tone (they should be just about equal).

This narrows the span and puts the center of the span on one of the tones.

Display the Average menu.

Highlight the number of averages.

Enter 20 averages.

1-36

GETTING STARTED

Press <Average Mode> to select Exponential.

Press <Averaging> to turn averaging on.

5. Press [MARKER MAX/MIN]

Press [MARKER REF]

Select exponential averaging so that the display is live.

Turn averaging on.

Notice how the noise floor has averaged to a stable value. RMS averaging averages the magnitude of the signal and reduces the fluctuations in the data. Note that distortion products may now be visible.

Intermodulation distortion (IMD) results in signal sidebands separated by the difference frequency. In this case, the two tones are at 9 and 10 kHz and the difference is 1 kHz. Thus, IMD products will show up at 8, 7, .. kHz and 11, 12, .. kHz.

Move the marker to the signal peak.

Use the knob to measure the distortion components.

Note that the distortion products are separated by 1 kHz increments from the two tone frequencies. They should also be very small (<-80 dB) relative to the tone amplitudes.

In this configuration, it is not possible to determine whether the distortion exists at the source output or the signal input. This measurement determines the sensitivity of any IMD measurement made using the two tone source.

1-37

GETTING STARTED

USING THE NOISE SOURCE

1. Turn the analyzer on while holding down the [<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A input.

Press [SOURCE]

Press <Noise>

Press [AUTO RANGE]

3. Press <Configure Source>

Press <Noise Level>

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Noise output.

Let the analyzer select the input range.

The spectrum is flat noise from 0 to 100 kHz at a level of approximately -30 dBV. The input range has auto-ranged to a much higher level, around 4 dBV. This is because the peak output voltage is greater than 1 V.

Display the noise configuration menu.

Select the noise level. The noise level is approximately the peak amplitude of the noise output. There will be occasional voltage excursions beyond this level. Because of the nature of noise, the peak amplitude is not a well defined quantity.

Press [1] [0] [0] <mV>

Press [1] [0] [0] [0] <mV>

4. Press [INPUT]

Press <Auto Offset> to select Off.

Press [AVERAGE]

Press <Number of Averages>

Press [4] [0] [0] [0]<Enter>

Press <Averaging> to turn averaging on.

Enter a noise level of 100 mV. This lowers the noise spectrum by 20 dB.

Restore the noise level back to 1000 mV.

Display the Input menu.

Turn off Auto Offset before we start averaging.

Display the Average menu.

Highlight the number of averages.

Enter 4000 averages.

Turn averaging on and wait until the average is complete. In Linear average mode, at the end of 4000 averages, the analyzer will stop.

Notice how the noise has averaged to a stable value. RMS averaging averages the magnitude of the signal and reduces the fluctuations in the data. Note that the noise spectrum is flat.

1-38

GETTING STARTED

Press [AUTO SCALE]

5. Press [MEAS]

Press <Measure Menu>

Change the display scale so that the fluctuations in the spectral flatness are visible.

The noise output spectrum covers 0 to 100 kHz, regardless of the analysis span. The source is digitally synthesized and passes through an output reconstruction filter. The Source Cal feature adjusts the input calibrations to compensate for this output filter's ripple. The actual signal at the Source Output is not affected. (The output ripple is measured at the factory and is not adjustable by the user.)

Source Cal only has an effect if the source is Noise or Chirp. Source Cal can be turned off in the Configure Noise or Chirp menu.

Never select Noise or Chirp source with Source Cal On and use an external signal source! The input calibrations are modified and will result in measurement errors unless the SR770 internal source is used as the test signal!

Display the Measure menu.

Go to the Measure submenu.

Press <PSD>

Press [AUTO SCALE]

Press [MARKER]

Use the knob to center the marker at 50 kHz.

Press [SPAN DOWN] twice to select a 25 kHz span.

Press [FREQ]

Press <Center Freq.>

Select Power Spectral Density.

Auto scale the graph.

The spectrum measures the amount of noise within a linewidth of each frequency bin. Since the linewidth is simply 1/400th of the span, the amount of noise signal in each bin will decrease with the span. Power Spectral Density (PSD) normalizes all measurements to a 1 Hz bandwidth (instead of the linewidth). Since white noise is Gaussian, it has a constant noise density (V/√Hz). Measurements with the same bandwidth will yield the same noise level. Thus, PSD will measure the same noise at any span.

Move the marker.

Read the PSD at 50 kHz. Remember this number.

Change the span to 25 kHz.

Go to the Frequency menu.

Highlight the Center Frequency.

Press [5] [0] <kHz>

Enter 50 kHz.

1-39

GETTING STARTED

Press [START] Take another spectra.

The final PSD level should be the same as the reading taken at full span at 50 kHz. (The reading is not exactly the same of course. That would take an infinite number of averages.)

Remember, the noise output covers the spectrum from 0 to 100 kHz, regardless of the measurement span. In this case, the measurement span extends from 37.5-62.5 kHz while the noise output is still full span.

The response of many systems is characterized using noise as the input. White noise (equal noise in equal bandwidth) is generally used in electronic measurements. Pink noise (equal noise per frequency octave) is preferred in acoustic systems, such as the natural response of an enclosure or speaker. White noise generates a flat Power Spectral Density curve. Pink noise generates a flat response in Octave Analysis.

1-40

USING THE CHIRP SOURCE

GETTING STARTED

1. Turn the analyzer on while holding down the

[<-] (backspace) key. Wait until the poweron tests are completed.

2. Connect the SR770 Source output to the A

input.

Press [SOURCE]

Press <Chirp>

Press [AUTO RANGE]

When the power is turned on with the backspace key depressed, the analyzer returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

Display the Source menu.

Turn on the Chirp output.

Let the analyzer select the input range.

The Chirp source provides an equal amplitude sine wave at each bin of the displayed spectrum. The phases of each sine wave are arranged so that they do not add in phase and the resulting output does not peak. Because of cancellation, not all sine waves are present during all portions of the time record. Since the input time record is windowed, some portions of the time record are attenuated, This results in certain frequency ranges in the spectra being attenuated. Hence the spectrum has peaks and valleys and is generally not useful when windowed.

3. Press [MEAS]

Press <Window Menu>

Press <Uniform>

4. Press [INPUT]

Press <Trigger Menu>

Press <Trigger> 4 times to select Source.

Display the Measure menu.

Show the Window menu.

Select a Uniform window (no windowing).

The spectrum becomes an almost flat line.

Since the chirp waveform consists of the 400 displayed bin frequencies, it is EXACTLY periodic over a time record. Every time record is the same (except for noise). An exactly periodic waveform requires no window. In this case, the window actually removes spectral information which we need.

Display the Input menu.

Go to the Trigger menu.

1-41

GETTING STARTED

same in EVERY time record. For this type of signal, this has the same effect as triggering.

We need to use source triggering in order to get phase information from this spectrum.

Press <Return>

Press <Auto Offset> to select Off.

5. Press [SCALE]

Press <Y/Div>

Turn the knob to change the vertical scale to

0.5 dB/div.

Go back to the main Input menu.

Turn off Auto Offset. In order to preserve the phase information from source triggering, we need to turn off Auto Offset calibration. The SR770 does not synchronize phase when turning the source on and off. Hence, the phase of the chirp components will be stable but arbitrary. The Auto Offset calibration interrupts the input and restarts data acquisition without synchronization. Thus, the phases of the components will change whenever Auto Offset calibration occurs, disrupting our measurements.

Show the Scale menu.

Highlight the Y (vertical) scale.

Zoom in so that the ripple in the spectrum is easily visible. This spectrum has very little ripple. If this source was the input to a device under test, the output spectrum would be the amplitude transfer function of the device.

The chirp source is digitally synthesized and passes through an output reconstruction filter. The Source Cal feature adjusts the input calibrations to compensate for this output filter's ripple. The actual signal at the Source Output is not affected. (The output ripple is measured at the factory and is not adjustable by the user.)

Source Cal only has an effect if the source is Noise or Chirp. Source Cal can be turned off in the Configure Noise or Chirp menu.

The amplitude of each frequency component is roughly -32dB relative to the peak output. If the individual frequency components were perfectly random, then each would be 1/√400 (-26 dB) of the peak. However, the chirp source is deterministic and each frequency component has a fixed phase relative to all the other components. This reduces the amplitude of each component by

1-42

GETTING STARTED

another 6dB (worsens the crest factor). Thus, the dynamic range of the measurement is reduced when using the chirp source.

5. Press [DISPLAY]

Press <Format> to select Up/Dn.

Press [ACTIVE TRACE] to select the lower trace.

Press <MEAS>

Press <Display Menu>

Press <Phase>

Press [SOURCE]

Press <Configure Source>

Press <Auto Phase>

Go to the Display menu.

Display two traces.

The active trace has its trace identifier (upper right of graph) displayed in inverse. Make the lower trace the active trace.

Go to the Measure menu.

Show the Display submenu.

Display the phase of the spectrum on the lower graph.

The phases of the frequency components of the chirp are stable (but seemingly random). To measure the phase response of a device under test, we need to calibrate the phase of the source.

Go back to the Source menu.

Show the chirp output configuration menu.

The Auto Phase function measures the current phase spectrum. This phase spectrum is stored in memory and subtracted from subsequent phase spectra to remove the phase of the chirp source.

Press [SPAN DOWN]

Press [SPAN UP]

6. Press [DISPLAY]

Press <Format> to select Single.

The phase spectrum now shows 0° at all frequencies. If a device under test is inserted between the source and the input, the lower graph will show the phase response of the device.

Auto phase is removed when the span is changed or the source type is changed.

Change the span.

Back to 100 kHz span.

The phase calibration is removed whenever the span is changed. Remember, if phase information is desired, Auto Phase must be performed after every span change. (Auto Offset also disrupts the phase.)

Go to the Display menu.

Display the phase as a full screen graph.

The phase spectrum of the chirp is quite

1-43

GETTING STARTED

complicated looking. The phase changes quite quickly from -180 to +180 degrees. Many filters exhibit similar large phase shifts. Sometimes it is more informative to view the phase spectrum as "unwrapped" phase. Unwrapping attempts to display the phase as a continuous unbounded curve instead of from -180 to +180 degrees.

7. Press [MEAS]

Press <Calculator Menu>

Press <Operation> 6 times to select unwrap.

Press <Do Calc.>

Press [Auto Scale]

8. Press <Operation> to select d/dx.

Press <Do Calc.>

Press <1.25%>

9. Press <Operation> to select / (divide).

Go to the Measure menu.

Select the Calculator menu.

Select the unwrap operation.

Do the calculation.

Scale the resulting graph. The phase curve is now a much simpler curve varying between very large phases. Unwrapping can make complicated phase curves more understandable if there is a relationship between adjacent frequency bins.

The derivative of the phase with respect to frequency (x axis) is the group delay.

Do the derivative calculation. This brings up a submenu to select the aperture. The aperture (as a percentage of 400 bins) over which the derivative is calculated needs to be selected.

Choose a narrow aperture.

The resulting group delay curve shows a linear group delay over several regions of the spectrum.

Group delay is actually dθ/dω where θ is in

radians. To convert the numerator to radians,

multiply by 2π/360. To convert from d/dx to d/dω

divide the curve by 2π * linewidth. Altogether, we need to divide the curve by 1/360*linewidth.

Press <Argument>

Press [9] [0] [0] [0] [0] <Enter>

Press <Do Calc.>

Press [AUTO SCALE]

Highlight the argument (denominator).

360 x 250 Hz = 90000 sec-1.

Convert the curve to group delay (in seconds).

The graph still reads out in degrees though the actual units are now sec. The calculator does math with numbers (not units). We need to remember the appropriate units to assign to the marker. The group delay varies between -1.9 ms to +1.9 ms.

Using the chirp source, the SR770 can measure amplitude and phase transfer curves for a device

1-44

GETTING STARTED

under test. The amplitudes are calibrated using the Source Cal mode. The phase spectrum can be calibrated to zero at any time using the Auto Phase function. Phases relative to the stored phase curve are displayed live. This gives the SR770 the functionality of a network analyzer with only a single input channel.

1-45

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.

1) Press the [START] key to make sure that

the indicator at the lower left of the screen displays RUN instead of STOP.

2) Check if linear averaging is on. When the

analyzer finishes a linear average of N spectra, the analyzer stops and the data is no longer updated. Press [START] to take another average.

3) Make sure the triggering mode is

CONTinuous. Otherwise, the analyzer may be waiting for a trigger (as shown by the Trg Wait indicator at the bottom of the screen).

4) If the unit is being triggered, check that the

arming mode is set to AUTO. If the arming mode is MANUAL, then the analyzer will only trigger once and then wait for the next manual arming command.

5) Check that the data is on scale by using

[AUTORANGE] and [AUTOSCALE].

6) Make sure that the analyzer is not in the

REMOTE state where the computer interfaces have locked out the front panel. Press the LOCAL key (the [HELP] key) to restore local control.

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 spectra.

1-46

ANALYZER BASICS

WHAT IS AN FFT SPECTRUM ANALYZER?

The SR770 FFT Spectrum Analyzer takes a time varying input signal, like 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 (as viewed on an oscilloscope) is periodic, then its spectrum is probably dominated by a single frequency component. What the spectrum analyzer does is represent the time domain signal by its component frequencies.

Why look at a signal's spectrum?

For one thing, some measurements which are very hard in the time domain are very easy in the frequency domain. Take harmonic distortion. It's 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 used to be done using analog spectrum analyzers. In simple terms, an analog filter was used to isolate frequencies of interest. The remaining signal power was measured to determine the signal strength in certain frequency bands. By tuning the filters and repeating the measurements, a reasonable spectrum could 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, similar to a digitizing oscilloscope. 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 SR770, sampling occurs at 256 kHz.

To make sure that Nyquist's theorem is satisfied, the input signal passes through an analog filter which attenuates all frequency components above128 kHz by 90 dB. This is the anti-aliasing filter. 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 FFT is simply a clever set of operations which implements Fourier's basic theorem. The resulting spectrum shows the frequency components of the input signal.

Now here's the interesting part. The original digital time record comes from discrete samples taken at the sampling rate. The corresponding FFT yields a spectrum with discrete frequency samples. In fact, the spectrum has half as many frequency points as there are time points. (Remember Nyquist's theorem). Suppose that you take 1024 samples at 256 kHz. It takes 4 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 128 kHz. The lowest frequency is just the period of the entire record or 1/(4 ms) or 250 Hz. Everything below 250 Hz is considered to be dc. The output spectrum thus represents the frequency range from dc to 128 kHz with points every 250 Hz.

Advantages and limitations

The advantage of this technique is its speed. The entire spectrum takes only 4 ms to measure. The limitation of this measurement is its resolution. Because the time record is only 4 ms long, the frequency resolution is only 250 Hz. Suppose the signal has a frequency component at 260 Hz. The FFT spectrum will detect this signal but place part of it in the 250 Hz point and part in the 500 Hz point. One way to measure this signal accurately is to take a time record that is 1/260 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). Not a good solution. In fact, the way to measure the signal accurately is to lengthen the time record and change the span of the spectrum.

2-1

ANALYZER BASICS

FREQUENCY SPANS

Before we continue, let's clarify a couple of points about our frequency span. We just described how we arrived at a dc to 128 kHz frequency span using a 4 ms time record. 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 100 kHz and then rolls off steeply from 100 kHz to 128 kHz. No filter can make a 90 dB transition instantly. The range between 100 kHz and 128 kHz is therefore not useable and the actual displayed frequency span stops at 100 kHz. There is also a frequency bin labelled 0 Hz (or dc). This bin actually covers the range from 0 Hz to 250 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 400 frequency bins. The first covers 0 - 250 Hz, the second 250 - 500 Hz, and the 400th covers

99.75 - 100.0 kHz.

Spans less than 100 kHz

So the length of the time record determines the frequency span and resolution of our spectrum. What happens if we make the time record 8 ms or twice as long? Well we ought to get 2048 time points (sampling at 256 kHz) yielding a spectrum from dc to 100 kHz with 125 Hz resolution containing 800 points. But the SR770 places some limitations on this. One is memory. If we keep increasing the time record, then we would need to store more and more points. Another limitation is processing time. The time it takes to calculate an FFT with more points increases more than linearly. The net result is that the SR770 always takes 1024 point FFT's to yield 400 point spectra.

Here's how it's done. The analyzer digitally filters the incoming data samples (at 256 kHz) to limit the bandwidth. This is similar to the anti-aliasing filter at the input except the digital filter's cutoff frequency can be changed. In the case of the 8 ms record, the filter reduces the bandwidth to 64 kHz with a filter cutoff of 50 kHz (the filter rolls off between 50 and 64 kHz). Remember that Nyquist only requires samples at twice the frequency of the highest signal frequency. Thus the digital filter only has to output points at 128 kHz or half of the input

rate (256 kHz). The net result is the digital filter outputs a time record of 1024 points effectively sampled at 128 kHz to make up an 8 ms record. The FFT processor operates on a constant number of points and the resulting FFT will yield 400 points from dc to 50 kHz. The resolution or linewidth is 125 Hz.

This process of doubling the time record and halving the span can be repeated by using multiple stages of digital filtering. The SR770 can process spectra with a span of only 191 mHz with a time record of 2098 seconds if you have the patience. However, this filtering process only yields baseband measurements (frequency spans which start at dc).

Starting the span somewhere other than dc

Besides being able to choose the span and resolution of the spectrum, we would also like the span to be able to start at frequencies other than dc. It would be nice to center a narrow span around any frequency below 100 kHz. Using digital filtering alone requires that every span start at dc. What is needed is heterodyning. Heterodyning is the process of multiplying the incoming signal by a sine wave. The resulting spectrum is shifted by the frequency of the sine wave. 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 frequencies of the actual signal, not the heterodyned frequencies.

Heterodyning allows the analyzer to compute zoomed spectra (spans which start at frequencies other than dc). The digital filter processor can filter and heterodyne the input in real time to provide the appropriate filtered time record at all spans and center frequencies. Because the digital signal processors in the SR770 are so fast, you won't notice any calculation time while taking spectra. The longest it can take to acquire a spectrum is the length of the time record itself. But more about that later.

2-2

THE TIME RECORD

ANALYZER BASICS

Now that we've described the process in simple terms, let's complicate it a little bit. The SR770 actually uses 512 point complex time records. Each point is a complex value (with real and imaginary parts) so the record actually has 1024 data points in it. But how does a real point get to be complex?

As we described in the previous section, the input samples are digitally filtered and heterodyned to produce a time record with the appropriate bandwidth and a constant number of samples. What we need to add to this is that the heterodyning is a complex operation. This means that the input points are multiplied by both sine and cosine to yield a real and imaginary part.

So instead of using 1024 real points, we use 512 complex points. The time records have the same duration so the complex record has half the sampling rate of the real record. Thus at full span, the real points would occur at 256 kHz and the complex points at 128 kHz. You can think of the complex record as two separate records, one real and one imaginary, each with 64 kHz of bandwidth. (1/2 of the sample rate). One covers 0 to +64 kHz and the other covers -64 kHz to 0 for a total bandwidth of 128 kHz (the same bandwidth as the real record). What a negative frequency means is beyond this discussion but suffice to say it works the same.

The time record display

What do you see when you display the time record? Clearly the time record is not as simple as the raw digitized data points you would see if this were a digital oscilloscope.

display to understand is Linear Magnitude. Remember that magnitudes are always positive. The negative parts of the waveform will be folded around zero so that they appear positive.

Because of the filtering and heterodyning, the time waveform may not closely resemble the input signal. For baseband measurements (when the start frequency of the span is 0.0 Hz) the waveform will resemble the signal waveform (with folding if magnitude is displayed). The bandwidth will be limited by the anti-alias filter and the digital filtering. For zoomed measurements (when the span start is not 0.0 Hz) the displayed waveform will not closely resemble the input signal because of the heterodyning.

Why use the time record?

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

Watch out for windowing!

The time display is not windowed. This means the time record which is displayed will be multiplied by the window function before the FFT is taken (see "Windowing" later in this section). Most window functions taper off to zero at the start and end of the time record. If the transient signal occurs at the start of the time record, the corresponding FFT may not show anything because the window function reduces the transient to zero.

The analyzer stores the 512 point complex time record described above. Because the display is designed for 400 point spectra, only the first 400 points of the time record are displayed. You can use the trigger delay to "translate" the time record to see the part not normally displayed.

The time record for every span has been digitally filtered and heterodyned into a complex record. You can display the magnitude, real or imaginary part as well as the phase. Normally, the easiest

Either use a Uniform window with transients, or use the trigger delay to position the transient at the center of the time record. (Remember that the display only shows the first 400 points of the record. The center is always at the 256th sample, which is not at the center of the display.)

To repeat, the time record is n ot a snapshot of the input signal. It is the output of the digital filter and the input to the FFT processor.

2-3

ANALYZER BASICS

MEASUREMENT BASICS

Now that we know that the input to the FFT processor is a complex time record, it should be no surprise to find out that the resulting FFT spectrum is also a complex quantity. This is because each frequency component has a phase relative to the start of the time record. If there is no triggering, then the phase is random and we generally look at the magnitude of the spectrum. If we use a synchronous trigger then each frequency component has a well defined phase.

Spectrum

The spectrum is the basic measurement of an FFT analyzer. It is simply the complex FFT. Normally, the magnitude of the spectrum is displayed. The magnitude is the square root of the FFT times its complex conjugate. (Square root of the sum of the real part squared and the imaginary part squared). The magnitude is a real quantity and represents the total signal amplitude in each frequency bin, independent of phase.

If there is phase information in the spectrum, i.e. the time record is triggered in phase with some component of the signal, then the real or imaginary part or the phase may be displayed. Remember, the phase is simply the arctangent of the ratio of the imaginary and real parts of each frequency component. The phase is always relative to the start of the triggered time record.

Power Spectral Density or PSD

The PSD is simply the magnitude of the spectrum normalized to a 1 Hz bandwidth. This measurement approximates what the spectrum would look like if each frequency component were really a 1 Hz wide piece of the spectrum at each frequency bin.

spectrum changes with the frequency span. This is because the linewidth changes so the frequency bins have a different noise bandwidth. The PSD, on the other hand, normalizes all measurements to a 1 Hz bandwidth and the noise spectrum becomes independent of the span. This allows measurements with different spans to be compared. If the noise is Gaussian in nature, then the amount of noise amplitude in other bandwidths may be approximated by scaling the PSD measurement by the square root of the bandwidth. Thus the PSD is displayed in units of V/√Hz or dBV/√Hz.

Since the PSD uses the magnitude of the spectrum, the PSD is a real quantity. There is no real or imaginary part or phase.

Octave Analysis

The magnitude of the normal spectrum measures the amplitudes within equally divided frequency bins. Octave analysis computes the spectral amplitude within 1/3 octave bands. The start and stop frequencies of each frequency bin are in the

ratio of 1/3 of an octave (2 analysis spectra will closely resemble data taken with older analog type equipment commonly used in acoustics and sound measurement.

To compute the amplitude of each band, the normal FFT is taken. Those bins which fall within a single band are rms summed together (square root of the sum of the squared magnitudes). The resulting amplitudes are real quantities and have no phase information. They represent total signal amplitude within each band.

We will have more about octave analysis later.

1/3

). The octave

What good is this? When measuring broadband signals such as noise, the amplitude of the

2-4

DISPLAY TYPES

Spectrum

The most common measurement is the spectrum and the most useful display is the Log Magnitude. The Log Mag display graphs the magnitude of the spectrum on a logarithmic scale using dBV as units.

Why is the Log Mag display useful? Remember that the SR770 has a dynamic range of 90 dB and a display resolution of -114 dB below full scale. Imagine what something 0.01% of full scale would look like on a linear scale. If we wanted it to be 1 centimeter high on the graph, the top of the graph would be 100 meters above the bottom. It turns out that the log display is both easy to understand and shows features which have very different amplitudes clearly.

Of course the analyzer is capable of showing the magnitude on a linear scale if you wish.

The real and imaginary parts are always displayed on a linear scale. This avoids the problem of taking the log of negative voltages.

ANALYZER BASICS

The phase is displayed in degrees or radians on a linear scale from -180 (-π) to +180 (+π) degrees (rads). There is no phase "unwrap".

The phase of a particular frequency bin is set to zero if neither the real nor imaginary part of the FFT is greater than 0.012% of full scale (-78 dB below f.s.). This avoids the messy phase display associated with the noise floor. (Remember, even if a signal is small, its phase extends over the full 360 degrees.)

Watch Out For Phase Errors

The FFT can be thought of as 400 bandpass filters, each centered on a frequency bin. The signal within each filter shows up as the amplitude of each bin. If a signal's frequency is between bins, the filters act to attenuate the signal a little bit. This results in a small amplitude error. The phase error, on the other hand, can be quite large. Because these filters are very steep and selective, they introduce very large phase shifts for signals not exactly on a frequency bin.

The PSD and Octave analysis are real quantities and thus may only be displayed as magnitudes. In addition, the Octave analysis requires the display to be Log Magnitude.

Phase

In general, phase measurements are only used when the analyzer is triggered. The phase is relative to the start of the time record.

On full span, this is generally not a problem. The bins are 250 Hz apart and most synthesized sources have no problem generating a signal right on a frequency bin. But when the span is narrowed, the bins move much closer together and it becomes very hard to place a signal exactly on a frequency bin.

2-5

ANALYZER BASICS

WINDOWING

What is windowing? Let's go back to the time record. What happens if a signal is not exactly periodic within the time record? We said that its amplitude is divided into multiple adjacent frequency bins. This is true but it's actually a bit worse than that. If the time record does not start and stop with the same data value, the signal can actually smear across the entire spectrum. This smearing will also change wildly between records because the amount of mismatch between the starting value and ending value changes with each record.

Windows are functions defined across the time record which are periodic in the time record. They start and stop at zero and are smooth functions in between. When the time record is windowed, its points are multiplied by the window function, time bin by time bin, and the resulting time record is by definition periodic. It may not be identical from record to record, but it will be periodic (zero at each end).

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 bin 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, it falls off from center rapidly. This means that even frequencies close to the bin may be attenuated somewhat. If the filter is wide, then frequencies far from the bin will contribute to the bin amplitude but those close by will probably not be attenuated much.

The net result of windowing is to reduce the amount of smearing in the spectrum from signals not exactly periodic with the time record. The different types of windows trade off selectivity, amplitude accuracy, and noise floor.

The SR770 offers four types of window functions Uniform (none), Flattop, Hanning and BlackmanHarris (BMH).

Uniform

The uniform window is actually no window at all. The time record is used with no weighting. A signal will appear as narrow as a single bin 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 affect every bin of the spectrum. These two cases also have a great deal of amplitude variation between them (up to 4 dB).

In general, this window is only useful when looking at transients which do not fill the entire time record.

Hanning

The Hanning window is the most commonly used window. It has an amplitude variation of about

1.5 dB (for signals between bins) and provides reasonable selectivity. Its filter rolloff is not particularly steep. 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.

Flattop

The Flattop window improves on the amplitude accuracy of the Hanning window. Its between-bin amplitude variation is about .02 dB. However, the selectivity is a little worse. Unlike the Hanning, the Flattop window has a wide pass band and very steep rolloff on either side. Thus, signals appear wide but do not leak across the whole spectrum.

BMH

The BMH window is a very good window to use with this analyzer. It has better amplitude accuracy (about 0.7 dB) than the Hanning, very good selectivity and the fastest filter rolloff. The filter is steep and narrow and reaches a lower attenuation than the other windows. This allows signals close together in frequency to be distinguished, even when their amplitudes are very different.

If a measurement requires the full dynamic range of the analyzer, then the BMH window is probably the best one to use.

2-6

AVERAGING

The SR770 analyzer supports several types of averaging. In general, averaging many spectra together improves the accuracy and repeatability of measurements.

RMS Averaging

RMS averaging computes the weighted mean of the sum of the squared magnitudes (FFT times its complex conjugate). The weighting is either linear or exponential.

ANALYZER BASICS

Peak Hold detects the peaks in the spectral magnitudes and only applies to Spectrum, PSD, and Octave Analysis measurements. However, the peak magnitude values are stored in the original complex form. If the real or imaginary part or phase is being displayed for spectrum measurements, the display shows the real or imaginary part or phase of the complex peak value.

RMS averaging reduces fluctuations in the data but does not reduce the actual noise floor. With a sufficient number of averages, a very good approximation of the actual random noise floor can be displayed.

Since RMS averaging involves magnitudes only, displaying the real or imaginary part or phase of an RMS average has no meaning. The RMS average has no complex information.

Vector Averaging

Vector averaging averages the complex FFT spectrum. (The real part is averaged separately from the imaginary part.) This can reduce the noise floor for random signals since they are not phase coherent from time record to time record.

Vector averaging requires a trigger. The signal of interest must be both periodic and phase synchronous with the trigger. Otherwise, the real and imaginary parts of the signal will not add in phase and instead will cancel randomly.

With vector averaging, the real and imaginary parts as well as phase displays are correctly averaged and displayed. This is because the complex information is preserved.

Peak Hold

Peak Hold is not really averaging, rather the new spectral magnitudes are compared to the previous data, and if the new data is larger, then the new data is stored. This is done on a frequency bin by bin basis. The resulting display shows the peak magnitudes which occurred in the previous group of spectra.

Linear Averaging

Linear averaging combines N (number of averages) spectra with equal weighting in either RMS, Vector or Peak Hold fashion. When the number of averages has been completed, the analyzer stops and a beep is sounded. When linear averaging is in progress, the number of averages completed is continuously displayed below the Averaging indicator at the bottom of the screen.

Auto ranging is temporarily disabled when a linear average is in progress. Be sure that you don't change the input range manually either. Changing the range during a linear average invalidates the results.

Exponential Averaging

Exponential averaging weights new data more than old data. Averaging takes place according to the formula,

AverageN = (New Spectrum • 1/N) +

(Average

where N is the number of averages.

Exponential averages "grow" for approximately the first 5N spectra until the steady state values are reached. Once in steady state, further changes in the spectra are detected only if they last sufficiently long. Make sure that the number of averages is not so large as to eliminate the changes in the data that might be important.

) • (N-1)/N

N-1

2-7

ANALYZER BASICS

REAL TIME BANDWIDTH AND OVERLAP PROCESSING

What is real time bandwidth? Simply stated, it is the frequency span whose corresponding time record exceeds the time it takes to compute the spectrum. At this span and below, it is possible to compute the spectra for every time record with no loss of data. The spectra are computed in "real time". At larger spans, some data samples will be lost while the FFT computations are in progress.

For all frequency spans, the SR770 can compute the FFT in less time than it takes to acquire the time record. Thus, the real time bandwidth of the SR770 is 100 kHz. This includes the real time digital filtering and heterodyning, the FFT processing, and averaging calculations. The SR770 employs two digital signal processors to accomplish this. The first collects the input samples, filters and heterodynes them, and stores a time record. The second computes the FFT and averages the spectra. Since both processors are working simultaneously, no data is ever lost.

Averaging speed

How can you take advantage of this?Consider averaging. Other analyzers typically have a real time bandwidth of around 4 kHz. This means that even though the time record at 100 kHz span is only 4 ms, the "effective" time record is 25 times longer due to processing overhead. An analyzer with 4 kHz of real time bandwidth can only process about 10 spectra a second. When averaging is on, this usually slows down to about 5 spectra per second. At this rate it's going to take a couple of minutes to do 500 averages.

The SR770, on the other hand, has a real time bandwidth of 100 kHz. At a 100 kHz span, the analyzer is capable of processing 250 spectra per second. In fact, this is so fast, that the display can not be updated for each new spectra. The display only updates about 6 times a second. However, when averaging is on, all of the computed spectra will contribute to the average. The time it takes to complete 500 averages is only a few seconds. (Instead of a few minutes!)

Overlap

What about narrow spans where the time record is long compared to the processing time? The analyzer computes one FFT per time record and can wait until the next time record is complete

before computing the next FFT. The update rate would be no faster than one spectra per time record. With narrow spans, this could be quite slow.

And what is the processor doing while it waits? Nothing. With overlap processing, the analyzer does not wait for the next complete time record before computing the next FFT. Instead it uses data from the previous time record as well as data from the current time record to compute the next FFT. This speeds up the processing rate. Remember, most window functions are zero at the start and end of the time record. Thus, the points at the ends of the time record do not contribute much to the FFT. With overlap, these points are "re-used" and appear as middle points in other time records. This is why overlap effectively speeds up averaging and smoothes out window variations.

Typically, time records with 50% overlap provide almost as much noise reduction as nonoverlapping time records when RMS averaging is used. When RMS averaging narrow spans, this can reduce the measurement time by 2.

Overlap percentage

The amount of overlap is specified as a percentage of the time record. 0% is no overlap and 99.8% is the maximum (511 out of 512 samples re-used). The maximum overlap is determined by the amount of time it takes to calculate an FFT and the length of the time record and thus varies according to the span.

The SR770 always tries to use the maximum amount of overlap possible. This keeps the display updating as fast as possible. Whenever a new frequency span is selected, the overlap is set to the maximum possible value for that span. If less overlap is desired, then use the Average menu to enter a smaller value. On the widest spans (25, 50 and 100 kHz), no overlap is allowed.

Triggering

If the measurement is triggered, then overlap is ignored. Time records start with the trigger. The analyzer must be in continuous trigger mode to use overlap processing.

2-8

INPUT RANGE

ANALYZER BASICS

The input range on the SR770 varies from a maximum of 34 dBV full scale to a minimum of 60 dBV full scale. A signal which exceeds the current input range will cause the OvrLoad message to appear at the bottom of the screen. A signal which exceeds the maximum safe range will turn on the HI V indicator.

The input range is displayed in dBV. The maximum and minimum range equivalents are tabulated below.

Max 34 dBVpk

31 dBVrms

50.1 Vpk

35.4 Vrms

Min -60 dBVpk

-63 dBVrms

1.0 mVpk

0.7 mVrms

Manual Range

The input range can be specified in the Input menu to be fixed at a certain value. Signals that exceed the range will overload and become distorted. Signals which fall to a small percentage of the range will become hard to see.

Auto Range

The input range can be set to automatically correct for signal overloads. When autoranging is on and an overload occurs, the input range is adjusted so that the signal no longer overloads. If the signal decreases, the input range is not adjusted. You must take care to ensure that the signal does not fall dramatically after pushing the input range to a very insensitive setting.

While the analyzer is performing linear averaging, the input range is NOT changed even if the signal overloads. The overload indicator will still light to indicate an over range condition. Changing the range during a linear average invalidates the average.

2-9

ANALYZER BASICS

THE SOURCE

The SR770 source provides a variety of test signals which allow the SR770 to measure the response of electronic, mechanical and acoustic devices, without the need for an external generator. In many cases, the SR770 source is better than an external source since it is synchronous with the input sampling.

Sine

Two Tone

Two low distortion sine waves can be generated simultaneously for intermodulation distortion tests (IMD). Each tone has independent frequency and amplitude settings.

Chirp

chirp waveform requires a uniform window to result in a flat spectrum. This is because the individual frequency components do not have a constant amplitude over the time record. Windowing will attenuate certain portions of the spectrum.

Source Trigger

The Sine, Two Tone, and Chirp sources can be triggered to measure phase response and/or vector average. The Source Trigger mode simply sets the overlap to 0% so each time record follows immediately after the previous one with no delay.

For the chirp source, each time record will be identical so the phase of each component will be stable. The absolute phase of the record is arbitrary since the source does not turn on and off synchronously with the input time record. Turning on and off the source, changing the span, performing Auto Offset can all change the absolute phase of the time record. The Auto Phase function can be used to set all the phase of each component to zero. Subsequent phase measurements will be relative to zero.

For sine and two tone, the source frequencies must be set to a multiple of the linewidth in order for a stable time records to be acquired. Random frequencies are not exactly periodic over a time record and do not result in a stable phase. Once again, the absolute phase is arbitrary since the source does not turn on synchronously with the input time record.

The Source Cal mode compensates for the ripple in the output filter by adjusting the input calibrations. This results in a flat chirp spectrum. Inserting a device under test will measure the device's amplitude transfer curve.

Windowing

The Sine, Two Tone, and Chirp sources can be used with or without a window function.

The sine and two tone frequencies can be set at exact bin frequencies of the spectrum, hence they can be exactly periodic in the time record. As long as the signal to noise at the input if high, windowing is not required.

The chirp waveform consists of 400 sine waves, each one perfectly periodic in the time record. The

Noise

Broadband noise is useful for characterizing circuits, mechanical systems or even the audio response of an entire room. W hite noise provides equal amplitude per root Hz from 0 to 100 kHz, regardless of the measurement span. White noise is useful in electronic applications. Pink noise rolls off at 3 dB/oct providing equal amplitude per octave. Pink noise is preferred in audio applications.

Since the signal is noisy and random, windows are always required when using the noise source.

Source triggering is not meaningful since there is no stable phase information in the source.

2-10

OPERATION

FRONT PANEL OVERVIEW

POWER BUTTON

The SR770 is turned on by pushing in the POWER button. The video display may take a few seconds to warm up and become visible. Adjust the brightness until the screen is easily readable.The model, firmware version and serial number of the unit are displayed when the power is turned on.

A series of internal tests are performed at this point. Each test is described as it is performed and the results are represented graphically as OK or NOT OK. The tests are described below.

RAM This test performs a read/write test to the

processor RAM. In addition, the nonvolatile backup memory is tested. All instrument settings are stored in nonvolatile memory and are retained when the power is turned off. If the memory check passes, then the instrument returns to the settings in effect when the power was last turned off. If there is a memory error, then the stored settings are lost and the default settings are used.

ROM This test checks the processor ROM. CLR This test indicates whether the unit is

being reset. To reset the unit, hold down the backspace [<-] key while the power is turned on. The unit will use the default

settings. The default setup is listed in a later chapter.

CLK This test checks the CMOS clock and

calendar for a valid date and time. If the there is an error, the time will be reset to a default time. Change the clock settings using the SYSTEM SETUP menu.

DSP This test checks the digital signal

processors and fast memory.

A/D This test checks the analog to digital

converter board.

RESET

Holding down the backspace [<-] key while the power is turned on resets the unit. The unit will use the default settings as listed at the end of the Menus section of the manual.

VIDEO DISPLAY

The monochrome video display is the user interface for data display and front panel programming operations. The resolution of the display is 640H by 480V. The brightness is adjusted using the brightness control knob located at the upper left corner. As with most video displays, do not set the brightness higher than necessary. The display may be adjusted left and

3-1

OPERATION

right using the Setup Screen function in the SYSTEM SETUP menu.

horizontal axis. The graph is continuously updated while the unit is in the RUN mode.

A complete description of the screen display follows in the next section.

SOFT KEYS

The SR770 has a menu driven user interface. The 6 soft keys to the right of the video display have different functions depending upon the information displayed in the menu boxes at the right of the video display. In general, the soft keys have two uses. The first is to toggle a feature on and off or to choose between settings. The second is to highlight a parameter which is then changed using the spin knob or numeric keypad. In both cases, the soft keys affect the parameters which are displayed adjacent to them.

KEYPAD

The keypad consists of five groups of keys. The ENTRY keys are used to enter numeric parameters which have been highlighted by a soft key. The MENU keys select a menu of soft keys. Pressing a menu key will change the menu boxes which are displayed next to the soft keys. Each menu presents a group of similar parameters and functions. The CONTROL keys start and stop actual data acquisition, select the marker and toggle the active trace. These keys are not in a menu since they are used frequently and while displaying any menu. The SYSTEM keys print the screen to a printer and display help messages. Once again, these keys can be accessed from any menu. The MARKER keys determine the marker mode and perform various marker functions. The marker functions can be accessed from any menu.

The main area of the display is occupied by the data trace display. Data is graphed as signal on the vertical axis and frequency or time bin on the

DISK DRIVE

The 3.5" disk drive is used to store data and instrument settings. Double sided, double density disks should be used. The disk capacity is 720k bytes formatted. The disk format is DOS compatible. Disks written by the SR770 may be read by PC compatible computers equipped with a

3.5" drive and DOS 3.0 or higher.

Only use double sided double density (DS/DD) disks. Do not use high density (DS/HD) disks.

BNC CONNECTORS TRIGGER

The rising or falling edge of the TRIGGER input triggers a time record. The input impedance is 10 KΩ and the minimum pulse width is 10 ns. The trigger level is adjustable from -5V to +5V with either positive or negative slope. The minimum pulse amplitude is 100 mV.

SIGNAL INPUTS

The input mode may be single-ended, A, or differential, A-B. The A and B inputs are voltage inputs with 1 MΩ, 15 pF input impedance. Their connector shields are isolated from the chassis by 1 MΩ (float) or 50 Ω (ground). Do not apply more than 50 V to either input. The shields should never exceed 3V.

SOURCE OUTPUT

The source can output either sine, two tone, chirp or noise waveforms. The output impedance is less than 5 Ω and is capable of driving a 50 Ω load. The output is ground referenced.

A complete description of the keys follows in the next section.

SPIN KNOB

The spin knob is used to adjust parameters which have been highlighted using the soft keys. Most numeric entry fields may be adjusted using the knob. In addition, functions such as display zooming and scrolling use the knob as well. In these cases, the knob function is selected by the soft keys. The [MARKER] key, which can be pressed at any time, will set the knob function to scrolling the marker.

3-2

SCREEN DISPLAY

OPERATION

DATA DISPLAY

Data is graphed with signal on the Y axis and frequency or time on the X axis. The physical size of the graph remains constant while the vertical and horizontal scales may be changed. The graph area has a dotted grid for reference. There are 10 horizontal divisions and either 8 or 10 vertical divisions. The frequency span consists of 400 frequency bins. The display normally shows all 400 bins. The X axis may be expanded and translated to display less than 400 bins. This expansion does not change the span or time record, it merely changes the display of the data.

SINGLE and DUAL TRACE DISPLAYS

There are two data traces being acquired at all times. The traces are labelled Trace0 and Trace1. The traces may be different measurements, such as spectrum and time record, or different displays, such as magnitude and phase. When the two traces are displaying live data, they have the same signal input, frequency span, window function, trigger, and averaging mode. If one of the traces is a recalled file, then it can have a span and window which differs from the live settings.

The display shown above is the SINGLE trace format. The [ACTIVE TRACE] key toggles the display between the two traces.

3-3

OPERATION

The dual trace or Up/Dn format is shown above. The display format is selected in the DISPLAY menu. Trace0 is always the upper trace. Each trace is annotated the same way as the single trace.

The left edge, center and right edge of the graph are labelled directly below the graph. When displaying spectral data with no horizontal

expansion, these values are the Start, Cen ter an d Stop frequencies of the frequency span in use.

When displaying time records, these values are

the Start, Middle and End of the time record.

These times are always relative to the start of the time record, they do not reflect any trigger delay which may be programmed.

To expand a graph, use the SCALE menu. When the display is expanded in the horizontal axis, the labels reflect the displayed span and time, not the actual acquisition span and time record. Expanded traces have an EXPAND indicator below the right hand edge of the graph as shown below.

The Top reference is the Y value of the upper

edge of the graph. The units can be Volts, dBVolts, or EU (user defined engineering units). The

Vertical scale is shown as the number of dB,

Volts, or EU per division. This value is changed whenever the vertical scale is adjusted.

The Window Function for the displayed data is

shown below the graph. In the case of a recalled graph, this window is the one used to calculate the recalled graph, not the window used for live calculations.

The File Type refers to the source of the data being displayed. Live means that the data is realtime, Calculated data is the result of Trace Math, and a "filename" is data recalled from a disk file.

At the upper right, the measurement and display type and trace number, 0 or 1, are shown. The measurement type can be Spectrum, PSD (power spectral density), Time record, or Octave analysis. The display types are Log Magnitude, Linear Magnitude, linear Real part, linear Imaginary part, and Phase.

On the active trace, the measurement type and trace number are highlighted in inverse.

3-4

OPERATION

MARKER DISPLAY

The Marker Region is the graph region between

the two heavy vertical dashed lines. The marker region may be set to 1 division (wide), 1/2 division (norm), or a single vertical line (spot). The marker region does not change with horizontal scaling.

The Marker is a small square which seeks the

minimum, maximum, or mean of the data within the marker region. When seeking min or max, the marker is located at the position of the data point which is the min or max. This allows peaks and valleys in the data to be easily read out. When seeking the mean, the X position of the marker is at the center of the marker region and the Y position is the mean of the data within the region. When a spot marker region is used, the marker is confined to a single frequency or time bin.

The Marker Position displays the X position

(frequency or time) and the Y data of the marker. Pressing the [MARKER] key will draw a box around the marker information. When the marker readout is surrounded by this box, the spin knob adjusts the position of the marker region. The marker region moves in increments of one frequency or time bin.

RUN/STOP/STOP-Invld

The RUN indicator is on whenever data is being taken and spectra are being calculated. STOP indicates that data is not being acquired and the data display is not being updated. STOP-Invld means that the data on the display may not match the graph parameters or the analyzer settings. For example, if the display is paused (using the [PAUSE CONT] key or some other means), and the span is changed, clearly the displayed data does not reflect the new span. In this case, the STOP-Invld indicator will turn on.

SETTLING

When changing between narrow frequency spans with long acquisition times, the digital filter requires some settling time before all of the data is replaced with new data. This time is longer than the record time. While this indicator is on, the filter is still settling and the displayed spectrum may not be accurate.

Input Range

The input range is always displayed. If the range is set manually, the display is in normal characters. If Auto Range is on, then inverse characters are used.

MENU DISPLAY

The Soft Key menu boxes define the functions of

the 6 soft keys to the right of the screen. The menu boxes are grouped into menus. Pressing each of the ten Menu keys will display a different menu of boxes. Related functions are grouped into a single menu. In general, pressing a soft key does one of two things. One is to toggle between 2 or 3 specific choices. An example is the Display Format box illustrated on the previous page. Pressing the first soft key toggles the display between Single and Up/Dn. The second soft key mode is to highlight an entry field and knob function. An example would be the Start Freq. Pressing the soft key will highlight the Start Freq. value. The Start Freq. may then be adjusted with the knob or entered as a value using the numeric entry keys. Each menu is described at length in a following section.

STATUS INDICATORS

In addition to the data display and menu boxes, there are a number of status indicators which are displayed at the bottom of the screen.

OvrLoad

This indicator turns on if the input signal overloads the analog amplifier or A/D converter.

No Avg./Avrging

This indicates whether averaging is in effect. Averaging affects both traces if they are live. If linear averaging is on, then the number below the

Avrging indicator is the number of averages

accumulated so far. If averaging is off or exponential, then no number is displayed.

Trigger/Trg Wait

If triggering is on, then the Trigger indicator

flashes on whenever a time record is triggered.

Trg Wait indicates that the unit is in triggered

mode and is waiting for a trigger to occur. Triggers received while acquiring data from a previous trigger are ignored.

Armed/Arm Wait

If triggering is on, the Armed indicator is on

whenever the unit is armed and awaiting a trigger.

Arm Wa i t means that the unit is in manual arming

mode and awaiting an arm command, either from the front panel or via the computer interfaces.

3-5

OPERATION

GPIB/RS232

Flashes when there is activity on the computer interfaces. This does not flash for printer or plotter activity.

SRQ

This indicator is on whenever a GPIB Service Request is generated by the SR770. SRQ stays on until a serial poll is completed.

REM

This indicator is on when the front panel is locked out by a computer interface. No front panel adjustments may be made. To return the unit to local control (if allowed), press the [HELP] key.

Pass/Fail

This indicates whether a trace passes or fails a limit table test.

ERR

Flashes whenever there is a computer interface error such as illegal command or out of range parameter is received. This does not flash for a printer or plotter error.

The High Voltage indicator turns on whenever an input greater than 50 V is detected. The analyzer immediately switches in an attenuator to protect the input circuitry. Any attempt to set the input range to a setting which would remove this attenuator will not be allowed until the input signal is reduced to a safe level.

ALT

Indicates that the ALTERNATE keypad is in use. The ALTERNATE keypad uses the alphabetic legends printed below each key. To enter the ALT mode, press the [ALT] key once. Pressing the keys will now enter alphabetic characters into the active entry field. The [0]...[9], [.], [-], [<-] and [ALT] have the same function in the ALTERNATE keypad. To return to the normal keypad, press the [ALT] key again.

3-6

KEYPAD

OPERATION

NORMAL AND ALTERNATE KEYS

The normal key definitions are printed on each key. In addition, each key also has an alternate definition printed below it. The [ALT] key toggles the keypad between the two definitions. The ALT screen indicator is on when the alternate definitions are in use. The [0]...[9], [.], [-], [<-] and [ALT] keys have the same definition in both modes. The alternate keys should only be used when accessing files on the disk drive or labelling plots.

MENU KEYS

All operating parameters of the SR770 are grouped into ten function menus. The ten menu keys select which menu of parameters is displayed next to the six soft keys. The soft keys then either toggle a parameter, highlight a parameter entry field (for numeric entry or knob adjustment), or display a submenu. The menus are listed below.

[FREQ] Sets the frequency span and start

and center frequencies.

[MEAS] Displays submenus for selecting

the measurement type, displayed quantity, units, window type, and calculator.

[DISPLAY] Sets the display format, marker

on/off, and grid modes.

[INPUT] Configures the signal inputs, sets

the manual input range and trigger setup.

[SCALE] Sets the graph scaling and

expansion and selects linear or log X axis.

[ANALYZE] Turns on harmonic, sideband and

band analysis as well as data and limit tables.

[AVERAGE] Turns averaging on and off and

selects the averaging type.

[SOURCE] Turns on and off and configures

built-in source.

[SYSTEM] Configures the computer interfa-

ces, sound, real time clock, plotter, printer, and screen location. The [TEST] submenu tests the keypad, external keyboard, knob, RS232 interface, printer interface, disk drive, video screen, and memory. The [INFO]

3-7

OPERATION

submenu displays various information screens.

[STORE RECALL]

This menu stores and recalls data and settings to and from the disk. Also contains a disk utilities submenu.

Detailed descriptions of each menu are provided in a later chapter.

ENTRY KEYS

The numeric entry keys can be used to directly enter parameter values. Parameters may be entered only if their menu box is displayed and their entry field is highlighted. For example, if the FREQ menu is displayed, the fifth soft key is next to the Start Freq. box. Pressing this soft key will highlight the entry field displaying the start frequency. The menu box will appear as below.

A new start frequency may now be entered using the numeric keys. For example, to set the start to -

1.25 kHz, press [1] [.] [2] [5]. As soon as the [1] is

pressed, the entry parameter is displayed in the upper left hand corner of the screen as shown below.

Entries may be made in exponential form using the [EXP] key. The entry above may be made by pressing [1] [2] [5] [EXP] [1] and then the Hz units soft key.

In general, whenever a parameter entry field is highlighted, the knob may also be used to adjust the value. If the knob is turned while making a numeric entry but before a units key has been pressed, the knob will adjust the marker position instead.

Some entry fields allow only knob adjustment or only numeric entry.

START and PAUSE/CONT

The [START] and [PAUSE/CONT] keys are used to start, pause and continue data acquisition. If the unit is in the RUN mode acquiring and displaying data, as indicated by the RUN indicator, then the [PAUSE/CONT] key will halt data acquisition. The RUN indicator switches to STOP and no new spectra will be taken and the display will not be updated. If averaging is off, then either the [START] key or the [PAUSE/CONT] key will resume acquisition. If averaging is on, the [START] key will reset the average and restart acquisition. [PAUSE/CONT], on the other hand, will continue the average where it was paused. In the case of linear averaging when the average is already completed, the [PAUSE/CONT] does nothing since there is no average to continue.

Note that the frequency menu is also replaced This menu shows the available units for the active entry field, in this case mHz, Hz, or kHz.

The entry field displays the characters as the keys are pressed. The '-' is the entry point. If an error is made, the backspace [<] key will erase the last character. Pressing the Escape soft key will abort the entry operation and leave the value unchanged. When the entry string is correct, press the kHz units soft key to change the start frequency to the new value.

with a units menu.

MARKER

Pressing the [MARKER] key highlights the marker information field by drawing a box around it. The knob will now scroll the marker region. The highlighted marker field appears below.

Any previously highlighted parameter field will become non-highlighted. Pressing a soft key to highlight a new parameter field will let the knob adjust the new parameter while the marker becomes unselected.

ACTIVE TRACE

Pressing [ACTIVE TRACE] toggles the active trace. In the single trace display format, the graph switches between Trace0 and Trace1. In the dual screen display, [ACTIVE TRACE] switches which

3-8

OPERATION

trace is active as indicated by the highlighted trace identification at the upper right of the graph. In both cases, the active trace determines which trace's parameters are displayed in the menus. For example, activating Trace0 and then selecting the Measure menu will allow you to select the measurement for Trace0. Pressing [ACTIVE TRACE] once allows you to select the measurement for Trace1 using the same menu. Only those parameters which are associated with an individual trace have differing values between the traces. Parameters such as input configuration, frequency span and window function are the same for any live trace.

AUTO RANGE

Pressing [AUTO RANGE] toggles the input ranging mode between Manual and Auto. In Manual mode, the input range is set within the INPUT menu. When the mode is toggled to Auto, the input range is stepped quickly from -60 dB towards +30 dB until no overload is detected. Any overload in the signal will cause the input scale to change to remove the overload. If the signal decreases, the input scale is not changed. The range can be autoranged at any time by toggling the mode from Auto to Manual and back to Auto. Switching back to Manual ranging leaves the input range at the current setting.

The Input Range indicator will be in inverse characters if Auto Ranging is on.

frequency depending upon which frequency field was most recently activated in the FREQ menu.

MARKER ENTRY

In the ANALYZE menu, pressing this key will enter the marker frequency into the Fundamental (Harmonic analysis), Carrier (sideband analysis), Band Start and Center (band analysis) frequency fields. This key also enters the marker frequency into the X Value field of the Data and Limit Tables.

MARKER MODE

The [MODE] key in the MARKER section of the keypad brings up a menu. This menu selects linked cursors in the dual trace display and allows marker offsets to be entered manually. The Peak find functions are also in this menu.

MARKER REF

The [MARKER REF] key toggles the marker offset or reference mode. Pressing this key once will turn on the marker offset and set the X and Y offset to the value of the current marker position. Subsequent marker readings are relative to the reference or offset values. The offset marker is indicated by a ∆ (delta symbol) preceding the marker readout above the graph as shown below. The [MARKER REF] key may be used in any menu.

AUTOSCALE

Pressing [AUTOSCALE] will automatically set the vertical scale and translation to display the entire range of the data. [AUTOSCALE] does not affect the horizontal scaling.

[AUTOSCALE] may be pressed at any time during or after data acquisition.

[AUTOSCALE] only operates on the data which is displayed on the graph. If the graph is expanded, data corresponding to frequency or time bins which are not shown do not figure in the autoscaling calculations.

SPAN UP and SPAN DOWN

The [SPAN UP] and [SPAN DOWN] keys increment and decrement the frequency span by a factor of 2. These keys provide a way of adjusting the span when any menu is displayed. The span is adjusted with either a fixed start or fixed center

The marker offset location on the graph is marked by a small star shaped symbol.

MARKER CENTER

The [MARKER CENTER] key sets the span center frequency to the marker frequency. If the span is large so that this operation would require a span which extends below 0 Hz or past 100 kHz, then the span is decreased to the largest span which allows the marker frequency to be the center.

MARKER MAX/MIN

Pressing [MARKER MAX/MIN] will center the marker region around the maximum or minimum data value on the screen. The Marker Seeks mode in the DISPLAY menu chooses whether this key finds the on-screen max or min. If the marker seeks the mean, then the [MARKER MAX/MIN] key finds the maximum on-screen point. The marker will be positioned at the Min, Max, or Mean

3-9

OPERATION

of the data within the region, depending upon the seeks mode. The [MARKER MAX/MIN] key only searches the data which is on the screen. If the max/min value occurs at more than one location, then the one closest to the left edge is found.

[PRINT] will print the currently displayed screen to a printer attached to the rear panel parallel printer port. The entire screen, including text and menus, is printed. The time and date will also be printed. The printer type needs to be configured in the SYSTEM SETUP menu before using [PRINT]. A "Printing in Progress" message will appear on the screen while printing occurs.

Pressing [<-] (backspace) will abort the printout. No other front panel operations may be

performed until printing is completed. If no printer is attached or there is a printer error, then the print operation is aborted after about 10 seconds. A "Print Aborted!" message will appear briefly on the screen.

HELP

[HELP] provides on screen help with any key or soft key. Pressing [HELP] followed by any key will display information about the function or use of that key. [HELP] with a soft key will describe the menu item next to the soft key. Pressing another key will exit the help screen.

The [PRINT] key is the one key for which no help is available. Pressing [PRINT] at any time will print the screen, including the help screens.

LOCAL

When a host computer places the unit in the REMOTE state, no keypad or knob input is allowed. To return to front panel operation, press the [HELP] key.

3-10

REAR PANEL

OPERATION

POWER ENTRY MODULE

The power entry module is used to fuse the AC line, select the line voltage, and block high frequency noise from entering or exiting the instrument. Refer to the first page of this manual for instructions on selecting the correct line voltage and fuse.

IEEE-488 CONNECTOR

The 24 pin IEEE-488 connector allows a computer to control the SR770 via the IEEE-488 (GPIB) instrument bus. The address of the instrument is set in the SETUP GPIB menu.

Also, a GPIB plotter with HPGL compatible graphics may be connected to the IEEE-488 port. In this case, the SR770 will control the plotter to generate plots of the screen graph. Use the SETUP PLOTTER menu to configure the SR770 for use with a GPIB plotter.

RS232 CONNECTOR

The RS232 interface connector is configured as a DCE (transmit on pin 3, receive on pin 2). The baud rate, parity, and word length are programmed from the SETUP RS232 menu. To connect the SR770 to a PC serial adapter, which is usually a DTE, use a straight thru serial cable.

Also, a serial plotter with HPGL compatible graphics may be connected to the RS232 port. The SR770 will drive the plotter to generate plots of the screen graph. Use the SETUP PLOTTER menu to configure the SR770 for use with a serial plotter.

PARALLEL PRINTER CONNECTOR

The [PRINT] key will print the screen to an Epson compatible graphics printer or an HP LaserJet compatible laser printer. Use a standard printer cable to attach the printer to the printer port. Use the SETUP PRINTER menu to choose the type of printer.

PC KEYBOARD CONNECTOR

An IBM PC or XT compatible keyboard may be attached to the keyboard connector. An AT keyboard may be in its PC or 8088 mode. Typing at the attached keyboard is the same as entering numbers and letters from the front panel keypad. Highlighted parameter entry fields will accept characters from the keyboard. Typing 'E' or 'e' is the same as [EXP]. In general, the keyboard is only useful for alphabetic fields such as file names or plot labels.

3-11

OPERATION

3-12

FREQUENCY MENU

Frequency

The Frequency menu is used to set the frequency span and location for the measurement.

Span Pressing the Span key selects the frequency span as the active entry field. A

new span may be entered from the numeric keypad or the knob may be used to adjust the span. The frequency span ranges from 191 mHz to 100 kHz in factors of 2. A numerically entered span is rounded up to the next largest allowable span.

If the new span is incompatible with the 0 to 100 kHz frequency range because the start or center frequency is close to the limits of the range, then the start or center frequency will be adjusted to accommodate the new span.

Changing the span will change the Linewidth (Span/400) and Acquisition Time (400/Span).

Linewidth The Linewidth key selects the linewidth as the active entry field. The linewidth

is defined as the span divided by 400. The linewidth ranges from .477 mHz to 250 Hz in factors of 2. A numerically entered linewidth is rounded up to the next largest allowable linewidth.

Changing the linewidth will change the Span (Linewidth*400) and Acquisition Time (1/Linewidth). If the new span is incompatible with the 0 to 100 kHz frequency range because the start or center frequency is close to the limits of the range, then the start or center frequency will be adjusted to accommodate the new span.

Acquisition Time The Acquisition Time key selects the acquisition time as the active entry field.

The acquisition time is defined as the reciprocal of the linewidth. The acquisition time ranges from 2097.1 s to 4.00 ms in factors of 2. A

4-1

FREQUENCY MENU

numerically entered acquisition time is rounded down to the next fastest allowable acquisition time.

Changing the acquisition time will change the Span (400/Acquisition Time) and Linewidth (1/Acquisition Time). If the new span is incompatible with the 0 to 100 kHz frequency range because the start or center frequency is close to the limits of the range, then the start or center frequency will be adjusted to accommodate the new span.

Full Span Pressing this key immediately sets the Span to 100 kHz, Linewidth to 250 Hz,

Acquisition Time to 4.00 ms, Start Frequency to 0.0 Hz, and Center Frequency to 50.0 kHz.

Start Frequency The Start Frequency key selects the start frequency of the span as the active

entry field. The knob adjusts the start frequency in steps equal to the linewidth. A numerically entered frequency is rounded to the nearest frequency bin (exact multiple of the linewidth). If the new start frequency is incompatible with the span because of the 0 to 100 kHz range limits, then the start frequency will be set to the closest allowable value.

Center Frequency The Center Frequency key selects the center frequency of the span as the

active entry field. The knob adjusts the center frequency in steps equal to the linewidth. A numerically entered frequency is rounded to the nearest frequency bin (exact multiple of the linewidth). If the new center frequency is incompatible with the span because of the 0 to 100 kHz range limits, then the center frequency will be set to the closest allowable value.

Note:

Activating the Start or Center Frequency fields fixes the start or center frequency for subsequent adjustments to the frequency span. Further adjustments to the span leave the span start or center untouched, even when the start or center frequency becomes de-activated as a menu choice. The most recently activated of the Start or Center Frequency fields sets the span adjustment mode.

Enlarging the frequency span may change the start and center frequencies. This is because these frequencies are always exact frequency bins or multiples of the linewidth. Larger spans have larger linewidths and thus the start and stop frequencies may need to be rounded to the nearest allowable bin of the new span.

4-2

MEASURE MENU

Measure

The Measure menu is used to select the measurement type, display type, units and window function. The Measure menu also activates the calculator for trace math.

Measure Keys Each Measure Key activates a sub menu. Each sub menu is described in

detail in the following pages.

4-3

MEASURE MENU

Measure

The Measure sub menu selects the type of measurement for the active trace.

Spectrum The SR770 filters the input data in real time to provide a time record with the

desired frequency span and then performs an FFT on this record. Pressing the Spectrum key displays this FFT on the active trace.

PSD The PSD or Power Spectral Density is the magnitude of the spectrum (the

square root of [the FFT times its complex conjugate]) normalized to a bandwidth of 1 Hz. This measurement approximates the amplitude within a 1 Hz bandwidth located at each frequency bin. The actual linewidth and window function are compensated for in this calculation. This allows measurements taken with different spans or windows to be compared.

Note:

PSD measurements are typically used to measure noise or noise density. The data values are read out in Volts/√Hz or dBV/√Hz. When measuring Gaussian noise sources, the noise in bandwidths other than 1 Hz may be obtained by multiplying the reading by the square root of the desired bandwidth. This is true only for Gaussian noise.

When measuring PSD, the Display may only be set to Log Magnitude or Linear Magnitude.

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MEASURE MENU

Time Record The Time Record is the minimum amount of filtered input data required to

generate an FFT with the desired span and linewidth. The SR770 filters the input data in real time to provide a stream of data points with the correct frequency span. The time record consists of 512 of these points, of which only the first 400 are displayed.

When averaging is on, only spectra are averaged. The Time Record shows the latest time record used to calculate a spectrum.

Note:

The SR770 is not a digital oscilloscope. The Time Record always shows

filtered data and does not resemble an oscilloscope trace of the same input.

The input data filter is a complex filter yielding complex outputs. Thus, the time record has a real and imaginary part as well as phase associated with each time bin.

Octave Analysis Octave Analysis computes the spectral amplitude within 1/3 octave bands.

The analyzer computes a normal FFT, then calculates the rms sum of the frequency components within each band. When Octave Analysis is on, only the Log Magnitude may be displayed. Also, the display is always logarithmic on the X axis, displaying evenly spaced octaves. The left and right most bands are labelled on the graph by center frequency and band number. The marker reads the center frequencies of the bands rounded to the nearest even frequency. The actual band frequencies are exact according to the ANSI standard.

Note:

When octave analysis is on for either trace, the FREQ menu will display the band menu for both traces. This is because both traces must have the same span. Thus, if one trace is measuring octave analysis, the other trace's span is determined by the bands displayed in the octave analysis.

Furthermore, in order to perform 30 band analysis accurately, the SR770 must combine spectra taken with two different overlapping spans. This is because the frequency range of 30 bands requires more than 400 linearly spaced frequency points. When 30 band analysis is chosen, the analyzer alternates between two different spans. If one trace is displaying 30 band octaves, then the other trace will show spectra taken with alternating frequency spans and is not very useful. In general, when using octave analysis, only the trace showing octaves is meaningful.

Only one trace may be measuring octave analysis at a time. The other trace must be measuring spectrum, PSD or time record.

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MEASURE MENU

To choose the number of bands displayed, the starting band and the weighting function, use the FREQ menu. The FREQ menu will display the band selection menu shown at the right whenever octave analysis is on.

# Bands The # Bands key toggles the octave analysis range between 15 and 30

bands. The bands are always 1/3 octave.

Starting Band Pressing this key activates the Starting Band number entry field. The SR770

can display bands -2 through 49. The starting band can range from -2 to 49 minus the number of bands (15 or 30).

Weighting The Weighting key toggles between no weighting and A weighting. A

weighting compensates for auditory sensitivity and can provide data comparable to that derived from analog analysis equipment.

Return The Return key will return to the main MEAS menu.

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MEASURE MENU

Display

The Display sub menu allows the user to choose the displayed quantity for the active trace.

Log Mag. This key displays the magnitude of the measurement on a logarithmic scale.

Only the active trace display is affected. Both the Time Record (as defined in this analyzer) and the corresponding FFT are complex quantities. The magnitude is the square root of the product of the measurement data and its complex conjugate.

Linear Mag. This key displays the magnitude of the measurement on a linear scale. Only

the active trace display is affected. Both the Time Record (as defined in this analyzer) and the corresponding FFT are complex quantities. The magnitude is the square root of the product of the measurement data and its complex conjugate.

Real Part This key displays the real part of the measurement on a linear scale. Only the

active trace display is affected. Both the Time Record (as defined by the SR770) and the corresponding Spectrum are complex quantities and thus have a real part. PSD and Octave Analysis are not complex and only display magnitudes.

Imag. Part This key displays the imaginary part of the measurement on a linear scale.

Only the active trace display is affected. Both the Time Record (as defined by the SR770) and the corresponding Spectrum are complex quantities and thus have an imaginary part. PSD and Octave Analysis are not complex and only display magnitudes.

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MEASURE MENU

Phase This key displays the phase of the measurement on a linear scale. Only the

active trace display is affected. Both the Time Record (as defined by the SR770) and the corresponding Spectrum are complex quantities and thus have phase. PSD and Octave Analysis are not complex and only display magnitudes.

In general, phase measurements are only used when the analyzer is triggered. The phase is relative to the start of the time record.

The phase is displayed in degrees or radians on a linear scale from -180 (-π) to +180 (+π) degrees (rads). The phase may be "unwrapped" using the Calculator in the Measure menu.

The phase is calculated starting with the left most bin. If neither the real nor imaginary component of a bin is greater than 0.018% of full scale (-75 dB below f.s.), then the phase is not calculated. In this case, the phase is set to the phase of the most recent bin which exceeded 0.018% of full scale. This avoids the messy phase display associated with the noise floor. (Remember, even if a signal is small, its phase extends over the full 360 degrees.) This display method also preserves cumulative phase rotation as a function of frequency.

Return The Return key will return to the main MEAS menu.

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MEASURE MENU

Units

The Units sub menu allows the user to choose the display units for the active trace.

Volts Pk (EU PK) This key chooses units of Volts Peak or Engineering Units Peak for the active

trace.

Volts RMS (EU RMS) This key chooses units of Volts RMS or Engineering Units RMS for the active

trace.

dBV (dBEU) This key chooses units of dBVolts Peak or dBEngineering Units Peak for the

active trace.

dBVRMS (dBEURMS) This key chooses units of dBVolts RMS or dBEngineering Units RMS for the

active trace.

dB units are not available when displaying Real or Imaginary parts of the spectrum. This is because the data values may be negative.

Volts/EU This key chooses whether the fundamental unit is Volts or user defined

Engineering Units (EU). Choosing EU will activate the EU definition menu shown below.

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MEASURE MENU

EU Label Pressing this key activates the EU Label entry field. Use the ALT keys to enter

a name for the engineering units.

EU/Volt Pressing this key activates the EU scaling entry field. Enter the number of

engineering units per Volt.

Return This key returns to the Units sub menu.

When Phase is being displayed on the active trace, the Units menu appears as shown to the left. Phase values are always between -180 and +180 degrees. The analyzer does not "unwrap" phase.

Degs This key chooses degrees for the Phase display.

Rads This key chooses radians for the Phase display.

Return The Return key will return to the main MEAS menu.

Note:

The choice of units does not affect the display scaling, whether linear or logarithmic. The Marker and data readouts reflect the choice of units but the graph remains unchanged.

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MEASURE MENU

Window

The Window submenu allows the user to choose the window function. Both traces use the same window function. A trace may be recalled from disk with a window different than the "live" window. This is the only case where the window on the graph is other than the "live" window shown in this menu.

Uniform This key selects no windowing (uniform or rectangular window function) of the

time record. This window provides high amplitude accuracy only for frequencies exactly on a bin and poor frequency selectivity making it a poor choice for continuous signals. It is primarily useful for analyzing impulses and transients which are shorter than a time record.

Flattop This key selects the Flattop window. This window has the least ripple and

thus the smallest amplitude errors for frequencies not exactly on a bin. It is most useful for precise amplitude measurements.

Hanning This key selects the Hanning window. The Hanning window has a relatively

narrow mainlobe and low sidelobes providing low leakage (spectral broadening) and good selectivity.

BMH This key selects the Blackman-Harris window. This window has the narrowest

mainlobe and the fastest roll-off for the best selectivity. This window is especially useful in measurements requiring the more than 70 dB of dynamic range since it has the lowest leakage and broadening of the skirts.

Return The Return key will return to the main MEAS menu.

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MEASURE MENU

Calculator The Calculator sub menu allows the user to perform arithmetic calculations

with the trace data. Operations are performed on the entire trace, regardless of graphical expansion.

Calculations treat the data as intrinsic values, either Volts, EU or degrees (radians). If a graph is showing dB, then multiplying by 10 will raise the graph by 20 dB and dividing by 10 will lower the graph by 20 dB.

Performing a calculation on the active trace will set the File Type to Calc to indicate that the trace is not Live. This is shown by the "File=Calc" message at the lower left of the graph. The analyzer continues to run, but the calculated trace will not be updated. To return the trace to live mode, activate the trace and press the [START] key. The File Type will return to Live.

Operation The Operation function selects the type of operation to be performed. The

add, subtract, multiply, and divide functions require a second argument which

may be a number, ω (2πf), or the other trace. The log (base 10), square root,

unwrap and d/dx functions require no argument.

Unwrap attempts to display the phase as a continuous curve rather "wrapping" around at ±180° (± π rads). Phase response curves (as measured using the chirp source) are generally more meaningful unwrapped.

The derivative function, d/dx, differentiates the curve with respect to the x

axis (in frequency bins). To convert to d/dω, divide the result by 2π times the

linewidth.

Do Calc Pressing this key starts the actual calculation. The "Calculating" message

appears below the graph while calculations are in progress. The calculation

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MEASURE MENU

uses the operation specified by the Operation key and uses the argument chosen by the Argument keys.

Note that many operations will require an AutoScale to display the result on the graph.

When the operation is d/dx, a submenu will appear to select the aperture. The aperture, expressed as a percentage of the span, is the how wide an area is considered when calculating the derivative. If the curve is noisy, a wide aperture may be required to yield meaningful results.

Argument Type The Argument Type function selects between a constant argument, ω (2πf),

and a second data trace. A constant argument adds or subtracts a constant,

or multiplies or divides by a constant. Choosing ω uses the argument

2π•frequency for each frequency bin. The other graph option uses the other (inactive) trace as the argument. There is no attempt to check whether the spans are the same, or even whether the measurement data are of the same type. In this case, calculations are performed on a bin by bin basis, i.e. bin #1 of one trace is added to bin #1 of the other trace, bin #2 is added to bin #2, etc. In the case of divide, the active trace is divided by the inactive trace. A disk file may be used as one of the traces by recalling a file into one of the graphs.

If the Argument type is a constant, then the Argument and Marker to Argument functions are displayed.

Argument Pressing the Argument key activates the constant argument entry field. Use

the keypad to enter a numerical argument. Integer (-3), real (-3.0), or floating point (-0.3E+1) formats are all allowed.

Marker to Arg. The Marker to Argument will copy the data value of the marker to the

constant argument field above. This is convenient when subtracting a baseline or normalizing to a data point.

Note:

This function takes the literal marker readout as shown above the graph and copies it to the argument field. This is true even if the marker is reading in dBV rather than Volts. The calculation will use the argument as if it were Volts and result in meaningless data. Use linear units when using the Marker to Arg. function to avoid this mistake.

Return The Return key will return to the main MEAS menu.

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MEASURE MENU

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DISPLAY MENU

Display

The Display menu is used to change the graph parameters and marker type.

The settings for the active trace are displayed in this menu. Note that marker movement is activated by the [MARKER] key and not by this menu.

Format The Format key toggles between single and dual trace screen formats. The

[ACTIVE TRACE] key toggles the active trace. If a single graph is displayed, the [ACTIVE TRACE] switches the graph between Trace0 and Trace1. If two graphs are displayed, [ACTIVE TRACE] selects either the upper or lower trace as active. When the format is switched back to Single, the active trace becomes the single displayed trace.

Marker On/Off/Trk This function turns the marker on and off or selects tracking mode. This

function only affects the active trace. Each trace has its own marker. It is sometimes desirable to turn off the marker before printing the screen. When the marker is set to Track, the marker automatically seeks the maximum or minimum point of the trace (according to the Marker Seeks selection).

Marker Width This function selects the width of the marker region defined by the vertical

dashed lines on the graph. Only the marker for the active trace is affected.

Normal width is 1/2 of a division, Wide is 1 division, and Spot is a single X position on the screen (the marker is a single dashed line).

The marker region moves to the left and to the right a single bin at a time.

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DISPLAY MENU

Marker Seeks The marker searches the data points within the marker region for the

maximum or minimum data value, or calculates the mean of the region. This key toggles between Max, Min, and Mean and only affects the active trace's marker.

When seeking minimum or maximum, the marker is located at the minimum or maximum data point. This allows peaks and valleys in the data to be read easily. When seeking mean, the X position of the marker is the center of the marker region and the Y position is the mean of the data within the region.

Grid Div/Scrn This function selects no grid or 8 or 10 vertical divisions per graph. The grid is

the set of dotted lines on the graph which mark each scale division. This affects only the active trace graph.

Graph Fill The active trace can be selected to display the spectrum as the envelope of

the X values (line), or to fill the solid region below the trace (fill).

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MARKER MODE MENU

Marker Mode

The Marker Mode menu is activated with the Mode key in the Marker area of the keypad. This menu is used to manually enter a marker offset as well as searching for peaks in the data.

Linked Markers This key links the markers on the two traces. When the dual trace display

format is used, linked markers means that the two markers are always at the same location on the graph. This is true even if one of the traces is showing the time record. This is strictly a graphical function. To move the markers, activate either trace and use the [MARKER] key to move the markers with the knob.

Marker Offset This function turns on the marker offset. W hen marker offset is on, a small

delta (∆) is displayed at the beginning of the marker readout above the graph. The marker readout is now relative to the marker offset. The marker offset location on the graph is marked with a small star shaped symbol.

The [MARKER REF] key toggles the marker offset on and off as well. When the [MARKER REF] key turns on the offset, the X and Yoffsets are set to the current marker position. Pressing the [MARKER REF] key again turns the marker offset off.

X Offset This key activates the marker X Offset entry field. This is the offset of the

marker along the X axis. Only numeric entry is permitted. The X offset is stored as unitless number. When displaying spectra, the offset is interpreted as a frequency. The X offset does not have to be a frequency which is within the current span.

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MARKER MODE MENU

Y Offset This key activates the marker Y Offset entry field. This is the offset of the

marker along the Y axis. Only numeric entry is permitted. The offset is stored as a unitless number. If the display units are changed, then the Y offset needs to be changed. The Y offset does not have to be a value which is currently within the vertical span of the graph.

Next Peak Left This function moves the marker to the next peak to the left of the current

marker position.

Next Peak Right This function moves the marker to the next peak to the right of the current

marker position.

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Stanford Research Systems SR770 User’s Manual

Specifications and Main Features

Frequently Asked Questions

User Manual