Stanford Research Systems SR785, Operating Manual And Programming Reference

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Measuring a Frequency Response Function 1-11
SR785 Dynamic Signal Analyzer
Measuring a Frequency Response Function
This example investigates the frequency response of the test filter (enclosed with this manual) using FFT
measurements. You will use the SR785 source to provide a broad band chirp and both input channels to
measure the input to and output from the device under test.
1. Press [System]
Display the System menu.
Press <Preset> Preset returns the unit to its default settings.
Press [Enter] to confirm Preset. Preset requires confirmation to prevent accidental
reset. Wait until the self tests are completed.
2. Use a BNC Tee to connect the Source Output to
the filter input and the Ch1 A Input.
Connect the filter output to the Ch2 A Input.
Frequency Response is defined as Ch2 response
divided by the Ch1 reference. Thus, Ch1 monitors
the filter input (source output) and Ch2 measures
the response of the device under test.
3. Press [Source]
Select the Source menu.
Press <Chirp> Choose Chirp output. The output is an equal
amplitude sine wave at each frequency bin of the
FFT spectrum.
Press <On> Turn the source On.
Press [Window] Select the Window menu.
Press <Window> Adjust the FFT Window function.
Select (Uniform) with the knob and press
[Enter]. The Chirp source requires the use of the Uniform
window since not all chirp frequency components
are present at all points in the time record. The
chirp is exactly periodic with the FFT time record
and does not ‘leak’ with the uniform window.
4. Press [Auto Range Ch1]
Press [Auto Range Ch2]
Let the analyzer automatically set the Input Ranges
to agree with the signals. Note that the Input Range
readouts at the top of the screen are displayed in
inverse when Auto Range is on.
5. Press [Freq]
Select the Frequency menu.
Press <Span> Adjust the FFT Span.