Stanford Research Systems SR865A, Operation Manual

Page: 143 / 221

Chapter 4 Programming 125
SR865A DSP Lock-in Amplifier
FFT Screen Commands
The SR865A has a basic FFT function accessable from the front panel by selecting the appropriate
[Screen Layout] display (remotely, by setting
DLAY FFT or DLAY BARFFT ). A more complete discussion
of FFT mode can be found in Appendix B .
Note that while FFT configuration commands and queries may be sent at any time, the FFT data query
(
FCRY? ) only returns valid data while the display is showing the FFT.
FFTR(?) { ADC | MIXer | FILTer | i }
The FFTR i command sets the source for the FFT to input ADC (i=0), post mixer PSD
(i=1) or post time constant filter (i=2).
Example
FFTR FILT Set the FFT source to post time constant filter
FFTR 2
FFTR?
Returns the FFT source i
FFTS(?) { i }
The
FFTS i
command sets the FFT vertical scale (dB/div) according to the table below.
i dB/div i dB/div
-8 2 m 0 1.0
-7 5m 1 2.0
-6 10 m 2 5.0
-5 20 m 3 10
-4 50 m 4 20
-3 100 m 5 50
-2 200 m 6 100
-1 500 m 7 200
The argument i is limited to −20 ≤ i ≤ 20. Vertical scale changes preserve the amplitude
of the top edge of the graph.
Example
FFTS 4 Set the FFT vertical scale to 20 dB/div
FFTS?
Returns the FFT vertical scale i
FFTO(?) { x }
The FFTO x command sets the FFT vertical offset x (dB). This is the amplitude
associated with the top edge of the graph.
Example
FFTO -20.0 Set the FFT vertical offset to
−
20.0 dB
FFTO?
Returns the FFT vertical offset