Stanford Research Systems SR785, Operating Manual And Programming Reference

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Capture Buffer 2-35
SR785 Dynamic Signal Analyzer
Normal Playback Speed plays back the capture buffer in real time, i.e. 1 second of
capture takes 1 second to play back. Normal Playback Speed looks like the ‘live’ real
time measurement. When the playback is in Octave Group, playback is always Normal
Speed.
When playback is in FFT or Time/Histogram Group, the Playback Speed can be either
Normal or Every Time Record.
Normal Playback Speed plays back the capture buffer in real time, i.e. 1 second of
capture takes 1 second to playback. Normal Speed playback looks like the ‘live’ real time
measurement. Normal playback is limited to the real time limitations of the equivalent
real time analog input measurement.
Not all time records are displayed during Normal playback though all time records
contribute to averaged measurements. For example, 1 second of capture contains 256 full
span FFT time records. Normal Speed playback at full span takes 1 second and updates
the display only about 8 times during playback.
Every Time Record playback displays the measurement result for every captured time
record. Since the data is stored in memory, Every Time Record playback is not limited
by real time considerations. For example, 1 second of capture can contain 256 full span
FFT time records. In this case, all 256 time records are measured AND displayed. The
display still updates at 8 Hz so playback takes about 32 seconds to complete. If the time
records are overlapped, there may be more than 256 measurements to display and
playback will take even longer.
When the measurement time records are very long (narrow spans), the time to process
and display each measurement is much less than the real time record length. Since the
data is already available in the capture buffer, Normal Speed playback means waiting
unnecessarily for a real time record to elapse between updates. In this case, Every Time
Record playback displays the measurements of all captured time records much faster
than Normal Speed real time playback.
When the playback is in Octave Group, playback is always Normal Speed .
Capture as the Arbitrary Source
The contents of the capture buffer can be used as the arbitrary source. This allows a
captured waveform to be output as the source. The output sample rate should be the same
as the capture sample rate to maintain the correct signal frequencies. If the output sample
rate is less than the maximum sampling rate (262 or 256 kHz), then the source output
will contain aliases. In this case, be sure to limit the measurement bandwidth to the
sampling rate/2.56 to eliminate the effect of these aliases on the measurement.
The amplitude of the arbitrary output depends upon the amplitude of the captured data
relative to the Input Range during capture. If the captured data was 100% of the Input
Range, then an output amplitude of 100% will be 1 Vpk.