Stanford Research Systems SR785, Operating Manual And Programming Reference

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4-144 Capture Menu
SR785 Dynamic Signal Analyzer
Command: CMOD (?) {i}
Capture Length
Set the Capture Length in 2 kPoint (2048 points) increments.
Memory is allocated in blocks. Each block is 2 kPoints. Thus 500 blocks of capture
memory allows a maximum Capture Length of 1000 kPoints for a single input and a
maximum Capture Length of 500 kPoints for two inputs.
The Capture Length can be less than the allocated memory.
Command: CLEN (?) {i}
Sampling Rate
Select the capture Sampling Rate. The Sampling Rate can be 1, 1/2, 1/4, 1/8, ... times the
maximum sampling rate.
The inputs are always digitized at the maximum sampling rate. The maximum sampling
rate is 262.1 kHz when the FFT Base Frequency is 102.4 kHz and 256 kHz when the
FFT Base Frequency is 100.0 kHz OR the Measurement Group is Octave.
If the selected capture Sampling Rate is the maximum rate, then the digitized input data
is stored in the capture buffer. If the Sampling Rate is less than the maximum rate, the
input data is filtered and downsampled to the desired rate and then stored in the capture
buffer. This allows the capture buffer to hold a longer time period of data at a lower
bandwidth. The captured data represents a frequency span from 0 Hz to 1/2.56 times the
Sampling Rate. Capturing at less than the maximum sampling rate restricts the playback
measurement span to 1/2.56 times the sampling rate.
In the order measurement group, the sampling rate cannot be directly set by the user.
Instead the analyzer calculates the sampling rate based on the Max Order and Delta
Order. Based on the computed sample rate and the capture length, the analyzer will
display the length of the capture buffer in seconds.
Command: CRAT (?) {i}