Stanford Research Systems SR785, Operating Manual And Programming Reference

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2-20 FFT Measurements
SR785 Dynamic Signal Analyzer
To measure from a region of the buffer, set the Playback Start and Length in the [Input]
menu. During playback, a marker at the bottom of the graph indicates the current time
record position within the buffer.
The capture data is filtered and down-sampled according to the capture Sample Rate.
Only baseband data (bandwidth starts at DC) are captured. The capture buffer resembles
a digital oscilloscope display. Signals at frequencies above the sample rate/2.56 have
been filtered out.
The capture buffer is not a continuous representation of the input signal. The data is
sampled and has a time resolution of 1/(sample rate). High frequency signals will appear
distorted in the time record. However, ALL of the spectral information (up to the
sampling rate/2.56) is preserved by the Nyquist theorem as long as the value of each
sample is accurate.
Amplitude calibration is performed in the frequency domain. Hence, the captured time
data amplitudes are not calibrated.
User Function
User Function displays the results of a user defined math function. User Functions
defined within the FFT Measurement Group may include FFT measurements. Use the
[User Math] menu to define a math function.
A User Function may not be selected as the measurement if it uses a Trace which is
empty or which contains data taken in a different Measurement Group.
See ‘User Math’ later in this section for more.