Stanford Research Systems SR780, Operating Manual And Programming Reference

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FFT Frequency Spans 2-7
SR780 Network Signal Analyzer
1. The FFT resolution (number of frequency bins in the spectrum) determines the
number of points required in the time record.
2. The duration of the time record determines the frequency resolution of the spectrum
(spacing of frequency bins in the spectrum).
3. The sampling rate determines the frequency span of the spectrum (Nyquist’s
theorem). The sampling rate is the number of points in the time record divided by the
duration of the time record.
The SR780 allows FFT resolutions of 100, 200, 400 or 800 bins (not counting DC).
Changing the resolution does not change the span, instead the time record length is
changed.
The various FFT resolutions are summarized below.
FFT Frequency
Resolution Resolution Time Record
100 bins
Span/100 100/Span
200 bins Span/200 200/Span
400 bins Span/400 400/Span
800 bins Span/800 800/Span
Starting the Span Above DC
Using digital filtering alone requires that every span start at DC. Frequency shifting is
accomplished by heterodyning. Heterodyning is the process of multiplying the incoming
signal by a sine (and cosine) wave. The resulting spectrum is shifted by the heterodyne
frequency. If we incorporate heterodyning with our digital filtering, we can shift any
frequency span so that it starts at DC. The resulting FFT yields a spectrum offset by the
heterodyne frequency. When this spectrum is displayed, the frequencies of the X axis are
the original frequencies of the signal.
Heterodyning allows the analyzer to compute zoomed spectra (spans that start at
frequencies other than DC). The digital processor must filter and heterodyne the input in
real time to provide the appropriate filtered and down-sampled time record at all spans
and center frequencies.