Operating instructions

University of Saskatchewan
Electrical Engineering Laboratory Equipment Manual
SR770 FFT Network Analyzer
( Standford Research Systems, 2000 )
A FFT spectrum analyzer takes a time domain input signal, samples it and
digitalizes it ( analog/digital converter ). The digital data samples ( time record ) are
converted to the frequency spectrum using the fast fourier transform. The maximum
frequency span of the SR770 is 0Hz to 100KHz. This FFT processor displays 400
frequency points (bins). It uses a 512 point time record. At full span (0 to 100KHz), the
bins are 250 Hz apart. The bins are at 250Hz, 500Hz and so on. The resolution or
linewidth is 250Hz. At a frequency span of 50KHz the bins are 125Hz apart. The
bandwidth ( frequency span ) is controlled by digitally filtering the data samples. The
digital filter is only capable of base-band measurements ( frequency spans starting at
DC ). Heterodyning allows the span to start at frequencies other than base-band.
Heterodyning multiplies the signal by a sine wave. The frequency span is shifted by the
frequency of the sine wave. This shifts the frequency span so that it starts at DC. This
allows the analyzer to display spans which start at frequencies other than DC. Spectral
components that have a negative magnitude are folded around zero and appear as a
positive magnitude.
The time record, which can be displayed by the measure menu, is not a true
representation of the input signal. It is the input to the FFT processor. The time record
has been digitally filtered and maybe heterodyned before the FFT processor receives it.
Real time bandwidth is the largest frequency span whose time record does not exceed