Stanford Research Systems SR780, Operating Manual And Programming Reference

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Swept Sine Measurements 2-43
SR780 Network Signal Analyzer
The signal connections are the same as for an FFT transfer function. The SR780 measures
Channel 2 response divided by Channel 1 as the transfer function. The input to the device
under test is measured by Channel 1 (typically this is the source output) and the device
output is measured by Channel 2.
Swept sine displays differ from FFT displays. The sweep span is not limited to factors of
two and the start and stop frequencies are not related to an FFT span. Also, the number of
points can range from 10 to 2047. Y axis scaling and X axis zooming are the same as
with FFT displays.
Each data point is graphed at its correct frequency and amplitude and connected to
adjacent points with a line. The marker only moves to the actual sweep points and the
Marker Position Bar shows the actual frequency of each point. If the marker position is
displayed with a ‘?’, the point is interpolated from nearby measured points. The actual
measurement of these points was skipped due to sweep Auto Resolution.
While a sweep is in progress, a small triangular marker moves across the bottom of the
display to show the current sweep point. The current frequency is displayed at the top of
the screen.
Swept Sine Measurements
Only Spectrum, Cross Spectrum, Transfer Function and User Function measurements are
available. Time record, capture and waterfall are not available in this measurement group.
Spectrum
The swept sine spectrum is simply the measurement of a single channel over a sweep.
The spectrum is complex (it contains phase and amplitude information). The phase is
relative to the source and is stable but arbitrary. Single channel phase is not generally
meaningful.
The spectrum measures the actual signal level at the inputs. If Source Auto Level is On,
then the spectrum will tend to be constant. Use Transfer Function to remove the effects of
a changing source level.
Cross Spectrum
The swept sine cross spectrum is a two channel measurement defined as
Cross Spectrum = conj( Spec1 ) • Spec2
The cross spectrum contains both magnitude and phase information. The phase is the
relative phase (at each frequency) between the two channels.
The magnitude is simply the product of the magnitudes of each spectrum. Frequencies
where signal is present in both spectra will have large components in the cross spectrum.
Transfer Function
The swept sine transfer function (sometimes called frequency response) is a two channel
measurement defined as
Transfer Function = Spec2 / Spec1