Stanford Research Systems SR780, Operating Manual And Programming Reference

Page: 202 / 497

4-20 Display Setup Menu
SR780 Network Signal Analyzer
Each Measurement has an associated View. Changing the Measurement changes the
View to the View last used with the new Measurement.
The two displays generally have different Measurements. This entry field can be linked
to both displays using the [Link] key. Choosing a two channel measurement (Transfer
Function, Cross Spectrum, Cross Correlation Or Coherence) links the frequency
parameters of the two displays.
Command: MEAS(?) d {, i}
The following Measurements are available in the FFT Measurement Group:
FFT
The FFT spectrum is the basic measurement of an FFT analyzer. It is simply the FFT of a
time record. The spectrum is a complex quantity (it contains phase as well as amplitude
information). This is sometimes referred to as the linear spectrum.
The phase of the spectrum is meaningful only if the time record is triggered with a fixed
relationship to the input signal. If the signal is periodic as well (the signal and trigger
repeat), then vector averaging can be used to reduce the noise level of the spectrum. The
vector averaged spectrum is still a complex quantity.
The rms averaged spectrum is sometimes called the power spectrum. The power spectrum
gives a stable (and usually more accurate measure) of the rms signal amplitudes as well
as the rms noise levels.
Time Record
A time record is simply a sequence of data samples. The length of the time record is the
FFT resolution/span.
For baseband spans (spans which start at DC), the time record resembles a digital
oscilloscope display. Signals at frequencies above the span have been filtered out.
Baseband time records are entirely real, they have no imaginary part.
For zoomed spans (spans which start above DC), the time record does NOT resemble the
original data. The data has been frequency shifted. Signals at the center of the span
appear at DC while frequencies at both edges of the span appear as high frequencies.
Zoomed time records are complex, they have both a real and an imaginary part. The
sampling rate is always half of the equivalent baseband span.
Remember, the time record 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 is
preserved by the Nyquist sampling theorem as long as the value of each sample is
accurate. A triggered time record will always jitter by 1 sample. This jitter is removed in
the computation of the phase of the spectrum relative to the trigger.
Averaging does not affect the time record. Averaging is performed on the FFT spectrum
and not on the time data.