Stanford Research Systems SR785, Operating Manual And Programming Reference

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4-28 Display Setup Menu
SR785 Dynamic Signal Analyzer
Orbit
The Orbit measurement is a two channel measurement whose real part is the real part of
Time Record Ch1 and whose imaginary part is the real part of Time Record Ch2. Orbit is
normally displayed with the Nyquist View (Time2 vertical vs Time1 horizontal).
For baseband spans, the time records are entirely real and the Nyquist view of the Orbit
measurement is a Lissajous figure.
Cross Spectrum
The cross spectrum (sometimes called cross power spectrum) is a two channel
measurement defined as
No Average
Cross Spectrum = FFT1* • FFT2
Vector Average
Cross Spectrum = <FFT1*> • <FFT2>
RMS Average
Cross Spectrum = <FFT1* • FFT2>
Peak Hold Average
Cross Spectrum = MAX(FFT2)* •
√
<FFT1*• FFT1>
The cross spectrum contains both magnitude and phase information. The phase is the
relative phase (at each frequency) between the two channels. Vector averaging can be
used to eliminate signals which do not have a constant phase relationship between the
two channels. In this case, triggering is not required for vector averaging.
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.
Frequency Response
The Frequency Response (sometimes called transfer function) is a two channel
measurement which ratios the spectrum of Ch 2 to the spectrum of Ch 1. Frequency
Response measures the response of a network or device under test . The reference
channel (1) measures the signal at the input to the device and the response channel (2)
measures the device output. The result is the complex Frequency Response of the device.
The definition of frequency response depends on the type of averaging which is
displayed.
No Average
Freq. Response = FFT2 / FFT1
Vector Average
Freq. Response = <FFT2> / <FFT1>
RMS Average
Freq. Response = (RMSAvg cross spectrum)/power spectrum 1