Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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2-18 FFT Measurements
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>
In these definitions the brackets <> represent averaging the real and imaginary part of the
enclosed quantity either linearly or exponentially as described below. 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 may not be required for vector averaging.
The magnitude is simply the product of the magnitudes of each spectrum. Frequencies
where signals are 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.
A broadband source (such as chirp or noise) should be used to measure frequency
response.
The definition of frequency response depends on the type of averaging which is
displayed.
No Average
Freq. Response = FFT2 / FFT1