Cross Spectrum (Trace Data)

Cross Spectrum trace data (also called cross power spectrum) is a two-channel measurement. Cross spectrum is seldom used as a separate measurement, but is used internally by the VSA to compute other measurements such as transfer function and Coherence.

This trace data is available only when more than a single input channel is selected.

The cross spectrum, Gxy, is defined as taking the FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. of two signals separately and multiplying the results together, as follows:

$image\Gxy_wmf.jpg$

The exact computation used by the VSA depends on the type of average used:

If the average is:	the VSA uses this formula:
None	A
Time	A
Time exponential	A
rms	B
rms exponential	C
peak hold/continuous peak hold	cross spec. unavailable

Term	Definition
L	number of averages
linspec,N	linear spectrum channel N
xavg	(1/L) * new + ((L-1)/L)* old
conj	conjugation