Coherence (Channel Quality)
is a two-channel measurement that indicates the similarity between two signals. In transfer functions, it indicates how much of the output power is coherent with the input power. In other words, coherence is a measure of the power in the output signal caused by the input.
The Channel Quality Coherence and Coherence Unbalanced traces are only computed when Computation Mode = Statistical.
is scaled from 0.0 (complete incoherence) to 1.0 (unity, or perfect coherence). A coherence of 1 indicates all the output power is caused by the input. A coherence of 0 indicates none of the output power is caused by the input.
Thus, coherence less than unity indicates the presence of external extraneous noise, system nonlinearities, or unexpected input signals.
There are two Coherence traces. One is computed by the Channel Quality algorithm (Channel Quality category, description above) and the other is a blind computation (no tracking/frequency error estimation/etc.) using two channels (Cross Channel trace category, description below). It is recommended to use the Coherence traces in the Channel Quality category.
The Cross Channel can be calculated only when averaging is on, there are at least 2 averages, and the average type is RMS (video). If averaging is off or the wrong average type is selected, the trace is blank. If only 1 average is specified, Coherence always 1.0 for all frequencies.
The VSA calculates coherence as follows:
