Coherence (Trace Data)

(Not available with all measurement types)

Coherence 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.

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

Coherence 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.

Coherence 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.

The VSA calculates coherence as follows:

$image\coherence_wmf.jpg$