13.9 FFT Analysis Modes
246
Waveform Example
Coherence Function
This function gives a measure of the correlation (coherence) between input and output signals. Val-
ues obtained are between 0 and 1.
Main uses:
• To evaluate transfer functions
• In a system with multiple inputs, to inspect the effect of each input on the output
See: "13.9.2 Analysis Mode Functions" ( p.254)
Axis Display Type Description
X axis
Linear Frequency is displayed with equal spacing
Log Frequency display of logarithm interval
Y axis Lin-Mag
Displays the causal relationship and degree of relationship between two input
signals, as a value between 0 and 1 (dimensionless units).
Normal display
X axis: Log
Y axis: Lin-Mag
• With a single measurement, the coherence function gives a value of one for all
frequencies. Spectrum (frequency-domain) averaging should always be per-
formed before measurement (analysis is not available with time-domain aver-
aging).
• The coherence function has two general definition formulas. For the definition
formulas, see"13.9.2 Analysis Mode Functions" ( p.254).
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