Frequency Response and Stimulus Signal

The FRF expresses the complex ratio of output-to-input in the frequency domain and fully characterizes linear, time-invariant systems. The magnitude and phase of the FRF characterize the frequency response. The magnitude is equivalent to the gain, or amplitude ratio, expressed as a function of frequency. The phase is equivalent to the relative delay, or phase shift, of each frequency component through the DUT.

Frequency response computes the transfer function of the response signal to the stimulus signal. You usually use the coherence to validate the frequency response results. The coherence quantifies the portion of the response signal that is linearly dependent on, or coherent with, the stimulus signal. The coherence ranges from 0 to 1. A coherence value of 1 indicates a perfect coherence.

Use the complex FRF, either real and imaginary parts or magnitude and phase components, to characterize the dynamic response of the DUT. Use the magnitude curve of the FRF to indicate the amplitude accuracy of the DUT. When the DUT reproduces the stimulus signal with no attenuation of any particular frequency band, the DUT has a flat magnitude curve.

Performing FRF measurements requires a signal source. Valid stimulus signals are pure tones, frequency sweeps, and broadband signals.