Peak Magnitude of Nonuniform Waveform Reported by Spectral Measurements Express VI is Lower than Maximum Amplitude

Updated Feb 9, 2021

Reported In

Software

  • LabVIEW

Issue Details

I'm using the Spectral Measurements Express VI to determine the Peak Magnitude of a signal without a fixed frequency, like a chirp signal. However, the reported peak magnitude is lower than the specified amplitude for my signal. This doesn't happen when I use a signal with a fixed frequency, like a sine wave. Why is this?

Solution

This lower output is actually expected, and is due to the construction of the non-fixed frequency signal and how FFTs determine peak magnitude. In order for the peak magnitude of a signal reported by an FFT to be equal to the maximum amplitude of the signal, there must be one cycle for each signal frequency component - signals with non-fixed frequencies vary with time, meaning that this cycle also varies, resulting in multiple peak magnitudes found that are lower than the maximum amplitude for the signal.

Additional Information

Information on chirp signals and how these values are calculated can be found here.

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