How do I get the correctly scaled amplitude of a power spectrum?

Question

I am taking data off of a afm probe with a resonant frequency.  I want to determine the amplitude of the signal at the resonant frequency.  I've tried using AutoPowerSpectrum (FFT(x)FFT*(x)/n^2) and just plain FFT, but neither are scaled correctly when I input a sine wave at a given amplitude and frequency.  What do I need to do to scale my spectrum, so that I can tell the actual amplitude of the signal at the resonant frequency?

Answer 1

The Help topic for the SpectrumUnitConversion Method describes ENBW as, “equivalent noise bandwidth of the selected window. You can use this value to compute the power in a given frequency span.” CoherentGain is defined as, ”peak gain of the window relative to the peak gain of the Rectangular window. Use this value to normalize peak signal gains to that of the Rectangular window.”
The Help topic also says, “The ENBW and CoherentGain parameters are output by the ScaledWindow function. You need this input only when you use the spectral density output formats (the last four DisplayUnit selections).” In other words, you only need to use these inputs if you have used the ScaledWindow method and are setting the DisplayUnit to 4, 5, 6, or 7. From what you and Caroline have discussed, I assume you will be setting the DisplayUnit parameter to 1.
Regards,
Eric