Using some real world measurement data that has an

underlying comb spectrum with specgram, I have noticed an issue with the amplitude

of narrowband signals being affected by the number of FFT points used (NTTF).

For areas where there is no signal (in this case near 0 Hz) I see the noise

floor (level) drop by about 3 dB as I double the value of NTTF, which is

expected. A narrower “resolution” bandwidth will provide lower

broadband noise amplitude. However, the signal has a comb spectrum

present, and as I increase the value of NTTF the amplitude values of each component

of the comb spectrum increases by about 3 dB when I double NFFT. I

would expect that the narrowband levels would remain the same and not increase.

I expect that the underlying FFT function in specgram scales by 1/NFFT,

or maybe it does not (the FFT function in MATLAB does not included the 1/NFFT

scaling). To get a “true” amplitude for each frequency bin

does the output of specgram need to be scaled by 1/NFFT (or 1/NFFT*1/NFFT since

it’s a power spectrum)?

Lee