[time-nuts] windows for FFT measurements of phase noise
john at miles.io
Sat Jun 11 14:57:16 EDT 2016
> I was thinking more of the sidelobes: if you're looking at a quiet
> oscillator (e.g. -140dBc @ 100Hz) , with a 1 second epoch, and you want
> to measure the noise at, say, 100Hz out, the window function needs to be
> down 140 dB at that bin.
> WIndows like uniform and Hamming are probably only down 50 dB that far out.
The segmented FFT helps with that. Ideally you have enough segments that there's rarely more than 30-40 dB of flatness variation within any one of them, which is why HFT95 works well and Hann is still usable.
Except in the presence of very strong spurs, most of the energy in the narrowband segments is going to reside in the first few bins. It's the HPF prior to each FFT stage that keeps that close-in noise from spreading, more than the choice of window function.
-- john, KE5FX
Miles Design LLC
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