[time-nuts] windows for FFT measurements of phase noise

Bob Camp kb8tq at n1k.org
Sat Jun 11 13:18:19 EDT 2016


> On Jun 11, 2016, at 11:42 AM, jimlux <jimlux at earthlink.net> wrote:
> On 6/10/16 4:09 PM, John Miles wrote:
>>> What sort of windows do folks use for making FFT measurements of
>>> phase noise.
>>> Say you have 1 second of sampled data (so the FFT resolution is 1
>>> Hz). If you're interested in the noise power at, say, 10 Hz away,
>>> a rectangular window isn't going to be very far down, unless you
>>> have a LOT of points in the FFT.
>>> Grove's paper from 2004 doesn't mention this detail.
>> As Bob suggests, a multisegment FFT chain is the usual approach.  By
>> the time you're displaying noise down to 10 Hz, you should have quite
>> a bit more than 1 second worth of data to draw from.
>> For measuring noise the choice of window function doesn't matter very
>> much as long as you correct for the noise bandwidth of the function
>> you use.
> 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.

Ummm ….. errrr … not so much.

If you are looking at phase noise, you are doing it with a system that has already taken the carrier out of the picture. 
Either you quadrature lock two oscillators (the 3048 approach) or you do an SDR to DC (TimePod) approach. 
The only thing you have to handle is the noise slope in the region you are working in. 


> WIndows like uniform and Hamming are probably only down 50 dB that far out.
> I did find a reference to some Blackman-Harris windows that are pretty wide for the main lobe, but the sidelobes are 100 or 150 dB down.
> However, for spur detection there are major
>> window-dependent differences that need to be considered.  There is
>> only one reference that's worth looking at, and that's the paper by
>> Heinzel, Ruediger, and Schilling.  (Google the authors' names and
>> it'll come up.)
> That's a real nice report.. Excellent
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