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

Richard (Rick) Karlquist richard at karlquist.com
Fri Jun 10 20:15:22 EDT 2016

I remember cobbling together a home brew phase noise system 35
years ago using an HP3582 FFT analyzer.  It had several available
windows:  IIRC: flat top, rectangular, and hamming (hanning?  I can 
never get them straight).  Basically, if you are looking for spurs, you
need to use the flat top.  If you are looking for noise, you use the
hamming, or whatever it was.  For most practical frequency sources,
the slope of the phase noise is gradual enough that the windowing
isn't a big issue.  If you think it is an issue in your case, as you
say, you can use a lot more points and see if the answer changes.
This is similar to decreasing the minimum time step in SPICE.

I am now putting together a modern homebrew phase noise system.
I am planning to use a ZRPD-1 phase detector driven by AMC-123
amplifiers.  The phase detector will drive a SpectraDAQ-200 digitizer
with SpectraPlus-RT FFT software.  I am not currently planning to
put a preamp between the phase detector and digitizer, as it
isn't needed for the particular measurements I am planning to make.
However, if you have a candidate for this preamp, chime in anyway.
I'm kicking around possible calibration modes.  I am thinking of
inserting a passive phase modulator.  It needs to be able to revert
to a mode where it contributes no phase noise of its own.  The
digitizer shown is the cheapest digitizer with believable performance.
The really cheap ones don't even use BNC connectors; they use
screw terminals :-(.  Much more expensive ones have bandwidth I
don't need, but fewer bits, that I do need.

I would appreciate any comments pro or con about this configuration.  I 
haven't bought any of this stuff yet, so if you have a better idea, fire 

I did a little looking on Ebay for bargains, but only no-name stuff
was cheap, and it was always "we don't have any way to test it".
Name brand stuff cost real money (or was "call for quote"; you know
that is always going to be expensive).

Does anyone know of a used equipment dealer in Silicon Valley?

Rick N6RK

On 6/10/2016 12:42 PM, jimlux 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.
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