[time-nuts] Spec An for phase noise measurements

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 21 17:50:31 EST 2008

David I. Emery wrote:
> 	Spectrum analyzer front ends often either have a blocking
> capacitor (to protect the mixer from DC) or don't.    The kind that
> don't usually start to roll off pretty significantly below 10 KHz, and
> are typically spec'd only to 9 KHz.   I suppose if one wants to live
> REALLY dangerously the cap  might be removable in a few cases (NOT
> 	But would I be too simple minded to suggest that maybe some form
> of A/D PC/workstation input device with high dynamic range and decent
> sample rate (certainly available in high end audio stuff to 192 KHz)
> would be the logical vehicle for close in measurement in a quadrature
> locked PLL type phase noise setup ?   Otherwise why would you care about
> performance below 9 KHz ?
> 	Why exactly does one need a wideband SA that goes down to 100 Hz
> (common spec) or 30 Hz (nicer and newer...) for this ?   Maybe I am missing
> something here.... (probably am, I often do...)
If one is measuring the phase noise of a good OCXO with a frequency of
less than 100MHz, there isnt usually much of interest above 10-100KHz.
However when measuring the phase noise of a VCO there may be significant
phase noise out to several MHz (limit depends on the VCO frequency phase
noise measurements at offsets above half the source frequency are
problematic.). The range of interest is even wider with microwave
sources, so the answer depends on what sources you are trying to measure.
For phase noise measurement of most OCXOs a soundcard system will
suffice and with accurate calibration a modern sound card will have
better performance over the [20Hz, 20kHz] range than most surplus
spectrum analysers. It is particularly important in the flicker phase
noise region that the noise bandwidth of the filter be significantly
less than the offset frequency. It is also important that a filter with
steep skirts be used for accurate measurement in this offset frequency
region where the phase noise spectrum is far from flat. These
requirements are easier to meet with an FFT filter than to implement
such a filter in hardware.

> 	And should one want an actual SA for these measurements instead
> of a soundcardish thing and FFT software, I know that LF/HF boxes with
> better specs than any of the affordable general coverage SA families on
> Ebay show up regularly for not too much money.
Thus for characterising the phase noise of OCXOs like an HP10811,
FTS1200 etc there is a good case for using a sound card based system.
Since sound cards have stereo inputs implementing a correlation phase
noise measurement system is also possible.


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