[time-nuts] Fast frequency counting question

Didier Juges didier at cox.net
Mon May 5 20:43:57 EDT 2008


> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Bruce Griffiths
> Sent: Sunday, May 04, 2008 7:27 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Fast frequency counting question
> John Miles wrote:
> > If you go nuts with sampling 
> > precision, aren't you just going to get more information than the 
> > underlying process is capable of generating?  (I'm probably not 
> > phrasing that very well, but you get the idea, hopefully.)
> >
> >   
> No, the device is an oscillator, everything of interest can 
> be obtained from the times at which zero crossings occur.
> In effect one is measuring the phase deviations of the 
> oscillator during startup.
> With a suitable measurement bandwidth, a measurement system 
> noise of a few tens of femtosec or less is possible.
> In most cases the oscillator zero crossing jitter will be 
> somewhat larger.
> The real problem lies in verifying the measurement system noise level.
> All one is attempting is to extend the phase noise 
> measurement range down to averaging times of a few tens of microsec.
> Bruce

I think the potential problem John is pointing out is that if the
performance down to that level is driven by process variations rather than
by circuit design (and therefore assumed to be randomly variable from unit
to unit), how useful is it going to be to measure the heck out of one or a
few unit if you cannot statistically extrapolate the results to the entire
population and be able to predict yield?

Didier KO4BB

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