[time-nuts] Fast frequency counting question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun May 4 20:27:23 EDT 2008

John Miles wrote:
> The last time I looked at those eval boards, though, they were pretty
> adamant about not releasing API specs, so you couldn't use them as platforms
> for your own DAQ applications.  Irritating, but I can see why, because
> they're probably just breaking even on the boards.
As long as you can get at the raw data the API is irrelevant.
> Would be nice if that's no longer the case.  Failing that, one of the GNU
> Radio USRP boards might be the most economical way to go.
Only if they have sampling jitter of a few tens of femtosec.
> Also: the effect to be studied must be coming from some type of circuit
> built with some standard semiconductor process, which in turn is driven by
> some sort of clock.  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 
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.


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