Re: Jitter measurement

On Fri, 20 Jan 2006 15:06:53 +0000 (UTC), kensmith@xxxxxxxxxxxxxxx (Ken Smith)
wrote:

>In article <cpn0t192v61ne7im7a829m43m81r0kuccj@xxxxxxx>,
>budgie <me@xxxxxxxxxxx> wrote:
>>Well, it's probably jitter and "wobble" as well ....
>>
>>I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference
>>source being a GPS receiver which outputs 10k. This all works very well, but I
>>was hoping to somehow quantify the residual phase variation, and without
>>requiring particularly elaborate gear.
>
>Can you make a second system?
>
>In theory, the 10k from two different GPSes should be the same frequency.
>The scope display of both outputs should have no slip action so you can
>blow up the timescale to see the jitter.

What is unknown is how the 10kHz signal is derived. It is possible that the
pulse count is 10,000 per sec but the duty cycle varies, or a number of other
possibilities.. The data *** isn't revealing, and the manufacturer doesn't
want to know legacy receivers, so there is no detail on the derivation. (Having
seen how the 50/60Hz was derived from colour-burst crystals in the old Fairchild
5369 devices, I am leery of making assumptions In that family, M cycles were
counted in the high state, and N in the low state, with M<>N. IIRC they also
varied M across one output cycle to achieve "proper" division - the 5369EYRN
produced 50Hz from a 3.579545 crystal by this form of "fudged" non-integer
division.)

For this reason, I see it as conceivable that two devices may show jitter but
still deliver zero slip.

That is why I used the OCXO from the frequency counter timebase in a CRO
comparison - the jitter or variation in the counter timebase would certainly be
independent of the GPS-derived oscillator's variation. That disclosed no
discernible perturbation to a slow but steady slip, indicating that neither was
particularly bad.

What I am trying to achieve is some improvements in the area of the loop filter
and the oscillator itself. With a methodology for quantifying the variation,
this becomes a more scientific process.
.

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