[time-nuts] Notes on tight-PLL performance versus TSC 5120A

Bob Q quenbob5 at pacbell.net
Thu Jun 3 21:19:29 UTC 2010

Maybe a pay per view event is in order.
Bob Q.

From: "John Miles" <jmiles at pop.net>
Sent: Thursday, June 03, 2010 2:02 PM
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Subject: Re: [time-nuts] Notes on tight-PLL performance versus TSC 5120A

>> I have already commented on this in another thread but to reiterate.
>> The test that John performed that for a range of Tau that was possible
>> to be calculated for the given measurement period, both methods
>> produced the same results for each and every value of Tau, not for a
>> single value of Tau.
> Keep in mind, though, that tau is only one axis.  I had three reasons for
> using a noisy HP 5062C cesium standard in the medium-term test
> ( http://www.thegleam.com/ke5fx/tpll/small/5062c.gif ):
> 1) To compare the instruments across a wide range of timescales from
> short-term to medium-term, watching for signs of accumulated error that 
> the
> software might exhibit in records >> 100K points;
> 2) To observe a familiar, repeatable source over multiple runs, looking 
> for
> general cases where one instrument might report different statistics 
> versus
> the other; and
> 3) To look at several different noise *slopes*, looking for specific cases
> where filtering or integration might cause the instruments to respond
> differently depending on the spectral characteristics of the noise in a
> given tau region.
> Bruce's concern is largely that of (3), whereas Warren historically has 
> been
> satisfied when condition (2) has been met.  Both criteria are important. 
> If
> you look back at the first test with only the two crystal oscillators:
> http://www.thegleam.com/ke5fx/tpll/small/10hz_with_resid.gif
> ... you'll see that while the blue and magenta traces agree pretty well,
> they are also almost flat, with a slight rise at longer timescales.  This 
> is
> characteristic of flicker frequency and random-walk frequency noise. 
> These
> are only two of roughly 5 different noise slopes that are likely to be 
> seen
> in various oscillators, so this plot by itself is useful mainly as a quick
> sanity check.
> The 5062C plot, though, has a characteristic 'hump' that you see in cesium
> and GPS sources, which is almost the opposite of the 'trough' you tend to
> see from undisciplined crystal oscillators.  This means the two plots are
> complementary to some extent: various noise characteristics that don't
> appear in crystal oscillators are present in the 5062C plot to some 
> extent.
> It is meaningful, I think, that the two instruments still exhibit good
> agreement.
> If you zoom in on the slope at the far left of the 5062C plot, though,
> you'll see the beginnings of some disagreement between the two.  It gets
> worse at shorter timescales, where the cesium loop is just starting to
> degrade the internal OCXO's stability.  This is an example of Bruce's
> concern, I think: whether it's due to Warren's hardware or my software,
> there *is* at least a slight dependence of measurement fidelity versus
> spectral content.
> In my opinion, this discrepancy is nowhere near large enough to be of any
> practical consequence, especially since it's occurring at timescales where
> I'd normally look at phase-noise measurements instead.  I would have no
> problem using either instrument in everyday work, tweaking GPS clocks and
> surplus Cs/Rb standards, comparing different crystal oscillators, 
> optimizing
> temperature controls, and such.
> But this list is for nuts, right?  If you're worthy of being called a nut,
> then the 2-3 pixels of separation in the blue and green traces near t=0.1s
> may be enough to keep you up at night.  I lack both the moral authority
> *and* the math skills to argue against that.
> -- john, KE5FX
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