[time-nuts] TIC resolution impact on GPSDO's performance

Ulrich Bangert df6jb at ulrich-bangert.de
Mon Dec 25 09:17:03 EST 2006


Poul,

> Your black line is bogus in the usual "teacher's bad example way".
> 
> We know that the hardware PPS signal from gps is 
> phasemodulated with a +/- N ns signal which has a box 
> distribution and upper frequency limit of 2 Hz and which, 
> subject to temperature stability and hanging bridges, has no 
> significant frequency components below < 1/500s.
> 
> It follows readily for this, that only teachers trying to 
> show a bad example would use the PPS signal for tau > 500 
> second without filtering the higher frequencies out, one way 
> or another.

I am not talking now about 2 vs. 0.5 Hz. It is the rest of your argument
that asks too much from me. Can you elaborate on that? I have no idea of
what you are talking about!

> No causal algorithm can allow you to implement:
> 
> 	if (tau < N)
> 		use OCXO
> 	else
> 		use GPS
> 

Come on, Poul! I have carefully tried to avoid such interpretation by
using terms like "starts to dominate" and so.

> But most fatal to your message: you look at the wrong kind of 
> stats for this particular kind of discipline.

Being still adaptive I wait for people who explain it to me.

> When you discipline an frequency source (OCXO, Rb, Cs) to a 
> phase source (GPS, Loran-C, WWV, DCF77, NTP etc), you have to 
> decide for which parameter you (optimize your) discipline:
> 
> 	Minimum phase offset.
> 	Minimum frequency offset.
> 	Best phase stability.
> 	Best frequency stability.
> 	Best holdover performance in phase.
> 	Best holdover performance in frequency.
> 
> All I have heard about here so far, is the first and a few 
> cases of the second kind, and neither of those shows their 
> performance particularly well on an ADEV plot.

I agree completely to you that anything concerning holdover is neglected
in the discussion but may this be due to the fact that holdover
performance is the least important one in amateur use? 

In my own GPSDO i discipline the LO's pps to have a MAXIMUM phase offset
of 500 ms against the receiver's pps. That avoids lots of ambiguity
problems and leaves the widest phase measurement range to make the loop
even lock when the OCXO is initially far apart its setpoint.

Otherwise you are correct that one can optimize for different
parameters. This gives immediate rise for the following publications:

1) A guide to discipline your frequency source to a phase source with
minimum phase offset

2) A guide to disipline  your frequency source to a phase source with
minimum frequency offset

3) A guide to discipline your frequency source to a phase source with
best phase stability

1) A guide to discipline your frequency source to a phase source with
best frequency stability

If you can deliver or send some links: I will surely devour stuff like
this! 

Best Regards
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Poul-Henning Kamp
> Gesendet: Montag, 25. Dezember 2006 00:51
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] TIC resolution impact on GPSDO's performance
> 
> 
> In message <000001c72769$8363beb0$03b2fea9 at athlon>, "Ulrich 
> Bangert" writes:
> 
> >For the most of you it will already now be kind of evident that the 
> >crossing point defines the magical value that we have to set 
> the loop 
> >time constant to but this fact can be formulated with a bit more of 
> >scientifical preciseness: At no observation time tau will it be 
> >possible to have an ADEV at the OUTPUT of the standard that is lower 
> >then BOTH Allan plots at this tau.
> 
> This is not true in general, but does hold true for the 
> example you have chosen.  The exact requirement for 
> truthfullness is that the noise-processes of your two sources 
> must be uncorrelated.
> 
> >What if we had not used the sawtooth corrected values but 
> the raw 1pps 
> >phase data?
> 
> Your black line is bogus in the usual "teacher's bad example way".
> 
> We know that the hardware PPS signal from gps is 
> phasemodulated with a +/- N ns signal which has a box 
> distribution and upper frequency limit of 2 Hz and which, 
> subject to temperature stability and hanging bridges, has no 
> significant frequency components below < 1/500s.
> 
> It follows readily for this, that only teachers trying to 
> show a bad example would use the PPS signal for tau > 500 
> second without filtering the higher frequencies out, one way 
> or another.
> 
> (In the initial capture phase, no filtering should be used to 
> get the best possible frequency response of the PLL, in the 
> "grab" phase where the integrator is clamped, a simple 
> exponential average should be used.  Once lock has been 
> aquired, linear regression offers a useful zero-latency 
> filtering model.)
> 
> Your black line should have reflected this.
> 
> 
> But your further argument has trouble as well.
> 
> No causal algorithm can allow you to implement:
> 
> 	if (tau < N)
> 		use OCXO
> 	else
> 		use GPS
> 
> For some interval of tau, both sources will affect the 
> result, if you do post-factum disciplines (ie: paper clocks) 
> you can do it a lot closer to optimal, but the statistics 
> gets increasingly nasty and the age of your data will 
> approach infinity as the fidelity increases.
> 
> But most fatal to your message: you look at the wrong kind of 
> stats for this particular kind of discipline.
> 
> When you discipline an frequency source (OCXO, Rb, Cs) to a 
> phase source (GPS, Loran-C, WWV, DCF77, NTP etc), you have to 
> decide for which parameter you (optimize your) discipline:
> 
> 	Minimum phase offset.
> 	Minimum frequency offset.
> 	Best phase stability.
> 	Best frequency stability.
> 	Best holdover performance in phase.
> 	Best holdover performance in frequency.
> 
> All I have heard about here so far, is the first and a few 
> cases of the second kind, and neither of those shows their 
> performance particularly well on an ADEV plot.
> 
> And most amateurs even forget to deal with quartz frequency 
> jumps and other 'point-like' upsets.
> 
> The theory behind a PLL is really no different from a PID 
> temperature regulation, and I highly recommend people read up 
> on those because they are generally explained much better 
> than when PLL's are the subject.
> 
> Before you get any good ideas: note that our measurement noise
> (jitter/resolution) only for very long tau permits meaningful 
> use of the D(ifferential) term.  It is possible to use a 
> hysteresis on the D term to catch frequency jumps in the 
> xtal, but it is of dubious advantage compared to just 
> detecting and resetting the PLL).
> 
> A less significant difference from PID regulations is higher order
> integrals:  They are not useful for temperature regulation, 
> but if you want to get really nasty with your PLL, you can 
> add another term to model the frequency drift, and another 
> one to model the change in frequency drift and another one to 
> model the change in the change of the frequency drift and ... 
> (you get the idea).
> 
> Be aware that floating point is necessary and that rounding 
> errors will mess you up if you are not very careful with your 
> sums and differences.
> 
> I can highly recommend writing a small program or big 
> spreadsheet to simulate a PLL so you can play with the 
> coefficients and get a feel for the dynamics by watching 
> plots of the phase and frequency deltas and ADEV etc.
> 
> Merry X-mas!
> 
> Poul-Henning
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by 
> incompetence.
> 
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