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

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 26 07:15:48 EST 2006

Magnus Danielson wrote:
> From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
> Subject: Re: [time-nuts] TIC resolution impact on GPSDO's performance
> Date: Tue, 26 Dec 2006 08:14:12 +0000
> Message-ID: <36958.1167120852 at critter.freebsd.dk>
>> In message <4590BB56.5070809 at xtra.co.nz>, Dr Bruce Griffiths writes:
>>> Poul-Henning Kamp wrote:
>>>> I think what you are trying to express is that the frequency from
>>>> the internal Xtal (at times) is in an overtone of the 1Hz PPS, which
>>>> gives rise to hanging bridges.
>>> Poul-Henning
>>> Surely you mean harmonic? An overtone is not necessarily a harmonic.
>> I meant overtone, becuase they are not in any harmonic relationship,
>> the XO is wandering around whereas the (ideal) PPS is, supposedly,
>> rock stable.
> The only real way to describe it is to say that the XO and the PPS is
> asynchronous to each other. Those who has freshen up on their greek for
> technicians will know that this means "not the same clock" which is quite
> accuratly what we have. ITU-T Rec. G.700 has a few interesting sections in it.
> The use of words such as harmonic or overtone should not be used since they
> is normally used to describe the various frequencies within one signal where
> as words as synchronous and asynchronous is to be used mainly for pair of
> signals and describes their relative timing. Naturally, for a pair of signals
> to be synchronous does not require them to have the same frequency, just a
> fixed ratio between those frequencies.
> It does happends that a single signal is said to be synchronous, but the
> wording which should have been used is isochronous (same clock). Then we have
> the lovely words of plesiochronous and mesochronous. :)
> Cheers,
> Magnus
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So the correct description of the cause of hanging bridges is something 

As the frequency of the clock which controls the timing of the receiver 
PPS pulse output drifts and approaches pleisiosynchronism with the 
"true" frequency of the PPS signal, the rate of change of the PPS timing 
error due to quantisation decreases reaching zero should the clock 
achieve synchronism with the "true" PPS frequency.
Thus hanging bridges occur when the clock frequency approaches 
pleisiosynchronism with the "true" PPS signal frequency and then drifts 
away from pleisiosynchronism with the PPS signal..

When the frequency of the clock controlling the timing of the receiver 
PPS pulse output is synchronous with the PPS frequency, then neither 
sawtooth corrections or hanging bridges occur, however there may be a 
fixed offset of upto 1/2 a clock cycle between the "correct" and the 
actual leading edge of the receiver PPS output pulse.

Please correct and simplify these statements as required.

The only problem that I can see is we still need to define how close to 
synchronism the timing clock frequency and the "true" PPS signal 
frequency system must be to be considered pleisiosynchronous.


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