[time-nuts] GPSDO TC

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 10 08:06:42 EST 2009

Hej Bruce,

>> As the PLL filters the noise of the OCXO and passes noise from the input 
>> side and the noise have several different components to it from either 
>> source.
>> You can't really extrapolate direction the results of an asynchronous 
>> reciever such as the M12+T to that of a synchronous receiver such as the 
>> Thunderbolt. The time-solution of thunderbolt is used in replacement of 
>> the time-interval counter fluff slapped onto a PPS based receiver such 
>> as the M12+T. Also, the Thunderbolt enjoys a much quiter and stable 
>> reference than the M12+T which allows for narrower filters in the 
>> sat-tracking as the phasenoise is lower. Notice how the Thunderbolt can 
>> be configured for different uses, they are direct hints to what the 
>> tracking loops may do as it reduces the physical dynamics of position as 
>> well as inflicted G effects on the OCXO.
> I wasn't attempting to do so.
> However the phase noise of the GPS receiver will still dominate for
> short tau whilst that of the OCXO is dominant for longer tau.

Agreed. I'm just trying to keep various error sources separate here.

The truncation noise isn't white noise one should recall.

>> With just two Thunderbolts and a reasonable TIC you can infact build a 
>> three-cornered hat. You have three clocks: GPS, OCXO1 and OCXO2 and the 
>> thunderbolts will measure GPS-OCXO1 and GPS-OCXO2 and the TIC will be 
>> able to measure the OCXO1-OCXO2. An interesting aspect of this is that 
>> when lockedup, the PPSes of the Thunderbolts will be confined into a 
>> rather small area. This arrangement will, as any other, give not the 
>> standalone OCXO noise when beeing steered, but it is not entierly lying 
>> for those longer taus.
> The 3 cornered hat technique only works well (even in the extended form
> where finite correlations between sources are included) when the noise
> of each of the 3 sources are comparable.
> That is this technique will only work well in the vicinity of the point
> where the GPS receiver and OCXO ADEVs crossover or equivalently near the
> drift corrected minimum of the ADEV as measured by the Thunderbolt when
> the OCXO is undisciplined. For shorter tau the GPS phase noise dominates.

Yes. I think it would be usefull to provide confidence intervals etc. to 
get a better "feel" of how good the measure is.

As a reference measurement the thunderbolts could be driven into 
holdover. I think the thunderbolts keep reporting timing errors.

>> True. However, I think there is still some more theoretical work to be 
>> done to give us better tools. These does not remove the need for 
>> measurements and I have never been foolish enougth to beleive so, but it 
>> could guide us in the right direction for selecting and steering our 
>> parameters.
> It would be helpful if the ADEV (and MDEV) plots for several
> Thunderbolts were plotted using the Thunderbolt's internal phase error
> measures obtained when the OCXO is undisciplined.
> This can easily be setup using the Trimble Thunderbolt Monitor program.

Indeed. We should recall that the PLL locking fades over from the OCXO 
to the GPS (as received) noise at about the BW frequency/tau of the PLL.

Finding the optimum crossover properties involves both balancing PLL 
bandwidth and damping.

>>> I had noted that your quoted damping factor was incorrect but I
>>> suspected that you would realise that.
>>> Actually according to Gardener critical damping factor is 1 ( minimum
>>> settling with no overshoot for a phase step).
>>> However a damping factor of 0.7071 is widely used.
>> It is interesting to clear up why this difference exists. Could be 
>> "critical" is judged different for different applications.
> The usual meaning of critical damping in a second order differential
> equation is for no overshoot to a step input.
> Thus critical probably isn't the appropriate term when optimising for
> other factors.
> Optimum damping for a particular criterion is perhaps better description.

Precisely. Critical damping is just a handy reference along the line, 
but should not be incorrectly interprented as optimum in some generic 
sense, there is several forms of optimum for different tasks.

I think best ADEV or best TDEV might be more relevant here.

>> My point was that regardless of implementation, second order type II 
>> loops seems to be the reference mark, which not necessarilly is a good 
>> one. Third order loops should be considered as it removes or reduces a 
>> type of problem and allows a more freer setting of parameters with less 
>> things to compromise between. When doing the loop in digital processing 
>> it is not that more expensive. There are re-tunable architectures which 
>> is being used in for instance GPS receivers which is not hard at all to 
>> use for both PI and PID controllers.
>> Cheers,
>> Magnus
> In particular the ability of a third order loop to track linear frequency drift can be very useful.

Indeed, this is why I prefer them for this application.

> The last time I let my Thunderbolt OCXO free run there was a very strong
> quasi parabolic component to the phase drift as measured by the
> Thunderbolt itself.
> However, this was soon after I first powered it up, its drift may have
> settled down somewhat by now.

You would expect a strong drift at that time, owing to the OCXO heating 
up. Actual drift in oscillators isn't a static value but a rather 
different curve.


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