[time-nuts] Thunderbolt versus Home made

Didier Juges didier at cox.net
Sat Feb 17 09:50:56 EST 2007

Angus wrote:
> On Fri, 16 Feb 2007 17:44:14 -0600, you wrote:
>> Richard W. Solomon wrote:
>>> I looked at the ACE-III on Trimble and do not see mention of a 10 KHz output.
>>> Maybe I miised it, but without it, the N1JEZ board will not do unless you modify
>>> it for 1 pps.
>>> I am trying to find more info, but since it reached EOL, Trimble may have deleted
>>> it.
>>> 73, Dick, W1KSZ
>> Dick,
>> You are correct, you need to patch another set of dividers in the chain. 
>> Keep in mind that the only benefit of a 10 kHz output on the Jupiter is 
>> to save 4 decade dividers. You should not use the fact that the Jupiter 
>> outputs 10 kHz to speed up the loop in a GPSDO. The loop bandwidth is a 
>> function of the crossover point between the VCXO and the GPS, which in 
>> most cases will be between many minutes (cheap VCXO) and a few hours (HP 
>> 10811). Whether you use 1 Hz or 10 kHz comparison frequency into the 
>> phase detector will not affect the filter.
> Hi,
> (As this is my first post here, I'll try not to mess up too badly :)
> I think that for a simple controller like the N1JEZ board, a 10KHz
> output on the GPS really is needed.
> I tried a circuit like that with 100pps once out of curiosity, but as
> I remember, the lower error sensitivity made things more difficult.
> BTW, with a standard GPSDO, would using the 100Hz/10KHz the pulse not
> reduce the need for a fast comparator clock, and also reduce sawtooth
> effects? 
> Angus.

One of the problems with the 10 kHz output us that it is only updated 
once per second, i.e. the period of the 10 kHz signal is the same for 1 
second, and every second, there is a small phase jump to update the 
phase of the 10 kHz to match the 1 PPS.

That means the 10 kHz has a one Hz component that is the correction 
signal. It must be filtered out. But even filtering the 1 Hz component 
is not sufficient, since the 1 Hz component has the same short term 
noise that is present on the 1 PPS output of any GPS receiver.

So while it's a lot easier to filter 10kHz down to a level where 10 kHz 
ripple does not appear on the OCXO EFC input, such a filter would be 
grossly insufficient to make a GPSDO. You would simply have an OCXO 
phase locked to a poor reference. You must filter the output signal from 
the phase comparator so that the short term instabilities in the GPS 
timing signal are also filtered out.

The 10 kHz is useful for experimentation, since you could artificially 
speed up the loop with one second or so time constant or even less so 
that you could easily verify with an oscilloscope that the OCXO is 
actually phase locked to the GPS, but you would not want to run the 
system that way because it would have terrible performance.

With a time constant of 20 minutes to an hour, it can be very 
frustrating to verify that the system phase locks and it would be best 
to have another hobby to attend to in the mean time :-)

Regarding the sawtooth effect, since the 10 kHz is only updated every 
second, it has no effect on the possibility of hanging bridges, and I am 
not sure what you mean by reducing the need for a fast comparator clock.


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