[time-nuts] Thunderbolt versus Home made

[Dr Bruce Griffiths]

Angus wrote:
> I don't know whether the outputs on a Jupiter exhibit any granularity
> or not (I've not seen anything to suggest that they do), but I think
> that with receivers that do, changing from 1PPS to a faster PPS output
> would normally change the granularity effects that are seen - assuming
> things are not in sync.
>
> As for possibly reducing the need for a fast clock; being able to
> measure a number of pulses in a second and then take the average count
> can allow a rather better resolution than just taking one measurement
> - so in some circumstances this could be useful.
>
> I've not really looked into it much, but was really just interrested
> if anyone here had come accross the 100/1K/10K, etc pulses being used
> like that.
>
>   
> Although that's not what I was talking about doing above, I think that
> it's pretty much what some of the hardware GPSDO's actually do using
> various types of oscillators (and to good effect too). As in these:
>
> http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm 
> http://www.frars.org.uk/cgi-bin/render.pl?parameter=&pageid=1285
>
> the loop filter is basically just an RC circuit. It's not quite raw
> GPS, but is quite different to using a digital filter as used in a
> Thunderbolt, Shera, etc., which can run to hours.
>
> When the N1JEZ board was mentioned, I assumed that it was this type of
> controller that was meant - and I think it does need 10KHz unless it
> gets much more of a mod than a couple of counters.
>
> Angus.
>   
Angus

The examples in the above links just illustrate how easy it is to fool 
yourself if you don't use the appropriate measurement techniques to 
measure the frequency stability of a GPSDO.

The 10kHz output is indeed phase modulated every second as indicated by 
Figure 11 (attached) on the datasheet. To remain locked to the PPS 
output as illustrated the 10KHz output must have the same timing jitter 
exhibited by the PPS signal.

The fact you were unable to detect just means that your measurement 
technique is inappropriate.
The phase modulation at 1Hz is somewhat pseudorandom.

Bruce
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