[time-nuts] Thunderbolt versus Home made

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Feb 25 19:51:26 EST 2007

Didier Juges wrote:
> Angus wrote:
>> 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,
> Don't get me wrong, the James Miller design does a great job, 
> considering it's simple and elegant implementation and the fact that for 
> so little money you can have a frequency standard that beats anything a 
> ham could make in his or her shop.
> This will do a great job for most applications where you want to be 
> within a few Hz or so around 10 GHz, and as long as the GPS signals are 
> good. For the intended market, if the GPS signals are not good, you 
> simply wait or move the antenna.
> A lot of the money spent on commercial GPSDO's is spent on improving 
> hold-over performance, because many commercial applications simply 
> cannot postpone using the system until the GPS signal is good.
> I just wanted to point out that the 10 kHz output can be misleading. The 
> GPS timing is such that if you have a quality OCXO, a loop faster than 
> at least 20 minutes will actually do a disservice to the OCXO, 
> regardless of the PLL reference frequency. If you use an inexpensive, 
> not temperature stabilized VCXO, you can probably speed up the loop 
> quite a bit, simply because the VCXO itself in free running mode will 
> not be doing that great.
> If your frequency and time needs are modest (and that probably covers 
> 99% of applications, including anything I could dream of personally), 
> this design will be perfectly satisfactory.
> But, and this is a significant caveat, this is TIME-NUTS where most 
> people are looking at that like the holly grail 1% :-)
> Didier
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There's no proof that the disciplined oscillator is in any way better 
than the undisciplined oscillator.
No before and after measurements seem to have been made.
The short term stability after disciplining with such a short time 
constant will invariably be several orders of magnitude worse than the 
undisciplined crystal.
Why bother?


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