[time-nuts] Thunderbolt versus Home made

Angus not.again at btinternet.com
Tue Feb 27 18:02:31 EST 2007


On Sun, 25 Feb 2007 09:52:55 -0600, you 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

Hi Didier,

Sorry, my original post was not very clear - I had only wondered if
faster PPS signals might ever help with the measurement each second; I
didn't mean to suggest that they might affect the required overall
loop time much. ( Thinking back to all the posts a couple of months
ago, I should probably resist any urge to comment on the effect of
sawteeth on stability for the moment...  )
Anyway, I was just curious. It's one of the many things I don't have
much time to mess with, so wondered if anyone else had - but
apparently not.

Angus.




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