[time-nuts] The VE2ZAZ GPSDO

Ulrich Bangert df6jb at ulrich-bangert.de
Sat Oct 21 06:24:59 EDT 2006

Hello Bruce, Hello Said

despite the fact that the author is a ham radio colleage of mine I agree
completly to what both of you said. 

>Obviously, the longer you integrate the 1PPS GPS signal for, 
>the better the accuracy will be. But the drawback of this 
>is that the longer the integration time (averaging cycle), 
>the more the VCXO will (or might) drift. Of course, the 
>better the VCXO, the more stability you will get. But this 
>is hard to specify, let alone quantify. 

Who writes something like this has no good idea about the theory! Draw
the tau-sigma of the receiver alone and draw the tau-sigma of the VCXO
alone and look where the lines cross. Then you know EXACTLY which time
constant you have to choose for the control loop without the slightest
guess. And yes, don't forget to throw the Garmin out of the window! 

Ulrich Bangert, DF6JB 

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von SAIDJACK at aol.com
> Gesendet: Samstag, 21. Oktober 2006 08:30
> An: time-nuts at febo.com
> Betreff: Re: [time-nuts] The VE2ZAZ GPSDO
> Hello Bruce,
> I agree. The DAC reference is an LM7805CT regulator with 
> 0.8mV/C  (typ.) 
> temperature coefficient. Considering that typical Oscillators 
>  (MTI units for 
> example) have about +-20Hz deviation per 5V - this alone  - 
> excluding the Opamps 
> tempco, would lead to a temperature drift  of 0.32Hz over a 
> 50 Deg C temp 
> range. That's about 3.2E-08, quite  a bit worse than a good 
> surplus oscillator.
> And the 7805 does not have a maximum tempco rating, so it 
> could be 10x  worse 
> than that...
> bye,
> Said
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