[time-nuts] First success with very simple GPSDO

Charles Steinmetz csteinmetz at yandex.com
Fri Apr 11 11:23:06 EDT 2014

I wrote:

> > (Note that a 256 cycles per second error is 51 PPM at 5 MHz.)

> > In the real world, you should  be able to trust that any oscillator that is
> > chosen for a GPSDO will  free-run within 1 PPM when it is warmed up, and
> > that your PPS will  always be within 1 PPM, as well.

Hal wrote:

>Why?  The original goal was low cost.  I could easily believe that an
>inexpensive oscillator would be off by more than 1 PPM.

Decent OCXOs are falling off the ebay tree for $20 every day (less, 
if you look and wait -- I've bought several for <$5).  I have bought 
dozens of them, and have yet to receive one that isn't within 
2e-7.  So I'm inclined to think any oscillator that is worth using 
for a GPSDO has a very good chance of being within 1ppm without any 
adjustment or selection.  Further, there are good ways to get an 
oscillator within 1e-6 without a counter -- for example, beating with WWV.

Indeed, if cost is the limiting factor, I submit that most people 
would be better served by spending their entire budget on the best 
OCXO they can buy and a box to put it in to reduce temperature 
fluctuations, then manually calibrating it periodically, than by 
buying a cheap oscillator and spending money on a GPS, uC, etc. in an 
attempt to make a silk purse out of it.  GPS can only help at long 
tau (or said differently, it can only help at short tau if the 
oscillator is too unstable to be worth using in the first place).

Best regards,


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