[time-nuts] First success with very simple GPSDO
csteinmetz at yandex.com
Fri Apr 11 11:23:06 EDT 2014
> > (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.
>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).
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