[time-nuts] RE: phase locking Rb to GPS
Tom Van Baak
tvb at leapsecond.com
Sun Oct 23 17:42:39 EDT 2005
> Indeed, but you can also handle that by using two oscillators, one for
> term stability and one for short-term and lock the short-term to the
> oscillator with a suitable PLL bandwidth. Comes at some additional cost,
> then you can optimize each end better rather than compromizing on one
> oscillator. That ain't a free lunch either. In the end, you should not
> the numbers, one must recall the intended application and make that work.
You've hinted at the scenario that I often use here
in my lab. I do use two oscillators. For short-term
I use a nice quartz OCXO -- free running. And for
long-term I use daily averages of 1PPS pulses from
a GPS receiver.
But they are not disciplined; it's just two completely
separate references. Neither helps the other but also
neither pollutes the other.
This works because, in most cases, for me at least,
absolute accuracy is not critical during short-term,
low-noise measurements. And similarly, undisciplined
averaged sawtooth corrected raw GPS 1 PPS timing
provides a slightly better long-term reference than a
GPSDO provides. So you get the best of both worlds.
Granted a GPSDO sort of gives you both in the same
box but do you lose a bit of performance on either end
as a result.
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