[time-nuts] GPS discipline oscillator vs phase lock

jimlux jimlux at earthlink.net
Sat Jun 17 09:29:02 EDT 2017

On 6/16/17 10:55 PM, Lifespeed via time-nuts wrote:
> Not too surprising to read locking two crystal oscillators together
> without using a physical cable is difficult to impossible.
> Essentially what I am looking for is the phase alignment accuracy
> (and phase noise) one would get PLL’ing one oscillator to the other
> using a cable, but over a longer distance.  Some modest phase noise
> degradation might be acceptable, but not an order of magnitude.
> Clearly not a trivial problem. Yes, the jitter (phase noise)
> typically accomplished from a PLL phase comparing at 100MHz is better
> than what one could get “locking” to GPS.  It was just a thought,
> apparently not a realistic one.  Thanks for disabusing me of that
> notion.

Well, at JPL we regularly lock two crystal oscillators together that are 
over a billion km apart with added Allan deviation of less than 1E-15 at 
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the 
distance and velocity to spacecraft (a few cm in range and mm/s in 
velocity) and from that figure out the gravitational fields (among other 

So it is *doable*

The performance depends ultimately on the noise within your tracking 
loop bandwidth.

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