[time-nuts] Fw: Optical transfer of time and frequency

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed May 4 19:14:37 EDT 2016


In the same vein if it takes 1000 seconds to measure the relative phase of a 
pair of clocks to within 500ps then the relative ADEV of the clock pair at 
1000 sec needs to be somewhat less than 5E-13.
For 100 s averaging the relative ADEV of a clock pair needs to be better than 
5E-12 @ 100sec.
For 10s averaging the relative ADEV of the clock pair needs to be better than 
5E-11 @ 10s.
Thus if the measurement takes too long the cost of the local clocks becomes 
unaffordable.
Comparison techniques that don't require more than 10-100 sec of averaging are 
preferable to keep the cost of the local clocks sufficiently low.

Bruce

On Wednesday, May 04, 2016 03:03:59 PM Tom Van Baak wrote:
> Hal,
> 
> > How close could you get if you brought two of them together, compared
> > phase, drove them to the site for a nights work, drove them back to the
> > same location and compared the phase again.
> 
> That's essentially asking what the ADEV (or, TDEV) is for tau 1 day. Rb
> isn't near good enough. Neither is Cs, for that matter.
> 
> See www.leapsecond.com/tmp/5071a-12-run8-5d-10d.gif for a plot of a bunch of
> 5071A Cs clocks. They are compared together for 5 days to determine their
> relative phase and frequency offsets and then go on a 5-day trip. You can
> see how the phase drifts as random walk does its thing. It's way more than
> 500 ps per day.
> 
> That's why the OP cannot use free-running clocks. He needs some method to
> actively keep them in tight phase lock or passively compare them to within
> 500 ps in order to adjust the timestamps in post-facto.
> 
> /tvb
> 
> ----- Original Message -----
> From: "Hal Murray" <hmurray at megapathdsl.net>
> To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and
> frequency measurement" <time-nuts at febo.com> Cc: <hmurray at megapathdsl.net>
> Sent: Wednesday, May 04, 2016 10:30 AM
> Subject: Re: [time-nuts] Fw: Optical transfer of time and frequency
> 
> > tvb at LeapSecond.com said:
> >> Any of these methods is going to be a challenge, given their 500 ps
> >> requirement and their $2k budget.
> > 
> > How stable are surplus rubidium oscillators?
> > 
> > How close could you get if you brought two of them together, compared
> > phase, drove them to the site for a nights work, drove them back to the
> > same location and compared the phase again.
> 
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