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

Magnus Danielson magnus at rubidium.dyndns.org
Thu May 5 02:36:45 EDT 2016


Hi,

On 05/05/2016 12:03 AM, 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.

Indeed. In this case the TDEV is of interest, as we have a 500 ps 
requirement. Notice that TDEV is a RMS-type of measure and I think we 
had the 500 ps given as a time error, you can't compare these values 
directly, as the noise will have a different scaling factor depending on 
the confidence interval around it that we want. The engineering factor 
of 3 sigma would give us 99,7 % of the time it would be within those 
limits, which may be good enough, this would mean that in normal 
conditions I would divide my 500 ps with 3 to get the limit value.
The trouble is that we don't follow the normal gaussian noise and hence 
error function but rather have the Chi-squared noise, so we need our 
confidence factor from that. The principle is the same thought.

A bidirectional link should not be too hard to set up, and with some 
care should be able to achieve this target.

Cheers,
Magnus

> 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.
>>
>>
>> --
>> These are my opinions.  I hate spam.
>>
>>
>>
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