[time-nuts] Optical transfer of time and frequency

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 30 02:06:29 EDT 2016


White Rabbit would be good in that a suitable TDC design with 1ns resolution already exists for White Rabbit. This TDC is used in the Tunka valley (near lake Baikal) Siberian Cherenkov telescope array. Note this TDC uses the SERDES receiver in the FPGA to implement a serial to parallel converter with a 1GHz clock synthesised from the 125MHz White Rabbit clock.
So far I haven't found an existing free space optical implementation of White Rabbit.
Bruce
 

    On Saturday, 30 April 2016 11:00 AM, Michael Wouters <michaeljwouters at gmail.com> wrote:
 

 So why not do White Rabbit free space ?

Cheers
Michael

On Sat, Apr 30, 2016 at 4:04 AM, Paul Boven <p.boven at xs4all.nl> wrote:
> Hi everyone,
>
> On 04/29/2016 03:28 PM, Magnus Danielson wrote:
>>
>> Phase/time transfer over fiber is shaping up, but White Rabbit is
>> starting to grow up and more reports for long distances is showing up.
>> ETFT is one of the placces to check for reports.
>
>
> And the White Rabbit workshops, with the presentations online:
> http://www.ohwr.org/projects/white-rabbit/wiki/Mar2016Meeting
>
> I happen to be working on time transfer via White Rabbit. The White Rabbit
> standard proscribes the use of 1000Base-Bx10 (10km reach bi-directional)
> SFPs, but it turns out to work just fine with longer reach SFPs. However,
> there are several effects that limit the accuracy that you can get on longer
> links:
>
> * Dispersion of the fiber (as the lasers change temperature, their
> wavelength changes, and they experience a slightly different index of
> refraction, hence propagation speed.
>
> * Change in index of refraction in the fiber itself. The propagation speed
> of both the uplink and downlink wavelength change, in absolute sense but
> also their ratio changes. This is something the WR protocol can't
> detect/correct for.
>
> There are several people working on these issues, trying to improve both the
> calibration and stability even further.
>
> Regards, Paul Boven.
>
>
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