[time-nuts] Optical transfer of time and frequency

Ilia Platone info at iliaplatone.com
Fri Apr 29 17:33:34 EDT 2016


Fiber is not what I need because the system will not be in fixed 
locations, and distances will be far more than 2km.

The requirements are to record photon arrival timestamps with a sampling 
clock of 400MHz, 2.5ns resolution. The two clocks are independent, and 
the timestamps will be the effective clock number at the photon arrival. 
An averaging algorithm will be used to normalize the timestamps.

Since the software I'll use can adjust some "small" time drift. Some 
clock cycle every houe is acceptable, so requirements, you see , are far 
to be impossible.

1 clock cycle = 2.5ns

max clock cycles @ 1 hour = can vary, 1024 would be a good value. 
Something achievable with commercial products and easily averagable by 
software.

So:

1280ns each 1H dT (less is well accepted).

Ilia.


Il 29/04/2016 22:14, Magnus Danielson ha scritto:
> Well, giving the conditions mentioned, doing ranging codes such as 
> those used by GPS is very easy and cheap. Doing this in bidirectional 
> isn't too hard. Doing a suitably high chip-rate should cost very little.
>
> The two-way time-transfer is relatively easy, but you will need to do 
> some calibration to get the precision needed.
>
> Now, what is the needed precision?
>
> Why can't you pull fiber?
>
> Still wonder how you use the time, to understand the timing requirements.
>
> Cheers,
> Magnus
>
> On 04/29/2016 08:50 PM, Ilia Platone wrote:
>> There is line of sight.
>>
>> The budget is around 2k€ by now, but can be increased.
>>
>> This project is for an amateur astro club, and the resources they would
>> give to me are limited, except the place where to do this and some 
>> optics.
>>
>> The setup must be mobile, I mean that I should be able to place the
>> telescopes in other places "easily".
>>
>> There is some material however. Material includes FPGA boards, VOCXO +
>> PLL boards, IR lasers and APD sensor boards, ARM boards, and consumer 
>> PCs.
>>
>> There is also the possibility to use some optics like small reflector
>> telescopes, as pointed before, they could be used as beam expanders for
>> IR lasers.
>>
>> Ilia.
>>
>>
>> Il 29/04/2016 09:36, Michael Wouters ha scritto:
>>> On Fri, Apr 29, 2016 at 1:18 PM, Bruce Griffiths
>>> <bruce.griffiths at xtra.co.nz> wrote:
>>>> Quoting Michael Wouters: "According to this,
>>>>
>>>> http://www.nist.gov/manuscript-publication-search.cfm?pub_id=912449
>>>>
>>>> there are many practical challenges  with a one way free-space
>>>> optical link."
>>>> That paper indicates that  one way transfer with noise of a few
>>>> picosec should be feasible using an IR laser.
>>> Oh, yes I see in Fig 2b that the short term, one-way noise is ca. +/-
>>> 5 ps. And probably with temperature measurements, the long term
>>> variation could be compensated.
>>>
>>> But we still don't know if there is line of sight.
>>>
>>> Cheers
>>> Michael
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>>
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-- 
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
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