[time-nuts] 10 MHz over optical fiber?

Magnus Danielson magnus at rubidium.dyndns.org
Thu Nov 27 21:14:55 EST 2008

Paul Boven skrev:
> Hi Didier, everyone,
> Didier wrote:
>> By the time you get the signal from the maser through 34 km of fiber optic,
>> what you get at the end might be not better than a decent GPSDO...
> That's exactly what I'm trying to work out before I'll actually start
> building something like that. The JPL device (thanks for the hint, James
> P. Lux) is very interesting, see:
> http://tycho.usno.navy.mil/ptti/1988/Vol%2020_02.pdf
> The JPL system aims to achieve better than 1E17 in 1000s, whereas the
> H-maser in their publication is quoted as 1E15 in the same averaging
> time. They also state that: 'when stabilities higher than 1E15 are
> required the link must be actively stabilized'. So, in a nutshell - If I
> were to build such a system, the first incarnation would be without such
> a compensation system.
> If such a compensation does turn out to be necessary I'd probably not
> use a 50% mirror but simply loop the received data back to the
> trasmitter of the SFP at the receiving end, and use a fiber pair instead
> of a single fiber.

You'd get a fiber-pair anyway, and the SFP provides both transmitter and 
receiver, so use them!

Infact, you should loop back the signal to the original site and monitor 
the signal there. If you use standard SFP testboards, you can take one 
of the signals from the SFP and loop it back to the SFP using a short 
semi-rigid SMA cable and put a 50 Ohm terminator on the other input 
signal. That's all the loopback you would need. Then at the transmitter 
end you use the output and send it to a counter and measure 10 MHz phase 
against the H-maser.

Logg against time and log outdoor temperature as well as lab temperature 
in both ends. Preferably SFP temperature in addition.


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