[time-nuts] Optical Distribution of 1PPS and IRIG

Hal Murray hmurray at megapathdsl.net
Mon Jul 14 15:25:08 EDT 2014


maflynn at theflynn.org said:
> I am working on a project where I need to transfer 1PPS approximately  120
> meters between buildings.    I cam borrow a pair of 62.5 fibers from  IT/
> telco to do so. 

Google for >fiber IRIG< gets plenty of hits.

Raw PPS is a nasty case for fibers.  You need some sort of modulation/encoding, or derive it from the IRIG signal.

The dynamic range that the receiver in a fiber setup can see is huge.  All the modules I've worked with include AGC.  That needs a signal to listen to.  I haven't seen any with a time-constant slow enough to work with 1 PPS.

If I wanted to send a PPS over fiber, I'd probably do something like this:
  send the PPS using Manchester encoding.
  that assumes you have a 20 MHz source clock synced to the PPS
  at the receiver, use a PLL to make a 20 MHz clock, and use that to decode the data

  The received PPS will be several clock cycles late.   You can undo that with with simple digital logic.  Just delay it by 20,000,000 - N cycles.
  Now it's only late by the fiber speed of light delay.  (plus various digital prop delays and clock-out delays)


  

-- 
These are my opinions.  I hate spam.






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