[time-nuts] inexpensive fiber optic distribution

[Hal Murray]

jimlux at earthlink.net said:
> It seems like you could probably figure out how to interface to these  things
> and use them to distribute timing signals. 

There are two big advantages of fibers:
  They work well for long distances.
  No ground loops.

The target market for those contraptions is networking.  The signal is 
digital, and they get to work with a "nice" signal, probably 8B/10B.  That's 8 
bits of data in 10 bits on the wire.  Roughly, they pick 256 out of 1024 
patterns that have minimal long strings of 0s and 1s and use a few more 
patterns for control - things like idle, and begin/end of packet.  It works 
fine if AC coupled.

I don't know how well they will work for something like a PPS.  Somebody 
should try it.  The signal pattern is mostly trying to be friendly to the PLL 
that does clock recovery and/or the AGC that sets up the switching level.

If I wanted to send a PPS, I'd use something simple like Manchester encoding.  
That assumes you can line your transmit clock up with the PPS.  It won't work 
if you just want a link-extender for a PPS.

If you are connecting to a FPGA, use whatever they support for high-speed 
serial interface.  Again, that assumes you can line your signal up with the 
transmit clock.

I'm pretty sure the signaling is differential, either PECL or one of the newer 
standards like LVDS.


> They take a standard singlemode duplex fiber and have a pinout suitable  for
> plugging into a Cisco, etc. switch. Apparently, there's some ...

I'm pretty sure none of the programming is on the high speed path.  Maybe 
changing the signal levels.  I don't know how much of it is total BS, aka 
vendor lock in, as compared to actually doing something useful.  I'd be 
surprised (but not much) if they didn't work fine with no programming.



-- 
These are my opinions.  I hate spam.