[time-nuts] 10 MHz over optical fiber?

[Magnus Danielson]

Paul Boven skrev:
> Hi everyone,
> 
> In message <20081124152247.DCDB0E91529 at mail.ebirds.it>, Marco IK1ODO -2
> writes:
>> Hi all,
>>
>> I have to carry a 10 MHz standard frequency signal inside an EMC 
>> screened room via fiber optic cable.
>>
>> Not willing to re-invent the wheel, do something like an optical 
>> standard frequency link exist on the market?
>> I think it is possible to use standard 100MB LAN transceivers, and 
>> POF. Phase noise requirements
>> are not very stringent, and the distance is in the order of some tens 
>> of meters.
> 
> I'm looking into something similar: transmitting an H-Maser signal
> (probably 10MHz) over some 34km using CWDM SFPs. At first glance this
> seems fairly uncomplicated: get some SFPs, and SFP connector + cage. Use
> a fast opamp/differential driver to drive the transmitting SFP, and use
> a similar setup at the other end to transform the received data back to
> 50 ohm unbalanced. How feasible would such a setup be?
> Possible problems might be that a 10MHz squarewave is simply too 'slow'
> to be transmitted by an SFP, which expects 1.25Gb/s 8/10 encoded data.

Not if you choose STM-1 SFPs. STM-1 has a baudrate of 155,52 MBd so a 
cycle will be 15,552 symbols, which is certainly not a problem as a 
STM-1 must be able to handle runs as long as 72 symbols of 0 or 1, and 
then the problem is really in the clock recovery PLL. The AGC will 
certainly work well. These SFPs are of the shelf components. You would 
like to drive them diffrentially and all that, but it is not too hard. 
There are nifty test boards with SMA connectors which would make it easy 
to hook up to form a nice rig.

> Another interesting question would be how much jitter/noise such a setup
> would add?

Consider that for SDH links as such, the jitter must be less than 0,07 
UI, where a UI is the length of a symbol. This turns out to be 450 ps 
for a STM-1. However, that includes ISI which would be essentially 
canceled out for a 10 MHz signal, so you should expect less than that.

It's off the shelf stuff. 34 km is not a big problem. You could pick up 
a pair of 70 km SFPs for better specs, but be sure to not overload the 
input, you might need to insert a damping pad in that case.

STM-4, GE or STM-16 SFPs can also be used, with increased requirements 
on jitter... GE SFPs will probably work very well even if they expect 
8B10B data according to standard, but they are not significantly 
different to the SDH SFPs these days. The expected jitter on STM-16 
would be about 28 ps, but can again be expected to be less...

So, jitter-wise it is a valid solution and off the shelf components will 
probably work well for you. I recommend you to use SFPs with monitoring 
functions, so you can monitor levels, temperatures etc. It's fairly 
simple I2C stuff so it is not big magic there.

Cheers,
Magnus