[time-nuts] Comparison of Logic Standards for Clock Distribution

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Oct 25 21:19:39 EDT 2006

Stephan Sandenbergh wrote:
> Hi Bruce,
> You mentioned that locking to a crystal at the receiving end as an option.
> Does this mean that signal transmission is primarily plagued by short term
> noise? 
> I have never really touched the topic of optical fibre, but I realise that
> it is superior to conventional methods. The superiority of optic fibre is
> probably not as pronounced at short distances, is it?
> Kind regards,
> Stephan Sandenbergh  
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> time-nuts at febo.com
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Distribution of clock frequencies via optical fibre (at least at low 
optical power levels) is indeed limited by white phase noise in the 
receivers for short averaging times.
For long averaging times the instability (white frequency noise etc.) of 
the clock source will manifest itself. Thus it can be advantageous to 
phase lock a low noise crystal oscillator to the received signal to 
cleanup the the received frequency.

Optical frequency combs are sometimes used to distribute microwave 
frequencies via an optical fibre.
In this case the photodetector (sensitive to the incident optical power) 
also acts as a mixer and its output will contain a frequency component 
at the difference frequency between adjacent comb signal frequency 

For short distances well screened coax is easier to use.
Fibre offers a lower propagation delay tempco which may be important if 
the cable/fibre temperatures varies significantly.
However the cost of the fibre transmitter, receiver and fibre may be 
relatively high compared to the cost of a piece of coax plus a driver 
and receiver for short distances.


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