[time-nuts] OCXO and 50Ω termination

Attila Kinali attila at kinali.ch
Wed Sep 27 13:52:10 EDT 2017


Most OCXOs have a 50Ω output, which suggests that they expect to be
terminated with 50Ω. Now on a normal PCB the wire from the OCXO to
the rest of the circuit is usually rather short (1-5cm) which means
that it is much less than the wavelength of the 10MHz output. Even 
when looking reflections, a 2*5cm path (ie forward and back again)
would be less than 500ps (in the order of 300ps for FR4). My guess
would be that a non-50Ω termination would not result in any adverse
effects as the paths are short and the reflections would be constant.
The changes that would affect the delay would be temperature and
humidity (mostly humidity in this case) which are both rather slow
and my guess would be that the added instability would be drowned
in the temperature and aging drift of the OCXO.

So, I'd say that it would be still ok to use 1-10kΩ termination
impedance without any problems. Is this assumption correct?
Is there anything I am missing?

				Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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