[time-nuts] Stepping up the output of an OCXO
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Feb 3 00:43:42 EST 2007
Don Collie wrote:
> Why not use an optocoupler as an isolation amplifier? - to the best of my
> knowledge it would provide infinite isolation.
> Cheers,.......................................Don Collie
And lots of noise.
You will need a cleanup PLL on the output side.
Also optocoupler isolation isn't infinite, there is a finite capacitance
between the closely spaced emitter and detector.
Not all optoisolators are fast enough.
The most effective way of using optical isolation is in fact to modulate
a stabilised laser and then couple the output into a fibre.
This technique is employed to distribute stable frequencies to Radio
telescope antennas especially when a large number are employed in an array.
A phase locked loo is used at the other end to cleanup the signal after
Fibre has the advantage in this and other applications where long runs
are involved that its propagation delay tempco is significantly lower
than that of coax.
It is also possible to use fibre stretchers and other techniques to
actively compensate variations in the fibre propagation delay.
Isolation becomes particularly important when one mixes a pair of
signals a few Hz apart and then analyses the zero crossing times of the
mixer output beat signal.
It is easier to achieve a high isolation between RF frequencies than at
frequencies of a few Hz, JPL found fibre optic isolation can be
invaluable in this case.
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