[time-nuts] opto coupler isolation?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 3 16:52:25 EST 2009


Henk ten Pierick wrote:
> Hi,
>
> An opto coupler seems  good idea for an isolation amplifier but maybe  
> not for the phase noise.
> Any experience?
>
> regards,
>
> henk
>
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>   
Henk

The better optoisolators typically have a random jitter on the order of
50-100ps rms.
This is considerably more than that for ACMOS logic (~1ps rms).
Consequently the phase noise floor will be about 34 to 40dB higher than
that when using ACMOS logic.

Thus for low phase noise OCXOs using an optical isolator will
significantly degrade the phase noise.
An RF transformer is a lower phase noise solution to providing low
frequency ground isolation.
Achieving a reverse isolation of more than 140dB isnt too difficult with
a few cascaded CB stages and a good layout combined with apprpriate
shielding.

Bruce




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