[time-nuts] Stepping up the output of an OCXO

Don Collie donmer at woosh.co.nz
Sat Feb 3 01:56:50 EST 2007

I am depantsd, Dr Bruce  ;-)................Don.

PS : How much reverse transconductance would a typical high speed opto have 
[ball park figure]?
    What sort of SNR is necessary to prevent an uncertaincy of 1part in 

----- Original Message ----- 
From: "Dr Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Don Collie" <donmer at woosh.co.nz>; "Discussion of precise time and 
frequency measurement" <time-nuts at febo.com>
Sent: Saturday, February 03, 2007 6:43 PM
Subject: Re: [time-nuts] Stepping up the output of an OCXO

> Don
> 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 
> the photodetector.
> 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.
> Bruce
> -- 
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