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

[Ulrich Bangert]

Don,

> What sort of SNR is necessary to prevent an uncertaincy 
> of 1part in 10^13?

I see, you are a non believer! But Bruce was talking about comparing
zero crossings of signals at a few (!) Hz, which needs indeed tremendous
amounts of SNR to get to an uncertainity of 1 part in 10^13! The reason
is that for precice timing not the signal-to-noise-ratio is the figure
of merit but the slope-to-noise-ratio. 

With a 10 MHz signal and a 1 Hz signal having the same SNR the 1 Hz
signal will be inferior 10E7 in terms of slope-to-noise-ratio which
gives raise to the necessity of an 140 dB improved SNR on 1 Hz against
the 10 MHz signal if you want to measure the zero crossings with the
SAME incertainity as on the 10 MHz signal.

If you are still critical about it: A zero crossing detector for a 1 Hz
signal giving a uncertainity of better than 1E-6 (some universes  apart
from your 10E-13) has been celebrated as an first grade scientifical
breakthrough some years ago.   

Best regards
Ulrich Bangert

P.S.

Of course the above applies only to sinusoidal signal forms where the
slope of the zero crossings depends on the signal's frequency. So one
might pretend he is going to measure the zero crossings of DIGITAL
signals of a few Hz which may be indeed easy to measure due to their
high slew rates. The people that argument this way must ask themselves
how to make a digital signal from a sine signal which is pretty much the
same problem as measuring the zero crossing with high precision.  

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Don Collie
> Gesendet: Samstag, 3. Februar 2007 07:57
> An: Dr Bruce Griffiths; Discussion of precise time and 
> frequency measurement
> Betreff: Re: [time-nuts] Stepping up the output of an OCXO
> 
> 
> 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 
> 10^13?
> 
> 
> ----- 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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> > 3:42 PM
> >
> > 
> 
> 
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