[time-nuts] New topics (was Re: He isa Time-Nut Troublemaker....)

[Didier]

John,

When you add two (statistically independent) 5 MHz signals and get a 10MHz
signal, the 10 MHz signal's *relative* noise and drift will be the average
of the *relative* noise and drift of the two 5 MHz signals. So as when you
average n signals, the noise and drift are reduced by sq.rt of n, in this
case, 1.4, or about 2dB (if I am correct), a modest improvement. 

Combining more than 2 signals that way (to get more than 2dB improvement)
gets complicated in a hurry.

I guess the idea behind differential locking was to simplify the circuit so
that a large n could be used to get meaningful improvement without too much
additional circuitry.

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Magnus Danielson
> Sent: Tuesday, December 23, 2008 12:03 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] New topics (was Re: He isa Time-Nut 
> Troublemaker....)
> 
> John Ackermann N8UR skrev:
> > Magnus Danielson wrote:
> > 
> >> This diffrential locking technique could be applied to atomic 
> >> standards, but then naturally require much improved solution than 
> >> simple oscillators. The diffrential locking technique does not 
> >> magically solve issues that is typically common mode, such as 
> >> temperature dependence. It can however even out individual 
> properties 
> >> like noise and systematic drift to some extent. It 
> essentially runs 
> >> the oscillators as a common constellation and attempts to 
> achieve the 
> >> average improvements of those oscillators in an 
> interlocked fashion. 
> >> In its simplicity it will do unweighed averaging. It is 
> fairly easy 
> >> to do weighed averaging by individualizing the feedback 
> gain to the 
> >> respective oscillators. Further refinements would 
> individualize the 
> >> proportional and integrate feedback terms, but as always, 
> the simplicity forms a limit.
> > 
> > Assuming that the atomic standards are correct for some 
> tolerance of 
> > "correct", I'm not sure why you would need to use a differential 
> > locking scheme (or anything else that moves one oscillator 
> versus the 
> > other) -- if you simply mix the two signals together you get a sum 
> > that contains both signals.  Apart from redundancy (what if 
> one unit 
> > fails), why not just use that sum to drive the clock?
> 
> Because they _WILL_ drift appart.
> 
> Interlocking them force them to a common frequency and average phase.
> 
> Cheers,
> Magnus
> 
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