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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Dec 23 17:25:44 UTC 2008


John Ackermann N8UR skrev:
> Magnus Danielson wrote:
> 
>> My intent is to get some stuff done in the lab during the vacation. 
>> (Desperatly trying to get some more on-topic discussions going).
> 
> Here are two questions that have been running around my head:
> 
> 1.  Following on from the discussion last week about trying to 
> synchronize multiple oscillators to improve phase noise, I've wondered 
> about a simpler tack:  take, for example, two 5 MHz atomic standards and 
> mix their outputs together, using the 10 MHz result to drive a time 
> scale.  Assuming the standards were of relatively equal quality, would 
> this provide a better time scale than using one of the standards alone?

I have been looking at mutual lock systems for say two 10 MHz 
oscillators. This mutual lock system behaves much as a PLL circuit (as 
contrary to the other mutual lock methods described previous) such that 
a phase detector detects the phase-difference between the oscillators, a 
loop filter filters this difference and then it is feed back with 
opposite signs to the respective oscillator. I envision that a common 
EFC input exists to the system. Such a system is kind of interesting in 
that within the PLL bandwidth, the two oscillators are locked and you 
see the average of frequency errors etc. Well above the PLL bandwidth 
the two oscillators is independent of each other and just above the PLL 
bandwidth they are essentially independent. As the two oscillators is 
phase-locked in, their outputs can be combined and noise reduction can 
be achived, which is the significant effect above the PLL bandwidth.

It is interesting to notice that the PLL operates on the difference 
between the oscillators, where as the EFC input is acting as a common 
mode action, which in the perfect world is not influenced by the 
diffrential PLL. Realities is however such that the EFC is not linear, 
and the need to regulate differently also ensures that the oscillator 
gain constant never really match, and therefore will also the 
diffrential PLL "leak-in" on the common mode behaviour.

I have not seen any reference to this type of diffrential PLL action to 
mutual locking. I've only seen discussions on mutual locking involving 
injection locking.

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.

I have intended to make some runs just for fun to see how this behaves 
in real life.

> Happy holidays!

I wish you all happy holidays!

Cheers,
Magnus



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