[time-nuts] Advantages & Disadvantages of the TPLL Method
didier at cox.net
Tue Jun 15 19:54:43 UTC 2010
I promised myself I would not get into this any more, but here we go again...
---- WarrenS <warrensjmail-one at yahoo.com> wrote:
> Charles posted:
> but the locked frequency will be different from both oscillators'
> free-running frequency and
> the EFC will not correctly indicate the test oscillator deviation
> because it isn't the only control input in the system.
> Good point and No argument (except for the deviation part)
> Because the EFC is the only control input THAT IS VARYING.
Any parasitic control input is a problem in that system, like any other system.
I thought the point of all this was to measure the noise of an oscillator?
If it is noisy (and they all are, to some level, otherwise you would not need to measure it), then its frequency (or phase) is varying.
If the test oscillator is coupled (via injection locking) to the reference oscillator, the test oscillator will force the ref oscillator to follow its noise without the need to move the EFC. The EFC voltage will be stable (because the oscillators move together), while you have two synchronously noisy oscillators. If you measure the EFC, you will be left to believe your oscillator is better than it is.
Please note that the effect is not simply a scaling factor, because injection locking is a non-linear effect, or rather it is a mostly linear effect over a typically very limited dynamic range. Small variations will be totally coupled, where larger ones could possibly unlock the oscillators, producing steps in the EFC voltage. Said another way, you cannot eliminate the effects of injection locking by post-processing the data.
Injection locking is a parasitic control input and it is a problem with ANY method that purports to measure noise. Ignore it at your own risk, but don't say it does not matter, unless you want to prove something we already know.
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