[time-nuts] Low cost alternate to Dual Mixer/DMTD
jmiles at pop.net
Sat Oct 3 06:25:26 UTC 2009
> > If there is another contributor to phase locking then the full frequency
> > instability wont be reflected by the EFC input fluctuations.
> Don't get me wrong, in the beginning the oscillators pretty much
> acted like they were sync locked
> because of poor ground, common PS, non isolated RF output,
> etc,etc. That did not go away by luck.
> What I'm saying is they do not now know that each other exist to
> the level that I can test to, which is about 5e-13.
It's been my experience with the HP 3048A, which is basically the same thing
as what you have built, that at least 20-30 dB of reverse isolation is
necessary when two OCXOs (all 10811s and most of the Wenzel units I have)
drive the LO and RF ports of a single mixer. Make no mistake, this is not
something you can blow off in the general case. It'll be good to see some
comparative results once you have some additional isolation in place.
Have you tried modulating your DUT oscillator with white noise and FFT'ing
the correction signal to visualize the actual loop bandwidth? My guess is
that your effective LBW is somewhat wider than your design target, thanks to
the injection-locking bugbear.
That said, yes, your setup sounds like a very reasonable approach to
better-than-TIC grade timing measurement, especially with a digital back
end. It can certainly win the price-performance contest over other
architectures. It's great to see several different approaches actually
being tried, as opposed to speculated upon!
> Something that some seem to of missed is that this is not the
> standard RF circuit configuration with the standard open loop errors.
> This is a closed loop Neg feedback "PID type" freq control system
> where the errors inside the loop are reduced by the loop gain.
> The Only significant error outside the loop is the Osc its self
> and Osc's internal EFC offset.
> The EFC range being used during a measurement period is typically
> less than 1/1000 of its range.
As Bruce points out, that doesn't matter in the least; in fact, I'd submit
that injection locking is the very reason why you are seeing such a small
EFC correction range. Ordinarily two oscillators won't track that well in
response to shared environmental conditions alone. Fortunately injection
locking is the easiest (and cheapest) malady to fix!
Another interesting challenge will be to find a way to determine your true
residual floor. Because your measurement is frequency-based, you presumably
can't just feed both DUT and reference ports from a splitter and watch the
residual phase error over time, correct?
-- john, KE5FX
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