[time-nuts] notch filter for close in phase noise measurement

Bob Camp kb8tq at n1k.org
Sun Oct 2 17:17:29 EDT 2016


If you do a power sweep on a crystal resonance, it’s a very predictable sort of thing.
Essentially you “chase” the resonance up (or down) in frequency with the sweep. At some
point, it “snaps” and drops back to the low power line. I suspect that what is being observed 
is the “snap” as the crystal stops accepting power.

In some crystals, you can get a very observable effect at a few hundred microwatts. With 
other designs it’s 10’s of microwaves or maybe even up around a milliwatt. 

Liquid nitrogen cooled crystals anyone? :) 


> On Oct 2, 2016, at 4:49 PM, Chuck Harris <cfharris at erols.com> wrote:
> That is a most interesting suggestion.
> Suppose the filter crystal was pulled to the DUT frequency, and due
> to the inertia of its very high Q, was able to show you the phase noise
> variations of the DUT better than one might expect?
> -Chuck Harris
> Bob Camp wrote:
>> .....One thing you may be seeing is the crystal shift frequency as it is tuned to “accept” power from the source. 
>> With milliwatts of power flying around, that would not be unusual. 
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