[time-nuts] OCXO and fluctuations after EFC adjustment

[John Moran, Scawby Design]

On Fri, 10 April 2020 14:31:53 -0700 Rick wrote:


>At this time, I will give my usual speech about IMHO the fact that

>since the invention of the DDS on a chip, EFC should no longer be used

>for high performance oscillators.

During my 50 years in the electronics industry I have always been puzzled about one aspect of crystal oscillators. They go to great lengths to use a precise piece of quartz as the heart, because of its unique properties, and then add standard external components - capacitors, varactors, Zeners, etc. to tweak its frequency. All these components vary far more than the original piece of quartz ... hence my confusion.

I know it is practically impossible to grind a crystal to exactly the frequency you want, and it then drifts over time, but what is the logic of using relatively wildly varying components to adjust the quartz? Are their temperature and ageing characteristics swamped by the superior crystal?

In all the papers I have ever read, the subject is never mentioned ... you just add a variable capacitor and/or an EFC circuit and job done.

I guess this is showing my total ignorance here, but I would like to know.

Maybe this is at the heart of Rick's usual speech?

John