[time-nuts] 10 MHz to 32.768 kHz converter

Attila Kinali attila at kinali.ch
Mon Mar 21 09:00:03 EDT 2016


On Sun, 20 Mar 2016 18:26:16 -0000
"Martyn Smith" <martyn at ptsyst.com> wrote:

> I  have a real time clock calendar chip that requires a 32.768 kHz crystal. 
> I want to feed it with 10 MHz signal instead, so it is synchronised to my
> main 10 MHz in a frequency standard I am designing.

Currently, all that has been discussed were digital solutions.
But what about using an analog approach?
If you have a 32kHz crystal oscillator, you can injection lock it
to the 10MHz signal, by dividing the 10MHz down to 128Hz, then use this
to form short (as in a couple of ns) pulses, which you then couple
to the crystal using a small (a couple of pF) capacitor.

Given that the crytsal has an accuracy of better than 100ppm, then
even a very weak coupling at 128Hz should be enough to keep it locked.
Upper bound on the jitter is 1/128Hz*100ppm=781ps (very simplified
calculation, but it should be definitly less than 1-2ns)

			Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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