[time-nuts] Yet another GPSDO project.

[Tobias Pluess]

By the way, I just did some more testing on stability.

I once tried to use the internal PLL of the microcontroller to boost the OCXO frequency to the max. 72 MHz which my microcontroller accepts. While this would greatly increase the frequency measurement resolution, it decreased stability because it appeared that the PLL had some frequency modulation at its output.
I just tested different PLL multiplication factors, and according to my tests, the stability of the PLL improves quite a bit if a power of 2 multiplication factor is used, e.g. if the OCXO's 10MHz are multiplied to 20 MHz or to 40 MHz.
So I let my GPSDO run with the PLL enabled and configured for 40 MHz, which increases the resolution by four. Then I compared the 10 MHz outputs of my two GPSDOs on the 'scope and made a time lapse video with my GoPro camera, which shows how the frequencies drift around.
The video covers around 2 hours with one picture taken every 10 seconds.

https://hb9fsx.ch/wordpress/index.php/2019/03/25/gpsdo-testing/

IMHO I have now a drift of max. 100ns in 2 hours, which gives a stability of 13.9e-12, right?

Tobias

________________________________________
From: time-nuts [time-nuts-bounces at lists.febo.com] on behalf of Leo Bodnar [leo at leobodnar.com]
Sent: Thursday, March 21, 2019 09:14
To: time-nuts at lists.febo.com
Subject: Re: [time-nuts] Yet another GPSDO project.

Tobias,
Have you measured resulting phase noise of the finished unit?
Thanks
Leo

> From: Tobias Pluess <tobias.pluess at xwmail.ch>
> sure, I believe you since my primary requirement was phase noise. This is because I'd like to use the OCXO as reference for my spectrum analyzer and also for my HP 8663 signal generator to do phase noise measurements
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