[time-nuts] Re: GNSS-disciplining a Ball FRS-C Rubidium Oscillator

[Bob Camp]

Hi

Turns out that getting one set on frequency is not as insane as you might think:

Grab a fairly cheap GPS module that puts out a PPS output. 

Feed one channel of a scope with it ( or at least trigger the scope with it).

Watch the 10 MHz sine wave move relative to the trigger 

You can (eventually) adjust the Rb to the point that the 10 MHz does not move.

Simple math: 

The Rb is likely to already be within 1x10^-8 of the correct frequency.

If it’s that far off, it will move 1/10th of a cycle per second ( 10 MHz period is 
100 NS and 10 ns would be 1x10^-8 ….). 

Watch it for 10 seconds and you can be reasonably sure it’s within 1x10^-8

Watch for 100 seconds and you are pretty sure of 1x10^-9 …. 

Since your GPS will have some jitter, things get exciting if you start to try to
go below 1/10th of a cycle. This isn’t a process that you can speed up a lot. 

At a bit over a day ( if you keep at it ) you get to 100,000 seconds and a cycle is
going to be 1x10^-12. For most folks, that is into the “good enough” range. For
the insane, you can keep at it for days / weeks / months. At the month level, the
resolution of your adjustment pot will become a pretty big issue….. Yes, there are
ways to attack that problem … :) :) 

Bob

> On Jan 14, 2023, at 4:49 PM, John Miller via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hello Everyone!
> A while back, a friend of mine who knows I'm into this hobby found a Ball FRS-C RuXO at an electronics surplus shop local to him. It works great, and outputs 10MHz with as many zeros as the best frequency counterI have access to can measure. I don't really know what to do with it, but I have thought about building a system to discipline it with a GNSS receiver. Any recommendations, thoughts, etc.? I suspect this thing may be out of my scope and I should find a new home for it.
> 
> Here are photos of the unit: https://photos.millerjs.org/?f=Ball+Efratom+FRS-C
> 
> Thanks,
> John
> KC1QLN
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