[time-nuts] Re: Testing GPSDOs

[Magnus Danielson]

Hi Hal,

On 2022-02-20 09:41, Hal Murray wrote:
> kb8tq at n1k.org said:
>> Can you build this or that from scratch? Sure you can. Being sure that it
>> does indeed work correctly .. not so easy.
> Let's change the discussion a bit.  Assuming I have a GPSDO, home built or
> eBay, how can I test it with a limited budget?
>
> There is another possible tangle in here.  What if I don't have a good antenna location?  Is there a simple way to measure/plot the goodness of an antenna?  How does the goodness of a GPSDO depend on the goodness of its antenna?
>
Well, it is usually hard to measure the absolute offset errors yourself, 
but you can get started with stability.

So, let's assume you have a rubidium clock, which is usually available 
for reasonable money for most hobbyists.

The phase and frequency will be off naturally, and we can assume that 
there is a linear drift in there too. There will be both environmental 
effects and random noise effects, but let's assume that we can live with 
those limits to start with.

In this context, we can assume that the GPSDO is nominally tied in 
frequency and drift to UTC over the GPS link, assuming we do not have a 
major design-flaw which would become apparent anyway, so we can then 
assign the detected frequency error and linear drift terms to the 
rubidium. Similarly we assume the phase error comes from there. By doing 
out measurement, then removing the quadratic trend from the measurement, 
we end up with the variations of the GPSDO and the variations of the 
rubidium. Having a reference trace of the rubidium alone will help to 
see what is reasonably additional instability from the GPSDO. You can 
view this as phase and frequency variations as well as the many ADEV 
variants of your liking. Essentially, this is exactly what we do with 
TimeLab in a straight setup.

Once you hit the floor of your rubidium, getting a better reference or 
visit a friend with a better reference becomes worth the effort. I'd say 
you can come fairly far in this approach. I strongly suggest to log all 
the state of the GPSDO into a InfluxDB database and illustrate it in 
Grafana. If you can include the rubidium phase measures in that, you 
have a lot of useful in-loop and out-of-loop data to ponder over. Toss 
in additional environmental sensor readings to help with characterizing 
environmental effects.

Pulling in GPS/GNSS receiver state into the Grafana can help to identify 
events happening there to deviations.

First you will find a number of bugs, some will be harder than others. 
You are bound to learn a number of practicalities of implementing 
real-time control systems. Most of which a rubidium would be just fine 
for a long time.

So, you can learn a lot this way, for reasonable money. Once you've 
covered enough of those corners, improved performance and corner cases, 
that's when you need to step up testing further.

Cheers,
Magnus