[time-nuts] Re: Testing GPSDOs

[Bob kb8tq]

Hi

If you go back in the archives, there have been a number of folks 
who have headed off on a “design my own” ( = not build a kit) GPSDO
projects. Many have fiddled a bit and dropped the idea. The ones that
have kept up and actually built something they know works have all 
wound up shopping for Cs standards and other similar gear. 

All of the work you do in the frequency or time domain requires a 
“better” standard to compare your design to. That is the unfortunate
truth of this field. 

So what to do:

If you are looking at 1 to 10 second ADEV, you grab a known better
OCXO. How do you know it’s better? You bought a pile of them and 
did cross comparisons. Compare A to B to C, keep the pair that looks
best, toss the other one. Step and repeat until you have sorted the group
of 50 to 100 OCXO’s. The pair that is reading X both do at least that well.
You now have a “home brew” standard for that range.

Once you get past 10 seconds you may well not find OCXO’s doing the job.
A lot depends on what you are after. You could start playing with Rb, Cs,
or a maser. Unless they are new / certified, you likely need more than one
(same process as above) to work out what you have. With the usual eBay
sourcing process, it depends a *lot* on how lucky you happen to get. 
( = you often get one with issues / needing repair …. )

If you went with an Rb, you will start wondering about what’s what out around
a day. That’s not to say at 25 hours it collapses, only that you are not as
sure of the result. A Cs or a maser now would be more useful. Depending on
the maser, the Cs will eventually win out ( is that days / weeks …. ). 

In all the cases above, the parts you want to work with likely aren’t easy to find.
You may well spend quite a bit of effort in the shopping process. Is that months
or years? Who knows. Something like a 5065 will be tough to grab and that’s 
only the start. 

Do you need all those standards? Do you care about all those time ranges? 
How many bugs do you want to track down? That’s all very much up to you.
This is something that once you are engaged, most will want to keep up with 
and find / fix the problems ….

So we’ve been comparing this to that. What did we use for the comparison?
A home brew single mixer setup is one quick / cheap way to go. With more 
time and debugging, you can do a DMTD. It might speed up the OCXO sort
process a bit. With a DMTD, you then get into needing two pretty good references.
That may or may not be a good thing. Something like a TimePod  would get
you going a *lot* faster. 

Phase noise likely needs to be checked along with ADEV and long term stability. 
Your mixer setup will get you the first two. A counter with long gate times might
help on the third. If indeed you are after frequency and are looking for the elusive
99.99% ( or whatever number) of samples < some number sort of goal, then 
a counter would be involved. 

Yes, the DMTD / single mixer has a bunch of gear associated with it. Tossing in
phase noise drags in decisions and gear. It all assumes this is on a single frequency
and you just care about that one output. 

So no, none of this is undoable. A number of list members have plenty of this
stuff in their basements. A smaller subset have fully working examples of this
and that in their basements. It’s a rabbit hole. Like most rabbit holes it can just 
keep on going and going ….

Bob

> On Feb 20, 2022, at 3:41 AM, Hal Murray <halmurray at sonic.net> 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?
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
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