[time-nuts] I've designed a GPSDO, but how "good" is it?

Bob Camp kb8tq at n1k.org
Sun Aug 16 21:04:40 EDT 2015


Here’s some of the “that depends” questions:

What is your stability goal? 

You talk about the NIST numbers on GPSDO’s. What level of stability are you after?

What is your end application? 

Is this intended as a lab standard, the reference for a radio, something else entirely? 

What is the destination? 

Is this heading towards a commercial venture or is it a basement project? 

What is the budget?

Do you have $200K to spend on this? Did the piggybank run dry at $100?

What is the timeline?

Does the project complete at the end of the summer, no matter what? Is it 
something that is worth another year or two of effort?

What is your background? 

Does all of the stuff we’ve been tossing around make perfect sense? (= you do
something like this for a living). Are we talking about a bunch of stuff that makes 
very little sense? (= you are just getting started at this sort of thing). 

Each of these twists and turns heads you off into a different set of further issues and 
likely some more questions. For a commercial venture, buying custom oscillators in 
bulk is a very normal thing to do. For a battery powered balloon carried reference, you
do things different than for a rack mount standard. Each of these projects people come up
with have its own unique drivers. 

Each of us in our replies, tries to guess what your constraints are or are not. In doing 
so we likely substitute our constraints for yours. The further our constraints  diverge from 
your constraints, the further off base our advice and answers will be.


> On Aug 16, 2015, at 3:39 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>> On Aug 16, 2015, at 12:31 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>> Anyway, to answer your question -- to measure its true performance you only need two things. 1) a phase meter (or time interval counter) that's good to 1 ns or better, and 2) a local reference standard that's maybe 10x better than the TCXO and the Adafruit GPS. Usually that means a cesium standard, or supremely qualified GPSDO, or equivalent.
> I have a frequency counter, but it’s not a phase meter. I have a scope, but I assume that trying to use a ruler with scope traces isn’t the textbook way of doing that. :D
> I have considered in the past buying a used rubidium standard off eBay, but have hesitated because I don’t know how much life there is left in the tube, and I just have to take it on faith that it’s stable and accurate. I have somewhat more faith in the GPS PPS, but clearly that has limits.
>> A number of us here on the time-nuts list have such equipment at home. And unlike professional labs, we will do it for free/fun if you loan the GPSDO for a week.
> I will happily *give* one to someone if they would be willing to help a relative newbie with this stuff.
> Just one though. They’re kind of expensive to build. :D
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