[time-nuts] GPSDO Alternatives

David davidwhess at gmail.com
Sat Dec 8 02:38:52 UTC 2012

On Fri, 07 Dec 2012 17:50:55 -0500, "Charles P. Steinmetz"
<charles_steinmetz at lavabit.com> wrote:

>John wrote:
>>What's *really* interesting, though, is the idea that collectively 
>>we might develop some standard measurement protocols that would be 
>>reproducible in a number of (amateur) labs.
>I agree, but I didn't dare to dream so large when I wrote:
>>>  From my perspective, the most interesting development would be an offer
>>>by someone with a very well equipped lab to test any DIY GPSDO with a
>>>consistent protocol and publish the results.  That way, we could all see
>>>how the various approaches compare with respect to the characteristics
>>>that are most important to each of us.
>At bottom, any such testing requires (i) a comparison standard at 
>least as good (and hopefully at least somewhat better) than the DUT 
>at all taus and offsets (which may, in reality, be several standards, 
>each doing part of that job), (ii) a reliable TIC (and, potentially 
>usefully, frequency counter) that can exploit the stability of the 
>comparison standard, and (iii) the capability to process the raw data 
>to produce meaningful information.  [Additionally, to characterize 
>poor-signal behavior one would presumably use attenuators and a 
>well-situated antenna.  Some may not have good antenna sites to begin 
>with, and in any case, it would be hard to standardize the signal 
>strength between locations.]
>My thoughts were (1) for many (most?) of the people who would want to 
>build a DIY GPSDO, it would likely be their first "really good" 
>standard, and therefore their best; and (2) the range of 
>TICs/frequency counters owned by the target base is so wide, and 
>covers such a large range of capabilities (to say nothing of whether 
>any given counter is in good repair and being used to best 
>advantage), that obtaining comparable results from one amateur lab to 
>another would be just as much if not more dependent on the individual 
>counters involved than on the GPSDOs under test.
>However, that is no reason not to push forward with standardized 
>measurement protocols, which would focus all of us on what the 
>relevant desiderata are and how to measure them.

For myself:

1. My current lack a comparison standard is the reason I would design
and build a GPSDO.  At best I might buy a used rubidium oscillator at
some point.   People talk about good deals on Thunderbolts but I have
yet to see one.  It seems peak Thunderbolt passed before I was
seriously looking.

2.  So far my best universal counter is a rebuilt Racal Dana 1992 with
a TCXO but it lacks GPIB.  It might be easier and cheaper for me to
duplicate my GPSDO phase detector and add a counter chain and trigger
so it can make and report its own time interval measurements against a
secondary asynchronous source but that would hardly be reproducible by
a third party.

3.  I am less interested in this since I will be at the mercy of
whatever timing GPS I use and my current antenna environment.

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