Hi
The only gotcha is accuracy.
If that is part of the equation, then even a pretty dumpy OCXO properly GPSDO’d
will beat one that is a very good OCXO indeed.
A darn good OCXO will age down in the 1x10^-11 per day range. In a bit over 10
days you may be past +/- 1x10^-10.
An OCXO based GPSDO that holds 1x10^-10 “some of the time” is one that gets
yelled at and kicked a lot .
It all depends on what you are after.
Bob
> On Nov 11, 2014, at 7:26 PM, Charles Steinmetz <csteinm...@yandex.com> wrote:
>
> Mark wrote:
>
>> I find the concept of occasionally adjusting a good OCXO which in turn is
>> used as a reference works well for me. I have some that haven't needed
>> adjustment for over 2 years (they are still well within one part per billion
>> of being on frequency.)
>
> A few of us have advocated this approach on the list, and there is good
> reason for it. A GPSDO offers two advantages: (1) it is self-adjusting,
> therefore easy to own and use; and (2) it has better stability at long tau
> than the OCXO alone. The price you pay for those advantages is poorer
> stability at low tau than the OCXO alone, which can be anywhere from slight
> with a good design (e.g., Thunderbolt, Z3801) to shockingly bad with a bad
> design (including many DIY attempts).
>
> If one does not need the very best performance at long tau -- and most
> time-nuts do not -- a free-running OCXO that you adjust manually every now
> and then can be the best reference available to the average time nut. ("Long
> tau" can be anywhere from 100 seconds to several thousand seconds, depending
> on the particular OCXO.) Plus, not spending money on GPS discipline allows
> you to spend more on the OCXO to get better stability at low tau, and a more
> extended upper limit on "low" tau (say, better than GPS all the way to 2000
> seconds instead of 200 seconds).
>
> Personally, I do use GPS discipline to keep my best OCXO in "perpetual
> adjustment," but that is mostly for convenience. Usually, I turn
> disciplining off when I'm taking data. Only when I'm doing something where
> the data are averaged for longer than about 3000 seconds do I leave it on
> (3000 seconds is based on the stability of my particular OCXO).
>
> Remember, GPS has a well-defined stability floor, and is not better than a
> good OCXO at averaging times (tau) less than 100 or even 1000 seconds -- so
> GPS discipline cannot do anything to help the stability of a good OCXO at
> shorter tau than that. (Yes, it may be able to help a lousy OCXO or TCXO at
> lower tau -- but you can get a better OCXO than that for $20, so why bother?)
> There is so much focus on GPSDOs that I think many time nuts do not realize
> this fundamental fact.
>
> A few rules of thumb:
>
> -- An OCXO is the best low-tau reference most amateurs can afford
> -- GPS discipline cannot help at low tau because it is noisy
> -- Most of us do not need extreme stability at long tau
>
> And some general conclusions:
>
> -- Get the best OCXO you can find
> -- Enclose it (thermally isolated from the enclosure)
> -- Don't try to whip a so-so OCXO into shape with GPS discipline
>
> Finding a really good OCXO may take some effort. Some models are more likely
> to be "really good" than others (like the BVA that Mark mentioned, and some
> others that have been vetted in large numbers), but even then there can be
> large differences from sample to sample. So, one may need to sort through a
> number of them to find a "really good" one. If one doesn't have access to a
> clearly better oscillator for comparison, using the "three-cornered hat"
> technique with one's best oscillators is probably the best method available
> to the amateur time nut. Note that quartz oscillators tend to exhibit best
> stability if they are left on continuously, and stability may improve for a
> long time (months, perhaps even many months) after they are turned on,
> depending on how long they were off and how much trauma they received before
> being powered up again).
>
> The point is that GPS discipline is not always (and maybe, not usually) the
> best way to get the best stability possibile over the range of tau that is
> most important to amateur time nuts. Further, it takes very well-designed
> GPS discipline to improve things at long tau without making them worse at
> shorter tau, so GPS discipline can easily be a net negative (particularly
> since most of us do not need extreme stability at very long tau). So, a good
> OCXO that is manually adjusted from time to time as required will likely have
> the best stability most amateur time nuts can obtain, at the range of tau
> that is actually important for the applications to which it will be put.
>
> Best regards,
>
> Charles
>
>
>
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