[time-nuts] 10 MHz OCXO recommendations

Charles Steinmetz csteinmetz at yandex.com
Tue Nov 11 19:26:55 EST 2014

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,


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