[time-nuts] 10 MHz OCXO recommendations

[Mark Spencer]

Sorry a few more points to mention.

If for some reason I am particularly concerned about the stability of an OCXO reference I will compare it to another OCXO and on occasion to a GPSDO as well while measuring a "Device Under Test."  This gives me some comfort that if am looking at the performance of a particular "Device Under Test" that any drift in the OCXO I am using as a reference would have been detected.   (It also gives me a reason to keep my stack of HP5370 and HP5335 counters running.) I don't expect this approach to give me absolute certainty of picking up drift or jumps in my reference but it does give me some comfort.

While I suspect this approach would not go over very well in a commercial lab vs buying a high performance cesium standard or H Maser (:  for hobby use it seems to work for me.   Timelab is also useful for collecting analyzing the data from the various counters.  I also trigger all the counters from the same 1pps source.

I typically compare my "best" OCXO's to my "best" GPSDO on a more or less continuous basis (from a time nuts perspective it's of some interest to look at their long term drift.)  From time to time I also cross check my "best" GPSDO against another GPSDO (:

Regards Mark Spencer

Sent from my iPad

On 2014-11-11, at 4:26 PM, Charles Steinmetz <csteinmetz at 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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