Re: [time-nuts] 10 MHz OCXO recommendations
Just to add a note on the original question : there are some brand new
never used Vectron 8091s available for a reasonable price now and it's
drift and jitter have been on par with my Wenzel.
I am not yet set up to measure it's phase noise or other general rf
characteristics but according to its cut sheet they're quite good.
On Tuesday, November 11, 2014, Mark Spencer mark@alignedsolutions.com
wrote:
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@yandex.com
javascript:;> 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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