[time-nuts] EFC info on Trimble 34310-T OXCO
EWKehren at aol.com
EWKehren at aol.com
Sun Aug 24 13:07:08 UTC 2014
Charles
I agree with every thing you wrote and I am implementing many of your
recommendations. Forty years ago I bought a 15 foot Alu channel to make small
frequency counter housings, always small, and at the time I did have access
to a machine shop so I made end plates. Still have five foot pieces now I
cut then off in 1 lb pieces and use them for tbolt, FE 405 B, FE 5650 and
even a HP 10811 taken out of the can. As I said before am waiting for the
small spheres and will see what happens. Working on a GPSDO for the FE 5680A
and the FE 405 B I did find out the hard way what moving air will do. When AC
season started my 405 tests showed the AC cycling it has a digital tuning
resolution of 5.7 E-15.. The nicely assembled packaged unit ended up in an
other R&S chassis with bubble pack on each end reduced AC influence but you
can still see it. If you like to see some data contact me off list file is
to large to post. Picture of my Alu channel is attached.
Bert Kehren
In a message dated 8/23/2014 10:20:19 P.M. Eastern Daylight Time,
csteinmetz at yandex.com writes:
Ed wrote:
>I agree with your statement regarding the determination of the
>optimum time constant, but, as Bob Camp mentioned, temperature
>change has a significant impact on setting the value. My 'lab' is a
>non-airconditioned bedroom. My Tbolt doesn't have any active
>temperature control. If I set the time constant to the point that
>Lady Heather thinks is optimum, I see large swings in PPS offset
>when I open the window and the temperature changes by a few degrees
>C. If I leave the time constant at the default of 100 seconds, the
>swimgs are drastically reduced. Active temperature control is on my
>'round tuit' list.
Bert wrote:
>As to Ed's and Bob's comments our projects are not able to compete
>with commercial products and I do not think that should be our
>goals. Having spend extensive time on temperature control, I limit
>my self to 10 C and use fans on all Rb's and passive on OCXO's.
>Concern about vibration induced noise on the OCXO made me remove
>the fan on the tbolt. Added a lot of mass and now ordered some foam
>balls from China to fill the enclosure as some one recommended.
Well, yeah, it goes without saying (or at least I thought it would)
that one must keep the rate of change of temperature of the OCXO low
enough that its oven can keep the crystal temperature within design
bounds at all times. I just assume that any time nut would do this,
since it is extremely simple and costs next to nothing (look in the
archives for my previous posts about "metal boxes," "metal
enclosures," and "thermal capacitance" in connection with
OCXOs). Active temperature control is NOT necessary. Which is not
to say it's a bad idea, it's just not necessary to stabilize any OCXO
worth owning by a time nut. (I'm not sure the MV-89 qualifies, even
if you are lucky enough to get a good one. There has been some
discussion on this list about the temperature control loop being
quasi-stable and tending to oscillate or even latch under some conditions.)
I also see no reason why amateur efforts cannot surpass the
performance of commercial products, particularly if we assume that
the environmental conditions are limited to those encountered in
living space, not a radio shelter exposed to the elements at a remote
tower. That is why I've been critical of designs that aim only to do
"the best that can be done for $5," or "the best that can be done
with a small ARM and 3 transistors." Given good design, there is no
reason why an inexpensive DIY GPSDO shouldn't handily outperform a
Thunderbolt (using the same OCXO), with two conditions: (i)
environmental conditions are limited to those encountered in living
space, and (ii) performance during holdover is neglected.
The reasons why most DIY designs do not work as well as commercial
designs, even if they use OCXOs of equal quality, is that their
designers evidently cannot design ADPLLs of sufficient performance to
do justice to the OCXO. (This includes implementing whatever means
of phase comparison and sampling are chosen, the DSP loop filter,
sawtooth correction, and the NCO or DAC/EFC design.) Doing all of
this right isn't particularly expensive, it just takes a designer who
has the skills and is willing to devote the effort. As a mentor once
told me, "Good thinking isn't any more expensive than bad thinking."
Some of the performance gain would be in reducing the rate of
temperature change seen by the OCXO, either passively as I have
advocated and described before, or actively. The other main
improvement would be setting the PLL crossover out where it belongs,
which becomes possible when the rate of change of temperature is
controlled. Avoiding a few common mistakes would provide some
additional performance gains.
While the foam peanuts, which I mentioned in a previous post, are
helpful in some circumstances, I have never seen the need for them in
the case of an OCXO inside a cast aluminum box. In that post, I
mentioned my gut feeling that spheres (balls) likely pack too tightly
to allow sufficient air circulation. I think irregularly-shaped
pieces of foam (like packing peanuts), which leave much more air
space between them, are required. The intent is NOT to impede air
flow, but to randomize it.
One point that I think gets lost in many of these discussions: The
quality of individual OCXOs, even of the same model, varies rather
widely, and you often won't know how good a particular OCXO is until
you have run it continuously for at least 90 days (preferably 180 or
more). The job of any GPS discipline is to gently keep the OCXO on
frequency, without lowering its xDEV performance at tau where the
OCXO is better than GPS.
The most effective thing you can do to construct a very stable GPSDO
is to start with a very stable OCXO.
Often, this means buying a bunch of OCXOs (even if you have to do it
one at a time for budgetary reasons), selecting the best one(s), and
moving the rest along. This can take a long time, since you need to
run each new oscillator continuously for at least 90 days before you
can know how stable it is. The odds of finding a good example are
improved if you stick to models that, after long experience,
knowledgeable time researchers have found to be consistently
good. Alternatively, you can hope that your sample of the $20 ebay
wonder of the week will live up to the anecdotal report someone
posted, but the odds do not ride with you.
Best regards,
Charles
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