Bob said: a lot of good points
except for a couple of little things?
"With the large increase in gain, since it's a simple controller,
you will need to re-set the control point.
There's no handy integrator in there to crank the offset out for you."
Does not seem to be true, at least on the schematics that I've seen for
the 10811.
There is a low leakage cap in series with the Feedback gain resistor which
does provide the near infinite gain integrator function.
What I see is a Full PI controller circuit, with integrator.
It would also be easy enough to retune the "PID" values if one did make
something that caused it to become unstable.
So Yes, certainly something to be aware of but No, does NOT need to be a
big problem.
ws
**************
----- Original Message ----- From: "Bob Camp" <lists at rtty.us>
To: "'Discussion of precise time and frequency measurement'" <time-nuts at
febo.com>
Sent: Tuesday, March 30, 2010 9:18 AM
Subject: Re: [time-nuts] Making a HP 10811 better
Hi
There are a couple of problems you will run into.
The first is that if you do a very good job with the dewar flask, the
"dead
heat" (power of the oscillator, regulator etc) can indeed raise the
temperature of the OCXO beyond the oven control temperature.
The second is that the gain of the control loop is in part determined by
the
thermal resistance. Increase the thermal resistance by a very large amount
and the control loop gain goes way up. The control loop gain is probably
pretty high already, so increasing it by a large amount is likely to make
it
unstable.
The next thing is that control loop gain and effective thermal gain to the
crystal are not the same thing. Even if it is stable, a large increase in
control loop gain probably will decrease the effective thermal gain. The
reason is a bit complex. The simple answer is that the thermistor is not
mounted on the crystal blank, thus it "sees" something different than the
crystal.
With the large increase in gain, since it's a simple controller, you will
need to re-set the control point. There's no handy integrator in there to
crank the offset out for you.
Simply put, you design a dewar flask / vacuum bottle OCXO in a very
different way than one that's conventionally heated. Miliwatts of
dissipation do mater in a vacuum design.
Reducing drafts is a good thing. Moderating ambient variation hour to hour
is a good thing. Burying an OCXO in the back yard works, dunking it in a
big
barrel of water also works. Both require you to remember the waterproof
bag
before you take the final step :)....
Bob
****************
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]
On
Behalf Of Steve Rooke
Sent: Tuesday, March 30, 2010 8:12 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Making a HP 10811 better
Has anyone tried putting one of these in a thermos flask? I have seen
wide necked flasks available for food storage which should allow the
device to be inserted and it could be wedged in place with something
like foam blocks inside. I appreciate that this does not let the item
loose heat but it should not make itself too hot in the first place.
At least this should protect it from external temperature swings.
Yes, I do know that they actually make OCXOs like this and that's
where I got the idea.
Cheers,
Steve (slowly recovering)
*******************
On 26 March 2010 08:10, WarrenS <warrensjmail-one at yahoo.com> wrote:
I was wondering if there is any interest for a single oven HP 10811
improvement design kit of some sort?
Background:
I am now using a double oven 10811 as the Freq reference for my own
projects.
Some recent off line conversations made me realize that there are many
advantages to using a single oven instead of a dual oven 10811 osc.
Among them are:
1) probable does not need separate floating PS to work good, due to it
having better grounds
2) has accessible freq offset adjustment, The EFC input can be set to
near
zero for 10.000 MHz output
3) Does not need an outer oven driver
4) It can be opened up for simple internal mods
5) And most important, they are much more common for the average time
nut.
The only disadvantages I know of is that they change freq more with room
temp changes. Up to 1e-10 is what I've seen.
My idea of a reference is something that one does not have to worry about
things like it's PS and room temp sensitivities.
I think where possible things like temp, PS. Load, Freq stability, etc
should be made to have effects on its freq that are below the noise
level,
which is around 1e-12.
Here is my thought;
I know how to make major improvements to the 10811 so that the standard
stuff does not have ANY measurable effect on its freq.
Nothing really magic, mostly simple things like secondary PS regulators,
an
outer oven heater wrap and controller, some internal span and reference
voltage adjustments,
tilt it on its Zero G axes, add an RF buffer, isolate or have a less
sensitive EFC input, a fine freq adjustment pot, and probable others once
I
get into it more.
The goal being if I can measure something that causes it to change freq
at
the a-12 range, fix it.
I am only taking about basic superficial things, I'm not knowledgeable
enough to fiddle with its RF stuff which seems OK as is.
But I do know from my DVM design experience, the way they offset the EFC
has to have some major contributing factor to its 1/F freq noise, at
least
on some units.
It has long been my suspicion, that part of their noise grading process
was
actually selecting units that happen to have a low noise internal 6.3
reference, The one used to offset the EFC.
AND YES I do know none of this takes care of aging. The best solution for
that is to select a good one and Let in run continuously.
In my case aging does not matter much because I have it disciplined to
the
GPS thru a very slow loop.
Any additions things to consider and comments welcome on or off line.
By the way, I already have a long list of thing one could do to it to
make
it worse, so probable don't need any more of those ideas.
ws
****************
