Not quite as simple as the PTC, an alternative may be:

http://shop.kuhne-electronic.de/kuhne/en/shop/accessoires/crystal-heater/Precision+crystal+heater+40%C2%B0+QH40A/?card=724

No it probably doesn't hold the crystal at it's optimum turn over
temperature, but it will keep the temperature of a crystal approximately
constant especially on a windswept hilltop.

On 05/06/2017 09:35, Ulf Kylenfall via time-nuts wrote:

--
Stephen Tompsett

There's a few 'OCXO' designs out there, I'm not qualified to comment on the
timenutty quality of them but someone else mentioned Hans Summers offerings
and I would offer Roman Black's simple design (if it's not been mentioned
already):

http://www.romanblack.com/xoven.htm

I've no idea if it's useful but it's ridiculously simple to implement.

Both Hans' and Roman's designs are 'on the list' of things to try for
myself at some point.

On 5 June 2017 at 09:56, Stephen Tompsett stephen@tompsett.net wrote:

--
Clint.

No trees were harmed in the sending of this mail. However, a large number
of electrons were greatly inconvenienced.

Hi

The posistor approach to heating a crystal was originally pioneered in Russia. Morion was
building vacuum insulated / PTC controlled OCXO’s long before the PTC parts started showing
up more generally in the 1970’s.

Bob

Here is a national new-technology of the art crystal oven from 1956:

http://tf.boulder.nist.gov/general/pdf/1642.pdf

Using the phase change properties of p-dibromobenzene it keeps temperature constant to 0.01C. It notes other organic compounds can be used for different temperature ranges.

Did this oven technology ever get beyond lab use and into the real crystal oven world?

I know in the past decade "thermowax" has been used in Honda lawnmower auto-chokes. I don't think it's ever supposed to be anything but solid, but it undergoes a phase change that causes it to expand by a large fraction.

Tim N3QE

Sent from my VAX-11/780

Wax is also used for thermostatic valves in engine cooling systems and
domestic heating systems.

On Mon, Jun 5, 2017 at 1:13 PM, Tim Shoppa tshoppa@gmail.com wrote:

Look at the dates on the uses of these exotic temperature controllers.
Organic compounds and positive temp coefficients and so on.  All these were
used before the current era.  Today all you need is a reliable way to
measure the error between the crystals' current temperature and the set
point.

Those exotic methods were good back when controllers were analog devices
build with op-amps and precision resisters and every math operation cost
you one more op-amp.  Today we can do a million floating point operations
per second for the cost of one kind good op-amp.

The trick with using a uP and it's built-in A/D converters is scale.  You
want the limited 10-bits of revolution to fall over the operating range
which is very narrow, like 1C.  Anything outside of that is either 0000 or
1111 and only seen at start-up.,  So at start up the the controller is on
"bang-bang" mode then later you have milli-degree resolution over your 1C
range.  Basically you are measuring noise. but your $2 uP can take 100,000
measurments per second and putt tour a digital filter.

Today we can do things the old-time designers even as recent as the 1980's
would not even dream of doing and it cost under $10.

On Mon, Jun 5, 2017 at 1:35 AM, Ulf Kylenfall via time-nuts <
time-nuts@febo.com> wrote:

--

Chris Albertson
Redondo Beach, California

Chris wrote:

That's all that's ever been needed.  But it is devilishly difficult to
measure the actual quartz temperature, or even to find a good proxy that
is easier to measure.

There is a fair body of published research on these topics, including
Rick's (et al.) work on zero-gradient ovens.

Keep this in mind when someone says they are controlling the "oven
temperature" to 0.001C (or even 0.1C).  They are measuring something,
and may even be holding whatever it is "constant" within fairly tight
tolerances.  But they have no idea what the quartz temperature is, and
no way to know with precision the relationship between the measured
temperature and the actual quartz temperature.

Some years ago, I consulted for a research group that was using a number
of non-contact technologies to measure the temperature of oscillating
quartz crystals.  The results were promising, but there were some issues
with measuring the temperature (which is, essentially, quantifying tiny
random molecular motions within the crystal lattice) against the
background of the hugely greater macro motion of the vibating quartz.  I
never knew the final conclusions, nor am I aware of any systems designed
using these principles or methods.  But it is something to think about
if you really want a temperature-stable oscillator.

Best regards,

Charles

On Mon, June 5, 2017 5:38 pm, Charles Steinmetz wrote:

In most cases what you really care about is the stability of the
frequency, and the temperature of the crystal is just a proxy for that,
correct?
I thought there was some effect where different modes of oscillation
shifted by different amounts with temperature, and if you had two
oscillation circuits running from the same crystal but different modes,
you could use the shift in difference frequency between the two modes to
infer the temperature change.

Found a reference in the Vig tutorial:
S. Schodowski, "Resonator Self-Temperature-Sensing Using a
Dual-Harmonic-Mode Crystal Oscillator," Proc. 43rd Annual Symposium on
Frequency Control, pp. 2-7, 1989, IEEE Catalog No. 89CH2690-6.

As is shown in chapter 4, see “Effects of Harmonics on f vs. T,” the f
vs. T of the fundamental mode of a resonator is different from that of
the third and higher overtones.  This fact is exploited for
“self-temperature sensing” in the microcomputer compensated crystal
oscillator (MCXO). The fundamental (f1) and third overtone (f3)
frequencies are excited simultaneously (“dual mode” excitation) and a
beat frequency fb is generated such that fb = 3f1 - f3 (or fb = f1 -
f3/3). The fb is a monotonic and nearly linear function of
temperature, as is shown above for a 10 MHz 3rd overtone (3.3. MHz
fundamental mode) SC-cut resonator.

The graph shows a line with slope of around 80ppm/deg C.  Not sure what
that translates to in terms of what you could realistically measure and
use for frequency compensation.  I guess you could use that information to
either control an oven or just let the crystal run free and control a
synthesizer for the used output.

--
Chris Caudle

On Mon, Jun 5, 2017 at 3:38 PM, Charles Steinmetz csteinmetz@yandex.com
wrote:

How much does this matter?  What we measure is the ambient temperature
inside the insulated box that contains the crystal.  The assumption is
that given some time the temperature will be uniform. OK, so the assumption
is not 100% correct but lets say the crystal is held to a range of 0.1C
This is a reasonable goal for a home shop made controller.

Here is another question:
Lets assume we place the operating point on the flat part of the curve with
say a 1.0 C absolute error and can hold the relative temperature to 0.1C
What does this mean in terms of frequency.?

This is a "Poor Man's" oven.  So really the question is this:  I have a
$10 budget, shouldI blow half my budget on a better sensor or is the 75
cent part good enough.  Or is it worth buying a second 75 cent sensor so I
can detect a temperature gradient  With a poor man's budget, I think the
trick is to use a good size thermal mass, chunks of scrap meter are cheap.

--

Chris Albertson
Redondo Beach, California

Hi

For further info on just why the crystal temperature is such a crazy thing to track, check out
Rick’s paper. At first glance it seems like it’s a trivial thing. In reality, gradients are a very
big deal.

Bob

Hi

That paper is the basis for the MCXO. It is an interesting way to do a TCXO.
The drift between the two modes makes it a difficult thing to master in an OCXO.
Plating a pair of electrodes (one pair per mode) is also an approach that has been
tried.

Bob

I have never been able to find a reference to them on the internet but
there was a similar product intended for TO-99 packages that could be
used with operational amplifiers.

On Mon, 5 Jun 2017 08:35:35 +0000 (UTC), you wrote:

Hi

For a while, a couple of outfits made TO-5 and TO-8 “cap heaters” with
PTC material. There are still a few obscure places that people do the same
sort of thing with a mini-pcb based design. In an OCXO design, the gotcha
is matching the PTC oven temperature to the crystal turn. You can do that if you
have a substantial inventory of material and custom fab the oven after the crystal
is bult.

Bob

In message A9E08DB0-2CB4-46EE-BC6F-3F3F9BC65931@n1k.org, Bob kb8tq writes:

Bob, you of all people must be able to answer this:

OCXO's have specified temperature ranges for instance
-40...+70°C for the heavy duty stuff.

But I cant imagine the ovens used are so perfect that they have the
same regulation performance at all temperatures.

I can choose the exterior temperature, which I should prefer ?

Disregard aging of electronics and materials, we all know that
stuff, what I'm interested in is at which exterior temperature
OCXO ovens work best ?

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

On 6/6/17 4:26 AM, Poul-Henning Kamp wrote:

Here's my guess...

1. you want minimal gradients across the device for a variety of reasons
2. therefore you want least amount of heat from the heater
3. therefore somewhat below the setpoint of the heater.

How far below? I've no idea.

On Tue, 6 Jun 2017 05:52:56 -0700
jimlux jimlux@earthlink.net wrote:

I counter that guess! :-)

1. You want the control loop as stable as possible
2. Stability is directly related to controllability
3. The larger the heat flow, the better the controllability
4. therefore the outside temperature should be as low as possible

Do we have to battle now, to see who is right? :-)

			Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson

Poul-Henning wrote:

Jim replied:

It depends on what you mean by "best."  "Best" can mean "minimizes the
wander in oven-regulated temperature at a constant (or slowly-changing)
ambient temperature," or it can mean "fastest recovery when the ambient
temperature changes more rapidly."

Minimum heater power tends to favor the first, but be careful -- this
means a low available (maximum) power, not just using a high-powered
heater at a lower output.  This is because the rate of temperature
change for full-scale heater swings is proportional to maximum heater
output, not to the ambient-to-oven differential, and this "granularity"
of the heater control function is what determines the oven wander at a
constant (or slowly-changing) ambient temperature.

A higher differential between the set point and ambient, and higher
maximum heater output, are necessary for fast recovery from larger
and/or faster changes in ambient temperature (i.e., to achieve a higher
dTemp/dTime).

Note that the above is why, for an oven controller with decent loop
gain, it is not necessary to control the ambient temperature very
tightly -- it is only necessary to slow down the rate of change in
ambient temperature to the point that the loop can track it to the
required tolerance.  See previous list discussions of "cast aluminum
boxes" and "thermal capacitance."

Best regards,

Charles

On Tue, Jun 6, 2017 at 7:34 AM, Attila Kinali attila@kinali.ch wrote:

I think you are correct but within reason of course.  It is easy to see
that the extremes can't work. If the internal set point is very close to
ambient the oven is uncontrollable.  because you only use the first bit of
the DAC to control the heater and after a few seconds you have overshoot.
Moving the set point up lets us use the full range of current on the heater
can gives us 8 or 10 bits of control and the rate of change is slow enough
that we have time take thousands of samples and see a rate of change in
temperature.  The PID algorithm needs something that is slow to change
compared to the control loop cycle.  So you want a good size thermal mass
compared to the amount of heat.

At the other extreme, where the set point to far above ambient we would
need to run the heater full time and also loose control.  So I disagree
with #4 above.  The heater would have to run full-on at 100% duty cycle.
(In other words avoid using liquid nitrogen baths)

There is an optimum were it peaks but I don't know how to find it.    Look
at the specific heat of the thermal mass (likely you are using aluminum)
and multiply that by the mass and I think you want that to be large
compared to the heat from a full-on heater so that the rate of change looks
slow compared to your control loop cycle.

--

Chris Albertson
Redondo Beach, California

Hi

As you point out, there really is no answer that is obviously better than all the others.
If you keep the outside warm (say 40C) you will reduce the strain on the heater devices
and likely not degrade the MTBF of anything outside the OCXO by very much.

Bob

On 6/6/2017 4:26 AM, Poul-Henning Kamp wrote:

My experience has been that, while ovens may not be perfect,
they are inherently linear.  So the exterior temperature is
a don't care.

I extensively characterized the E1938A oven.  By adjusting
the ratio of heat to the top/bottom vs edge, I was able to
get the thermal gain into the millions.  At that point,
finally a modicum of non-linearity showed up and the thermal
gain varied with ambient temperature.  It might be 2 million
at the ambient where I adjusted it, and drop to 1.5 million
when well away from that temperature.  Or it might change
sign at some ambient.  (Yes, you can have negative thermal
gain).  You shouldn't need to worry about this for any ordinary oven.

Rick N6RK

On Tue, 6 Jun 2017 11:00:57 -0700
Chris Albertson albertson.chris@gmail.com wrote:

You are looking at one minor issue here. Of course, if your control loop
is only of the bang-bang kind, then you will have a hard time to keep
the system parameter stable. But that is easy to deal with if one knows
the overall system.

I was talking about the control therotical "controllability":
https://en.wikipedia.org/wiki/Controllability

For a simple system like an OCXO that is mostly dependent on
thermal mass vs thermal flow (both in and out) which translates
into "delay" between the heating element and the temperature sensor.
A lot of people assume that adding more thermal mass is going to make
the control more stable. But in reality this might make it unstable
(aka cause oscillations) or make the control error larger.
What happens here is that the mass takes time to heat up. During heat
up you can describe it like a (very slow) transmission line. It will
take time until the signal (heat) reaches the other end (center of the mass).
When the heat reaches the sensor, the control electronics will dial the heater
down. But there is still a heatwave traveling inwards, ie the core will get
warmer and warmer. Thus the heater will be dialed down more and more until
it doesn't heat enough. This will cause a cold wave traveling inwards..

Having a larger thermal flow vs mass helps against this problem.
Good thermal conductivity between heater and sensor helps as well.
(that's why you will read often, that the sensor should be placed
close to the heater and not to the quartz)

The control loop does not need something slow to change. You need to
factor the termal mass, its insulation etc into PID parameters so you
get a stable loop. As I have shown above, if this is not correctly done
you will get oscilations. One way to avoid them, if your physical system
is fixed, is to lower the loop bandwith and thus make the system respond
much slower to changes than the time it takes to conduct the heat from
the heater to the sensor. But this means also that the control loop will
be slow to react to changes in the environment.

There are more sophisticated control loop designs that can handle this
better, eg by using two temperature sensors, one at the crystal and
one at the heater. But designing them correctly is more difficult
than the normal PID loop.

		Attila Kinali

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

On Mon, 5 Jun 2017 20:21:10 -0400
Bob kb8tq kb8tq@n1k.org wrote:

That's the first time I hear of modes drifting respective to eachother.
Do you have any references I could read on this?

I always wondered why the MCXO approach was not used more often.
Or why none of the OCXOs used a dual mode approach to sense
the temperature of the crystal directly instead of using a
thermistor.

		Attila Kinali

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

Hi

If you do the classic MCXO with two oscillator circuits and one resonator, the issue is
pretty simple. You have a load capacitance on the fundamental. You have a load capacitance
on the third overtone. Even if it is the exact same capacitor, the tuning sensitivity on
the fundamental is different than the sensitivity on the third overtone. As the load impedance
changes (parts do drift) the delta between the two modes will show up as an offset between
them. If you run through the math, it gives you a delta temperature. How much? How fast? Obviously
that depends. When I brought this up at the time with the authors of the paper, the reply was that
a recalibration of the MCXO was provided for for this reason.

Bob

Keeping with the thread topic, I think this is the key.    For the cost of
only one more cheap sensor you gain a lot.  Harder design as you say but
getting help on-line seems to be free.

I have gotten PID to work myself with linear systems (motor speed) and I
reading up on Kalman Filters as I need them for navigation using multiple
sensors.

I guess one could use the crystal frequency as a measure of its temperature
to tune the system.  Is there a name to Google to read up on using two
sensors and a pid-like algorithm?

Chris Albertson
Redondo Beach, California

Hi

In the case of a second sensor, “at the crystal” effectively means “inside the crystal package”.
That heads you into all sorts of “interesting” problems. Better to just read the papers and do
it the “old fashioned” way.

Bob

In the E1938A oscillator, we used a PIDI^2 loop.  IOW, a PID
plus a double integrator.  This was Len Cutler's idea.
Once the constants were dialed in, this worked phenomenally
well in terms of transient response.  Even dumping in liquid
nitrogen full throttle into the environmental test chamber
barely wiggled the crystal temperature/frequency.

Rick N6RK

On 6/6/2017 3:16 PM, Bob kb8tq wrote:

I don't understand what you are talking about here.  The tempco
difference between modes is unrelated to load capacitance.  The
dual mode idea would work just as well if the oscillators
operated at series resonance.

[I attended this talk in person ~25 years ago; it got a lot of
interest].

The reason why the SC cut mode C and mode B dual mode patent
from HP fell out of favor was the problem with activity dips
in mode B.  Otherwise, it was a great idea.  It would still
be fine for an OCXO, where you just avoid activity dips.
However, the circuit design is very complicated.

Rick N6RK

Hi

The circuit that Stan Shadowski presented is a fundamental / third overtone dual. The example
below is based on that circuit.

Let’s say both modes are running into a 32 pf load and it is a single capacitor.

The capacitor changes due to aging by 1 pf, you now are at 33 pf load.

The fundamental changes frequency ~ 3X as much (in ppm) as the third overtone.

The beat frequency shifts since the two modes do not tune identically.

Beat frequency shift = temperature error.

Yes the example is a little contrived. The real numbers would depend a bit on the design of
the crystal used.

Bob

On Tue, 6 Jun 2017 19:07:29 -0700
"Richard (Rick) Karlquist" richard@karlquist.com wrote:

The E1938 is kind of special in several ways, IMHO.
Beside having a nice zero-gradient design (I really love
this idea :-) it has also a quite large surface vs volume.
This means that there is a lot of heat flow out of the
can and at the same time the (face) heater is large, which
makes the PI loop better behaved. Also the thermal mass of the
crystal holder is quite small. Especially compared ot the 10811.

Due to the flat puck design, I assume that the majority of the
heat going to the crystal holder is due to radiation (unless I
missed some insulation around the crystal). Radiation, albeit being
a high "resistivity" transport mechanism, has very low inherent
"capacitance". And thus the delay associated with this transport
mechanism is low.

There are probably a lot more small design decisions in the E1938
that make it such a superb oven. More than I probably will ever
be able to figure out. And, I still am astonished how well it works.

		Attila Kinalid

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor