[time-nuts] Improving ocxo temp control

Azelio Boriani azelio.boriani at gmail.com
Mon May 21 16:15:27 EDT 2018

First, be sure not to measure your  HP52132A stability with the OCXO.
What is your reference source?

On Mon, May 21, 2018 at 7:12 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> Hi
> There are a lot of reasons an OCXO drifts. Temperature control is rarely the issue.
> More likely you are looking at the drift / wander characteristics of the crystal ( and
> components) in the OCXO. The simple answer is to leave it on for a while ( like weeks)
> to allow things to settle out a bit.
> The paper that Rick tossed up last week is a pretty good one in terms of temperature
> control issues in an OCXO and what the issues are.
> This all assumes you are in a fairly benign environment. If you have lots of drafts, put a
> cardboard box over the unit to shield it. If you room temperature is all over the place, then
> there are a lot of ways to get to ~ 1 or 2C sort of stability in a lab. What you do depends
> a *lot* on what your situation is.
> =====
> So, assuming you *do* want to improve the temperature control:
> First you need to take out what’s in there now. If it’s wandering around get rid of it. This
> involves tearing apart the OCXO.
> Now take a look at Rick’s paper and redesign the thermal enclosure. Get the heater placement
> and sensor placement right and feed that into a simple controller.
> Run some tests over temperature and check out the data. It’s likely your first guess at things
> is not going to be correct.
> Try to optimize the heat sources and sensors and re-test the result. Everything interacts so
> this is not a quick process.
> Once you are reasonably happy with where things are, start looking at a more fancy controller.
> A simple approach would be feeding thermistor voltage into some 24 bit ADC’s and then
> processing the result with an MCU.
> Ok so that’s all a bit much.
> =====
> What happens if you mess with the OCXO from the outside of the package?
> You change the heat loss out of the package. This increases the thermal gain. ( less power
> to increase the oven temperature by 1 C ). Assuming the original circuit was balanced
> out, you have made things worse rather than better.
> Ok so you do an enclosure with a fan it it so the heat loss doesn’t get less.
> You now have more heat loss and the same issue applies. In addition the fan and it’s
> nonsense probably haven’t done the poor little OCXO any good.
> When one designs a double oven, the inner oven is optimized for performance *with*
> the outer oven present. Equally, the outer oven is optimized for performance with the
> heat load (and dynamics) of the inner oven.
> =====
> Assuming you still want to head down this road, temperature controllers are no different
> than any control loop. The first place to start is a textbook on control loops and control
> theory. The basics of what a loop does and the terminology are what you are after. Anything
> advanced will assume you understand this part first.
> Next up are temperature sensors. Simple answer here is that a glass bead thermistor is
> the way to go. For heat, transistors are the normal go-to device. The controls loop takes
> in the thermistor output and spits out a voltage to change the current through the transistor.
> If you have the money for software licenses, the next stop is some good mechanical CAD
> that will feed into thermal modeling. From that you can work out a proper heat flow and
> gradient design. Assuming that is a bit to expensive, you are back to trial and error. There
> are no “just duplicate this” designs that I know of.
> Once you have the structure, sensors, heaters, and control you toss it into a temperature
> test chamber. That may be something fancy or something you put together. You run the
> gizmo over temperature and observe what it does. You then optimize the P,I,D coefficients
> in your control loop. Indeed you may not have all of them or you may have an extra one.
> =======
> Of course one could simply shop for a $20 OCXO on eBay. Even if you have to buy a
> dozen before you find a good one, it’s still cheaper / faster / easier / more likely to succeed
> than all the nonsense above. If this is a commercial design for a product you are going
> to sell, that does not work very well. The same fundamental answer applies. If you need
> better performance, shop for a better oscillator.
> Lots of fun !!!!
>> On May 21, 2018, at 12:23 PM, Club-Internet Clemgill <clemgill at club-internet.fr> wrote:
>> Thanks for your interesting replies.
>> What I am actually trying to do is the following:
>> I bough a small ocxo (size of half a ping-pong ball) that performs well (Abracon / AOCJY3_B 10Mhz)
>> Reaching about 5*10E-11 kind of MDEV at low point ("kind of"… because a I use an HP52132a as input to Timelab)
>> But it’s frequency is slowly drifting with time, with a quasi linear slope.
>> I wondered if placing it in a third ovenized enclosure could improve things.
>> I tried a few experiments but seems that the temp needs to be very accurately controlled.
>> Any similar experience ?
>> Could you suggest papers describing high performance analog or digital controllers ?
>> Thx,
>> Gilles.
>>> Le 19 mai 2018 à 16:09, Bob kb8tq <kb8tq at n1k.org> a écrit :
>>> Hi
>>> One key point about the need for “zero gradient”:
>>> Crystals and many other components are quite sensitive to thermal gradients. Very small
>>> fractions of a degree (as a gradient ) can have significant impact on the frequency of an
>>> oscillator.
>>> One of many “interesting things” about fiddling about OCXO’s.
>>> The equally frustrating thing about this is that unless you can tease kind paper authors
>>> into posting things ( thanks Rick !!) the papers are behind pay walls. I pretty much despise
>>> that practice. Referencing papers that send people off to spend money ….not so much.
>>> Bob
>>>> On May 19, 2018, at 12:03 AM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
>>>> In my experience, the oven temperature controller is rarely
>>>> the determining factor for static oven performance.  This article
>>>> explains what the real determining factors are:
>>>> http://www.karlquist.com/oven.pdf
>>>> An analog oven temperature controller will be limited in
>>>> its dynamics by how much capacitance you are able to
>>>> design with.  Digital controllers get around this as well
>>>> as having the capability of double integration for much
>>>> better transient response.
>>>> Rick
>>>> On 5/18/2018 11:03 AM, Gilles Clement wrote:
>>>>> Hi,
>>>>> I am trying to improve performance of an OCXO.
>>>>> Could you point me at a good design of a high resolution oven temperature controler please ? Preferably analog.
>>>>> Thx much,
>>>>> Gilles.
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