DIY precision ovenized crystal oscillator

[ddavidebor]

hi guys and girls.

i'd like to make, only for fun, a diy double-oven cristal oscillator, with PD microcontrolled-based logic and datalogging.

i've a lot of nice idea for isolation, heating element and thing like this.

but i've one question, maybe the most important:

what crystal buy?

in a crystal oscillator the most important thing is the crystal, so what tipe of crystal oscillator should i buy for get the best performance, at a good price? (10Mhz)
thankyou guys

[Rerouter]

the one with the lowest long term drift you can find, as you will be removing the temperature component that is the most likely to bite you,

[alm]

HP generally used SC-cut crystals in ovens (eg. the HP 10811). You might find some useful information in the Time-nuts archives.

[ve7xen]

You want one with a turning point at a high enough temperature that you can maintain it near the turning point and still not be impacted by environmental changes. SC-cut is good for this, but they're not exactly readily available.

AT-cut have a turning point near room temperature, but someone else on the board suggested using a peltier heat pump to maintain the temperature at the turning point. That might be interesting to investigate.

[ddavidebor]

Good idea, i'm full of peltier...

I'm going to do some test.

[JackOfVA]

If the object of the exercise is to learn about ovens, etc. about any crystal will work.

If the purpose is to build a highly stable oscillator, be prepared to pay a significant sum of money for the crystal. Typical off-the-shelf crystals priced at a few dollars are not hermetically sealed nor are they assembled with the fanatical care required of a precision crystal. Over time, contaminants leak into the typical soldered crystal case and they also outgas from the contaminants of the crystal holder material, such as the insulating base.

A high quality time base crystal will be hermetically sealed, perhaps cold welded at the base and will be built with material that will not outgas onto the quartz plate. Or, more accurately, will not materially outgas such that the crystal ages out of tolerance before the specified period.

Heating a crystal increases the outgassing rate, by the way.

Perhaps it would be worth looking for a defective high quality oven oscillator module and salvage the crystal, assuming it's still functioning. If you are lucky, the problem is defective component other than the crystal and the crystal can be salvaged. If the oscillator still runs, but has drifted beyond the ability of the trimming function to bring it  back into tolerance you might - emphasis on might - be able to use it in a slightly changed circuit and have it be on-frequency.

[ve7xen]

Your post inspired me to tear down a broken commercial OCXO I had tossed in the bin (luckily still there!) just last week. Perhaps it will be of interest:

https://www.eevblog.com/forum/reviews/trimble-ocxo-%27extreme%27-teardown/

[jmole]

I could do some research on this, but I wonder, it cooling a crystal just as effective as warming it in terms of temperature stability?  Seems like you could use a Peltier to cool the crystal down sufficiently to ensure a stable temp.

[gxti]

Crystals have an innate temperature (sometimes called turning point) where the temperature coefficient is closest to zero. That's the temperature that the oven will be set to, because it means slight fluctuations in the oven have the least possible impact on frequency. Cheap crystals will be cut for room temperature. OCXO crystals will be cut for temperatures that an oven can maintain, well above ambient.

There probably aren't crystals cut for cooled environments. Also, thermoelectric coolers are super inefficient and unlike an oven the waste heat isn't contributing constructively.

[ddavidebor]

I could do some research on this, but I wonder, it cooling a crystal just as effective as warming it in terms of temperature stability?  Seems like you could use a Peltier to cool the crystal down sufficiently to ensure a stable temp.

No, temperature coefficent increas as temperature decrease...