Simple xtal oven for accurate clocks


From the comments on our earlier HP 5350B OCXO repair post, a temperature controlled OCXO xtal oven:

This is a very simple and easy to make temperature controller and heater to be attached to a xtal (crystal). The xtal is normally used to clock a microcontroller (PIC or ATMEL etc).
Normally xtals provide an accurate clock to 50 or 100 PPM (parts per million) making them useful for real-time clocks, like in your wristwatch. However their frequency output changes with temperature.

This circuit keeps the xtal at a constant temperature – commonly called a “xtal oven” also called an “Oven Controlled Xtal Oscillator” or “OCXO”. Hence the xtal error is reduced to 1 – 10 PPM and the clock will keep almost perfect time.
This circuit can be built with very common cheap parts and means you don’t have to find or buy an expensive xtal oven.
Coupled with my 1-sec PIC timer algorithm HERE you can use ANY value of xtal to build a very high accuracy clock.

Via the comments.  Thanks Nerd Ralph!


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1 Comment

  1. A nice project with good performance! Note that if you are putting styrofoam around it, probably the major heat leak is through the power and signal leads (copper wire is a good conductor of heat). So you can use thin gauge wire, and wrap the wire around the body of the device several times before exiting through the insulation. The old OCXO designs from HP have taken this to a high art form with symmetric connections, double shielded ovens etc.

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