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Re: Crystal Ovens

> Does anyone know of a source for reasonably-priced crystal ovens
> that would fit a HC/18 crystal and run on 12 volts?

First off, you probably really don't need a crystal oven to be pretty happy 
with the results.  However, it's not hard to improve stability of this kind of
crystal oscillator, and if we can do it cheaply, why not?  :-)

The cheap hack I like the best right now is to take one of the button-sized PTC
thermistors, unsolder the lead from one side, sweat solder it to one of the 
flat faces of the crystal can, and then feed it from an 8V or 9V three-
terminal voltage regulator, grounding through the crystal can's ground lead 
(or adding one if the crystal doesn't have the third lead).  In fact, I often
use the lead I've removed from the PTC to ground the crystal can.

This, in effect, uses the crystal can itself as the oven, with the PTC as both
heater and thermostat.  Very cheap, very easy, and my experience doing this 
with various oscillators has been very good.  Just make sure you let the unit
run with the PTC on for a few days before you get too frantic about trying to
measure or adjust the frequency.  Once the crystal settles in at the new
operating temperature, it'll stay there.  If you're really finicky, or if the
oscillator in question is likely to be out in the wind, then notching a small
block of styrofoam to fit over the whole mess to insulate it some might help.
I've never bothered, since my RF widgets tend to run with the covers on, 
unlike my computers...  :-)

Down East sells these as the PTC-60, "60 deg Thermistor, RL 3006-50-60-25-PTC",
for $2.75 each.  Their web page is:


The web page they reference for docs on the part is:


The last page of that document discusses using the PTC thermistor as both a 
heater and thermostat in one device.  Similar devices are available from 
places like Digikey.  If you need one or two, Down East is a fine place to
get them.  If you need a bunch, it would probably be worth wading through the 
online distributor catalogs to get a better price per unit.

Bdale, KB0G
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