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Re: Crystal Ovens

I can second this motion!.. I use these in many of my downconverters
including the old DEM Mode-S (which I still am not conclusively sure if it 
is working).

Even though they are spec'd at 9V.. I run them at 13.8V no problem.  Their 
resistance goes up with temperature, so they self heat until
they hit a steady state.  Not sure if running them at 13.8V makes them run a 
degree or too hotter... but it really doesn't matter as they run
hotter than the hottest day in Iowa anyway!! :-)

They DO draw a couple of amps of current at start up but idle at maybe
100 mA max at steady state.  Stuffing the downconverter with fiberglass 
insulation helps a bit from the wind cooling things down too rapidly.  Never 
noticed much of a performance differance with the "fiberglass dielectric" 
near the no tune filters.  I'm sure styrofoam
would work as well if you are a purist.

I got my PTC thermistors at DigiKey.. they are about $1.50 a piece if I 
remember correctly.

Fred W0FMS

>From: Bdale Garbee <bdale@gag.com>
>To: amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] Crystal Ovens
>Date: 28 Dec 2000 15:56:07 -0700
> > 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 
>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 
>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 
>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 
>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 
>run with the PTC on for a few days before you get too frantic about trying 
>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 
>oscillator in question is likely to be out in the wind, then notching a 
>block of styrofoam to fit over the whole mess to insulate it some might 
>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 
>for $2.75 each.  Their web page is:
>         http://www.downeastmicrowave.com/
>The web page they reference for docs on the part is:
>         http://www.thermometrics.com/assets/images/ptcnotes.pdf
>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 
>online distributor catalogs to get a better price per unit.
>Bdale, KB0G
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