[time-nuts] poor-man's oven

Chris Albertson albertson.chris at gmail.com
Sun Jun 4 16:49:29 EDT 2017

Look at the temperature coefficient of your XO.  Then figure a very simply
control loop and a thermistor will keep a block of aluminum within 0.1C of
a set point.  Use a decent size block and insulation

We drilled a deep hole then epoxied the thermistor.  I think this step is
important as you want to measure the center of the black, not the surface..
   Then epoxied a TEC (aka Peltier device) to the Al Block.  TECs are nice
because they can both cool and heat.   On the other side other TEC was a
rather large heat sink (or heat source depending on the polarity) a uP and
a PID loop controlled the output current.   Place an insulator over the
controlled end of the block.   I think a stainless steel vacuum insulated
coffee mug works well.

I think cheap thermistors are OK as they will never, in use, see
temperature swings of more then 0.1C so who care if they are linear or not.
They are all linear enough over a short range.  What you pay for is being
perfect over a 100C range, you don't need that.

We used the pelter because we preferred a cool "oven" to a hot one.   The
theory has that we get less electronic noise so we ran the TEC in cooling
mode.   But for your use a resistive heater would be cheaper.   But in
either case you see a coffee mug with a round chunk of Al shoved in and
heat sink fins showing.

When I was doing some contraction work, I thought of it would be fun to
toss an XO in the big hole that was going to get a truckload of concrete
poured into it.  The temperature down there would be very stable.

On Sun, Jun 4, 2017 at 5:13 AM, jimlux <jimlux at earthlink.net> wrote:

> I recall some years ago folks were talking about putting a PTC thermistor
> on the TCXO of a FlexRadio SDR1000 to stabilize the frequency as a sort of
> poor-man's OCXO.
> It's also referenced at
> http://www.setileague.org/askdr/xtaloven.htm
> where he says "order of magnitude improvement" with no numbers (from 1% to
> 0.1% or from 1 ppb to 0.1 ppb?)
> I wonder how well that actually works.
> Say you bought an inexpensive (perhaps non TC) XO and an equally
> inexpensive thermistor, glued on on the other, hooked em both up to 3.3 or
> 5V.
> Yeah, there's issues with room air blowing on it, and tolerances in both
> the XO and thermistor, so your absolute frequency accuracy may not be so
> hot. But what sort of medium to long term performance can one expect.
> I did some searches, because I'm sure we've discussed this before, but I
> couldn't find it.  There was some stuff from Oct 2007, but that was in the
> context of a more complex circuit, and the thermistor was the sensor.
> (discussions of TE devices too)
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Chris Albertson
Redondo Beach, California

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