[time-nuts] Improving the stability of crystal oscillators

Brooke Clarke brooke at pacific.net
Wed Oct 10 15:46:47 EDT 2007

Hi Brian:

I used to use "Balco" (as far as I can remember) positive coefficient resistors 
that had specs similar to the Digikey units you linked to.  These were only 
available in values under 200 Ohms and have a positive linear coefficient.

The more common negative coefficient type like:
have a much larger change with temperature and so are a very common way to 
sense a set point.  When a resistor with the same value is used to make a half 
bridge the voltage change around the balance point is nearly linear.

There's a way  to fit a third order polynomial to the data and once you have 
the coefficients it's only a linear equation to solve for converting resistance 
back to temperature.

Have Fun,

Brooke Clarke
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

wa1zms at att.net wrote:
> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+brooke=pacific.net at febo.com RETRY
> Try DigiKey pn:  235-1123-ND  for a simple PTC Thermistor that can be
> soldered to the xtal case if one were so inclined. Do so at your own
> risk to the xtal!
> -Brian
> -------------- Original message ----------------------
> From: John Ackermann N8UR <jra at febo.com>
>>SAIDJACK at aol.com said the following on 10/10/2007 02:17 PM:
>>>In a message dated 10/10/2007 11:13:21 Pacific Daylight Time, jra at febo.com  
>>>>We  first built a small proportional heater circuit that we could shove
>>>>in  next to the crystal, and that worked OK.  We finally found a  source
>>>>for thermistors spot welded to a spring clip that would mount  snugly
>>>>directly to the crystal case.  With 12 volts applied, these  heated the
>>>>crystals very nicely.  With the heaters, we were able  to keep the
>>>>systems running year round with about 1/10 the service  calls.
>>>Hi John,
>>>interesting anecdote!
>>>Would you know if the thermistors are still available? Who made  them?
>>We bought them as a replacement part from Yaesu USA.  This data is 10
>>years or more old, but here you go:
>>Part Name	Yaesu Part Number	Murata Part Number
>>Posistor	G9090019		PTH507A01BG330N020
>>They used to be about $7.00 each.  Yaesu's parts-order phone number at
>>the time was (800) 255-9237.  I'm not sure if there's another source for
>>the Murata part; back then, we couldn't find one.
>>Amazingly, I still have a web page documenting all that we learned about
>>those cursed radios, including the schematic of our original homebrew
>>heater design: http://www.febo.com/hamdocs/d4art.html.  The quality of
>>the heater schematic isn't very good, but it's all I have.
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