AW: [Re: thermal tips re expo converters:]

[terry michaels]

Message text written by INTERNET:ijfritz at earthlink.net
>I assume you are saying that the HP xtal osc is in an oven?  It would be
interesting to see exactly how they do this.  Remember, though, that
this is not a critical temperature control application(!) These units
use a specially oriented quartz crystal whose oscillation frequency has
a zero temperature coefficient. (Developed by a genius working down the
hall from me -- I saw it happen. He now has his own company making xtals
for frequency control.) So the oven just needs to hold the xtal near the
compensation temperature. Also, how long does it take the unit to
stabilize? And, what is the relative thermal coupling between the heater
and the xtal vs. the coupling between the heater and the sensor?  For a
typical oven, of course, the heater and sensor are tightly coupled, and
any small oscillations in the control loop tend to be smoothed by the
lag to the object being heated.  This is not the case for the chip
heater.  You can actually hear the servo-loop noise feeding to the audio
output in the SSM 2033 VCO if you tune the output frequency to above the
audible range.
<

Hi Ian:

The oscillation frequency of a quartz crystal has a zero tempco at only one
specific temperature.  HP and others cut the crystal so the zero tc point
is well above ambient temperature, then elevate the crystal to this
temperature and hold it there with a feedback loop, heater, and sensor.  I
don't know the specific parameters for the HP units.  I do have a schematic
for the control loop for a ovenized oscillator that drives a local
oscillator in a microwave radio transmitter.  I could send you the
schematic for this if you want to determine the loop characteristics.  This
unit uses a TO-126 type transistor as the heater element, which is screwed
to a small metal block.  The crystal is clipped into a machined cavity in
the metal block, and a thermistor is glued into a hole drilled into the
block.  The whole thing goes inside a larger block of insulating foam.  The
thermal coupling between heater, sensor, and crystal appears to be very
tight.  The power transistor heats everything up fast, probably less than a
minute.  I don't know how relevant this is to what we are doing, because I
don't know how critical the crystal temperature is or how closely it is
maintained, and I don't know the heat loss rate.

I just threw it out there for something to think about.

The analogy to a regulated power supply was just to point out you can
achieve a stable control loop with a highly non-symetrical charge/discharge
rate, possibly equivilent to a fast heating, slow cooling thermal control
loop. Of course, if I am proven wrong, then I disavow everything I just
wrote.

Terry Michaels