[time-nuts] MHM-A1 maser temperature stabilization

[Skip Withrow]

Hello Time-Nuts,
I apologize in advance as this may be a long post, but I hope one that you
find interesting.

I have now had the Sigma Tau MHM-A1 maser running (most of the time) for
about a year now.  There are several problems that still need to be dealt
with, but it has become a reasonably useful instrument.  I have not
adjusted the synthesizer since October 2022 and it seems to be staying
within 1-3 nanoseconds per week (with a month delay in seeing the data).

One issue that I have been up against is the temperature variations of the
lab location.  The diurnal excursions of the oven heaters were clearly
visible.  My solution was to build an environmental box (1.5" foam
insulation board) around the unit and try to control the maser ambient
temperature with a simple proportional controller running a fan that moves
air through the chamber (fan at the bottom, hole at the top).

The maser has a temperature controlled (resonant) cavity inside the vacuum
chamber.  You can think of it as the inner oven.  Nominal heater voltage is
2 to 4 volts (about 25-100 milliwatts).  The vacuum chamber itself is
temperature controlled (Outer Oven heater) with a nominal heater voltage of
2 to 23 volts.  [Actually there is a Top Plate heater and Lower Support
heater too, but where one goes the others follow]  My tact was to look at
the OO heater voltage and try to keep it constant by controlling the speed
of the chamber fan.

This helped quite a bit.  However, the vacuum chamber is contained within a
layer of insulation, a magnetic shield, another layer of insulation, and
finally the magnetic shield of the maser box itself.  There is a
considerable thermal delay.  If I wanted to get fancy I'm sure the
controller could be improved.

Attached are the plots of the inner (cavity) and outer ovens for August
2022 (before the environmental box) and December 2022 (after box).  There
are still diurnal variations in the outer oven voltage, but they are much
reduced in magnitude.  Summer variations are due to open windows and house
fan turned on at night, winter variations are due to heat being turned down
at night.  Both months have periods when the house was not occupied, which
really helps to calm things down (Aug. 18-25, Dec. 21-27).  The variations
in the cavity voltage have almost disappeared now.

There was also an unexpected benefit that was not anticipated.  Because all
of the heaters have a smaller daily excursion in voltage the maser voltage
busses are more stable now as well.  This makes all of the sensitive
electronics much happier (and stable) I'm sure.

Regards,
Skip Withrow
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