Regardless of isolated anecdotal data on one oscillator, it
is probably not advisable to change the set point. The majority
of 10811 crystals do NOT have a turnover, only a region of
low tempco around 82 degrees.
Instead of that, change the circuit to B-mode and optimize
the heat between the two heater resistors for maximum thermal
gain (you should be able to get 1000). Then change the circuit back to
normal.
Rick Karlquist N6RK
On 10/6/2010 6:59 PM, Perry Sandeen wrote:
Gents,
Some on this list mentioned having had less-than-optimal performance with their
HP 10811 series oscillators. James Miller G3RUH made a slight mention of
optimizing performance by readjusting the set point temperature in one of his
phase detector articles. He said though time consuming the results were worth
the effort.
Looking at the schematic it shows that a thermistor was used in a bridge
circuit. The final set point was obtained by using a selected resistor from
the HP chart. The value went from 1.33K for 80.0 C to 34 ohms for 83.8 C.
Although HP probably selected the best thermistor possible they drift over
time. Also complicating the issue is that the drift is non-linear. At that
time the only other choice for an accurate non-drifting sensor was a platinum
100 ohm sensor. While highly accurate the rate of change per degree is very,
very low which would require complicated amplification. Because of the
platinum sensor cost and the associated circuitry that choice would have been
cost prohibitive.
I suppose if it were designed somewhat more recently some sort of SS sensor
would have been used.
I don’t know how James Miller did his turning point process but perhaps this
might point someone in the right direction.
Regards,
Perrier
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