Hewlett-Packard 10544A Crystal Oscillator
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Notes found on the internet: (see http://home.teleport.com/~oldaker/10mhz_construction.htm)
There are two basic internal differences in the design of the HP 10544 series and the HP 10811 series OCXO.
The major change is how the heat is generated in the oven heater and in the cut of the 10mHz crystal.
The crystal oven heater in a HP 10544 uses a heater control circuit to drive a Darlington transistor Q4 that controls the current flow to a 47 ohm Wire Heater Resistor (a film wire heater wrapped around the crystal aluminum casting) and this provides a variable heat source for the crystal oven.
In the HP 10811 series, the Wire Heater Resistor has been replaced with two Darlington transistors Q7 and Q8. These transistors do not control current flow to a heater resistor. They are the oven heaters. The heater control circuit drives these heater two transistors (Q7 & Q8) and the heat generated by these transistors is dissipated into a large heat sink surrounding the 10Mhz crystal. The oven heater control circuit maintains the oven temperature at approximately 85ºC (185ºF). This can burn your fingers.
The 10544A oscillators use a switch mode oven controller. An LC filter using a series 10 mH, 0.75A, inductor and a shunt 200 uF capacitor is recommended to isolate the oven controller switching transients from the oscillator when the oscillator and oven controller employ the same 10.6-11.7V power supply." This will come into effect if you share the same supply for the Oven Control with the Oscillator supply.
The other change involves the crystal cut.
The HP 10811A/B, using an SC-cut resonator, is very similar physically to the HP 10544 that used a AT-cut resonator. The SC-cut crystal has several advantages compared to the AT cut. It has a much smaller temperature coefficient, the resonator can be operated at a higher drive level, which improves the signal-to-noise ratio and short-term frequency stability without degrading the aging rate; it has faster warm up with less frequency overshoot. The SC cut is a doubly rotated resonator and requires much tighter angular tolerances when the crystal is cut from the quartz bar. The SC-cut resonator came into commercial production around 1980. The use of SC-cut resonators in oscillators immediately improved their performance of the stand alone HP 10811 series OCXO installed in their test equipment.