This is frontend for fine oven temp control when two crystals from oven get in phase at 10Hz range the xor produce their pwm dutty 0-100% or 0-3.3V resolution. When booth crystals is in phase "phase locked" pwm out dutty is 100%, vout is 3.3V and current is zero. This way the temperature should respond very nicely. The load resistors-heaters are arranged in a circle around the edge of the oven, fine heater is about 0.5W. The rough temperature will be controlled by stm32 and an external temperature sensor and two PLLs until the frequency of both enters the phase range where further control is taken over by XOR and pid.
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