Abstract: A compact temperature controlled crystal oscillator employs a high conductivity heat spreader bonded to one side of a printed circuit board into the crystal. Heaters are arranged at edges of the circuit board and a temperature sensor for the temperature regulation circuitry is centered in the circuit board with components of high thermal sensitivity being placed in zones closer to the temperature sensor. An operating temperature of the heat spreader is selected by measuring multiple operating temperatures at different ambient temperatures and picking an operating temperature that causes a least absolute frequency deviation.