Abstract: A differential mixer oscillator employing two transistors in a differential pair as the sustaining amplifier for a quartz crystal resonator at some first high frequency. The bias current of the differential pair is modulated at a second high frequency derived from some second frequency generating source, typically a second quartz crystal resonator, and the product terms are available at the collectors of the two transistors for selective filtering. The differential mixer oscillator may be employed in multiples, and with a variety of current sources, such as a Colpitts oscillator circuit.