A circuit for high frequency crystal oscillators

This paper describes a circuit for high frequency crystal oscillators using an AT-cut resonator in the UHF frequency range. This circuit can be built based on a conventional Colpitts oscillator circuit with a feedback capacitor between the collector terminal and the emitter terminal of a transistor. The new circuit has the characteristic features of low drive level and high negative resistance, while maintaining the equivalent input capacitance of the circuit. Analyzing the equivalent input impedance of the circuit clarifies the effects of the proposed circuit compared with the conventional Colpitts oscillator circuit. The performance of the circuit is verified by an experimental evaluation of a voltage controlled crystal oscillator (VCXO) with a 622 MHz high frequency fundamental (HFF) crystal resonator. This VCXO offers a frequency deviation of +/- 160 ppm or greater versus a control voltage change of 1.65 V +/- 1.65 V, a temperature frequency stability of +/- 30 ppm or less over the range from -40 degreesC to 85 degreesC, and a frequency stability of +/- 5 ppm or less versus a supply voltage change of 3.3 V +/- 0.3 V. A phase noise level of greater than -100 dBc/Hz at 1 kHz carrier offset frequency is demonstrated.