Novel Circuit for a Stable Variable Frequency Oscillator

In the past, quartz crystals have been used extensively to control single frequencies. This paper describes a circuit where a quartz crystal exercises considerable control over a continuously variable frequency spectrum. The circuit is characterized by multiloop feedback, which enables oscillation to be maintained at three frequencies, with the corresponding voltages confined to certain branches of the circuit and coupled to each other through three mixers. The frequencies of oscillation, to be called f1, f2, and f3, are determined by zero phase shift conditions in the feedback loops and by one frequency condition, f1+f2 = f3. The frequency f3 is restricted by a quartz crystal resonator, whereas the frequencies f1 and f2 are tunable in such a manner, that as f1increases, f2 decreases and vice versa. The LC resonators which control f1 and f2 drift in the same direction when the ambient conditions change. Such a drift cannot influence f1 and f2, since the latter can change only when the LC resonators shift in the opposite direction. The presence of a quartz crystal in the feedback loop also improves the phase stability of the circuit. An experimental model has been tested, and shows a thirty to hundredfold reduction of the frequency drift with temperature, excellent short and long term stability and a remarkably small initial frequency drift with switching on.