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Abstract

A voltage controlled push-push oscillator is provided having a variable frequency output over a range of frequencies. Usually, the range of frequencies is in the microwave range. The configuration is such that the collectors of a pair of transistors are tied together, and an inductive reactance is provided across the base and collector of each of the transistors, with the emitters of the pair of transistors being each connected to opposite phases (at the fundamental frequency) of a resonator which may comprise of one or more elements, bisected to provide an output tap at which an RF null at the fundamental frequency and an anti-null at the second harmonic exists, whereby the second harmonic output frequency of the push-push oscillator is derived. Particularly when the push-push oscillator operates at microwave frequencies, the resonator element is a microstrip line, having the output tap at the centre thereof. A source of DC voltage is connected to the commonly connected collectors, a source of variable DC tuning voltage is connected to each of the bases of the transistors, and a DC current source is connected to each emitter whereby a common current source can supply the sum of the emitter currents, whereby the RF voltage null (at the fundamental frequency) at the resonator centre is utilized to isolate the fundamental frequency signal from the current source, which is connected through a bias choke which in turn isolates the current source from the second harmonic. The output is twice the operating frequency of each transistor and is produced by each transistor to have the same phase relationship (push-push). The second harmonic is produced by the nonlinearities of the transistor pairs which oscillate at the fundamental oscillating frequency whereby the fundamental signal is opposite in phase with respect to each transistor (push-pull). Output power is derived from the transistors so as to maintain the same phasing at the second harmonic and opposite phasing at the fundamental oscillating frequency whereby the fundamental is cancelled and second harmonic added, such as when a centre tapped resonator with even symmetry about the output tap is used as the output.

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