Electronically-tuned thin-film resonator/filter controlled oscillator

Abstract

An electronically controlled oscillator capable of operating in the rf/microwave frequency range, and using a stacked crystal filter as the frequency determining element in the oscillator. A non-linear element including an appropriately biased high frequency amplifier has the stacked crystal filter connected in its feedback path and provides a loop gain of greater than 1 to meet one aspect of the Barkhausen criteria. An electronically variable impedance, such as a hyperabrupt junction varactor, is connected in the feedback loop along with the stacked crystal filter to controllably insert a phase adjustment into the feedback path, to be compensated by a phase adjustment by the stacked crystal filter, thereby to controllably maintain a loop phase shift which is an integral number of 2.pi. radians at the oscillator frequency, and to vary the oscillator output frequency about the frequency of the stacked crystal filter in a controllable fashion. In a further implementation of the electronically controlled oscillator, an overmoded stacked crystal filter is utilized which has a comb response, and a second delay stacked crystal filter is also employed in the feedback path for providing an adjustable phase delay for causing the oscillator to controllably operate at the respective responses in the comb of responses of the overmoded stacked crystal filter.