Phase locked loop variable oscillator with switched capacitors

Abstract

A variable oscillator, suitable for integration as part of a phase lock loop (PLL) clock source in a complementary metal oxide semiconductor (CMOS) integrated circuit, includes an amplifier and terminals for connection to a tank circuit, for example a crystal resonator. Capacitors are alternately coupled and decoupled in relation to the amplifier to cause oscillatory operation at lower and higher frequencies. In the CMOS circuit the reactance is conveniently provided by conductive layers of predetermined dimensions being carried by an oxide layer. Each layer provides a capacitive reactance which is arranged in series with a field effect device being controlled by associated PLL control circuitry. The control circuit includes a strobe generator for strobing a phase detector in response to a data signal received by an associated receiver. Each time the phase detector is strobed it registers one of an early or late phase indication. The phase indication is used to control the field effect devices. The phase indication is applied via a register at an instance of consistent phase angle of the oscillatory signal from the output of the amplifier whereby glitch free operation is achieved. In one example, the variable oscillator is controlled by fast and slow control signals to improve convergence toward the desired frequency of operation.