Ring oscillator type voltage controlled oscillator

Abstract

A voltage controlled oscillator includes plural cascade-connected unit circuits supplied with selection signals corresponding to an oscillation frequency. Each unit circuit includes a voltage controlled delay circuit, selection circuit and adder circuit. The selection circuit has a first input terminal supplied with an output signal of the voltage controlled delay circuit and a second input terminal supplied with the selection signal. The adder circuit has a first input terminal supplied with an output signal of the selection circuit, a second input terminal supplied with a feedback signal from a next-stage one of the unit circuits and a third input terminal supplied with the selection signal. The adder circuit adds signals supplied to its first and second input terminals to form a feedback signal. The output signal of the voltage controlled delay circuit in each unit circuit is supplied to the voltage controlled delay circuit in the next-stage one of the unit circuits. The output signal of the voltage controlled delay circuit in the unit circuit selected by the selection signal is supplied to the adder circuit via the selection circuit, sequentially supplies the output signal of the adder circuit to the adder circuit in a preceding-stage one of the unit circuits and feeds back the feedback signal output from the adder circuit in the first-stage one of the unit circuits to the input terminal of the voltage controlled delay circuit of the first-stage unit circuit.