Clock extracting circuit

Abstract

A clock extracting circuit for use in a data recorder or the like to extract clock pulses included in input data transmitted by a self-clocking system. The circuit comprises a data window generator for generating data window pulses each having a predetermined duration and rising or falling in synchronism with the rise and/or the fall of the input data; a voltage-controlled oscillator for generating reference clock pulses of a predetermined frequency in accordance with a control voltage; and a phase comparator for comparing the phase of the data window pulse with that of the reference clock pulse, then generating the control voltage proportional to the phase difference, and feeding back the control voltage to the voltage-controlled oscillator to thereby control the frequency of the reference clock polses. The duration of the window pulses and the frequency of the reference clock pulses are selectively switched in accordance with the input data transmission rate, so that a locked state is immediately achieved in response to any change of the transmission rate to consequently ensure stable extraction of the clock pulses included in the input data.