Phase synchronization circuit

Abstract

Voltage controlled oscillator 40 has an oscillation stoppage cancelling circuit 46 and a current/frequency converter circuit 44 which is a ring oscillator made by connecting inverters forming 3 stages like a ring. Oscillation stoppage canceling circuit 46 stops and releases oscillation of the ring oscillator by a control signal RS. One-shot circuit 3 has a pulse width adjusting circuit 60 which is made by cascade-connecting the inverters constituting the ring oscillator of current/frequency converter 44 and inverters having the same characteristics in 3 stages. When the PLL enters a synchronization field, a synchronization field detector 1 issues a detection signal C and an input switch signal SC; a selector circuit 2 selects read data; and oscillation control signal RS starts upon the rise of a pulse S.sub.IN. Oscillation stoppage cancelling circuit 46 thus stops oscillator 40 and keeps an output V.sub.OUT at H level. After a few bytes of the synchronization bit have passed, detection signal C rises and oscillation control timing circuit 50 starts control signal RS upon the rise of the read data. Oscillation restarts, but the time at which the output V.sub.OUT falls from H level to L level is delayed by half of the oscillation cycle T due to the delayed quantity of the ring oscillator. On the other NAND, the output SIN starts in synchronization with the rise of the read data RD but it is adjusted to a pulse width which is equal to the half of oscillation cycle T due to the presence of pulse width adjusting circuit 60. Thus, the time of fall of the read data and the time of the restart of oscillation substantially coincide. It is possible to bring the PLL into a synchronization-locked state at high speed without loop filter 30 of high gain.