AFC LOCKING DECIDER

PURPOSE:To detect the locking state of an AFC loop by providing a lock discrimination means monitoring a timewise rate of change of a control voltage and detecting it that the timewise rate of change of the control voltage is smaller than a prescribed value so as to discriminate the lock state of an automatic frequency control means. CONSTITUTION:A locking deciding section 10 consists of an averaging section 4, a comparison section 5, and a threshold level latch section 6, and the locking state is detected by detecting that the output of a frequency error detection section 1 is smaller than the predetermined threshold level epsilon. Since a signal (x) is fed to a VCO 3 via a loop filter 2 in an automatic frequency control (AFC) loop, the oscillating frequency of the VCO at a succeeding time is varied with the signal (x). When the threshold level epsilon is selected in advance to be the level at which the AFC loop is locked, the locking state is decided by detecting the time when the output of the averaging section 4 is smaller than the threshold level epsilon.