Phase locked loop circuit for demodulating suppressed carrier signals

A phase locked loop circuit for receiving a suppressed carrier input signal for use in a heterodyne receiver, comprising a mixer, a local oscillator, a reference oscillator oscillating at a frequency corresponding to an intermediate frequency, a first and a second phase shifter shifting the phase of the output signal of said reference oscillator by +45.degree. and -45.degree. respectively, a first and a second phase detector, a first comparator and a second comparator, a first and a second multiplier, a subtractor, a third phase detector and a change-over circuit, the operation of said phse locked loop circuit being such that the suppressed carrier input signal and the output of said local oscillator are applied to said mixer, the output of said mixer is applied to said first, second and third phase detectors, the output of said reference oscillator is applied directly to said third phase detector and applied through said first phase shifter to said first phase detector and applied through said second phase shifter to said second phase detector, the outputs of said first and second phase detectors are applied to said first and second comparators respectively, the outputs of said first phase detector and the output of said second comparator are applied to said first miltiplier, the output of said second phase detector and the output of said first comparator are applied to said second multiplier, the output of one of said first and second multipliers is subtracted in said subtractor from the output of the other, and the resultant output of said subtractor and the output of said third phase detector are applied to said change-over circuit changing over between the output signal of said third phase detector and the output signal of said subtractor, the output signal of said change-over circuit being applied to said local oscillator for controlling the oscillation frequency of said local oscillator.