Abstract: A muting circuit for an AM stereophonic receiver of the type in which a received signal is phase modulated with a sub signal and amplitude modulated with a main signal and in which noise generated under certain adverse conditions is eliminated. A first detection signal is generated when the level of a received AM stereophonic signal is lower than a predetermined value, a second detection signal is generated upon detection of a non-locked state of a phase-locked loop circuit used to detect the sub signal, a third detection signal is generated when a frequency deviation detection senses is that a free-running frequency of a voltage controlled oscillator in the phase-locked loop circuit and a frequency of the AM stereophonic signal are different in frequency by more than a predetermined value; and a fourth detection signal is generated when a stereo pilot signal is absent. A muting circuit is activated in response to any of first through fourth detection signals.

Abstract: In a current-controlled type division circuit, an auxiliary transistor is employed in conjunction with a division control transistor to allow a divisor signal, which normally flows through the division control circuit, to flow through the auxiliary transistor in the case when a division controlling signal applied to the base of the division control transistor falls below a predetermined level. By the provision of the auxiliary transistor, the divisor signal of a predetermined minimum is adapted to be flown therethrough to prevent the gain of the division circuit from exceeding a predetermined maximum.

Abstract: A quadrature receiver circuit in which the output frequency of a local oscillator is controlled to follow the free-running frequency of a voltage-controlled oscillator so that accurate four-phase demodulation is maintained independent of oscillator drift. The output control voltage of a phase comparator in a phase-locked loop circuit in the demodulator is coupled through a low-pass filter and a DC amplifier to the frequency control input of a voltage controlled local oscillator.