Patent ID: 6803815
Filing Date: 2004-10-12
Classification: H03D

Abstract:
An amplitude modulation demodulation circuit comprising:an amplitude modulation detector which produces an amplitude modulation detection signal based on an amplitude modulation signal and an amplitude modulation detection reference signal and outputs the amplitude modulation detection signal; a first automatic phase control detection circuit which generates an automatic phase control detection signal having a normal polarity from the amplitude modulation signal and an automatic phase control detection reference signal; a second automatic phase control detection circuit which generates an automatic phase control detection signal having an inverted polarity with respect to the normal polarity signal generated by the first automatic phase control detection circuit from the amplitude modulation signal and the automatic phase control detection reference signal; an automatic phase control filter which smoothes the automatic phase control detection signal having the normal polarity or the inverted polarity to generate a control voltage; a voltage controlled oscillator which produces a signal having a frequency controlled by the control voltage output from the automatic phase control filter to output an oscillation signal; a first phase shifter which outputs the amplitude modulation detection reference signal based on the oscillation signal output by the voltage controlled oscillator; a second phase shifter which outputs the automatic phase control detection reference signal based on the oscillation signal output by the voltage controlled oscillator; a voltage comparator which compares a no-signal potential, which is a signal potential of a detection output when the amplitude modulation signal is not modulated, to the amplitude modulation detection signal; a first switch with one output terminal and two input terminals, the output terminal being connected to the automatic phase control filter, a first of the input terminals being connected to the first automatic phase control detection circuit, and a second of the input terminals being connected to the second automatic phase control detection circuit; and a switch control circuit which controls the first switch in response to an AND signal of an output from the voltage comparator and a lock detection signal of a phase locked loop so that the first of the input terminals of the first switch is selected in normal modulation and controls the fist switch based on the AND signal so that the second of the input terminals of the first switch is selected in overmodulation.