This invention relates to an abnormal synchronization preventing device used in a multiple value quadrature modulation type radio device and the abnormal synchronization preventing device includes a monitoring circuit for monitoring the carrier pull-out based on demodulated base band signals to output a carrier pull-out alarm signal, an abnormal synchronization detection circuit for sampling the reception signal level at a speed twice the transmission speed of reception data according to identification data and detecting abnormal synchronization according to whether or not the detection area of sampled data lies in a specified area, an abnormal synchronization preventing circuit for generating a signal for a preset period of time when abnormal synchronization is detected in a case where the carrier pull-out alarm signal is not output, and a selection circuit for receiving a reference signal used for generating a reference carrier frequency signal having a frequency close to the frequency of the carrier wave and a phase difference signal, selecting the phase difference signal as the control signal in the normal state, and selects the reference signal as the control signal while receiving an output signal of the abnormal synchronization preventing circuit and supplying the selected signal to an oscillator.
Quadrature amplitude modulation type digital radio communication device and method for preventing abnormal synchronization in demodulation system