System for interrupting a transmitter output wave

Abstract

In a transmitter, a modulator switchably operates on one of two modes of operations, that is, in a first mode of the operation, such as an offset QPSK, FSK or .pi./4 shift QPSK, an amplitude of an output signal of the modulator does not become zero at any instant, and in a second mode of the operation, such as two-phase FSK or four-phase PSK, the output signal of the modulator is 100% amplitude-modulated or 180 phase-modulated. Second mode operation. The operation mode is switched by changing the mode of the parallel input signals to the modulator, where the parallel signals have been converted from a serial input data signal to be transmitted. After the first mode operation is switched to the second mode operation the output signal is interrupted preferably on a first moment that the amplitude of the output signal becomes zero, as well as, before the second mode operation is switched to the first mode operation the output signal is resumed preferably on a second moment that the amplitude of the output signal is to become zero. The interruption of the modulated output signal may be done by cutting the local frequency or modulated signal, or by the parallel input signals. Thus, spurious wave is not generated on the transition of interrupting or resuming the modulated transmittal wave, and the period of the interrupting can be short enough.