MODULATION CIRCUIT

PURPOSE:To form a modulation signal independently from 2 kinds of signals by providing an adding circuit to apply FM modulation to a carrier signal with a 2nd signal before the input to a PSK modulator and fixing the 2nd signal to a prescribed value when the carrier signal is subjected to PSK modulation and outputted by a 1st signal. CONSTITUTION:A 1st switch circuit 17 is closed to switch the movable contact of a 2nd switch circuit 22 to a fixed contact at the earth side and an analog voice signal AS is supplied to an input terminal 16. A carrier signal Asin((omega+delta)t) FM-modulated by an analog voice signal AS is generated from a voltage controlled oscillator 14 and fed to modulators 24, 25. In the case of transmitting a digital data DD, the 1st switch circuit 17 is opened to switch the movable contact of a 2nd switch circuit 22 to the fixed position at a high level logic '1' to supply the digital data DD to the input terminal 18. Since regular carrier signals Asinomegat and Acosomegat are supplied respectively to the modulators 24, 25, a QPSK modulation signal 27 whose center frequency is 140MHz is generated from a band pass filter 28.