PURPOSE:To make a frequency deviation at modulation equal to that at direct phase modulation to an input signal by differentiating the input signal, applying spectrum scramble to the result and then applying phase modulation after integration. CONSTITUTION:A signal whose band is limited to f1-f2 is inputted to an input terminal 11 and the result is differentiated at a differentiation circuit 5, then the spectrum of the signal is scrambled at a spectrum scramble circuit 6. A radio wave ciphered by spectrum scramble is transmitted by integrating an output of the circuit 6 at an integration circuit 7 and applying the integrated signal to a phase modulation circuit 8. Observed waveforms at an input point and output points d-g of the circuits 5, 6, 7 are as shown in Fig. and the frequency deviation after modulation at the circuit 8 is made equal to that at direct phase modulation of the input signal, allowing to improve the secrecy of a transmitted wave sufficiently depending on the application of scramble. This method is applied directly to an existing PM transmission system and introduced economically.
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