AbstractThe continuous‐phase FSK system is a remarkably beneficial digital modulation and demodulation system due to its good transmission performance over satellite communication circuits and others which has some band restriction elements and nonlinear components. However, no report has been made yet on its practical circuit construction or design method. Here, studies are made on practical circuit structure based on the principle of modulation and demodulation in connection with the binary continuous phase FSK, sometimes called MSK, FFSK, etc., with 0.5 modulation index. Presentation is made for the circuit structures for a modulation system by using the off‐set‐bipolar‐modulation‐and‐synthesis, and for a demodulation system by using the synchronous‐detective‐wave‐coherent‐detection and 2‐steps‐up carrier synchronous system. These are applied appropriately to high‐speed modulation and demodulation over several hundred M bit/s in the radio frequency range in or beyond the microwave frequency. As for the coherent detection, quantitative study is made on impairment figures due to the optimum reception band restriction over the Gaussian transmission system, phase errors of regenerated carriers, and timing discrimination errors. It is concluded that its error characteristics are almost equal to those of the optimum detection system at the BT product of reception, 0.5‐0.6, and its phase error takes a deterioration figure of the two‐phase PSK system. Furthermore, experiments were conducted on modulation and demodulation at the carrier frequency of 1.7 GHz and the data transferring speed of 100 M bit/s to confirm the basic characteristics and theoretical results. The result showed that it could have satisfactory featur