A modulation scheme in which the transmission path length of the mmwaoe signal is digitally modulated by means of a PIN diode switch has been proposed for the waveguide transmission system. To demonstrate the feasibility of this proposal, a complete transmitter has been developed in the 50 ~ 60-GHz range, consisting of an IMPATT oscillator, circulator, PIN diode switch, and its driver circuit. The model can handle signals at SOO Mb/s and deliver 100 mW of modulated power at the output. This paper describes the design considerations of each component in detail, together with the experimental results.