A frequency shift keyed (FSK) data transmission system utilizes existing power lines to transmit data by superimposing a high frequency FSK signal on the power line carrier. A transmitter includes a phase locked loop which is locked on the power line carrier frequency and which synthesizes the FSK signal...http://www.google.ca/patents/US4556866?utm_source=gb-gplus-sharePatent US4556866 - Power line carrier FSK data system
