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Abstract

A transmission system, for transmitting signals over an electrical power distribution circuit, comprises a transmitter arranged to produce at least two carrier signals of different frequency and to modulate each of them with the same digital signal. The respective frequencies of the carrier signals are derived by frequency division from a common master oscillator, and are thus phase-coherent. The system includes a receiver for receiving the signals, comprising a respective input circuit for each modulated carrier, the input circuits each being tuned to the frequency of their respective carrier signal. A plurality of oscillator signals of different frequencies, derived from a variable frequency oscillator, are used to beat the received carrier signals to respective signals at a common frequency, which signals are summed. The summed signal at the common frequency is applied to one input of a phase detector, whose other input is connected to receive another oscillator signal, also at the common frequency, from the variable frequency oscillator. The output signal from the phase detector is applied to the variable frequency oscillator to lock its frequency to those of all the carriers and therefore to that of the master oscillator in the transmitter. The digital signal is derived from the output of the phase detector by digital correlation techniques.

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First Claim