By I. Switzer, Maclean-Hunter Cable TV Limited
Phase locking techniques have been used for some time in a variety of communications applications. The term may be generally applied to locking the phase of one signal source to another. Such sources will then operate at the same frequency and their relative phase will remain constant or nearly so. The principle application in communications has been suppressed carrier communications systems, both wire and radio, and in the transmission of colour television. Colour television subcarriers are operated in a suppressed carrier mode. Each individual colour receiver has a local subcarrier oscillator which must be phase locked to the original colour subcarrier in order to permit the colour information to be demodulated. Similarly, other suppressed carrier communications systems require a locally generated carrier to permit the transmitted information to be demodulated, The local carrier is often generated by phase locking to a pilot signal of some kind.