Techniques for Close Channel Spacing at VHF and Higher Frequencies

Physically separated radio transmitters can be frequency-division multiplexed to obtain a great economy of bandwidth utilization at vhf and higher frequencies, by use of a common reference signal at both transmitter and receivers in lieu of locally generated high-frequency oscillations. In an individual transmitter, a modulated low-frequency subcarrier would be used to modulate a harmonic of the reference signal, and the resultant sum (or difference) frequency-band selected for transmission. Amplitude-modulated voice signals spaced only 5 kc apart at frequencies above 100 mc, have been separated with ease in a two-channel laboratory model of such a system by use of a standard low-frequency communication receiver preceded by a wide-band mixer, the local oscillator for which was synchronized with the transmitter reference signal.