Echo Distortion in the FM Transmission of Frequency-Division Multiplex

The composite multiplex signals generated by frequency-division methods long standard in telephone communication, can be transmitted by the new transcontinental broad-band FM radio relays. Signal intermodulation by echoes must be minimized. Such intermodulation is investigated in this paper experimentally and analytically. Two types of echoes are considered: (1) Weak echoes with delays exceeding 0.1 microseconds, caused mainly by mismatched long lines; and (2) Powerful echoes with delays shorter than 0.01 microseconds, caused by multipath transmission, and leading to selective fading. Using random noise signals, the distortion is evaluated as a function of various parameters of the echo, the baseband and the rf modulation.