Abstract :
The paper discusses some of the difficulties encountered in the reception of frequency-shift and amplitude-modulated telegraph signals over radio links. Comparison between both methods of signalling leads to the conclusion that greater transmission efficiency can be obtained by the use of f.s. signalling. Fading and interference affect both systems and give rise to distortion characteristics which depend not only on the type of modulation used but to some extent on the particular shape of the modulating waveform. The three main components of distortion are defined, and bandwidth requirements before and after demodulation are discussed. The use of direct printing circuits introduces difficulties during long marking periods and some methods of overcoming this problem are given. The need for exceptional frequency stability in relation to the reduction of errors for both methods of signalling is stressed, and methods of minimizing the effects due to frequency drift by the use of automatic frequency control and other means are discussed. The means of obtaining the greatest benefit from a diversity system are considered, and mention is made of an effective method of diversity working using a.m. signalling. Some observations are made on the effects of noise, and the main features of a satisfactory system for the reception of high-speed signals using either method of signalling.
