A review of wide band frequency modulation technique

Abstract

The paper provides a review of the whole field of frequency modulation. Commencing with aa historical introduction and a short discussion on the general nature of frequency modulated waves, the author passes on to a comprehensive and yet thoroughly practical examination of the factors which result in the greatly reduced level at which noise is reproduced in a frequency modulation receiver. After treatments of the suppression of weaker stations and pre-emphasis, there is a graphical presentation of the total improvement in signal to noise ratio.The improvement obtained when using frequency modulation for a radio telephone link is shown to be some 26 db. This figure is based on a deviation ratio of 5 and includes the 8 db. additional noise which the human ear can tolerate due to the triangular distribution. It is also shown that the improvement on a high fidelity broadcast system employing preemphasis can be as high as 31 db.The distortion produced by different types of fading is dealt with in a section on propagation. After a description of the turnstile aerial and figures of gain due to its use, there follows an examination of the various types of F.M. transmitter. The reactance valve and Armstrong's frequency modulator are discussed along with the various methods of monitoring an F.M. station. After a section on F.M. signal generators, the receiver is dealt with in some detail. Under this heading are sections on the limiter stage, the different types of discriminator and tuning indicators.