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.
