This book contains a thorough treatment of phase noise, its relationship to thermal noise and associated subjects such as frequency stability. The design of low phase noise signal sources, including oscillators and synthesisers, is explained and in many cases the measured phase noise characteristics are compared with the theoretical predictions. Full theoretical treatments are combined with physical explanations, helpful comments, examples of manufactured equipment and practical tips. Overall system performance degradations due to unwanted phase noise are fully analysed for radar systems and for both analogue and digital communications systems. Specifications for the acceptable phase noise performance of signal sources to be used in such systems are derived after allowing for both technical and economic optimisation. The mature engineer whose mathematics may be somewhat rusty will find that every effort has been made to use the lowest level of mathematical sophistication that is compatible with a full analysis and every line of each mathematical argument has been set out so that the book may be read and understood even in an armchair. Due to a novel approach to the analytical treatment of narrow band noise, the book is simple to understand while simultaneously carrying the analysis further in several areas than any existing publication.