The Basic Principle of Frequency Synthesis

AS a result of the steadily increasing congestion in communication bands of the electromagnetic spectrum, efforts are being made to utilize new ranges, particularly microwave and optical frequencies, and to better exploit existing frequency allocations. The latter tasks have led to improvements in communication techniques (SSB modulation, multiplexing, digital systems, spread spectrum application, etc.). Simultaneously, a perfection of frequency generators has been attempted in transmitter exciters and local oscillators in receivers, in most cases with the assistance of frequency synthesizers, that is, devices changing a given standard frequency, f5, into the desired output frequency, fx. This approach is preferred to single-frequency oscillators (Xtals) because of larger tuning ranges, better shortand longterm stabilities (lower output noise), microprocessor-aided tuning or programming, and many other advantages. The present status of frequency synthesis techniques in the whole range of the electromagnetic waves, from audio and RF to the visible frequencies, is shown in Fig. 1-1. The lesson one learns from Fig. 1-1 is that different approaches to frequency synthesis exist depending on the desired output frequency. Nevertheless, one basic principle is common to all systems. THE BASIC PRINCIPLE OF FREQUENCY SYNTHESIS