TABLE OF CONTENTS There are many ways to realize synthesizers; possibly the most common is based on a PLL [1 7]. PLL-based synthesizers can be further subdivided by which type of a division is used in the feedback path. The division ratio N can be either an integer or a fractional number. If the number is fractional, then the synthesizer is called a fractional- N synthesizer. This type of synthesizer can be further distinguished by the method used to control the divide ratio, for example by a ? ? controller or by some other technique. In this chapter, analysis is done with a general N without giving the details of how N is implemented; thus, the analysis is applicable both to integer- N and fractional- N synthesizers. An example of a synthesizer not based on a PLL is the direct-digital synthesizer.
Figure 3.1 shows a block diagram for a PLL. In brief, the PLL is a feedback system that forces the divided-down VCO output phase to follow the reference signal phase. That is, it is a negative feedback loop with phases as the input and output signals. The loop is composed of a phase detector, a lowpass filter, a VCO, and a divider. The phase detector, which is the summing block of the feedback system, is used to compare output phase ? o to reference phase ? R. The LPF controls the loop's dynamic behavior, which...
