A frequency synthesizer by itself is a complicated system consisting of many building blocks connected in a feedback loop. Specifically, as shown in Fig. 2.7, these building blocks include a voltage-controlled oscillator (VCO), dividers, a phase-frequency detector, a charge pump, and a loop filter. Each of these building blocks affects the overall synthesizer's performance differently and thus have different design issues and criteria. This chapter will focus on reviewing and discussing these design considerations. In addition, design issues for passive components like inductors and varactors, which are critical for high-frequency VCOs and dividers, will also be presented.

Voltage-controlled oscillators (VCOs)

One of the most critical building blocks in any phase-locked loop or frequency synthesizer is the voltage-controlled oscillator. In general, the oscillator needs to operate at the highest frequency, and its phase noise is dominant in the whole system at frequency offsets beyond the synthesizer's loop bandwidth. In current CMOS technologies, both ring oscillators and LC-tank oscillators can be fully integrated on-chip (Lee, Kim and Lee, 1997; Momtaz et al., 2001), but both types have their own advantages and disadvantages. Depending on the specifications and applications, one may be more suitable than the other. The following sections will discuss and compare the design issues for the two types of VCOs.

Ring oscillators

Ring oscillators are typically easier to implement, and so are more desirable in standard CMOS technologies because it is not necessary to employ passive devices such as inductors and varactors.