Phase lock loops

Phase lock loops (PLL) are not a recent invention. Their use became widespread with the availability of high-quality integrated circuit operational amplifiers (opamps) in the 1960s. This versatile circuit has found applications across the frequency spectrum in consumer, commercial, deep space, and military projects. Tracking Voyager through the solar system and tuning a car radio are made to order uses for a PLL. To understand a PLL, a good working knowledge of RF techniques, oscillator design, closed loop control theory, analog circuit design, and digital circuit design is required. A comprehension of each of the components and its place in the system is essential. Fortunately not all of this knowledge is required at once. The books listed at the end of this chapter can each provide an in-depth insight into areas beyond the present scope [1¨C5]. This discussion will begin with the basic concepts and rapidly expand these ideas into practical considerations.

Discussion of the PLL draws heavily on many other areas of analysis, which includes an understanding of the principles of closed loop control theory. From control theory comes the concept of negative feedback to tailor the performance of closed loop systems. Response time, transient performances, bandwidth, damping ratio, and phase margin are used to describe PLL operation. The type and order of a closed loop system define the complexity and response to a stimulus. In most PLL¡¯s, at least two of the components, the voltage-controlled oscillator (VCO) and phase detector, are high-frequency components. There may also be amplifiers, mixers, frequency multipliers, and other oscillators. To use these items, a familiarity with RF design practices and terminology is important.

Frequency multiplication may require digital integrated circuits (ICs) within the PLL. These ICs require digital control words to set the desired frequency. Many integrated circuits are presently available that combine many of the PLL functions on a single chip. Most of the interface control is digital. Analog circuit design is perhaps the most demanding of the circuit areas within a PLL. Op-amps are used in many of the filtering circuits used within a loop. Inverting and noninverting circuits are required for loop filters and search circuits. Integrators, dc amplifiers, Schmitt triggers, and offset circuits are used to set the loop operation. Resistor/capacitor circuits provide phase shift for stability. The oscillator is an intrinsic part of a PLL, and its design in itself is a specialized and technically challenging area.