TABLE OF CONTENTS One of the most important applications of the mixed-signal PLL is frequency synthesis. In the design of PLL frequency synthesizers we are confronted with a number of problems that are specific for that topic. For this reason PLL frequency synthesizers will be discussed in a separate chapter (Chap. 3). In this section we are going to consider some other typical PLL applications.
The PLL is used in almost every digital communication link. Digital data can be transmitted over serial or parallel data channels. In any case, the data are clocked; i.e., they are sent in synchronism with a clock signal, which is normally not transmitted. Thus the receiver is forced to extract the clock information out of the received signal.
There are basically two different principles of transmitting digital data, namely, baseband and carrier-based transmission. In baseband transmission, the digital signal is directly sent over the link; when a carrier system is used, however, the digital signal is first modulated onto a carrier, usually a highfrequency signal. Amplitude (AM), frequency (FM), and phase (PM) modulation are used here, or it is even possible to combine different modulation techniques, such as AM + PM.20 In carrier systems, a number of digital signals can be mod-ulated onto different carriers; hence the bandwidth of carrier systems can be much larger than the bandwidth of baseband systems. An overview of these digital modulation schemes is given in Chap. 9.
In this section...
