ike any other electronic circuits and devices, PLLs have also been evolving. Designers of modern communications equipment are incorporating the latest PLL devices into them, circuits that now contain multiple PLLs (analogue or digital). To reduce real-estate requirements on circuit boards, newer-generation devices can operate with lower-frequency oscillators that can replace expensive high-frequency oscillators. They also output signals with ultra-low levels of jitter that is required for the tight jitter specifications required by some equipment designs (remembering that jitter is high-frequency noise). 5.1.1 Low-Pass and High-Pass Filters A low-pass filter (LPF) is a filter that filters out (removes) signals that are higher than a fixed fre- quency, which means an LPF passes only signals that are lower than a certain frequency-hence the name low-pass filter. For the same reasons, sometimes LPFs are also called high-cut filters because they cut off signals higher than some fixed frequency. Figure 5-5 illustrates LPF filtering where sig- nals with lower frequency than a cut-off frequency are not attenuated (diminished). The pass band is the range of frequencies that are not attenuated, and the stop band is the range of frequencies that are attenuated. Meanwhile, the range up to the cut-off frequency becomes the clock bandwidth, which also matches the width of a filter's pass band. For example, as shown in Figure 5-5, in the case of an LPF, the clock bandwidth is the range of frequencies that constitute the pass band. The clock band- width is typically a configurable parameter based on the capabilities of a PLL. Cut-off frequency Pass band Stop band Bandwidth Frequency Figure 5-5 Low-Pass Filter Similarly, a high-pass filter (HPF) is a device that filters out signals that are lower than some fixed frequency, which means that an HPF passes only signals that are higher than a certain frequency. And again, HPFs are also sometimes called low-cut filters because they cut off lower than a fixed frequency signal. Figure 