5-6 depicts an HPF, showing that the pass band and stop band are a mirror image of the LPF case. Cut-off frequency Stop band Pass band Bandwidth Frequency Figure 5-6 High-Pass Filter It is interesting to note that combining both filters (LPF and HPF) on a circuit, you could design a system to allow only a certain range of frequencies and filter out the rest. Such a combination of LPF and HPF behaves as shown in Figure 5-7 and is called a band-pass filter. The band-pass name comes from the fact that it allows a certain band of frequencies (from lower cut-off to higher cut-off) to pass and attenuates the rest of the spectrum. Lower Higher cut-off frequency cut-off frequency Bandwidth Stop band Stop band Pass band Frequency Figure 5-7 Band-Pass Filter These filters are of the utmost importance when trying to filter errors out from a clock signal. To understand this process in more detail, this chapter next defines these errors, primarily jitter and wander, more technically. 5.1.1 Jitter and Wander Jitter and wander are phase variations in the clock signal timing instants, as illustrated in Figure 5-8. Also, refer to the introduction to jitter and wander in Chapter 3. This variance and error, commonly called clock signal noise, can be caused by many factors, one being the quality of the clock com- ponents. Another factor is the noise accumulating from one node to the next when distributing tim- ing signals through a chain of clocks. Jitter Wander Figure 5-8 Jitter Versus Ideal Versus Wander Low rate (less frequent, slower) changes are a gradual wander in the signal, whereas high rate changes are a very short-term jitter in the signal. The ITU-T specifies (in G.810) that 10 Hz is the dividing line between jitter and wander (and has been a convention in the telecom industry for some time). And so, phase variations occurring at rate higher than 10 Hz are described as jitter, and the variations lower than 10 Hz are described as wander. When all the phase variations of a clock signal as compared to its reference signal ar