e jitter and the wander of a clock signal. Signal with jitter and wander present + 400 ns 80 ns/div 480 ns 0 hours 2.259 hours 10 minutes/div Figure 5-10 Plot of Phase Deviations of a Clock Signal Without Any Filters In order to clearly visualize (and analyze) the jitter and wander, filters are applied to the phase vari- ation measurements. In Figure 5-11, you can see that after the HPF is applied (which filters out the wander), the remaining noise is jitter (frequency of phase variations higher than 10 Hz). Jitter: Filter out low frequency components with HPF + 400 ns 100 ns/div - 500 ns 0 hours 2.259 hours 10 minutes/div Figure 5-11 Plot of Phase Deviations of a Clock Signal After HPF Applied Similarly, in Figure 5-12, after the LPF is applied (which filters out the jitter), the remaining noise is wander. Wander: Filter out high frequency components with LPF +60 ns 0 ns/div - 100 ns 0 hours 2.259 hours 10 minutes/div Figure 5-12 Plot of Phase Deviations of a Clock Signal After LPF Applied After reading the preceding section about clock bandwidth and the PLL loop filter, there are two questions that could arise. First, why not keep the LPF cut-off frequency very low to filter the jitter and also limit the wander? Recall that jitter is filtered with the LPF. And if the LPF cut-off fre- quency is kept low, it could also filter some range of wander. Of course, this means that the pass band for the LPF becomes very small. Secondly, why not do the same on every clock node in the chain? To understand the answer to the second question, you first need to appreciate the following aspects of PLLs: Not all PLLs can keep LPF cut-off frequency very low. Wander is classified as low-rate phase variations, which can reach extremely small values-10 Hz down to microhertz. So, there will always be some phase noise (in this case wander) within the LPF clock bandwidth and SO will always be tracked by the PLL. A PLL combines two signals: 1) the input reference signal and 2) the clock signal from the local oscillator (VCO) to output a