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U. Glaeser

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FIGURE 10.13 DLL output jitter sensitivity to sine-wave supply or substrate noise.<br />

FIGURE 10.14 DLL output jitter sensitivity to square-wave supply or substrate noise.<br />

FIGURE 10.15 PLL output jitter sensitivity to input jitter.<br />

Figure 10.14 shows the output jitter sensitivity to square-wave supply or substrate noise for a DLL<br />

with a log-log plot of the peak absolute output jitter magnitude as a function of the noise frequency.<br />

With fast rise and fall times, the square-wave supply noise causes the delay line delay to change instantaneously.<br />

The peak jitter is then observed on at least the first output transition from the delay line after<br />

the noise signal transition, independent of the loop bandwidth. Thus, the output jitter sensitivity is<br />

independent the square-wave noise frequency. Overall, the output jitter sensitivity to supply and substrate<br />

noise for DLLs is independent of the loop bandwidth and the reference frequency for the worst-case of<br />

square-wave noise.<br />

PLL Supply/Substrate Noise Response<br />

Figure 10.15 shows the output jitter sensitivity to input jitter for a PLL with a log-log plot of the absolute<br />

output jitter magnitude normalized to the absolute input jitter magnitude as a function of the input jitter<br />

frequency. This plot shows that the normalized output jitter magnitude decreases asymptotically at 20 dB<br />

per decade for noise frequencies above the loop bandwidth and is constant at one for noise frequencies<br />

below the loop bandwidth. It also shows that for underdamped loops where the damping factor is less<br />

than one, the normalized jitter magnitude can be greater than one for noise frequencies near the loop<br />

bandwidth leading to jitter amplification. This overall behavior directly results from the fact that the PLL<br />

is a low-pass filter to input phase noise as determined by the closed-loop frequency response.<br />

Figure 10.16 shows the tracking jitter sensitivity to input jitter for a PLL with a log-log plot of the<br />

tracking jitter magnitude normalized to the absolute input jitter magnitude as a function of the input<br />

© 2002 by CRC Press LLC<br />

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