Page 149 - Phase-Locked Loops Design, Simulation, and Applications

Source : Phase-Locked Loops: Design, Simulation, and Applications, Sixth
Edition Ronald E. Best 93

PLL Performance in the Presence of Noise

Noise is an extremely important issue in the wide field of PLL applications. Noise has
two entirely different aspects, however. In a first category of applications, the PLL is used
to extract weak signals from an extremely noisy environment. The PLL has been very
effective in space communications where it is used to detect signals buriNed in noise. In
this situation, the input signal of the PLL is a low bandwidth signal with large
superimposed broadband noise. In a quite different category of applications—for
instance, frequency synthesizers—the PLL is required to generate an output signal with
high spectral purity. Here, the PLL does not “receive” a noisy input signal but
generates the noise itself. As we will see in Chaps. 6 and 7, the output signal of such
PLLs contains a broadband phase noise spectrum. Outside of that, it may show spurious
frequencies (unwanted spectral lines), something also called “spurs” or “tones,” as
mentioned earlier in the book. In integer-N frequency synthesizers, these spurs are
generated by the phase detector (more about this in Chap. 6). In fractional-N frequency
synthesizers, additional spurs are created by the sigma-delta modulators that are used to
switch the divider ratio of the down scaler from one value to another. This will be
discussed in greater detail in Chap. 7.
In this chapter, we will deal with the first category of applications, where the PLL must
lock onto signals that are corrupted by noise. Before entering into details, we first want to
identify the sources and types of noise in PLL systems.

Sources and Types of Noise in a PLL

In most communications where the PLL is used, digital signals are transmitted—that is,
the baseband signals consist of pulses or square wave signals. A binary sequence of
10101010 …, for example, would be represented by a symmetrical square wave. Due to
the harmonics, the bandwidth requirements would become excessive if the pure square-
wave signal were sent. To save bandwidth,

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