Page 9 - Phase-Locked Loops Design, Simulation, and Applications

INTRODUCTION TO PLLS Ronald E. Best 7
In analogy to filters, PLLs can also be implemented “by software.” In this case, the
function of the PLL is no longer performed by a piece of specialized hardware, but rather by a
computer program. This last type of PLL is referred to as SPLL.
Different types of PLLs behave differently, so there is no common theory which covers all
kinds of PLLs. The performance of LPLLs and DPLLs is similar, however; thus, we can
develop a theory that is valid for both categories. We will deal with LPLLs and DPLLs in
Chaps. 2 and 3. The term “mixed” indicates that these PLLs are mostly hybrids built from
linear and digital circuits. Strictly speaking, only the DPLL is a mixed-signal circuit; the LPLL
is purely analog.
The ADPLL behaves very much different from mixed-signal PLLs; hence, it is discussed in
a separate chapter (Chap. 11).
The software PLL is normally implemented by a hardware platform such as a
microcontroller or a digital signal processor (DSP). The PLL function is realized by software.
This offers the greatest flexibility because a vast number of different algorithms can be
developed. For example, an SPLL can be programmed to behave like an LPLL, a DPLL, or an
ADPLL. We will deal with SPLLs in Chap. 13.

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