This section describes basic PLL (Phased Locked Loop) operation and introduces
terminology that will be used throughout this book. The PLL starts with a stable crystal
reference frequency (XTAL). This frequency is divided by R to a lower frequency, which is
called the comparison frequency (Fcomp). This is one of the inputs to the phase detector. The
phase-frequency detector outputs a current that has an average DC value proportional to the
phase error between the comparison frequency and the output frequency, after it is divided by the
N divider. The constant of proportionality is called Kf.? Note that this constant turns out to be
the magnitude of the current that the charge pump can source or sink. Although it is technically
correct to divide this term by 2p, it is unnecessary since it is canceled out by another factor of
2p which comes from the VCO gain for all of the equations in this book. So technically, the
units of Kf are expressed in mA/(2p radians).