PLL Performance, Simulation, and Design 3rd Edition,Basic PLL Operation and Terminology
This section describes basic PLL (Phase-Locked Loop) operation and introduces terminology that will be used throughout this book. The PLL starts with a stable crystal reference frequency, XTAL, which is divided down to a lower frequency by the R counter. This divided frequency is called the comparison frequency (Fcomp) and 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 Kφ. 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 2π, it is unnecessary since it is canceled out by another factor of 2π which comes from the VCO gain for all of the equations in this book. So technically, the units of Kφ are expressed in mA/(2π radians).
If one takes this average DC current value from the phase detector and multiplies it by the impedance of the loop filter, Z(s), then the input voltage to the VCO (Voltage Controlled Oscillator) can be found. The VCO is a voltage to frequency converter and has a proportionality constant of Kvco. The loop filter is a low pass filter, often implemented with discrete components. The loop filter is application specific, and much of this book is devoted to the loop filter. This tuning voltage adjusts the output phase of the VCO, such that its phase, when divided by N, is equal to the phase of the comparison frequency.