4

LOOP COMPONENTS

4-1 OSCILLATOR DESIGN

The phase-locked loop generally has two oscillators, the oscillator at the output frequency and the reference oscillator. The reference oscillator at times can be another loop that is being mixed in, and the voltage-controlled oscillator is controlled by either the reference or the oscillator loop. The voltage-controlled oscillator is one of the most important parts of the phase-locked loop system because its performance is determined inside the loop bandwidth by the loop and outside the loop bandwidth by its design. To some designers, the design of the voltage-controlled oscillator (VCO) appears to be magic. Shortly, we will go through the mathematics of the oscillator and some of its design criteria, but the results have only limited meanings. This is due to component tolerances, stray effects, and, most of all, nonlinear performance of the device, which is modeled with only a certain degree of accuracy. However, after building oscillators for awhile, a certain feeling will be acquired for how to do this, and certain performance behavior will be predicted on a rule-of-thumb basis rather than on precise mathematical effort. For reasons of understanding, we will deal with the necessary mathematical equations, but I consider it essential to explain that these are only approximations.

4-1-1 Basics of Oscillators

An electronic oscillator is a device that converts dc power to a periodic output signal (ac power). If the output waveform is approximately ...

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