5.1 Introduction of Frequency Control
Practically, all modern telecommunication equipment and test equipment uses frequency-control techniques based on frequency synthesis, the production of the many frequencies involved in a radiocommunication system's modulation, transmission, reception, and demodulation functions through the combination and mathematical manipulation of a very few input frequencies. Although the frequency synthesis techniques now ascendant in wireless systems—phase-locked loops and, less commonly, direct digital synthesizers—are fundamentally different, all are ultimately based on RF oscillators. This chapter covers oscillator theory, evaluation, and design.
Two types of oscillators are needed in a phase-locked-loop system (Figure 5.1). One, typically a crystal oscillator, generates the synthesizer's reference signal. As Figure 5.1 reflects, the reference oscillator may be built into the synthesizer IC in highly integrated systems. The other oscillator, a voltage-controlled oscillator (VCO), is varied in frequency by the system to produce the synthesizer's output signal. Although designing good oscillators remains somewhat like black magic or a special art, we will shown that mathematics and CAD tools, applied in conduction with practical experience, can keep the oscillator design process well under control.