资料介绍
高速电路设计技巧sect6SECTION 6
HIGH SPEED DACs AND DDS SYSTEMS
Walt Kester
INTRODUCTION
A frequency synthesizer generates multiple frequencies from one or more frequency
references. These devices have been used for decades, especially in communications
systems. Many are based upon switching and mixing frequency outputs from a bank
of crystal oscillators. Others have been based upon well understood techniques
utilizing phase-locked loops (PLLs). This mature technology is illustrated in Figure
6.1. A fixed-frequency reference drives one input of the phase comparator. The other
phase comparator input is driven from a divide-by-N counter which is in turn driven
by a voltage-controlled-oscillator (VCO). Negative feedback forces the output of the
internal loop filter to a value which makes the VCO output frequency N