Chapter 10: Direct Digital Synthesis

10.1 Introduction

Modern communication systems are placing increasing demands on the frequency resolution, channel switching speed, and bandwidth requirements of frequency synthesis. For instance, spread-spectrum applications require a frequency synthesizer that is capable of tuning to different output frequencies with extremely fine frequency resolution and switching speed of the order of nanoseconds. The resolution and switching-speed requirements of many systems are surpassing the performance capabilities of a conventional analog phase-locked loop (PLL). The conventional PLL-based frequency synthesizer has difficulty meeting these requirements due to internal loop delay, low resolution, and the limited tuning range of the VCO. In contrast, a direct digital synthesizer (DDS) generates a digitized waveform of a given frequency by accumulating phase changes at a higher clock frequency. DDS is a digital technique for frequency synthesis, waveform generation, sensor excitation, and digital modulation and demodulation. Since there is no feedback in a DDS structure, it is capable of extremely fast frequency switching or hopping at the speed of the clock frequency. DDS provides many other advantages, including fine frequency-tuning resolution and continuous-phase switching. In addition, as will be discussed in Chapter 11, DDS can provide quadrature signals with accurate I/Q matching. DDS can directly provide various modulations. DDS can also generate arbitrary waveforms in the digital domain. The increasing availability of ultra-high-speed digital-to-analog converters (DACs) allows a DDS to operate at clock frequencies of more than 10 GHz.

The DDS has many advantages; however, it has two major deficiencies that are related to the inadequacy of the...