Modulator Using Direct Digital Synthesis Technique

摘要

The aim of this work is to find possible applications for the direct digitalsynthesis in the radio communication system, where the direct digital synthesizer (DDS) could be used in the modulator and the local oscillator. In Chapter 2, a description of conventional direct digital synthesizer is first given. In Chapter 3, the blocks of the DDS is investigated as well as Phase Accumulator, Phase to Amplitude Converter (conventionally a sine ROM), and D/A-Converter. In Chapter 4, additional digital techniques (for example dithering, error feedback methods) may be incorporated in the DDS in order to reduce the presence of spurious signals at the DDS output. In Chapter 5, three upconversion possibilities are introduced: a DDS/PLL hybrid, a DDS/mixer hybrid, and a DDS quadrature modulator. In Chapter 6, a description of a modulator for burst-by-burst carrier frequency hopping in TDMA systems is given. The performance of the digital frequency synthesizer/CPM modulator is analyzed theoretically and simulated by a computer program. In the computer simulations, the effect of the digital frequency synthesizer/CPM modulator parameters on the power spectrum and the phase error are investigated.