| Even though the term digital modulation signifies modulating an analog time-continuous carrier-wave based on digital binary data, the signal processing is still carried out by analog signal-processing blocks. In this thesis, a novel Direct Digital Frequency Synthesizer (DDFS), targeted for CMOS implementation as an alternative to traditional PLL used for digital modulation, has been proposed. The novel DDFS integrates a discrete-sinewave generation-logic, based on the quarter-wave symmetry of a sinusoidal wave. It provides spurious improvement comparable to that of a DDFS incorporating a full-wave sine-LUT. The proposed DDFS architecture allows one to design distinctive modulator circuits for BPSK, DBPSK, QPSK, pi/4-DQPSK, BFSK, and MFSK modulation schemes. Such modulators perform most of the signal-processing in the digital domain, without having to rely on multiple frequency-generators and other analog processing blocks which are used in traditional digital modulation schemes. In other words, the thesis aims to show how the proposed DDFS structure can be exploited to implement digital modulators. The thesis makes a successful attempt to render the digital modulation process as "all-digital" or "truly-digital" as possible without having to rely much on analog parts. Thus, the work presented here can be considered as an approach to transforming or embedding the digital modulators on to a single chip. Furthermore, at the end of the thesis, a novel architecture for a frequency-hopping pi/4-DQPSK spread-spectrum transmitter has been modeled. |
