Book excerpt: This dissertation concerns the investigation of current problems associated with the analysis and design of tunable continuous time bandpass (CT BP) sigma-delta (CA) modulators. This specific modulator group is particularly promising within the context of softwaredefined radios. However, due to the nonlinear sampling element within a closed-loop sdomain system, the high level of analytical complexity makes current CT BP CA modulators difficult to implement. Specific problems addressed in this research were the fundamental principles of the CA modulation process, analytical and design methodology, loop delay compensation techniques, monolithic implementation and possible application areas. Theoretical general closed form solutions for the center frequency tunable CT BP CA modulator with fractional delays were derived, defining a new sub-class of fractional CT BP CA modulators. The developed subclass offers numerous possible solutions to existing problems in CA modulator based circuits, such as loop delay compensation and signal upconversion. A theoretical CT BP CA design methodology was then modified to be suitable for mixed-signal integrated circuit (IC) design flow, currently used in both industrial and academic environments. In order to experimentally demonstrate these new analytical concepts, an IC prototype of the proposed fractional CA modulator was designed, manufactured in SiGe technology, and tested. This research showed that the developed fractional CT BP CA modulator concept is a feasible option for future wireless networks; thus providing the crucial element required for software-defined radios.