Modern cellular phones and base stations have to serve a multitude of wireless communication standards with each standard using different frequency bands. For such applications, a universal programmable hardware is desirable which is often referred to as software-defined radio (SDR). This book describes agile frequency synthesizers representing key building blocks for an SDR transceiver. In this work, an innovative frequency generation scheme is derived to provide a multi-octave tuning range with very low phase noise and low spurs. A PLL phase noise model including a nonlinear phase detector is thoroughly discussed. Design techniques for low phase noise VCOs with mixed analog/digital tuning are presented. Further, the book investigates both single-loop and dual-loop PLL architectures in fractional-N synthesizers. An unconventional two-transistor based charge pump in a dual-loop PLL improves phase detector linearity and reduces charge pump noise. This results in a low in-band phase noise similar to an integer-N synthesizer. Finally, the book presents the design of fully integrated integer-N and fractional-N frequency synthesizers with detailed measurement results.