ABSTRACT
This chapter reviews the key ideas and the main issues related to the implementation of directly modulated phase-locked loops (PLLs) in digital radio-frequency (RF) transmitters in modern complementary metal-oxide semiconductor processes. It describes alternative architectures of a RF transmitter, starting from the conventional Cartesian transmitter to the polar and out-phasing (OP) schemes. The chapter deals with the possibile architecture of phase modulators to be used in a polar or OP transmitter. It shows how a digital PLL can be used as a direct frequency modulator and compares the two main techniques to achieve wide modulation bandwidth: the two-point injection and the pre-emphasis scheme. The chapter discusses impact of the digitally controlled oscillator non-linear characteristic and means to linearise that block. It also describes a practical implementation of a low-power high-linearity 3.6 GHz frequency synthesiser, able to generate arbitrary phase and frequency modulations. The chapter also discusses both polar and OP transmitters rely on a wideband phase modulator.
