CORDIC-Less Digital Polar Transmitter Architecture Based on Delta-Sigma Modulator

Conventional digital polar transmitters (TXs) suffer from limited power efficiency and linearity due to the multi-bit nature of intermediate signals. This work proposes a digital polar TX architecture that avoids such drawbacks by reducing the bit count of TX signals. The proposed TX oversamples and quantizes multi-bit I/Q inputs into three-level signals using simple delta-sigma modulators (DSMs), followed by a nine-state lookup table (LUT) to directly generate amplitude modulation (AM) and phase modulation (PM) signals. The complex coordinate rotation digital computer (CORDIC) that is widely used in polar TXs is not required. The AM signal is truncated to two levels for <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small>/<sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> control of an intrinsically linear 1-bit power amplifier (PA). The PA avoids the linearity and switching loss issues of conventional switched-capacitor PAs (SCPAs). The PM signal is represented with 3 bits for eight-phase generation via edge selection from a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times $</tex-math> </inline-formula> carrier local oscillator (LO), which offers better linearity and power efficiency than phase modulators based on multi-bit digital-to-time converters (DTCs). Therefore, the proposed approach enables simultaneous improvements in power efficiency and linearity. Fabricated in a 65-nm CMOS process, the proposed TX achieves a 33.5% peak power-added efficiency (PAE) at 24.2-dBm output power and supports wideband 50-MSymbol/s 256-quadratic-AM (QAM) signals without digital predistortion (DPD). A maximum system efficiency (SE) of 24.5% is achieved at −25.1-dB error vector magnitude (EVM), demonstrating superior performance compared to state-of-the-art DPD-less digital polar TXs.