A 1-to-4GHz Multi-Mode Digital Transmitter in 40nm CMOS Supporting 200MHz 1024-QAM OFDM signals with more than 23dBm/66% Peak Power/Drain Efficiency

To support wideband complex modulated signals and comply with the stringent requirements of modern communication standards in an energy-efficient manner, recently, digital transmitters (DTXs) have been explored to fully benefit from the high-speed switching and integration capabilities of nanoscale CMOS technologies [1]–[5]. These DTXs are primarily exploiting a polar or Cartesian architecture. In a polar DTX [1], [2], two eigenvectors of amplitude (p) and phase $(\phi)$ are generated from the in-phase (I) and quadrature (Q) baseband signals using non-linear coordinate rotation transformations (i.e., CORDIC). Provided that $\rho$ is constant, the achievable drain efficiency (DE) is constant (Fig. 1 top). However, polar $\text{DTXs}$ cannot manage large modulation bandwidth due to their non-linear $\mathrm{I}/\mathrm{Q}$ to $\rho/\phi$ conversion. Moreover, their phase and amplitude paths must recombine at the output stage without any delay mismatch to maintain linear operation. In contrast, Cartesian DTX variants can handle signals with large modulation bandwidth [3]. Nevertheless, their DE is lower than their polar counterparts owing to the linear combination of orthogonal I/Q vectors, yielding a 3-dB worst-case output power loss at the orthogonal (I/Q) axes. Alternatively, a multi-phase operation can be utilized that compromises polar and Cartesian features by mapping the I/Q signals into two non-orthogonal basis vectors with 45° relative phase difference and magnitudes of $\mathrm{I}_{\text{MP}}=\mathrm{I}-\mathrm{Q}$, QMP $=\sqrt{2}\mathrm{Q}$ [4]. This architecture inherits the advantages of the cartesian DTX, such as wideband operation, symmetrical, and synchronized $\mathrm{I} /\mathrm{Q}$ paths along with a DE behavior that imitates the polar case. This paper presents a multi-mode DTX that uses both Cartesian and multi-phase operation modes to target applications requiring large modulation bandwidth, decent spectral purity and average efficiency.