TR2018-065

Compact Analytical Description of Digital Radio-Frequency Pulse-Width Modulated Signals



Radio frequency pulse-width modulation (RFPWM) has been used as a power coding method in all-digital transmitters, which employ highly efficient switched-mode power amplifiers (SMPA). The main drawback of RF-PWM is the high level of in-band harmonic distortion when digitally implemented. In order to reduce spectral aliasing effects and produce acceptable levels of harmonic noise, ultra-fast clock speeds are required, making it commercially infeasible. In this paper, we derive a novel compact analytical model of a multilevel digital RF-PWM, driven by an arbitrary bounded baseband signal. We show that the spectral aliasing effects are equivalent to a particular amplitude quantization of the input baseband signal. This result implies that highly linear digital RF-PWM can be realized with modest clock speeds if and only if the input baseband signal is prequantized according to the inherent quantization process. We provide full description of this quantization process and describe its dependence on RF-PWM design parameters. Presented results enable a complete understanding of the nonlinear behavior of digitally implemented RF-PWM, and therefore can aid in optimal transceiver design. Numerical simulations in MATLAB were used to verify the derived analytical expressions.