Quad-phase lowpass delta-sigma modulator for switching-mode power amplifiers

This work presents a transmitter frontend based on lowpass delta sigma modulators. In contrast to the conventional approach where two ΔΣ modulators are utilized - one for the inphase, I, an one for the quadrature, Q, component - the new system is based on four lowpass ΔΣ modulators. By adding two additional modulators to the conventional concept with a phase-shift of 45° compared to the I and Q phase the overall quantisation noise level can be decreased. The feasibility of the concept is demonstrated by an implementation on an FPGA for verification and measurements. The measurement results show a reduction of the peak noise by 6dB and in improvement of the near carrier signal to noise ratio of 3dB.