Manchester encoded bandpass sigma-delta modulation for RF class D amplifiers

An analysis of a continuous-time bandpass sigma-delta modulator in a configuration with an upconverter is given for a RF class D amplifier application. The upconverter multiplies the modulator pulse train with a synchronised clock signal and maps each modulator bit to an integer multiple k of a (+1, -1) or (-1, +1) pattern depending on the sign of the modulator bit. The upconversion is equivalent to an extension of Manchester encoding, which is usually defined for k = 1. The analysis focuses on evaluating the impact of upconversion on the modulator coding efficiency and the average pulse period. A design equation is derived, which shows that coding efficiency is dependent only on the upconversion frequency ratio, while the average pulse period depends only on k. The equations provide a designer with a way of evaluating the trade-offs in the amplifier system and show that encoding with k = 1 is the most efficient configuration for maximising coding efficiency and minimising switching power loss.