Most published research related to RF switch-mode amplifiers has used sigma-delta modulation techniques to generate encoded pulse trains for amplification in the switching stage. A constraint in sigma-delta modulation is that zero-crossings are restricted to occur synchronously with a clock edge in the quantizer. In this paper, the zero crossing constraint is relaxed and a new encoder is analyzed that is based on noise shaped pulse position modulation. A comparison is made between the two encoders. Comparative examples of a fourth order noise shaping loop filter show that coding efficiency is increased by up to 2.1 dB which will directly translate into an improvement in power efficiency in the switch mode amplifier. The work motivates further research into the design of better time encoders for RF switch-mode power amplifier applications.
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