2) Transmitters using ΣΔ modulators: The development of silicon technologies allows the design of high efficiency switched PA. For non constant envelope modulations, one can consider the use of a RF class S power amplifier. The principle consists in transforming the RF signal into a Pulse Width Modulated (PWM) signal which is then amplified by the switched PA and filtered, as presented on Fig. 2. The PWM transformation is preferably proceeded with a ΣΔ modulator to reduce in-band spurious levels. Various architectures going by that principle are proposed, some based on bandpass RF Sigma Delta modulator [5], fully digital [6] or based on the multiplication of a phase modulated signal and a PWM coded envelope signal [7]. For all these versions, the limiting point is the output spectrum. Indeed, the ΣΔ modulator reshapes and rejects the quantification noise outside the bandpass of the signal and the spectrum is replicated (sampling system). The output has then to be strongly filtered outside the bandpass with as a drawback the filter losses. Consequently, these architectures move the constraint from the PA to the filter.