Filter considerations in polar transmitters for multi-mode wireless applications

Polar based transmitter architectures are attractive for future multi-mode wireless communication systems. This paper presents some considerations on the filters needed in polar transmitters to meet the spectrum mask requirements. A 1-bit ΣΔ modulator in the envelope path can push the quantization noise outside the channel bandwidth, but will unfiltered cause problems for fulfilling the spectrum mask. This noise can be removed by a low-pass filter in the envelope modulator. The paper shows that a 3^rd order Butterworth filter with a bandwidth three times the signal bandwidth can meet the system requirements. The delay caused by the filter must be compensated in the phase path to correctly align the phase and envelope in time. This compensation can be integrated into the base-band DSP block.