Filtering of a Group Delay Equalized Delta-Sigma Modulated Envelope Signal in an EER Architecture

Опубликовано 7 сентября 2016, 16:30

The polar RF transmitter architecture, Kahn Envelope Elimination and Restoration (EER), aims at achieving linearity, while amplifying non-constant envelope signals efficiently by means of switch mode amplification. A Delta-Sigma Modulator (??) is proposed to be used before the switch mode amplifier of the envelope signal. Due to its noise shaping characteristics, sharp analog filtering is compulsory afterwards. As a consequence, spectral regrowth appears in the output signal fed to antenna. This happens due to two reasons: (i) the filter group delay variation cannot be accurately compensated with a frequency constant delay module in the phase signal path, and (ii) the reduction of the envelope signal bandwidth, which is spread by the cartesian to polar conversion. In this talk, the equalization of the group delay variation in the envelope path is studied and the improvement achieved is directly related to the relaxation of the ?? design requirements. It is shown that with a 8th order Butterworth analog low pass filter with cutoff frequency of 24 MHz, the improvement in EVMRMS and in ACPR @ 30 MHZ offset from the carrier is approximately 7 dB and 2 dB, respectively, considering a 2nd order ?? sampling at 1.28 GHz.