A Low-Current Digitally Predistorted Direct-Conversion Transmitter With 25% Duty-Cycle Passive Mixer

A low-current direct-conversion transmitter (TX) is obtained by combining an efficient single-balanced 25% duty-cycle passive mixer with a low-bias single-ended driver. The nonlinearity of such architecture is aggravated by strong harmonics at the mixer output folding down to the fundamental frequency due to driver nonlinearity. To address this, a baseband mathematical model of the passive mixer plus nonlinear driver is derived. Then, based on this model, a novel digital-predistortion approach is disclosed. Implemented in 40-nm CMOS, the modulator and driver combined consume 45 mW when delivering a +3-dBm Release 99 WCDMA signal with 0.9% error vector magnitude (EVM) and -59-dBc adjacent-channel leakage power ratio (ACLR). In long-term evolution (LTE) mode, the TX delivers +0-dBm 10-MHz LTE 16QAM with -53-dBc ACLR and 1.2% EVM.