A UHF RFID reader transmitter with digital CMOS power amplifier

This paper presents a low TX noise transmitter with digital modulated power amplifier (DPA) for UHF RFID reader System-on-Chip. With I/Q digital signals directly modulated on DPA, the transmitter eliminates the pulse shaping filter, up-conversion mixer and power amplifier driver, which are dominant noise contributors in a conventional RFID transmitter. A switch-mode power amplifier cell is adopted in the DPA, which suppresses the amplitude noise in the TX chain. An LO signal buffer strategy is proposed to minimize uncorrelated phase noise which cannot be cancelled by self-cancellation in the RFID receiver. A low noise self-adaptive LDO is designed for lowering TX noise and stable output power. The transmitter achieves a TX noise density lower than -145dBc/Hz and provides a peak output power of 24 dBm with 48% drain efficiency when transmitting CW signal. Through pre-distortion, the transmitter can achieve a spectrum mask with -50 dBc for ACPR1, -62 dBc for ACPR2 and -65dBc for ACPR3. The proposed transmitter is fabricated in a standard 0.18-μm CMOS process with an area of 1000×640μm².