A watt-level 2.4 GHz RF I/Q power DAC transmitter with integrated mixed-domain FIR filtering of quantization noise in 65 nm CMOS

This paper presents an S-band RF I/Q power DAC transmitter with embedded mixed-domain finite-impulse-response (FIR) filter to suppress out-of-band (OOB) quantization noise for FDD/co-existence. Watt-level output power and low output OOB noise level is achieved through (1) power combining (2) device stacking with thick oxide devices in a switching-class power amplifier (PA) and (3) embedded RF FIR filtering. To combat the strong conductance-to-amplitude/phase nonlinearity of switching-class digital PAs, a transformer-based FIR architecture is proposed that ensures completion of filtering after the nonlinearity. Implemented in 65 nm CMOS, the transmitter has a measured peak output power of 29.1 dBm with 42% system efficiency at 2.25 GHz. The measured noise floor at 100 MHz offset from the 2.4 GHz carrier is -134 dBc/Hz for a 20 MHz 64-QAM signal at 200 MSa/s including 8 dB noise floor suppression from transformer-based two-tap third-order FIR filtering, yielding an ENOB of 8.18.