13.10 A >1W 2.2GHz switched-capacitor digital power amplifier with wideband mixed-domain multi-tap FIR filtering of OOB noise floor

Digital power amplifiers and transmitters have drawn significant interest in the recent past due to their reconfigurability, compatibility with CMOS technology scaling and DSP, and potential for automated design synthesis [1–5]. While significant progress has been made in achieving moderate output power levels in CMOS, wideband modulation, and high efficiency under back-off, out-of-band emissions remain an unsolved problem. The elimination of the analog reconstruction filter that follows the DAC in a conventional analog transmitter implies that broadband DAC quantization noise appears at the output of the transmitter unfiltered. Quantization noise can be suppressed by increasing resolution and/or sampling rate, but to meet the challenging −150 to −160dBc/Hz out-of-band (OOB and specifically RX-band) noise requirement of FDD with conventional duplexers, nearly 12b at 0.5GS/s is required. Such a high effective number of bits (ENOB) is extremely challenging in digital PAs given their strong output nonlinearity. Consequently, while low-power modulators are able to approach −150dBc/Hz RX-band noise floor and below [6], state-of-the-art digital transmitters achieve −130 to −135dBc/Hz RX-band noise, nearly 20dB or 100× away [2–4]. Embedding mixed-domain FIR filtering into digital transmitters to create notches in the RX band has been proposed [4,7], but, while successful in low-power modulators [7], nonlinearity significantly limits notch depth to <10dB in digital PAs [4]. Further, notch bandwidth (BW) is far less than 20MHz, the typical LTE BW, in the simple two-tap FIR structures that have been explored [4].