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

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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].