13.4 All-digital RF transmitter in 28nm CMOS with programmable RX-band noise shaping

Abstract

The crowded radio spectrum allocated for 3G/4G mobile communication, together with the growing demand for higher data-rates, has led to the situation where transceivers need to support FDD operation in multiple frequency bands with different TX-RX duplex spacing. In order to reduce costs and enable SAW-less operation, many recent transmitter implementations have thus targeted stringent out-of-band (OOB) emission levels. Analog-intensive TX architectures achieve low OOB noise at the price of large area consumption, as complex reconstruction filters are used to suppress DAC quantization noise and image replicas [1,2]. On the other hand, due to the lack of analog filtering, digital-intensive TX architectures need 12–14b DAC resolution for low OOB noise, which complicates DAC design and typically requires DPD or calibration [3–5]. This work presents an RF transmitter implementing a fully digital solution to the aforementioned challenge. Instead of using bulky analog filtering or high resolution DAC, the disclosed TX employs digital ΔΣ modulation and mismatch shaping to attenuate the DAC noise at a programmable duplex distance. This solution enables −160dBc/Hz noise in the RX-band, by using only a 10b DAC without DPD, calibration or analog filtering.