A Cellular Multiband DTC-Based Digital Polar Transmitter With −153-dBc/Hz Noise in 14-nm FinFET

A digital polar transmitter is presented that uses a digital-to-time-converter (DTC) for wide-channel phase modulation with multiband support. DTC-based phase modulation enables high-efficiency polar operation with high-efficiency RFDAC. The transmitter uses coarse division inherent in DTC operation to generate TX output frequencies from 0.7 to 2.2 GHz with a limited DCO tuning range of only 7.3-8.7 GHz (±8.7%). Phase modulation of narrowband signals (e.g., one resource block) using DTC could result in spurs in output spectrum due to the almost periodic nature of DTC errors. A theoretical analysis is presented that predicts the location of these spurs and a technique to eliminate such spurs by appropriately adjusting the PLL frequency is introduced. A new digitally controlled-edge-interpolator architecture is also presented to minimize dynamic errors, which is necessary in order to meet stringent cellular noise requirements. The DTC was implemented using digital synthesis and auto-place-and-route (APR) tools, requiring very limited analog layout resources. The TX was fabricated in 14-nm FinFET and it achieved a noise of -153/-150 dBc/Hz for LTE5/LTE20 in Band1 at +4-dBm output power with DTC power dissipation of 27 mW.