A Multi-Band Low-Supply-Voltage 16-QAM Transmitter Based on a Digital Fractional-N PLL With TDC Calibration for Capsule Endoscopy

This paper presents a highly digital transmitter capable of multi-band 16-QAM modulation for capsule endoscopy, which requires a high data rate with high energy efficiency and relatively high output power to meet receiver sensitivity specifications. A counter-assisted digital phase-locked loop (CDPLL) is utilized for generating clocks across multiple bands with low noise, resulting in good error vector magnitude (EVM) performance. The EVM generated by the CDPLL is less than 1% by using the proposed time-to-digital converter (TDC) calibration. A digital power amplifier with high power efficiency helps reduce the power consumption of the entire endoscopy capsule system, powered by a battery with limited capacity. Fabricated in a 180-nm CMOS process, the transmitter achieves a maximum data rate of 40 Mbps using 16-QAM modulation. At a 1.1-V supply, it achieves a 0.27/0.26-nJ/bit efficiency with a data rate of 40 Mbps, and the measured EVM is 3.24/3.7% while delivering a $\boldsymbol{-}$5-dBm output power within a 10-MHz narrow bandwidth at 435/415 MHz for the wireless medical capsule endoscopy band in Europe and the biomedical band in China, respectively. The corresponding figure of merit is 0.86/0.82 nJ/(bit$\cdot$mW) in each band.