A 0.12-nJ/b 20-Mb/s 700-MHz GFSK Transmitter With a Subsampling PLL Employing 8-Bit Class-D DCO

A 700-MHz Gaussian frequency-shift keying (GFSK) transmitter (Tx) for the Internet of Things (IoT) application is presented in this letter. The proposed Tx adopts direct open-loop oscillator modulation to reach 20-Mb/s data rate, overcoming the rate limitations of conventional architectures. To the author’s best knowledge, this is the highest data rate for the GFSK Tx operating in the sub-1-GHz band. A subsampling phase-locked loop (SSPLL) is duty-cycled and activated just before packet transmission. An 8-bit low-power low-phase-noise class-D digitally controlled oscillator (DCO) is employed to achieve high-frequency resolution, thereby enhancing spectral efficiency. Besides, the Tx utilizes an inverter-based push-pull power amplifier (PA) to accomplish high efficiency and full on-chip integration. The Tx operates with a 0.8-V supply voltage except DCO, and the DCO is powered at 0.25 V. Fabricated in 65-nm CMOS, the DCO achieves a phase noise less than −118 dBc/Hz @ 1 MHz, and the proposed Tx achieves an energy efficiency of 0.12-nJ/b for 20-Mb/s data rate.