# A Crystal-Less Clock Generation Technique for Battery-Free Wireless Systems

Ziyi Chang, Yunshan Zhang, Changgui Yang, Yuxuan Luo, Sijun Du, Yong Chen, Bo Zhao

Research output: Contribution to journalArticlepeer-review

## Abstract

The size of wireless systems is required to be reduced in many applications, such as ultra-low-power sensor nodes and wearable/implantable devices, where battery and crystal are the two main bottlenecks in system miniaturization. In recent years, battery-free radios based on wireless power transfer (WPT) have shown great potential in miniature wireless systems, while a reliable on-chip clock without a crystal remains a design challenge. Conventional methods utilized the RF WPT tone as the reference for clock generation, but the high RF frequency leads to high power consumption. In comparison, using a lower WPT frequency results in an antenna with a larger size. In this work, the 2<inline-formula> <tex-math notation="LaTeX">$^{\rm{nd}}$</tex-math> </inline-formula>-order inter-modulation (IM2) component of the two RF WPT tones is extracted to lock an on-chip oscillator, providing a low-jitter PVT-robust clock. In this way, the wireless systems can benefit from: 1) The clock recovery circuits operate at a low IM2 frequency, reducing the power consumption. 2) The WPT can be set to a high RF frequency to minimize the antenna. Fabricated in 65 nm CMOS process, the proposed crystal-less clock generator takes a small area of 0.023 mm<inline-formula> <tex-math notation="LaTeX">$^2$</tex-math> </inline-formula> in a wireless system chip. Measured results show <inline-formula> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula>92 dBc/Hz@10 kHz phase noise and 6.8 <inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula>W power.

Original language English 1-12 12 IEEE Transactions on Circuits and Systems I: Regular Papers https://doi.org/10.1109/TCSI.2022.3201196 E-pub ahead of print - 2022

## Keywords

• Antennas
• Battery-free
• clock generator
• Clocks
• Crystals
• Generators
• injection locking
• inter-modulation