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

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 2nd-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(2) in a wireless system chip. Measured results show -92 dfic/Hz@10 kHz phase noise and 6.8 mu W power.

Automated summary

In many applications, it is necessary to reduce the size of wireless systems, which are limited by the battery and crystal in system miniaturization. Recent advancements in battery-free radios based on wireless power transfer (WPT) have shown great potential in miniature wireless systems, but the design of a reliable on-chip clock without a crystal remains a challenge. This study proposes a method of locking an on-chip oscillator using the 2nd-order inter-modulation (IM2) component of two RF WPT tones, providing a low-jitter clock that is robust to variations in process, voltage, and temperature (PVT). The proposed crystal-less clock generator achieves a small area footprint in wireless system chips and demonstrates low phase noise and power consumption.