All Digital Phase-Locked Loop for Ultra Low Power Internet of Things Radios

Imec, Holst Centre and ROHM presented an all-digital phase-locked loop (ADPLL) for Internet-of-Things (IoT) radio transceivers, at the 2017 International Solid-State Circuits Conference in San Francisco (US). PLL's are traditionally one of the major power consumers in a radio systems and can take up to 30% of the RF system real estate. The new ADPLL proposed by the researchers features a small area (0.18mm² in 40 nm CMOS), low power consumption (0.67 mW) and excellent performance. With all spurs lower than -56 dBc and jitter below 2 ps, which is beyond state-of-the-art digital PLLs, the new ADPLL shows an excellent robustness.

The Internet of Things relies on tiny sensor nodes, invisibly embedded in our environment and wirelessly connected to the internet. As billions of IoT devices are set to be deployed, battery replacement becomes impossible and therefore, power consumption reduction, especially in wireless connectivity, is one of the leading concerns and challenges on low power radio design to address.

The PLL is the radio component for frequency synthesis and has traditionally been an analog component. All-digital PLLs enable a smaller footprint, better control and testability, and improved scaling to advanced CMOS nodes. However, they have lagged behind in terms of performance, compared to analog solutions.

Imec and ROHM's all-digital PLL is a first to offer low power consumption of only 0.67 mW, with state-of-the-art performance. It supports all specifications of Bluetooth Low Energy (BLE) radios while significantly reducing cost and power consumption below any of today's solutions. This divider less fractional-N digital PLL features a power-efficient spur-mitigation technique and a digital phase unwrap technique. Both approaches contribute to its excellent performance.

At ISSCC2017, imec presents four papers addressing key building blocks for ultra-low power connectivity. The innovations serve power reduction in active, standby, sleep and transient operation of standard radios like Bluetooth, or newcomers in the sub-GHz communication space.