At the 2023 International Solid-State Circuits Conference, imec, a research and innovation hub in nanoelectronics and digital technologies, presents a novel digitally calibrated charge-pump (CP) phase-locked loop (PLL) that can generate high-quality frequency-modulated continuous-wave (FMCW) signals for mmWave radars at low power consumption. The novel PLL is a critical building block for future short-range automotive (in-cabin and out-of-cabin) and industrial (e.g., on-cobot sensing) radar applications.  

FMCW mmWave-based radar sensors are becoming increasingly popular for multiple automotive, healthcare and industrial applications. FMCW radars transmit a sinusoidal wave whose frequency increases linearly in time, a sweep referred to as ‘chirp.’ After being reflected by the object, the signal is picked up by the receiver and mixed with the original chirp signal, after which the object’s distance and speed can be extracted. The quality of the radar signal depends largely on the performance of the PLL used to generate the chirp signal.  

Imec’s novel PLL generates highly linear, high-quality chirp signals centered around 16GHz with a chirp bandwidth of 1.5GHz. A key achievement is reaching chirp speeds down to 12 µs, with as low as 41 kHzrms error in frequency modulation (rms-FM-error). The PLL enables extremely fast startup (below 1 µs) and minimal reset time between chirps (1 µs reset time). The chirp generator operates in duty-cycled mode—synthesizing N chirps in one burst before powering down—providing significant power savings. For example, the PLL consumes only 9.2  and 1.48 mW when operating in a 50 percent and one percent duty-cycled mode, respectively. Even after power-down mode, the rms-FM-error of the first chirp remains below 41 kHz. The duty cycling performance was enabled by adopting a CP-based PLL architecture, fabricated in 28 nm CMOS technology and extended with a phase-offset compensation time-to-digital convertor (POC-DTC) to facilitate fast self-calibration.

These excellent performance parameters make the PLL highly suitable for short-range automotive radar applications ranging up to several tens of meters. “Applications include in-cabin radar sensors to monitor presence, movements and well-being of driver and passengers, as well as out-of-cabin sensors for parking assistance or vehicle detection,” explains Ilja Ocket, program manager at imec. “Our PLL also opens doors to robotics radar applications—think of on-cobot radar sensors to enhance safety and efficiency of human-robot interaction in industrial environments—as well as to radar sensors mounted on small moving objects or vehicles such as drones. At ISSCC, we are presenting a functional demo that integrates our CP-PLL with imec’s existing 140 GHz radar receiver and transmitter blocks to showcase the potential of the technology for future automotive and industrial applications. The PLL can also be used for up-conversion to mmWave radar signals with other carrier frequencies, e.g., 80 GHz.”