October 14, 2010
4 PM EST (1 PM PST)
Millimeter-wave (MMW) frequencies have wavelengths small enough to offer sufficient spatial resolution for certain imaging applications, such as automotive radar and concealed weapon detection. Advances in silicon processes have developed devices that are capable of operating at these frequencies, enabling low-cost MMW imaging, provided that the circuit design can meet the challenging performance specifications for these applications. In this research, we investigate key components for an active imaging system operating at 77GHz that performs purely digital beamforming. Each element in the phased array has an antenna and processor that measures the phase and amplitude of the received signal. The main focus is the phase measurement and the quality of the measurement. Phase noise from the receiver's front-end circuits will degrade the overall image integrity.
The system requires a MMW phase-locked loop (PLL) to generate the local oscillator. The PLL is a significant contributor of phase noise. A MMW PLL was designed in a 0.13um silicon-germanium BiCMOS technology for low phase noise and power consumption while maintaining enough output power to robustly drive a mixer load. Measurement results show a de-embedded single-ended output power of -2dBm, a phase noise of -81dBc/Hz at 1MHz offset from the carrier, and a total power dissipation of 107mW.
A new technique called digital phase tightening reduces phase noise from receiver front-end circuits to allow precise phase estimation for digital beamforming. This is achieved by leveraging the large ratio between the MMW carrier frequency and the relatively low frame rates in imaging applications. By mixing down to an intermediate frequency and then averaging over many samples to estimate the phase, we reduce error caused by phase noise. A test chip demonstrating the phase tightening concept was designed and fabricated using 0.13um CMOS devices, and we show that femtosecond levels of RMS error are feasible with this technique.
Massachusetts Institute of Technology
Khoa Nguyen received his BS degree in Electrical Engineering and Computer Science from UC Berkeley in 2004. He received his MS degree in Electrical Engineering and Computer Science from Massachusetts Institute of Technology in 2006. He is currently working towards his PhD at the Massachusetts Institute of Technology. His research interests lie in millimeter-wave and mixed-signal integrated circuit design for millimeter-wave imaging.