Design and Implementation of 24-GHz CMOS FMCW Radar Transmitter

With the increasing emphasis on automobile safety, the 24-GHz collision avoidance radar emerges as a product that is likely to enjoy commercial success. Presently, due to output power and reliability requirements, the transmitter front-end of the radar is mainly fabricated with III-V process. However, the resulting high implementation cost impedes mass deployment. Therefore, a low-cost solution is desired to make the radar affordable.
In this thesis, the cost reduction is achieved through system integration, and CMOS technology is chosen to carry out the challenge. The CMOS process is known for its unparalleled versatility, but its break-down voltage decreases with the shrinking of the feature size, leading to insufficient output power from a single device. Therefore, the power-combining techniques are introduced in the implementation of the CMOS front-end circuit to join the output power from several devices, which guarantees the detection range of the radar. The fabricated front-end circuit provides above-17-dBm output power in the allocated K band and reaches a maximum power-added efficiency (PAE) of 29%.
Based on the front-end circuit, a highly-integrated transmitter is realized with the addition of a fractional-N frequency synthesizer. The synthesizer is capable of producing the frequency-modulated continuous wave (FMCW) signal that is commonly employed in the collision avoidance radar application. The spurious tones that would present in a simple accumulator-based fractional-N function realization are suppressed by the incorporation of multi-stage noise shaping (MASH) technique. The synthesizer provides a digital control interface, and the system processor is allowed to change the sweep range and sweep time of the FMCW signal through this interface.
In order to further reduce the cost of the transmitter, the bulky and out-of-chip crystal oscillator (XO) that is essential in the realization of frequency synthesizer is proposed to be removed. The FMCW signal is generated by a triangular waveform generator directly modulating a linear-tuning voltage-controlled oscillator (VCO). The precise frequency definition provided by the XO is replaced by a temperature-variation- insensitive calibration loop, which monitors the output signal frequency and provides feedback through the band-switching function of the VCO. Thus, the output signal of the crystal-less transmitter will remain within the allocated frequency band.