24-GHz 互補式金氧半導體連續波雷達發射機設計與實作

Abstract

隨著車載安全設備日漸受到重視，24 GHz 防撞雷達有機會成為各式車輛的標準配備。目前，由於輸出功率與可靠度考量，該種雷達的發射機前端電路大多以三-五族製程實現。然而，該製程過高的成本讓防撞雷達無法普及，也使得低價的版本成為值得發展的目標。 在本篇論文中提出了以互補式金氧半導體製程達成系統整合的方法，以減低總製造成本。互補式金氧半導體製程已被用在多樣化的電路中，但隨著製程世代演進，其崩潰電壓也逐步下降，致使電晶體的輸出功率不足。在本次的前端電路設計中採用了功率結合的技巧，以得到較大的總輸出功率與足夠的雷達偵測距離。實作的前端電路可在K band提供17 dBm以上的輸出功率與最大29%的功率增加效率。 將此前端電路與一除小數的頻率合成器結合後，便可得到一個高整合度的雷達發射機。該頻率合成器可以合成出防撞雷達常用的頻率調變連續波波形，而多級雜訊整型的技巧則用來抑制以簡單累加器達成除小數功能時，在輸出頻譜上會出現的混附訊號。雷達系統的處理器可經由頻率合成器提供的數位控制介面控制連續波的調變速率與頻率範圍。 在前述發射機中，石英振盪器是唯一的非整合元件。若將其省去，則可造出更低成本的雷達發射機。在新的電路架構中，發射的連續波由一三角波產生器直接控制線性壓控震盪器產生。石英振盪器所提供的精準頻率由一校正迴圈取代，而此校正迴圈具有溫度變異不敏感的特性。此迴圈監視發射機的輸出頻率，並由壓控振盪器的調頻帶接腳進行回授控制。以此方法製造的發射機，其輸出訊號頻率將能落在所指定的頻率範圍中。

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.