射頻壓控震盪器使用多重靜電驅動電極之微機械式可變電容

Abstract

摘要 本篇論文提出了一應用於射頻壓控震盪器之以CMOS-MEMS製程的線性可變電容。根據靜電力學與彈力學的延伸可得到控制電壓與電容之非線性關係。使用多重靜電驅動電極，藉由改變極板之數目可得到電容之工作區間，亦可得到壓控震盪器頻率變化之工作區間。因此，藉由此工作區間，可使用線性控制設計之演算法來決定電壓所加之極板的數目與次序。 現代的無線通訊系統中，鎖相迴路在射頻前端扮演相當重要的角色。而以鎖相迴路為基礎的頻率合成器中，最重要的部分即為壓控震盪器。傳統如電容電感式的壓控震盪器，具非線性控制電壓對頻率變化之問題。使用多重靜電驅動電極可變電容之CMOS-MEMS壓控震盪器提供一不使用線性化電路而可電壓線性控制震盪之解決方案。再者，機械改變方式可避免掉高損耗，並且使用空氣介質取代其他介電材料可使電容之Q值提高，因而改善壓控震盪器之相位雜訊。

Abstract In this thesis, a linearly tunable capacitor applied to radio frequency (RF) voltage-controlled oscillator using CMOS-MEMS process is proposed. According to the extension of electrostatic theory and elastic theory, the nonlinear relationship between capacitances and applied voltages is obtained. Using multiple electrostatic driving electrodes, the working space of capacitor can be obtained by varying the number of electrodes, so as the frequency tuning range of the voltage-controlled oscillator. Hence, with those working spaces, a linear control design algorithm can be used to determine the number and the order of multiple electrodes that the control voltage is applied to. Phase-locked loop (PLL) plays an important role in RF front-ends of the modern wireless communication systems. Voltage-controlled oscillator (VCO) is the main part of phase-locked loop based frequency synthesizers. Conventional VCO such as the LC-tank VCO has the problem of nonlinear frequency tuning range versus control voltage. A CMOS-MEMS VCO with tunable capacitor that uses multiple electrostatic driving electrodes offers a solution for linear voltage-controlled oscillation without a linearization circuit. Furthermore, mechanical tuning avoids high losses and the use of air gap instead of other dielectric materials makes high-Q capacitors possible. Hence, the phase noise of VCO can also be improved.