Study Of RF MEMS Variable Capacitor And Voltage Controlled Oscillator

In recent years, increased demand for wireless communication systems motivates a growing interest in low phase noise voltage controlled oscillator (VCO). The tunability of VCO is normally provided by a variable capacitor. Achieving low phase noise requires a high-Q on-chip variable capacitor. However, it is very difficult to realize a high-Q on-chip variable capacitor by standard integrated circuit (IC) process. Hence, current VCO is often implemented by an external PN junction varactor. The off-chip device not only increases final system area, but also increases package complexity, power consumption and cost. Therefore, a high-Q on-chip variable capacitor is needed stringently which can be integrated monolithically with VCO circuit.The quality factor of concave shape RF MEMS variable capacitor is optimized by full-wave electromagnetic field simulation software, and the goal of optimization is to receive the highest quality factor at the frequency of 2GHz. The parasitic capacitance is also discussed. One-port lumped parameters equivalent circuit model is then found based on above analysis. According to the optimized structure, we have calculated pull-in voltage and the first-order resonant frequency.The nonlinear analysis is conducted for parallel-plate variable capacitor. The second-order dynamic equation is solved by iterative method, then the dynamic model is built. We have discussed detailedly the effect of air squeeze film damping and direct current (DC) bias on the motion of suspended plate. The bistable state phenomenon which exists in variable capacitor is discovered for the first time, furthermore, the reason for this phenomenon is also explained. At last, the characteristic of bistable state is applied to investigate the tuning range of variable capacitor, and we draw a conclusion that the tuning range may be larger than 50% theoretically.Based on existing process of RF MEMS switch, the surface micromachining process is proposed which is the same with concave shape variable capacitor, and RF MEMS variable capacitor samples are successfully made out. The test results show that quality factor is the...