Study Of RF MEMS Switched-line Phase Shifter Based On Interdigital Coplanar Waveguide

With the employment of MEMS technology in radio frequency(RF) field, RF MEMS, a new research field appears which makes the RF devices' micromation possible. Compared with the traditional RF devices, the components and systems fabricated by RF MEMS technology have smaller volume, easier integration and higher performance than traditional RF devices. RF micromachining and MEMS technology promise to provide an innovative approach in the development of effective and low-cost circuits and systems, and are expected to have significant impact on existing RF architectures in radar and communications by reducing weight, cost, size, and power dissipation. MEMS technologies provide good solutions for realizing miniature, low-power and high-frequency wireless communication systems.Transmission line is a primary component of microwave devices and many kinds of novel transmission lines have been designed since MEMS technology was introduced into RF field. As to coplanar waveguide which is widely used in MMIC, several new structures such as Elevated CPW, Overlay CPW and Microshield CPW were designed and fabricated. These structures have better performance in some aspects, reducing loss or widening available impedance range, but they still have much limitation in their practical application for their complicated fabrication process and can not minimize the size of devices effectively. To overcome these disadvantages, the structure of traditional CPW was modified and a novel interdigital CPW was designed in this paper.Phase shifter is a vital component of modern communication and radar systems and now it is mainly realized based on ferrite material, PIN diode and FET switch. These phase shifters may have high loss, or high cost, or high power consumption which limit their application. RF MEMS phase shifter was designed to avoid these disadvantages which has low loss,low cost and weight. Since then, several kinds of phase shifter, such as switched-line phase shifters, reflect-line phase shifter and distributed phase shifter, were designed and fabricated. In this paper a X-band 3-b switched-line phase shifter was designed using shunt capacitive switch, based on the study of interdigital CPW.The interdigital CPW has a periodic bend structure which can minimize the size in the longitudinal direction. Its ground lines were thickened through MEMS technology which can improve the distribution of the surface current density on the signal line and consequently reduce the conductor loss. It also can limit the distribution of the electric field in the space which can reduce the radiation loss. Another merit is that the thickened ground lines can reduce the interference between the adjacent parallel sections of the signal line. This design was evaluated by high frequency electromagnetic simulation software. Studied the simulated results and measured results, the effects of interdigital CPW's sturcture parameters on its performance were ayalysised. The measured insertion loss can be lowered to -0.33dB/cm and the return loss has wave hollows within 5～20GHz and its position can be moved with the structure parameters. Based on the study of the wave hollows' position, the interdigital CPW with ground lines partly buried was designed and simulated. From the simulated results and measured results we found that its positons has distinctly been moved and the interference between the adjacent parallel sections of the signal line can be reduced above and under it.The switched-line phase shifter consists of interdigital CPW and shunt capacitive switch which has卍-type movable ploar plate. The plate is supported by four L-shape beams whose effective elastic coefficient was calculated by FEM and the effect of structure parameters on it was studied. Also the electrostatic actuated principle and electromagnetic model was built and analysised. The 3-b phase shifter consists of four 1-b phase shifter with 0/450, 0/900 and 0/1800 phase shift respecitvely and has 9 switches and the whole size is 5.5mm×7mm. The measured pull-in voltage is 36.5V and its isolation is higher than 25dB within 6～15GHz. The measured phase shifts at 10GHz are 38.10, 94.70 and 188.30 and their insertion losses are -6.6dB, -8.6dB and -9dB respectively and their return losses are all higher than -31dB.The related micromachining technology has been studied for interdigital CPW and switched-line phase shifter, and the proper method, parameter and materials have been selected. The basic micromachining technologies used include sputtering, electroplating, photolithography, etching, grinding and sacrificial layer technology. In order to ensure the devices being fabricated successfuly, some normal micromachining technologies have been modified according to the design and actual requirement.