Modeling And Analysis Of 5-bit Spiral Distributed RF MEMS Phase Shifters

In view of the existing problems such as low frequency,low precision,lots of MEMS switches,large planar area,high loss,high manufacturing difficulty and high cost in RF MEMS phase shifters,this paper proposes a 5 bits spiral distributed RF MEMS phase shifter with high frequency,small size and high precision.The characteristic of the phase shifter is that it can realize 31-phase shifts,each phase shift is 11.25°,the phase shift error is less than5°;the central operating frequency is 40 GHz;and the area is less than 3×3 mm~2.This paper contains the following works:1.A 5 bits spiral distributed RF MEMS phase shifter is proposed for the requirements of Phased Array Radars system in frequency,volume and precision.It uses a spiral CPW structure and 31 MEMS switches are loaded on the spiral CPW.The phase shifter has the advantages of high frequency,small size,high precision,low loss and low cost.The structure model of the phase shifter is clearly demonstrated using Pro/E software.Then the phase shifting process of the phase shifter is described in detail.2.The phase shift,Bragg frequency and pull-down voltage of the distributed RF MEMS phase shifter is derived theoretically.The following conclusions can be obtained.1)The Bragg frequency determines the operating frequency of the phase shifter.Moreover,the Bragg frequency increases with the reducing of the distance between the MEMS switch units.2)The capacitance ratio of the switch in the off state and the on state determines the amount of phase shift.High capacitance ratio is helpful to increase the phase shift.3)The pull-down voltage affects the size of CPW,the material and the size of MEMS switch bridges.3.The CPW and the MEMS switches structure are analyzed in ADS,Ansoft HFSS and ANSYS.The following conclusions can be obtained.1)The substrate material is high-resistive silicon and thickness is 500?m,the width of CPW signal line is 100?m,the distance between CPW signal line and ground line is 148.5?m,the thickness of CPW is 1?m.2)All conductor materials are gold.The length,width and thickness of MEMS switch bridges are 400?m,110?m and 2?m respectively.The distance between all MEMS switches is 120?m.The distance between bridge and the CPW is 2.5?m and the distance is0.1um when the switch is off.3)The pull-down voltage is approximately 42.5 V.4.The spiral distributed RF MEMS phase shifter was modeled and simulated in Ansoft HFSS.The following conclusions can be obtained.1)The volume of the phase shifter is2.6×2.6×0.505 mm~3.2)The simulation results of phase shift are 11.43°,23.04°,44.53°,91.73°and 182.65°respectively when the central operating frequency is 40 GHz.The deviations of the design target are 0.18°,0.54°,0.47°,1.73°and 2.65°respectively.3)Insertion loss and return loss are-15.1092 dB/-8.9120 dB,-14.2271 dB/-9.9053 dB,-15.1092dB/-8.9120 dB,-13.2439 dB/-11.6679 dB and-16.3046 dB/-10.2057dB respectively.4)The simulation results and phase shift accuracy of the 31 phase shifts of RF MEMS phase shifter are given,and the deviation of the design target is within 3.72°.5)The cause of the error between simulation and design of phase shifts is analyzed.5.According to the simulation model of the spiral distributed RF MEMS phase shifter,the planar micro-machining process flow of the phase shifter is designed.According to the shape characteristics of the MEMS switches bridge,the process flow is divided into two parts.These are the sides of the H shape and the middle part of the H shape,respectively.Then it is suggested that there may be invalidation of adhesion and reliability of MEMS switch membrane bridge during the manufacture of phase shifter.