Design and development of a CPW-based 5-bit switched-line phase shifter using inline metal contact MEMS series switches for 17.25 GHz transmit/receive module application

Abstract

A radio frequency micro-electro-mechanical system (RF-MEMS) phase shifter based on switchable delay line concept with maximum desirable phase shift and good reliability is presented in this paper. The phase shifter is based on the switchable reference and delay line configurations with inline metal contact series switches that employs MEMS systems based on electrostatic actuation and implemented using coplanar waveguide (CPW) configuration. Electromechanical behaviour of the MEMS switch has been extensively investigated using commercially available simulation tools and validated using system level simulation. A detailed design and performance analysis of the phase shifter has been carried out as a function of various structural parameters with reference to the gold-based surface micromachining process on alumina substrate. The mechanical, electrical, transient, intermodulation distortion (IMD) and loss performance of an MEMS switch have been experimentally investigated. The individual primary phase-bits (11.25°/22.5°/45°/90°/180°) that are fundamental building blocks of a complete 5-bit phase shifter have been designed, fabricated and experimentally characterized. Furthermore, two different 5-bit switched-line phase shifters, that lead to 25% size reduction and result in marked improvement in the reliability of the complete 5-bit phase shifter with 30 V actuation voltage, have been developed. The performance comparison between two different CPW-based switched-line phase shifters have been extensively investigated and validated. The complete 5-bit phase shifter demonstrates an average insertion loss of 5.4 dB with a return loss of better than 14 dB at 17.25 GHz. The maximum phase error of 1.3° has been obtained at 17.25 GHz from these 5-bit phase shifters.

RF MEMS True-Time-Delay Phase Shifter

“…Furthermore, signal leakage through a TiW bias line also contributes to deviations between simulation and the measured results. Finally, all individual phase bits are cascaded together in a proper arrangement to build a complete 5-bit phase shifter [2]. The SEM image of the complete phase shifter is shown in Fig.…”

“…The length and width of the reference and delay arms are similar for the individual phase bits. Finite-ground CPW lines with uneven ground plane widths may cause a higher loss and noise of localized points in CPW circuits [2]. The CPW ground plane width (300 lm) is three times that of the t-line width (100 lm).…”

“…In general, the coupled slotline mode is excited in the CPW circuits if there is a discontinuity or an asymmetry in the transmission line. It is for this reason that air-bridges are used to equalize the voltages on the two ground planes of the CPW lines at the discontinuities and also at periodic intervals along the uniform CPW line [2].…”

“…To overcome this resonant behavior, the connecting line length (l c ) needs to be optimized using the FEMbased simulator HFSS. In this work, l c has been chosen to be 1400 lm that is k g /5, where k g is the guide wavelength [2].…”

“…11). The total area of the phase shifter is 9 9 4 mm 2 [2]. The phase shifter exhibits a measured return loss of better than 14 dB and the worst-case insertion loss of 5.48 dB over 13 to 17.25 GHz.…”