Phase error and nonlinearity investigation of millimeter-wave MEMS 7-stage dielectric-block phase shifters

Abstract

This paper reports on phase error and nonlinearity investigation of a novel binary-coded 7-stage millimeter-wave MEMS reconfigurable dielectric-block phase shifter with best performance optimized for 75–110-GHz W-band. The binary-coded 7-stage phase shifter is constructed on top of a 3D micromachined coplanar waveguide transmission line by placing λ/2-long high-resistivity silicon dielectric blocks which can be displaced vertically by MEMS electrostatic actuators. The dielectric constant of each block is artificially tailor-made by etching a periodic pattern into the structure. Stages of 15°, 30° and 45° are optimized for 75 GHz and put into a coded configuration of a 7-stage phase shifter to create a binary-coded 15°+;30°+5×45° 7-stage phase shifter with a total phase shift of 270° in 19×15° steps. The binary-coded phase shifter shows a return loss better than −17 dB and an insertion loss less than −3.5 dB at the nominal frequency of 75 GHz, and a return loss of −12 dB and insertion loss of −4 dB at 110 GHz. The measurement results also show that the binary-coded phase shifter performs a very linear phase shift from 10–110 GHz. The absolute phase error at 75 GHz from its nominal value has an average of 2.61° at a standard deviation of 1.58° for all possible combinations, and the maximum error is 6° (for 240°). For frequencies from 10–110 GHz, all possible combinations have a relative phase error of less than 3% of the maximum phase shift at the specific frequencies. The 7-stage binary-coded phase shifter performs 71.1°/dB and 490.02°/cm at 75 GHz, and 98.3°/dB and 715.6°/cm at 110 GHz. From the measured self-modulation behavior the third-order intermodulation (IM) products level are derived to −82.35 dBc at a total input power of 40 dBm with the third-order IM intercept point (IIP3) of 49.15 dBm, employing a mechanical spring constant of 40 N/m. In contrast to conventional MEMS phase shifters which employ thin metallic bridges which limit the current handling and show fatigue even at slightly elevated temperatures, this novel phase-shifter concept is only limited by the power handling of the transmission line itself, which is proven by temperature measurements at 40 dBm at 3 GHz and skin effect adapted extrapolation to 75 GHz by electro-thermal FEM analysis.