Design of an x-band 3-bit rf mems constant phase shifter


Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Electrical and Electronics Engineering, Turkey

Approval Date: 2016

Student: AHMET KUZUBAŞLI

Co-Supervisor: TAYFUN AKIN, ŞİMŞEK DEMİR

Abstract:

This thesis presents a 3-bit 180° constant phase shifter design implementing Co-Planar Waveguide (CPW) and RF MEMS variable capacitors with ±1.8% accuracy at 10 GHz and ±5.8% maximum peak error between 9-11 GHz. The phase shifter with minimum phase errors is determined by considering exemplary circuit simulations of different phase shifter types designed with a novel in-house RF MEMS fabrication process [1] parameters. Due to its wide-band characteristics and CPW compatibility, the selected topology is the reflect-type phase shifter employing RF MEMS capacitor banks and 3-dB couplers. Phase shifts are provided by the change in the reflection phase of RF MEMS capacitor banks. At first, a 2-bit 90° phase shifter is designed to cover 0°-90° phase shifts. Successive addition of two 2-bit phase shifters allows to obtain a 3-bit operation up to 180° phase shift. The capacitor bank design includes various configurations of RF MEMS capacitors, and they are compared to provide minimum phase errors between 9-11 GHz. The design procedure is managed while having regard to the in-house novel RF MEMS fabrication technique [1] and its design rules. Two different options are considered for the design of 3-dB couplers, namely regular Branch-Line Coupler (BLC) and Double Branch-Line Coupler (DBLC), comparing their bandwidths which are defined by phase difference and power imbalance between coupled and thru ports. Circuit simulations suggest to use a miniaturized double branch-line coupler (M-DLBC) implementing CPW Lines shortened with lumped air-bridge capacitors, and therefore this coupler and RF MEMS capacitor banks are modelled and simulated by using a 3D Electromagnetic (EM) solver software. The proposed 3-bit phase shifter provides phase shifts with a ±1.8% accuracy at 10 GHz and ±5.8% maximum peak error between 9-11 GHz. The mechanical parameters such as spring constant and pull-in voltages of RF MEMS bridges used in the phase shifter are also calculated.