RF MICROWAVE SPDT SWITCH

Abstract

This project report presents design simulation, fabrication and testing of Single Pole Double Throw (SPDT) microwave switch. SPDT is frequently used in RF and microwave applications like radar, telecommunication, satellites and test and measurement equipment. The microwave switch was designed for an operating frequency of 10 GHz, insertion loss of ≤ 2.5dB, and isolation of ≥ 18dB. A silicon beam lead PIN diode DSG 9500-000 was selected as the solid-state switching device due to its low series resistance and bandwidth up to 18Ghz. The simulations were performed on Keysight ADS. As nonlinear model of this device was not available, therefore packaged PIN diode model was used. Values of series resistance Rs and parallel capacitance Cj from datasheet were substituted in packaged PIN diode model for simulation in ON and OFF state. During design process, a draft layout was made with 50-Ohm transmission lines between all the lumped components. The draft layout was simplified to two parallel loads fed by transmission lines from input port. The impedance of parallel loads will change as diodes turn on or off. Single stub tuning was done to match the on-state impedance to 50 Ohm transmission line. Feed line to single stub was used to maximize off-state impedance. Finally, return loss of input port was minimized to allow maximum power to enter the device. The design was implemented on Rogers RO4350 substrate due to its low loss tangent (⸹=0.0037) and excellent temperature performance. Through hole platting was done to provide dc ground to cathode of both diodes and to make a short circuited stub. The dielectric constant (εr=3.6) makes it possible to achieve reasonable dimensions of the circuit. The final circuit dimensions are 6 cm x 4 cm. The circuit was fabricated using LPKF laser PCB prototyping machine. The PCB was mounted on an aluminum plate with the help of screws. The aluminum plate also provided a means to hold SMA connectors. The device has three ports. One port is specified as input and other two ports as output ports. Both output ports are interchangeable. The fabricated circuit was measured using Network Analyzer PNA E8363B (Agilent Technologies). The measured results of the SPDT switch exhibit an insertion loss of -3.8dB and an isolation of -18.6dB between input and output ports.