June 15, 2019
June 15, 2019
October 19, 2019
Experimentation and Laboratory-Oriented Studies
A new semester design project was created for a graduate-level course in High-Frequency Circuit Design at California State University, Chico. This course covers topics including but not limited to transmission lines, component behavior at high frequencies, Smith Charts and Vector Network Analyzers, impedance matching, and radio-frequency (RF) behavior of diodes and transistors.
This project was developed to give students experience in designing RF filters and verifying their results using both software simulations and physical implementation. Concepts in filter design covered at an undergraduate level including frequency response and filter types, bandwidth, roll-off, and Bode plots are extended to filters implemented for operation in the gigahertz range. Issues to consider when designing filters to operate at high frequencies are highlighted, including unrealistic values and non-ideal frequency responses of lumped components, measurement techniques, and performance of printed circuit board (PCB) substrates.
Many issues that affect lumped-component filters at high frequencies can be satisfactorily addressed with the implementation of distributed-element filters consisting solely of transmission line segments with controlled dimensions. All passive filter designs undertaken for this project originate with a normalized lumped-component LC low-pass prototype. The first portion of the project requires students to design a 5th-order band-pass or band-stop filter using lumped components and analyze the performance of the resulting filter circuit using LTSpice in terms of parameters such as passband ripple, upper and lower cutoff frequencies, and bandwidth. For the second portion of the design project, students design a 3rd-order lumped-component LC low-pass filter and then convert it into a distributed-element filter consisting of microstrip shunt stubs implemented on FR-4 PCB substrate. Richard’s Transformation is used to convert the lumped LC low-pass prototype into a distributed element filter consisting of only short-circuit series and open-circuit shunt stubs. Kuroda’s Identities are then used to convert all series stubs into shunt stubs for microstrip implementation. The characteristic impedances and dimensions of each microstrip segment needed to emulate the lumped-component filter are then calculated. After the distributed-element filter design has been completed, students then use Sonnet Lite to evaluate the filter’s performance in terms of scattering parameters (S11, S21). The primary goal of this portion is to verify that the filter’s frequency response matches the design requirements.
Students then design a PCB for their distributed filter element using Autodesk EAGLE. Each student’s PCB is manufactured in-house on ½ oz. copper FR-4 substrate, using an LPKF ProtoMat E34 PCB milling machine. Students are required to solder SMA connectors on the input and output ports of their filters. Finally, students use a Vector Network Analyzer to verify the frequency response of each filter prototype then compare theoretical vs. physical results.
A detailed description of the design process, mathematical equations, simulation procedures, measurements, and performance limitations of materials and manufacturing methods will be provided. A summary of theoretical vs. physical results will be presented, along with a description and assessment of student learning outcomes.
Silveira, D. (2019, June), Integrating Theory and Hands-On Implementation in RF Distributed-Element Filter Design Paper presented at 2019 ASEE Annual Conference & Exposition , Tampa, Florida. 10.18260/1-2--32993
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