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The Design, Fabrication and Measurement of Microstrip Filter and Coupler Circuits

Today’s microwave designers rely on many tools to help create effective circuits and systems. They use their libraries of published references, along with powerful EDA design tools and electromagnetic (EM) analysis tools, combined with the lessons of their own experience. Their work is verified with the construction and testing of a finished circuit. This article describes two microstrip designs that were developed using different methods, fabricated quickly using a p.c. board milling machine, then measured to determine the accuracy of the design methods. The example designs are a classic hairpin filter with a bandwidth of 3.7 to 4.2 GHz, and a 1 to 8 GHz directional coupler using the Schiffman sawtooth, or zig-zag, technique to reduce the size. The hairpin filter was designed and simulated using Agilent ADS 1.3 [1], with planar EM analysis using Sonnet Lite [2]. The coupler used a design-rule-based transformation, starting from an existing stepped-line coupler design. Both circuits were fabricated on a Protomat C100HF from LPKF Laser & Electronics [3], with measured results obtained using an HP (Agilent) 8753E network analyzer.