microstrip filter design

These practical microstrip examples provide a valuable tutorial on the use of many different engineering resources: published references, comprehensive EDA tools, EM analysis and rapid prototyping equipment.
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.
Design example #1: A 3.7 to 4.2 GHz hairpin filter This filter was designed for a flat response over the 3.7 to 4.2 GHz band, with low insertion loss and return loss better than 16 dB across the band. The filter’s application is image rejection at the input of a synthesized block downconverter. A classic hairpin design was chosen, since experience has shown that it would meet the performance and size requirements for this design. The filter was designed using ADS 1.3, with the resulting layout shown in Figure 1. This, of course, is the familiar hairpin configuration. The area occupied by the filter is approximately 500 by 1200 mils (0.5 x 1.2 in.), plus sufficient area beyond the hairpin loops to maintain consistent dielectric properties. Figure 2 shows the design and optimization setup in ADS. Since this topology has symmetry around the center, it was designed as two sections, connected in a “back-to-back: configuration. With this reduction in the size of the mathematical problem, calculation time is significantly reduced. The optimization was set up to obtain a minimum 16 dB return loss within a passband of 3.55 to 4.4 GHz, and a minimum stopband attenuation of 28 dB below 3.2 GHz and above 4.7 GHz. The optimization was set up for a frequency range of 3.0 to 5.0 GHz. A wider range is not required to obtain the desired results.
Coupler performance After fabrication with the LPKF milling machine, the coupler was evaluated for the degree of coupling, directivity across the 1 to 8 GHz band. In Figure 11, the coupled port transmission is the smooth line. The horizontal line at the center of plot is –18 dB and the grid is 2 dB per division. Coupling is –19 dB ±1.5 dB over the measured frequency range. In the same figure, input return loss is plotted at 5 dB per division, referenced to 0 dB at the second line from the top. Worst case return loss is 16 dB at the lowest frequencies.
Reverse coupling is plotted in Figure 12, along with output port return loss. Both plots are 5 dB per division. For reverse coupling, the center line is the reference, again at –18 dB, and coupling is –28 dB or better across the band, better than 31 dB at all but the high frequency end. The output port return loss is plotted using the same scale as input return loss in Figure 11, and also shows the same 16 dB worst case performance at 1 GHz.

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