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Design of parallel-coupled microstrip filters with suppression of spurious resonances using substrate suspension

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3 Author(s)
Jen-Tsai Kuo ; Dept. of Commun. Eng., Nat. Chiao Tung Univ., Hsinchu, Taiwan ; Meshon Jiang ; Hsien-Jen Chang

Substrate suspension is used to suppress the spurious response of microstrip bandpass filters at twice the passband frequency (2fo). It is known that a proper height of substrate suspension can be used to equalize the even- and odd-mode phase velocities for coupled microstrip lines. In this paper, this property is applied to design the coupled stages of a parallel-coupled line filter so that the spurious response at 2fo can be completely suppressed. The individual image impedance for each coupled stage is changed accordingly. Required filter design formulas are derived for a series of coupled stages having different image impedances. Several filters made on a substrate of relative high dielectric constant are designed and fabricated. The measured results show that a rejection level of better than -40 dB to the spurious resonance at 2fo can be obtained.

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Microwave Theory and Techniques, IEEE Transactions on  (Volume:52 ,  Issue: 1 )