Abstract
A dual-mode dual-band bandpass filter with two transmission poles in both of dual-passbands is presented with the proper stub-loading and external-excitation of a single microstrip ring resonator. By placing two excited ports at the 135°-separated positions along a uniform ring resonator, two 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> -order degenerate modes, TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> , can be simultaneously excited and they are split to produce two transmission poles in the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> passband. Meanwhile, two 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> -order degenerate modes, TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> , are split by attaching four identical open-circuited stubs to the ring in a symmetrical manner with respect to two external ports. Thus, two transmission poles can be generated in both of dual passbands as demonstrated in theory. Finally, a compact dual-band ring resonator filter is designed and fabricated to operate at 2.3 and 4.8 GHz and its predicted results are verified in experiment.
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