Miniaturized planar filters exhibiting flexible placement of three transmission zeros suitable for duplexer design

In this work, we show that a stub‐loaded open‐loop double resonator filter can produce three transmission zeros at finite frequencies close to the passband when designed using our proposed asymmetric feed point topology. We demonstrate that by selecting the length of the stub, the placement of double transmission zeros on either the high side of the passband or the low side of the passband can be selected. This selection can be achieved by using a single design parameter: the stub length. The flexibility in the double zero placement makes these filters particularly advantageous for duplexer designs. Two duplexers are fabricated using the proposed filter topology in a microstrip on a duroid substrate and substrate‐embedded stripline via Low‐Temperature Cofired Ceramic technology (LTCC). The measurements show steep attenuation of approximately 30 dB, close to the filters' passband as well as a compact size down to 0.075λ × 0.17λ.