Design of tri-band bandpass filter using uniform impedance resonators loaded with different impedance stubs

Abstract

ABSTRACT This paper presents a design of a compact tri-band bandpass filter simply using a pair of the uniform impedance resonator loaded with an open-circuited stub. By introducing the variable impedance of the open-circuited stub, the proposed design has additional design parameter of the impedance ratio to adjust the resonant modes of the used resonator, so as to more easily achieve the different frequency bands. The resonant frequencies of the oddmode and evenmode of the used resonator are analyzed first in this study. A tri-band filter example is designed at the center frequencies of 1.9, 3.58, and 5.75 GHz, which can be used in 4 G system, 5 G communication system, and wireless local area network, respectively. The designed filter is simulated, fabricated, and then measured. The measurement shows that the insertion loss |S21|, return loss |S11|, and 3 dB fractional bandwidth for the three passbands are less than 0.25/0.37/0.9 dB, larger than 30/15/15 dB, and 17.5/12.2/8.9%, respectively. Moreover, by using the interdigital coupling of the used resonator, four transmission zeros are generated near the passband edges at 1.47, 2.56, 4.05, and 6.48 GHz, realizing high selectivity and high isolation. The measurement is consistent with the simulation, confirming the design concept.