A Tri-Band Bandpass Filter Realized Using Tri-Mode T-Shape Branches

Abstract

Abstract-In this paper, we present a tri-band filter design using tri-mode T-shaped branches connected by λ/4 transmission lines. By analyzing the input admittance of a T-shape branch with commensurate electrical lengths, three resonant modes with two transmission zeros between are found and design formulas are derived. The filter can be regarded as a combination of three bandpass filters with only one set of coupling elements. To realize different bandwidths for each, the admittance slope of each resonating mode is set as required. A genetic algorithm is used in solving related equations to obtain the impedance of each line in a T-shape branch, followed by a final optimization. A three-pole tri-band filter having passbands of 0.6-0.9, 1.35-1.65 and 2.1-2.4 GHz, is designed, fabricated and measured with low passband insertion losses of < 0.7 dB and high rejection of > 60 dB between the passband regions. As a generalization, necessary to achieve a tri-band filter with arbitrary passbands, a non-commensurate version of the T-shape branch is introduced. An example filter design Liu, Dou, and Zhao is given with the passbands asymmetrically located at 0.7-1, 1.65-1.95, and 2.2-2.3 GHz. This technique is able to achieve good design flexibility with respect to bandwidth ratios. This is validated by studying the maximum impedance variations of a T-shape branch when the bandwidth ratios vary.

Compact Dual-Mode Tri-Band Microstrip BPF With Three Sets of Resonators

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A Novel Triple Passband Filter Design Method Based on Stepped Impedance Resonators

“…Resonators with controllable second and third harmonics and stubloaded resonators have been used to generate three passbands. And some other design procedures about multi-band filter have been discussed in paper [7][8][9][10][11][12][13].…”

“…Multi-band filters become quite popular. Therefore, they had been widely studied in several papers [2][3][4][5][6][7][8][9][10][11][12][13]. In [2], a tri-band filter was designed using three sets of resonators with a common feed circuit, but it has a large size due to a large number of resonators.…”

A novel tri-band filter realization via band-splitting technique

“…A kind of stub-loaded tri-mode resonator is proposed to construct tri-band filter with transmission zeroes between the passbands [5]. T-shape tri-mode resonators connected by transmission lines are used to realize tri-band filters with good features such as wide bandwidths and configurable bandwidth ratios [6].…”

“…To reduce circuit size, the number of resonator sets are decreased to two, one set of dual-mode resonators operate at two passbands and one set of single mode resonators at the third [2,3]. Tri-mode resonators are adopted to tri-band filter design for higher circuit efficiency [4,5,6]. In [4] tri-section stepped impedance resonators (SIR) technique is utilized to design tri-band filter with compact dimension.…”