IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL 19, NO 8, AUGUST 2009 497 A Novel Dual-Mode Dual-Band Bandpass Filter Based on a Single Ring Resonator Sha Luo, Student Member, IEEE, and Lei Zhu, Senior Member, IEEE Abstract—A dual-mode dual-band bandpass filter with two transmission poles in both passbands using a single ring resonator is proposed Two excited ports are placed at the 135 -separated positions along the ring resonator and coupled with the ring via parallel-coupled lines, leading to synchronous excitation of two transmission poles in dual passbands After the principle of this initial filter is described, an improved ring resonator with periodic loading of open-circuited stubs is constructed and studied to achieve compact size and adjustable spacing between the two passbands Finally, a dual-band ring resonator filter with center frequencies at 2.4 and 5.8 GHz is designed and fabricated Measured results verify the design principle Index Terms—Bandpass filter (BPF), dual-band, dual-mode, open-circuited stubs, ring resonator I INTRODUCTION M ICROSTRIP ring resonators have been widely used in designing microwave components, such as antennas, bandpass filters (BPFs), baluns, couplers, mixers and oscillators [1] In 1972, Wolff firstly reported that there were two degenerate modes coexisting at the two resonant frequencies [2] These two modes can be split by disturbing the symmetry of a ring resonator so that the two transmission poles in the primary passband can be excited To meet the requirement in the recent development of advanced multi-band wireless systems, there is high demand to explore various dual-band BPFs In particular, the dual-band filters based on the dual-mode ring resonator [3]–[7] have been attracting much attention in the recent years due to their compact size and good roll-off skirt In this aspect, a dual-band filter is constructed in [3] using the first and second resonant modes of a stepped-impedance ring resonator, but it fails to generate two transmission poles in the second passband In [4]–[7], two dissimilar ring resonators with different shapes or diameters are properly formed in a single- or two-layer substrate In this case, the dual passbands with two poles in each individual band are realized by virtue to two different sets of two degenerate modes in two individual ring resonators To our best knowledge, there has been no reported work that implements a dual-band filter with two transmission poles in both passbands using a single ring resonator In this paper, a novel dual-mode dual-band BPF with two transmission poles in two passbands is designed based on a Manuscript received March 01, 2009; revised April 13, 2009 First published July 28, 2009; current version published August 07, 2009 The authors are with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (email: luos0002@ntu edu.sg; ezhul@ntu.edu.sg) Digital Object Identifier 10.1109/LMWC.2009.2024826 Fig Proposed dual-mode dual-band BPF using a single uniform ring resonator (a) Schematic (b) S-parameters versus electrical length ( ) with Z = 30 , Z = 73 , Z = 108 and Z = 30 single microstrip ring resonator on a single-layer substrate As shown in Fig 1(a), the two excited ports are placed along the ring with a separation of 135 and they are capacitively coupled to this ring via parallel-coupled lines The remaining parts of this work describe the principle of the proposed ring resonator dual-band filter and demonstrate its dual-band performance via an equivalent circuit model Finally, a compact dual-BPF with periodically loading of opened stubs is designed for 2.4/5.8 GHz wireless local area network applications, and the predicted results are confirmed experimentally II PRINCIPLE AND ANALYSIS OF THE PROPOSED RING FILTER Fig 1(a) depicts the schematic of the proposed dual-mode dual-band microstrip ring resonator, where is the input and and are the inner and outer radii output port impedance, is the characteristic impedance of the ring of this ring, and In our design, the parallel-coupled lines are one quarter of the and a spacing of length of the ring, with a width of or As illustrated in Fig 2(a), a three-port parallel-coupled line , a voltage transcan be treated as a capacitive impedance former with turns ratio and two parallel-connected lines at and denote the evenport and as discussed in [8] 1531-1309/$25.00 © 2009 IEEE 498 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL 19, NO 8, AUGUST 2009 Fig (a) Equivalent-circuit diagram of three-port parallel-coupled lines (b) Complete equivalent-circuit model for the filter in Fig 1(a) (c) Normalized frequencies of the two poles (f =f and f =f ) in the first passband versus spacing (s) with Z = 30 , Z = 73 Substrate: " = 10:8 and thickness = 1:27 mm and odd-mode characteristic impedances of this parallel-coupled line, while is their effective electrical length Follow the work in [8], the relationship between all the element parameters of the two networks in Fig 2(a) can be derived as (1a) (1b) As such, the equivalent-circuit model of the filter in Fig 1(a) can be derived as shown in Fig 2(b) Fig 1(b) plots its simulated -parameters versus electrical length As can be observed, the first and second passbands with two transmission poles at and , respectively each band appear at and ) are symmetrically located at The two lower ones ( the low sides of the two higher ones ( and ) with respect In addition, there exist three transmission zeros, to , and , between the two passbands We can analyze this proposed ring resonator filter based on Fig 2(b) According to the transmission theory, transmission zeros of this ring filter occur at the frequencies where the overall mutual of the network inside the dash square in Fig admittance 2(b) equals to 0, such that (2) and where By solving (2), all the zeros can be determined as (3a) (3b) Equation (3a) determines the first and third transmission and while the second zero, , is derived under zeros, in (3b) Under the even- and odd-mode excitations at two ports, the symmetrical plane in Fig 2(b) becomes perfect magnetic wall (M.W.) and electric wall (E.W.) Thus, its bisection becomes a one-port network with open- and short-circuited ends at the and reprecentral position, respectively In Fig 2(b), sent the two input admittances at the port, looking into the left and right sides Under the even- and odd-mode resonances, i.e., and , , and , , can be determined Fig 2(c) plots the first and the second normalized freand , with quencies of these transmission poles, respect to In our design, the filter is formed on the RT/D6010 and As can be substrate with found in Fig 2(c), when increases from 0.1 mm to 0.5 mm, gradually moves towards This means that the first and second poles in the first passband or third and fourth poles in the second passband become close to each other as the coupling degree of the parallel-coupled lines is reduced Next, a modified ring resonator with periodic loading of eight identical opened stubs, that have a width of and a length of , is constructed as displayed in Fig 3(a) to make up a size-reduced and dual-passband controllable dual-band fiter Fig 3(b) plots , the normalized frequencies of the transmission zeros, , and , and poles, , , , and , versus normalized stub length Herein, is the second zero without stubs and is the electrical length of the stubs As increases from to 1.0, the first and second poles are simultaneously reduced At the same time, the first zero moves closely to the right side of the first passband and the second zero works a certain distance beis excited by the yond the first zero An additional pole opened stubs With the increment of , the third and fourth poles move close to each other and merge to one pole around The fifth pole quickly moves towards the third and fourth poles, and it forms the second passband together with the merged pole The third zero always stays close to the left side is stimulated of the second passband An additional zero when From to 1.0, moves towards to the second passband and locates at its right side Furthermore, as the stubs are stretched, the ratio between center frequencies of the first and second passbands is gradually reduced from 3.0 to 2.3 III RESULTS AND DISCUSSION Based on the above analysis, a modified dual-mode dual-band BPF is designed and implemented The center frequencies of the two passbands are designated at 2.4 and 5.8 GHz To get a 12% fractional bandwidth for the first passband, is chosen as 73 , is 108 and is 30 Meanwhile, is selected to achieve the center frequencies ratio of 2.42 that is required in the design of a 2.4/5.8-GHz dual-band filter To achieve good impedance matching in the second passband of the fixed ring needs to be reduced to 30 Fig 4(a) shows its resonator, layout with all the dimensions denoted In our final design, the two stubs placed at the two feeding points are slightly shortened to compensate for the unexpected effects caused by T-junctions LUO AND ZHU: NOVEL DUAL-MODE DUAL-BAND BPF Fig (a) Schematic of the dual-mode dual-band BPF using a single ring resonator with eight periodically-loaded open stubs (b) Normalized frequencies of transmission poles (f =f , f =f , f =f , f =f and f =f ) and zeros (f =f , f =f , f =f and f =f ) versus normalized stub length (t = = ), with Z = 30 , Z = 73 , Z = 108 and Z = 30 Furthermore, to be connected with two coaxial cables in the experiment, two transmission line transformers with a width of 2.0 mm and a length of 13.2 mm are installed at its two feeding lines to transform 30 into 50 Fig 4(b) plots the simulated results from the ADS fullwave simulator [9] and the measured results of a fabricated filter circuit Both of them are in reasonable agreement with each other Visibly, the two expected transmission poles exist in both of the first and the second passbands at the required center frequencies of 2.4 and 5.8 GHz From Fig 4(b), the measured insertion losses in the two passbands are lower than 1.4 and 3.2 dB, respectively, whereas the measured return losses in the dual passbands are both higher than 20 dB With the help of the second transmission zero, the isolation between these two passbands is better than 10 dB from 2.55 to 5.52 GHz IV CONCLUSION In this paper, microstrip dual-mode ring resonator BPFs with uniform and periodically stub-loaded configurations have been presented and implemented The principle of the proposed dualband filters is explained and discussed via the equivalent circuit models Afterwards, a modified dual-band BPF based on a single microstrip ring resonator with loading of eight opened 499 Fig Modified dual-mode dual-band BPF for fabrication and measurement (a) Layout (b) Simulated and measured frequency responses stubs is designed and fabricated Our experiment has verified that a dual-band filter with two poles in both passbands can be constructed using a single ring resonator REFERENCES [1] K Chang and L H Hsieh, Microstrip Ring Circuits and Related Structures New York: Wiley, 2004 [2] I Wolff, “Microstrip bandpass filter using degenerate modes of a microstrip ring 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