J.-Z Chen, N Wang, Y He and C.-H Liang A tri-band bandpass filter (BPF) using a novel square ring loaded resonator (SRLR) is presented The SRLR can generate a tri-band response by tuning its geometric parameters Moreover, it can build the highorder tri-band BPFs using the proposed resonators because of the sufficient coupling between adjacent resonators A fourth-order Chebyshev tri-band BPF centred at 2.4, 3.5 and 5.2 GHz is designed and fabricated Measurement results agree well with the full-wave EM simulated results Introduction: In recent years, the tri-band BPF has become one of the most important RF devices to meet the increasing communication requirements Many different methods have been introduced and reported to design tri-band BPFs A widely used method is to utilise a single resonator with controllable resonant frequencies, such as the stepped impedance resonator (SIR) [1-3] and the stub loaded resonator (SLR) [4-6], applying their multiband behaviours The three desired frequencies can be conveniently controlled by the tri-section SIR [2, 3] For the SLR, it is also attractive in dual-band [4, 5] and tri-band [6] BPF design owing to its flexibility Although these designs are effective, all of them are just second-order BPFs and they are hard to extend to high-order BPFs In this Letter, a novel square ring loaded resonator (SRLR) is proposed for high-order tri-band BPF applications The novel SRLR is flexible for high-order tri-band BPF designs To verify its performance, a fourth-order Chebyshev tri-band BPF that can be operated at 3.5 GHz WiMAX band and 2.4/5.2 GHz WLAN bands is designed, fabricated, and tested square ring fodd1 feven1 –10 feven1 fodd1 fodd2 W2=0.4 W2=1.4 W2=3.4 W2=5.4 W2=7.4 L2=4mm L2=5mm L2=6mm L2=7mm L2=8mm –30 –40 frequency, GHz a b Fig Simulated insertion loss of odd symmetrical SRLR for varying L2 and W1 a L2 b W1 db –2 db –4 db –6 db –8 db –10 db –12 db –14 db –16 db –18 db –20 db –22 db –24 db –26 db –28 db –30 db –32 db –34 db –36 db –38 db –40 db L3 L3 L2 Z0 L2 L1 a L1 b L3 L1 Z L2 L2 Z0 c 2.4GHz W1 3.5GHz 5.2GHz d Fig Simulated electric current density at 2.4/3.5/5.2GHz Fig Layout of square ring loaded resonators a Original SRLR b Odd-mode equivalent circuit c Even-mode equivalent circuit d Odd symmetrical SRLR L8 L3 W3 c 3c √ , fodd2 = √ 4(L1 + L2 ) 1e 4(L1 + L3 ) 1e (1) where c is the speed of the light in free space, and 1e denotes the effective dielectric constant of the substrate For even-mode excitation, the required resonant frequencies can be written as: feven1 c = √ 2(L1 + L3 ) 1e (2) These three frequencies can be used to design a tri-band BPF However, it is found the original SRLRs are not suitable for building high-order tri-band BPFs, because the couplings between the resonators are not so efficient Therefore, the odd symmetrical SRLR is put forward in Fig 1d Along the dashed line shown in Fig 1d, a microstrip line is added in the SRLR for fine-tuning resonant frequencies It is found that the proposed new odd symmetrical SRLR can offer the required couplings while it can keep the similar frequency characteristics with the original SRLR Fig shows the EM simulated frequency responses ELECTRONICS LETTERS 21st July 2011 Vol 47 G3 L4 L 10 L9 G2 W6 Properties of square ring loaded resonator: As shown in Fig 1a, the original SRLR consists of two open folded microstrip lines and a square ring Since the original SRLR is even symmetrical to the dashed line, the resonator frequency can be extracted by the even and odd-mode method The odd-mode and even-mode equivalent circuits are shown in Figs 1b and c, respectively The odd-mode equivalent circuit contains two resonant circuits The two resonant frequencies can be obtained as follows: fodd1 = frequency, GHz Zin,even Zin,odd fodd2 –20 folded line L3 L1 Z of the odd symmetrical SRLR From Fig 2a, it can be seen that fodd1 can be kept constant while feven1 , fodd2 change when tuning L2 Correspondingly, as shown in Fig 2b, fodd1 , fodd2 change in the same direction while feven1 changes in the opposite direction when tuning W2 According to the above analysis, when we design a tri-band BPF, basic structure parameters (such as L1, L2, L3) of the SRLR can be first decided by sovling (1) to (2), and then the resonant frequencies can be determined by adjusting L2 and W2 slightly Therefore, the required three frequencies can be simultaneously obtained Fig shows the simulated electric current density at f1(2.4GHz), f2(3.5GHz), and f3(5.2GHz) It can be seen that the electrical current density level is higher at the vertical parts of the SRLR, which is mainly atributed to a stronger magnetic coupling between the odd symmetrical SRLRs |S|21, dB Fourth-order tri-band bandpass filter using square ring loaded resonators L1 W4 L2 W5 L6 L5 W2 L5 G1 L7 W1 Fig Layout of tri-band BPF Circuit dimensions (in mm): W1 ¼ 2.7, W2 ¼ 0.3, W3 ¼ 1.0, W4 ¼ 2.8, W5 ¼ 3.3, W6 ¼ 6.2, L1 ¼ 30.9, L2 ¼ 7.8, L3 ¼ 6.2, L4 ¼ 7.3, L5 ¼ 6.6, L6 ¼ 26.1, L7 ¼ 9.7, L8 ¼ 6.9, L9 ¼ 6.8, L10 ¼ 5.9, G1 ¼ 0.2, G2 ¼ 0.4, G3 ¼ 0.8 Tri-band bandpass filter design and result: A fouth-order Chebyshev tri-band BPF applying the proposed novel SRLR is designed on a substrate (1r ¼ 2.65 and h ¼ 1.0 mm) Given the three operating frequencies are centred at 2.4, 3.5 and 5.2 GHz, the 3dB fractional bandwidths are 8.4, 8.0 and 4.8%, respectively Fig describes the layout of the proposed filter After an efficient optimisation process using IE3D software, the dimensions for this tri-band BPF are found Its simulated and measured results are shown in Fig Good agreements are obtained The simulated results are centred at 2.4/3.5/5.2 GHz, their 3dB fractional bandwidths are 8.5, 8.0 and 5% In the measurements which are performed on an Agilent 8917ES network analyser, the three passbands are centred at 2.4/3.5/5.2 GHz, with the 3dB fractional bandwidths of 8.6, 7.8 and 4.9%, respectively The measured insertion losses at the No 15 magnitude of S-parameters, dB passband centre frequencies are 1.57, 1.60 and 1.77 dB, respectively The return losses of the three bands are below 215 dB J.-Z Chen, N Wang, Y He and C.-H Liang (Science and Technology on Antenna and Microwave Laboratory, Xidian University, Xi’an, People’s Republic of China) E-mail: xjtucjz@gmail.com S11 –10 References –20 S21 –30 –40 simulation measurement –50 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 frequency, GHz Fig Simulated and measured results of tri-band BPF Conclusion: A novel square ring loaded resonator is proposed and has been used to design a tri-band BPF A fourth-order BPF with tri-band performance centring at 2.4/3.5/5.2 GHz is designed, fabricated and measured to verify performance From the measured results, the designed filter has exhibited good tri-band bahaviour This filter can be applied to various tri-band designs such as wireless local area networks Weng, M.-H., Wu, H.-W., and Su, Y.-K.: ‘Compact and low loss dualband bandpass filter using pseudo-interdigital stepped impedance resonators for WLANs,’ IEEE Microw Wirel Compon Lett 2007, 17, (3), pp 187– 189 Hsu, C.-I.-G., Lee, C.-H., and Hsieh, Y.-H.: ‘Tri-band bandpass filter with sharp passband skirts designed using tri-section SIRs,’ IEEE Microw Wirel Compon Lett., 2008, 18, (1), pp 19– 21 Lin, X.-M., and Chu, Q.-X.: ‘Design of triple-band bandpass filter using tri-section stepped-impedance resonators’ Proc Int Microwave and Millimeter Wave Technology Conf., Guilin, China, April 2007, pp 1– Zhang, X.-Y., Cheng, J.-X., Xue, Q., and Li, S.-M.: ‘Dual-band bandpass filters using stub-loaded resonators,’ IEEE Microw Wirel Compon Lett 2007, 17, (8), pp 583– 585 Zhou, M.-Q., Tang, X.-H., and Xiao, F.: ‘Compact dual band bandpass filter using novel E-type resonators with controllable bandwidths’, IEEE Microw Wirel Compon Lett., 2008, 18, (12), pp 779–781 Chen, F.-C., Chu, Q.-X., and Tu, Z.-H.: ‘Tri-band bandpass filter using stub loaded resonators’, Electron Lett., 2008, 44, (12), pp 747–749 # The Institution of Engineering and Technology 2011 17 March 2011 doi: 10.1049/el.2010.3724 One or more of the Figures in this Letter are available in colour online ELECTRONICS LETTERS 21st July 2011 Vol 47 No 15