Switchable bandpass filter with two-state frequency responses The input impedance for even-mode and odd-mode can be expressed as Zine = jZ1 B Lui, F Wei and X Shi A novel switchable bandpass filter with two-state frequency responses is presented, where dual-band bandpass and single bandpass characteristics can be conveniently switched by turning pin diodes on and off The switchability results from the effect of different working modes The filter can work as a dual-band bandpass filter obtaining two centre frequencies, 1.8 and 3.5 GHz, which could be appropriately used for GSM and WiMax systems In the meantime, it also can operate only at 3.5 GHz, rejecting signal at 1.8 GHz The proposed filter consists of two dual-mode resonators, using pin diodes to switch the frequency responses An experiment was carried out to validate the design concept, and the measured results agree well with simulated results Introduction: Electronically switchable and reconfigurable microwave devices such as filters are in great demand for existing wireless communication systems, and this trend will continue for future systems To meet the increasing requirement of modern multi-functional systems, bandpass filters (BPFs) should have multi-frequency response Switchable BPFs can be used to control the spectrum of proposed signals and support multiple information channels [1, 2] In some cases, there is a requirement for switchable BPFs to have dual-band bandpass and single bandpass characteristics Much research work has been performed and various methods have been designed to realise dual-band behaviour Among them, there are two typical methods The first category is to combine sets of independent resonators with common input/output ports [3, 4] The second is to utilise stepped impedance resonators [5, 6] However, dual-band BPFs designed by these methods cannot be conveniently switched to operate in one passband In this Letter, a novel structure to implement dual-band behaviour is proposed, and the filter employing this structure can be easily switched to operate in one passband With the pin diodes on, a shortcircuited stub is loaded in the centre of the open-loop resonator to obtain a dual-mode dual-band BPF By turning the pin diodes off, the stub is disconnected with the open-loop resonator, and the filter has only one passband The passband frequencies can be easily controlled by tuning the length of the stub and the open-loop resonator Based on the above principle, the switchable BPF operating at 1.8/3.5 and 3.5 GHz is designed and fabricated The EM simulated and measured results are present to demonstrate the performance of the proposed filter L1 DC s Zino = −jZ1 cot(bL1 ) (1) (2) where b is the propagation constant, and it is equal for even-mode and odd-mode The resonance condition is Zine 1, Zino (3) By solving (3), the fundamental resonant frequency can be determined as follows: feven = c √ , 4(L1 + L2 ) 1e fodd = c √ 4L1 1e (4) Apparently, the odd-mode resonant frequency fodd is larger than the even-mode resonant frequency feven fodd is only determined by the length of the open-loop resonator and the length of the short-circuited stub only influences feven In our design, the odd-mode resonant frequency fodd is 3.5 GHz, and the even-mode resonant frequency feven is 1.8 GHz When the pin diodes are in the ‘ON’ state, the short-circuited stubs are connected The existence of both even-mode and odd-mode in the resonators results in a dual-band bandpass response Under this state, the filter obtains two passbands ( f1 ¼ 1.8 GHz, f2 ¼ 3.5 GHz) When the pin diodes are in the ‘OFF’ state, the short-circuited stubs are disconnected with the open-loop resonators There is one mode in the resonators, and thus the filter has only one passband ( f1 ¼3.5 GHz) The bandwidths of the two passbands are mainly affected by the coupling structure, and the desired bandwidths can be acquired by choosing proper dimensions of the coupling structure Based on the above analysis, the filter is designed to operate in two different states by turning the pin diodes on and off Infineon pin diodes BAR63-02V in a SC79 package are used to connect the stubs, which have a capacitance of 0.22 pF, a parasitic inductance of 0.6 nH, and a resistance of 1.2 V at 100 MHz The filter is fabricated on a substrate with dielectric permittivity 1r¼ 2.65 and thickness h ¼ 0.8 mm The fabricated filter with DC bias is shown in Fig The final optimised parameters of the filter are listed as follows: L1 ¼ 1.6 mm, L2 ¼ 5.2 mm, L3 ¼ mm, L4 ¼ 2.5 mm, L5 ¼ 11.8 mm, w0 ¼ 2.2 mm, w1 ¼ mm, w2 ¼ 0.9 mm, w3 ¼ 0.5 mm, g1 ¼ 0.7 mm, g2 ¼ 1.1 mm, s ¼ 0.18 mm, L¼ 27 nH, R ¼ kV The overall size of the proposed filter is 30 × 30 mm2 L2 g1 L3 L5 Z1 tan(bL1 ) + 2Z2 tan(bL2 ) Z1 − 2Z2 tan(bL1 ) tan(bL2 ) L4 D2 g2 D1 w2 w0 w3 w1 L R DC Fig Schematic of proposed filter Z2 , L2 2Z , L Zine Z1 , L1 Z1 , L1 a Zino Z1 , L1 b Z1 , L1 Fig Photograph of fabricated filter c Fig Structure and equivalent circuit of dual-mode resonator a Dual-mode with short-circuited stub b Even-mode equivalent circuit c Odd-mode equivalent Filter design: The schematic of the proposed BPF is shown in Fig The filter is composed of a pair of coupled microstrip open-loop resonators and two short-circuited stubs The external coupling is established by edge coupling As depicted in Fig 2a, the resonator with the shortcircuited stub loaded is symmetrical, and then odd- and even-mode analysis can be applied to characterise it For even-mode and oddmode excitation, the equivalent circuit is shown in Figs 2b and c ELECTRONICS LETTERS 6th January 2011 Vol 47 Simulated and measured results: Simulation and measurement were carried out with EM simulation software HFSS 11.0 and an Agilent vector network analyser N5230A Fig shows the simulated and measured scattering parameters when the pin diodes are in the ‘ON’ state It can be seen that the proposed filter can operate at 1.8 and 3.5 GHz, and the measured fractional bandwidths are 9% in the lower passband and 7% in the upper passband Fig shows the simulated and measured results when the pin diodes are in the ‘OFF’ state According to Fig 5, the filter obtains only one passband The measured results agree with the simulated results, which validates the design Insertion loss is nearly dB in the lower passband, a little higher than that of the simulated results, which mainly results from the resistances of the pin diodes No tuned in two states, operating as a dual-band bandpass filter, or just having one passband Experimental results agree well with simulated results, and show good performance Because of its simple structure, compact size, and good performance, the proposed filter is attractive for use in GSM and WiMax systems |S11| and |S21|, dB S11 –20 S21 # The Institution of Engineering and Technology 2011 November 2010 doi: 10.1049/el.2010.3156 One or more of the Figures in this Letter are available in colour online –40 simulated measured B Lui, F Wei and X Shi (National Key Laboratory of Science and Technology on Antennas and Microwaves, Xidian University, Box 223, No South Taibai Road, Xi’an, 710071 Shaanxi, People’s Republic of China) –60 frequency, GHz Fig Simulated and measured results with pin diodes on E-mail: baby2884419@163.com References |S11| and |S21|, dB S21 –20 S11 –40 simulated measured –60 frequency, GHz Fig Simulated and measured results with pin diodes off Conclusion: A switchable bandpass filter with two-state frequency responses has been developed The proposed filter can be conveniently Wong, P.W., and Hunter, I.C.: ‘Electronically reconfigurable microwave bandpass filter’, IEEE Trans Microw Theory Tech., 2009, 57, (12), pp 3070–3079 Tu, W.-H.: ‘Swithable microstrip bandpass filters with reconfigurable onstate frequency responses’, IEEE Microw Wirel Compon Lett., 2010, 20, (4), pp 208–210 Zhang, X.Y., and Xuei, Q.: ‘Novel dual-mode dual-band filters using coplanar-waveguide-fed ring resonators’, IEEE Trans Microw Theory Tech., 2007, 55, (10), pp 2183–2190 Dai, G.-L., Guo, Y.-X., and Xia, M.-Y.: ‘Dual-band bandpass filter using parallel short-ended feed scheme’, IEEE Microw Wirel Compon Lett., 2010, 20, (6), pp 325– 327 Shi, J., and Xuei, Q.: ‘Novel balanced dual-band bandpass filter using coupled stepped-impedance resonators’, IEEE Microw Wirel Compon Lett., 2010, 20, (1), pp 19– 21 Tsengand, C.-H., and Shao, H.-Y.: ‘A new dual-band microstrip bandpass filter using net-type resonators’, IEEE Microw Wirel Compon Lett., 2010, 20, (4), pp 196–198 ELECTRONICS LETTERS 6th January 2011 Vol 47 No