IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 5, MAY 2014 309

Microstrip Multiplexer and Switchable Diplexerwith Joint T-Shaped Resonators

Ming-Lin Chuang, Senior Member, IEEE, and Ming-Tien Wu, Member, IEEE

Abstract—This work presents a four-channel multiplexer thatuses joint T-shaped resonators. The multiplexer contains foursecond-order bandpass filters that are combined by two jointT-shaped resonators. The joint T-shaped resonators are alsoinherently the resonant elements and frequency selective signalsplitter. Therefore, the proposed multiplexer has compact sizebecause of fewer resonators and no matching/combining circuit.The T-shaped resonator also makes the presented structure suit-able for multiplexers having very close bands. The multiplexer isthen modified to obtain a switchable FDD diplexer which is animportant device in dual-band wireless communication system.The mechanism of band switching is discussed. Only two diodesare required to control the switching between the two FDD bands.Good agreement between the simulated and measured responsesvalidates the proposed structure.

Index Terms—Bandpass filter (BPF), multiplexer, switchablediplexer, T-shaped resonator.

I. INTRODUCTION

M ULTIPLEXER is an important device in wirelesscommunication system, especially for the increasing

demand of multi-band or multi-service mobile communicationsystem in recent years. Most multiplexers are constructedfrom several bandpass filters (BPFs) with different centralfrequencies and a combining circuit. The performance of themultiplexer depends on both the BPF and the combining circuit.A ring manifold [1] is used to combine several BPFs whilereducing intersection between the individual filters. In [2], theauthors add stepped-impedance stubs to the combining circuitto enhance the isolation at the cost of increasing the circuitsize. To reduce the overall size, a common resonator has beenused as a combining circuit [3]. Distributed coupling technique[4], which arranges BPFs along a single feeding line, has beenpresented for applications with many channels. Instead of de-signing the BPFs and combining circuit separately, an iterativeapproach that considers the responses of individual filters andthe star-junction concurrently was proposed [5].This work proposes a multiplexer without any combining

or matching circuit. Two T-shaped resonators are the commonresonance elements of the filters and inherently the combining

Manuscript received November 27, 2013; accepted January 30, 2014. Date ofpublication March 12, 2014; date of current version May 06, 2014. This workwas supported by the National Science Council of Taiwan, under Contract NSC-102-2221-E-346-001.The authors are with the Department of Communication Engineering, Na-

tional Penghu University of Science and Technology, Makung, Penghu, Taiwan(e-mail: morris@npu.edu.tw; morris-chuang@ieee.org).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LMWC.2014.2309084

Fig. 1. Coupling scheme of the proposed multiplexer with four second-orderBPFs composed of two joint T-shaped resonators.

circuits. Because T-shaped resonators is utilized, the proposedmultiplexer can deal with a smaller relative band separation thancan other structures [6]. Furthermore, by connecting only twodiodes in the central branches of the T-shaped resonators, theproposed multiplexer is easily modified to become a switchableFDD diplexer with different TX/RX bands.

II. QUAD-CHANNEL MULTIPLEXER

This work presents a four-channel multiplexer, or quadru-plexer, that comprises four second-order coupled BPFs. Fig. 1shows the coupling scheme of the proposed multiplexer. Theblack node denotes the resonator and the white node denotes I/Oport. The design concept is using a joint resonator to combinetwo BPFs such that avoids additional matching and combiningcircuit. This joint resonator, which is shared by two bandpasfilters, is also a resonance element of the BPF. Therefore, thecircuit size is reduced. Fig. 2 displays the corresponding layoutusing microstrip circuit. The filters of bands 1 and 2 are com-bined by a joint T-shaped resonator, , and the filters of bands3 and 4 are combined by another joint T-shaped resonator, .Each T-shaped resonator has two resonance modes with veryclose frequencies and can be used as a splitter that separatessignals of two different frequencies [6], which makes it suitablefor combining or splitting two signals of different frequencies.The parameters of the T-shaped resonators are obtained by

solving the resonance condition [6]

(1)

where represents the resonance mode number and repre-sents the ratio of the fundamental resonance frequency to thefirst harmonic frequency.Once the T-shaped resonators have been determined, the four

second-order BPFs are designed following the procedure of tra-ditional single-band coupled resonator filter design [7]. Becausethe T-shaped resonator is the common resonator of two filters,two physical parameters, coupling length and coupling gap, are

1531-1309 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

310 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 5, MAY 2014

Fig. 2. Layout of the proposed multiplexer.

Fig. 3. Simulated and measured results of the multiplexer. The photograph ofthe fabricated multiplexer is inserted.

used to meet the required external quality factors of the twobands concurrently.To validate the proposed structure, a multiplexer that is fab-

ricated on a 60 mil-thick Roger RO3003 substrate with a di-electric constant of 3.0 and a loss tangent of 0.0012 is designedand measured. The operating frequencies of the four channelsare 2.0, 2.2, 2.4, and 2.6 GHz. The four BPFs have Chebyshevresponses of a 0.5 dB ripple and bandwidths of 80 MHz. Therequired coupling coefficient and external quality factor pairsfor the four channels are (0.0402, 35.0736), (0.0365, 38.5810),(0.0335, 42.0883), and (0.0309, 45.5957), in that order. De-tailed dimensions are listed in Table I. The size of the circuitexcluding the feeding lines is . Fig. 3 comparesthe numerical results simulated by Zeland IE3D 14.0 with theexperimental results measured using Agilent E5062A networkanalyzer. Both simulations and measurements meet the channelrequirements. Fig. 4 shows the magnified passband response.The simulated insertion losses of the four channels are 1.13 dB,1.01 dB, 1.06 dB, and 1.26 dB and the measured insertion lossesare 1.12 dB, 1.42 dB, 1.30 dB, and 1.54 dB in that order. Table II

Fig. 4. Magnified passband response of the multiplexer.

TABLE IDIMENSIONS (mm) OF THE FABRICATED MULTIPLEXER

TABLE IIPROPOSED MULTIPLEXER AND OTHER MULTIPLEXERS

is the frequency ratio of the -th band to the lowest band.

and are the guided wavelengths of the lowest band and the highest

band.

Estimated results.

Fig. 5. T-shaped resonator with switching diode and bias circuit.

lists the performance of the proposed multiplexer and previousstudies. Although the proposed structure is not the smallest one,this work has the smallest frequency ratio which is the generalcase for multiplexers. Good insertion losses are also noted.

III. SWITCHABLE FDD DIPLEXER

The proposed four-channel multiplexer can be modified tobecome a switchable FDD diplexer. Switchable FDD diplexerscan be found in dual-band wireless communication equipments,such as the DCS/WCDMA cellular system.Most FDD diplexershave very close uplink band and downlink band. For example,FDD WCDMA system has uplink of 1.95 GHz and downlinkof 2.14 GHz. The frequency ratio is 1.097 which is suitable forusing T-shaped resonator [6]. In this dual-band application, theupper part in Fig. 2 is used as the lower-band FDD diplexer

CHUANG AND WU: MICROSTRIP MULTIPLEXER AND SWITCHABLE DIPLEXER WITH JOINT T-SHAPED RESONATORS 311

Fig. 6. Simulated and measured responses of the fabricated switchablediplexer. (a) off and on (b) on and off.

whose ports 1 and 2 are connected to a transmitter and a receiver,respectively. The lower part in Fig. 2 is the higher-band FDDdiplexer whose ports 3 and 4 are connected to a transmitter anda receiver respectively. These two FDD diplexers operate at dif-ferent TX/RX frequency pairs but use the same antenna or com-munication channel. Two p-i-n diodes are connected to the openend, P, of the central branch of the T-shaped resonators withproper bias circuit [8] as shown in Fig. 5. The high-impedanceresistor plays a role of RF choke, and the gap between res-onator and feeding line inherently blocks the DC bias voltage.When the diode is reverse-biased, point P remains open be-cause of high-impedance resistor and resonator operates inthe original resonance mode. The upper diplexer is then turnedon. When the diode is forward-biased, point P is groundedand the resonance frequencies of resonator change. Theresonance condition becomes

(2)

In this study, the first two resonance frequencies of theT-shaped resonator change to 0.67 and 2.09 GHz. Thus,the passbands at 2.0 GHz and 2.2 GHz are destroyed and theupper diplexer is turned off. Diode has similar mechanism.If the p-i-n diode is forward-biased, the central branchof the T-shaped resonator is grounded and the first tworesonance frequencies change to 0.69 and 2.46 GHz. Hence,the passbands at 2.4 and 2.6 GHz are destroyed and the lowerdiplexer is turned off. Therefore, a switchable FDD diplexer isobtained using only two diode switching circuits.Fig. 6 shows the simulated and measured results of the fabri-

cated switchable FDD diplexer. In the passbands, the simulatedand measured off-to-on attenuation of the two channels for theupper diplexer are around (34 dB, 33 dB) and (27 dB, 23 dB),respectively. For the lower diplexer, the simulated and mea-sured off-to-on attenuation are approximately (22 dB, 21 dB)and (26 dB, 21 dB), respectively.

IV. CONCLUSION

This work proposes a microstrip four-channel multiplexer.The multiplexer consists of four BPFs which can be designedindividually. The multiplexer has compact size because thejoint T-shaped resonators reduces the number of resonators andeliminates the need for a matching or combining circuit. TheT-shaped resonators also make the multiplexer having closebands to meet general applications. Adding only two p-i-ndiodes turns the multiplexer into a switchable FDD diplexerwhich can be used in switched dual-band system such asDCS/WCDMA handset. The mechanism of band switching isanalyzed and discussed. Simulations and measurements agreeclosely with each other and validate the proposed structure.The proposed configuration can be applied to multiplexers withmany channels by placing more T-shaped resonators alongboth sides of the single feeding line.

REFERENCES[1] M. Zewani and I. C. Hunter, “Design of ring-manifold microwave mul-

tiplexers,” in IEEE MTT-S. Int. Dig., Jun. 2006, pp. 689–692.[2] P. H. Deng, M. I. Lai, S. K. Jeng, and C. H. Chen, “Design of matching

circuits for microstrip triplexers based on stepped-impedance res-onators,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 12, pp.4185–4192, Dec. 2006.

[3] C. F. Chen, T. M. Shen, T. Y. Huang, and R. B. Wu, “Design ofmultimode net-type resonators and their applications to filters andmultiplexers,” IEEE Trans. Microw. Theory Tech., vol. 59, no. 4, pp.848–856, Apr. 2011.

[4] S. J Zeng, J. Y. Wu, and W. H. Tu, “Compact and high-Isolationquadruplexer using distributed coupling technique,” IEEE Microw.Wireless Compon. Lett., vol. 21, no. 4, pp. 197–199, Apr. 2011.

[5] G. Macchiarella and S. Tamiazzo, “Synthesis of star-junction mul-tiplexers,” IEEE Trans. Microw. Theory Tech., vol. 58, no. 12, pp.3732–3741, Dec. 2010.

[6] M. L. Chuang and M. T. Wu, “Microstrip diplexer design usingcommon T-shaped resonator,” IEEE Microw. Wireless Compon. Lett.,vol. 21, no. 11, pp. 583–585, Nov. 2011.

[7] J. S. Hong and M. J. Lancaster, Microstrip Filter for RF/MicrowaveApplications. New York: Wiley, 2011.

[8] P. H. Deng and J. H. Jheng, “A switched reconfigurable high-isolationdual-band bandpass filter,” IEEE Microw. Wireless Compon. Lett., vol.21, no. 2, pp. 71–73, Feb. 2011.