Compact Microstrip Diplexer Using Triple-mode Stub ... - IEEE Xplore

2017 IEEE MTT-S International Conference on Microwaves for Intelligent Mobility (ICMIM)

Compact Microstrip Diplexer Using Triple-mode Stub Loaded Resonators Sarun Choocadee1, Nattapong Intarawiset2, and Sugchai Tantiviwat3 1

Faculty of Industrial Technology, Songkhla Rajabhat University, Songkhla 90000, Thailand. 2 Department of Teacher Training in Electrical Engineering, King Mongkut's University of Technology North Bangkok, Bangkok 10800, Thailand. 3 Faculty of Industrial Education and Technology, Rajamangala University of Technology Srivijaya, Songkhla 90000, Thailand. E-mail: [email protected], [email protected], [email protected]

In this paper, the triple-mode resonators are introduced, featuring diplexers with two three-pole bandpass filter. Therefore the number of resonators in a filter can be reduced by one third, thus resulting in a compact configuration. The characteristics of the proposed resonators are designed and confirmed by experimental and simulated results, which shows good agreement. The operating frequencies are independent because the two fundamental even-mode can be conveniently tuned while the fundamental odd-mode is fixed, which does not provides influence to the resonant frequency. The theory and guidelines for selecting the geometric parameters of triple-mode resonators are presented in section II. The design techniques for diplexer is given in section III. The results of simulation and measurement are illustrated in section IV. Finally, a conclusion is written in section V.

Abstract—This paper presents the design of compact microstrip diplexer using triple-mode resonators. This diplexer consists of two triple-mode stub loaded resonators which have the same size but dissimilar resonance frequencies for combining the diplexer. The proposed diplexer features compact size and can be tuned to be functioned as together two resonators such that the pair of even-mode resonant frequencies can be flexibly controlled while the odd-mode resonant frequency is reserved at the fundamental frequency. T he diplexer operating at 1 .7 5/1 .9 5 GHz, and exhibits sharp attenuations near its passband is implemented and the area of the circuit is only 0.35λg × 0.40λg . An experiment was done for this diplexer, and the measurement results are in good agreement with the simulation predictions. Keywords—Diplexers, triple-mode resonator, microstrip filters.

I. INTRODUCTION Diplexers are crucial component in the RF front-end of both transmitter and receiver for RF/microwave wireless communication systems. They are widely demanded to separate or combine two different RF signals. The development of multiservice and multi-band communication systems, high performance diplexers with compact resonators have been established as well [1 -3 ]. To satisfy the strict system requirements, good performing diplexers that are compact, low loss, frequencies can be flexibly controlled and high isolation are necessary. For a planar diplexer design, it is required to select proper resonator types since resonators are the basic components of a filter. Several types of resonators such as the hairpin resonators [ 4 ] , open loop resonators [5], stepped impedance resonators [6] and hybrid resonators [7]. Although various approaches of several resonators have been investigated in past to achieve good selectivity of microstrip diplexer, there are a number of resonators that have not led to achieve miniature size. To reduce the resonator size, multiple mode resonator [8-11], as for the example dual-mode resonators, triple-mode resonators and quadruple-mode resonators, are attractive resonators, since each of them can be used as a multiple tunable resonators. Accordingly, the total number of degrees required in a bandpass filter can be reduced by half for dual-mode, one third for triple-mode and to one fourth for quadruple-mode, generating the multiple mode resonators as the common technique for designing compact filters respectively.

(a) Structure of the triple-mode stub loaded resonator

(b) Odd-mode ( f odd )

(c) Even-mode ( f even1 )

(d) Even-mode ( f even 2 ) Fig. 1. Schematic of the proposed resonator

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II. THE PROPOSED TRIPLE-MODE RESONATORS

feed the proposed dual-mode stub loaded resonator by using loose coupling. The length of L1 is fixed at 60.30 mm and width of Z1 = 1.00 mm, while L2 is varied from 22.50 to 24.50 mm as shown in Fig. 2. As can be seen in Fig. 2, the open-stub loaded length does not affect the S21 response at odd-mode resonant frequency at 2 GHz, while the even-mode resonant frequency are flexibly controlled.

The schematic model of the proposed triple-mode resonator is shown in Fig. 1(a), consist of a half-wave length resonator and two open circuit of stub loaded transmission line, where Z1 , Z 2 , Z 3′ , Z 3 and L1 , L2 , L3′ , L3 denote the characteristic impedances and lengths of the microstrip line respectively. The two open stub are shunted at the midpoint of microstrip line. Since the stub loaded are symmetrical in structure, an odd-mode and even-mode analysis can be applied to analyze the proposed structure due to symmetrical structure of the circuit. The all corresponding equivalent circuits structure are shown in Fig. 1 can be composed into the superposition of an odd-mode and two even-mode citation as shown in Fig. 1(b), Fig. 1(c) and Fig. 1(d), respectively. Based on the mechanism of the resonator resonance, the structure is resonant when its input admittance is zero for both even- and odd-modes, i.e., Yodd = Yeven = 0 . The odd-mode admittance in Fig. 1(b) can be expressed as Y1 Yin, odd = (1) j tan(θ1 / 2) From the resonance condition of Yin , odd = 0 , where θ1 = β L1 is the electric length of the half-wave length resonator, we obtain the answer of θ1 = π , the fundamental odd-mode ( f odd ) resonant frequency can be expressed as c f odd = (2) 2 L1 ε eff

Fig. 2. Simulated response with different length of stub loaded resonator

The triple-mode resonator is designed to achieve the desired resonant frequencies, while L3 is varied from 39.50 to 42.50 mm. Fig. 3 illustrates the simulated results of the amplitude S21 (dB) when the length of L3 is varied while other parameters are fixed to demonstrate the variation in lower even-mode resonance. The response of proposed triple-mode resonator which has the benefits of compact size is developed, the resonator is formed by a parallel two open-stub loaded resonator. It is shown from calculated expression (1) to (6) that the resonant frequencies of the resonator can be controlled by adjusting the odd-mode and even-mode structural parameters.

The even-mode admittance in Fig. 1(c) can be given as 2Y tan(θ1 / 2) + Y2 tan(θ 2 ) Yin,even1 = jY1 1 (3) 2Y1 − Y2 tan(θ1 / 2) tan(θ 2 ) The even-mode of a higher resonant frequency ( f even1 ) in Fig. 1(c) for special case 2Y1 = Y2 can be given as f even1 =

c ( L1 + 2 L2 ) ε eff

(4)

The result of input admittance for even-mode in Fig. 1(d) can be expressed as  2Y tan(θ1 / 2) + Y2 tan θ 2 + 2Y3 tan(θ3 / 2)  Yin ,even 2 = jY1  1  (5)  2Y1 − (Y2 tan θ 2 + 2Y3 tan(θ 3 / 2)) tan(θ1 / 2)  The even-mode of a lower resonant frequency ( f even 2 ) in Fig. 1(d) for special case 2Y1 = Y2 = 2Y3 can be expressed as f even 2 =

c ( L1 + 2 L2 + L3 ) ε eff

(6)

From the above analysis, for the example, the operation of resonant frequencies against the length of resonator has been investigated using full-wave EM simulations. The dual-mode resonator is designed to achieve the desired resonant frequencies. The fundamental frequency is fixed by the length of L1 and even-mode characteristic can be achived by

Fig. 3. Resonant properties of the triple-mode with different length L3

III. DESIGN OF THREE-POLE DIPLEXER Fig. 4 shows the configuration of the proposed diplexer. It consists of two triple-mode resonators of two difference bandpass filters. The resonators are geometrically folded in triple-mode resonators of similar size to achieve compact size as

adjusting the length of open circuit stub loaded L2 . Two microstrip lines with 50 Ω characteristic impedance are used to

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well as symmetrical structure. The transmission path from port 1 to port 2 is working at lower frequency 1.75 GHz, while the higher band transmits effectively between port l and port 3. The three-pole diplexer is designed using copper metallization on Diclade Arlon 880 substrate with relative dielectric constant of 2.20, a thickness of 0.80 mm, and a loss tangent of 0.009. The specification for the diplexer is designed to operate at 1.75 GHz and 1.95 GHz and the fractional bandwidths are 5.71% and 5.13%, respectively. The physical dimensions of the diplexer are determined using a full-wave electromagnetic simulator that simulated the coupling coefficients and external quality factors against physical structures, as described in [12-13]. In this resonator design, (for the first passband) the three resonant frequencies are chosen as 1.67 GHz, 1.75 GHz, and 1.82 GHz. The optimized geometrical dimensions of the proposed diplexer are found as: L1 = 64.10, W1 = 1.00, L2 = 29.45, W2 = 2.00, L3 = 55.30, W3 = 1.00, where L3′ = 6.25 , (for the second passband) the three resonant frequencies are chosen as 1.86 GHz, 1.95 GHz, 2.02 GHz. L1 = 57.90, W1 = 1.00, L2 = 26.60, W2 = 2.00, L3 = 50.30, W3 = 1.00, where L3′ = 6.30 (all in mm). The length and width of port input are 28.25 mm, 1.00 mm, respectively, and the gap diameter between port input and resonators is 0.40 mm.

Fig. 5. Photograph of the proposed diplexer

(a) Measured and EM simulated S-parameters of the diplexer.

Fig. 4. Schematic layout of the proposed diplexer.

(b) Comparison of isolation (S23) response of diplexer Fig. 6. Comparison between the simulated and the measured response of the proposed diplexer

IV. SIMULATED AND MEASURED RESULTS Fig. 5 is the photograph of the fabricated diplexer. The overall size of this filter is 43.50 mm × 56.60 mm, i.e., about. 0.35λg × 0.40λg , where λg is the guided wavelength of 50 ohm line on the substrate at 1.75 GHz. The measured and simulated results of the filter are shown in Fig. 6. The measured in-band return loss is better than l0 dB in the first passband (1.75 GHz) and 12 dB in the second passband (1.95 GHz) respectively. The insertion loss are approximately 1.95/1.80 dB at the two passbands. The insertion losses are mainly attributed to the conductor loss of copper. The measured isolation between the channels of better than 20 dB is achieved. Totally, the simulation and measurement results are in good agreement.

V. CONCLUSION A compact diplexer using triple-mode stub loaded resonator structure is proposed in this study. As can be seen, the diplexer is formed by two third-order BPFs. The characteristics of the triple-mode resonator have been demonstrated. Its resonant frequencies of two even-mode can be flexibly controlled, whereas the odd-mode is fixed, with no effect on the resonant frequency. Furthermore, all triple-mode resonator are folded to be compact to satisfy the miniaturization requirement. Good agreements between the simulated and measured results are obtained, which validates the simulation.

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