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Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 2730, 2012 1407

A Novel Passive Dual-band Bandpass Microwave Filter Using

Microstrip Loop Resonators

Sholeh Jahani Maleki1, Samaneh Sadi2, Kambiz Sadat Najafi3, and Massoud Dousti1

1Department of Electronic Engineering, Islamic Azad University (IAU)Science and Research Branch, Tehran, Iran

2Department of Electrical EngineeringIslamic Azad University (IAU), South Tehran Branch, Tehran, Iran

3Departement of Electrical EngineeringIslamic Azad University (IAU), Islamshahr Branch, Tehran, Iran

Abstract In this paper, a novel passive dual-band bandpass filter using microstrip loopresonators is proposed, which allows two transmission paths to microwave signals. Each of themusing dual-mode resonators results in respective passband. This can provide convenience to easilychange one passband frequency, while another keeps almost the same. Several attenuation poles

in the stopband are realized to improve the selectivity of the proposed bandpass filter. It isdesigned and simulated on RT/Duroid6010 substrate with permittivity of 10.8 and thickness of1.27mm. Compact size, low loss, dual band and high selectivity characteristics are realized bythis type of filter structure. The filter of this type has 0.9% and 1.4% bandwidth at 1.8 GHz and2.4 GHz, respectively.

1. INTRODUCTION

Modern wireless communication systems demand microwave devices operating in multiple separatedfrequency bands. For example, for Some mobile phone networks, like global systems for mobilecommunications (GSM) and Wireless LAN protocols (WLAN), such as Bluetooth and the IEEE802.11b specifications, the RF transceiver must be able to receive and transmit 1.8 GHz and 2.4 GHzsignals. Therefore, the bandpass filter, as a key component filtering unwanted frequency in RFsystems [1], is necessitated to generate two or more frequency bands. A number of publications haveprovided a variety of solutions to the realization of dual-band bandpass filter [24]. A dual-bandbandpass filter has been proposed using stepped impedance resonators (SIRs) [1, 46] and equal-length coupled-serial-shunted lines [7, 8]. However, it is difficult to generate two adjacent passbandsusing SIR. Recently, The dual-mode resonator using patch [9] or square loop [10, 11] has attractedmuch attention in design of single frequency filter, which meets the demands for compact size,low radiation and high-performance filters. The microstrip dual-mode bandpass filter permits thecompact filter development with narrowband and high-Q for modern telecommunication systems.

In this paper, a dual-band bandpass filter using loop resonators with square patches is presented,which needs only single input and output. The filter provides two transmission paths to an RFsignal, and each of them is realized using a dual-mode square loop and meander loop resonators.The patches used in resonators are called perturbation element. Each path generates its ownpassband, and there is good isolation between them. Compared with the conventional dual band

filter, the filter is smaller in size and better in performance.

2. MICROSTRIP MEANDER LOOP RESONATOR

Based on a variety of symmetric dual-mode resonating structures, dual-mode microstrip bandpassfilters have been investigated by many researchers for applications in both wired and wirelesscommunication. The schematic layout of the dual-mode meander loop resonator are shown inFigure 1, which shows that the meander loop resonator has the advantage of compact size. Thisresonator is designed at 1.8 GHz. For dual-mode operation, a perturbation is introduced in theresonator in order to couple its two degenerate modes. The response of the dual-mode filter canbe changed from elliptic to Chebyshev by simply adjusting the size of the perturbation. Theappearance of the two transmission zeros in the response, is due to the presence of a parasiticcoupling between the input and output. Frequency response is shown in Figure 2. It is clearlyobserved that two poles in stopband are realized, which can improve the selectivity of the filter.

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Figure 1: Layout of meander loop resonator withouter corner patch perturbation.

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Figure 2: Simulated frequency response of meanderloop resonator.

Figure 3: Layout of square loop resonator with innercorner patch perturbation.

Figure 4: 3-D view of proposed dual-band bandpassfilter.

3. MICROSTRIP SQUARE LOOP RESONATOR

Figure 3 shows layout of a dual-mode microstrip square loop resonators with an inner perturbationpatch. For dual-mode operation, a perturbation is introduced in the resonator in order to coupleits two degenerate modes. The square loop consists of four identical arms in length. Different filterresponses can be obtained with different positions and size of the perturbation, which is analyzed

in detail in [10]. The fundamental resonance occurs when a g4

, where g is guided wavelengh

g = cfeff

(1)

where c is the velocity of light in free space, and eff is the effective dielectric constant of thesubstrate. According to (1), for a fixed resonant frequency, a = g/4 is decreased to realize sizereduction as eff increased. Similarly, for a fixedeff, the resonant frequency f is decreased as aincreased. So the fundamental resonant frequency is shifted down. Therefore, to obtain 2.4 GHz,this square resonator is designed at 2.66 GHz. It is designed and simulated on a substrate withpermittivity of 10.8 and thickness of 1.27 mm.

4. PROPOSED BANDPASS FILTER DESIGN

The 3-D view of proposed dual-band bandpass filter is shown in Figure 4. The substrate 2 withoutground plane at its bottom side is stacked on the mid-layer loop, which increases theeff effectively.

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Figure 5: Simulated frequency response of dual-band bandpass filter.

So the fundamental resonant frequency is shifted down from 2.66 GHz to 2.4 GHz. The bottom sideof substrate 1 is ground plane. The substrate with dielectric constant r = 10.8 and thicknessh = 1.27 mm are used for our proposed bandpass filter design. The size of substrate 2 on themid-layer loop is 22 22mm2. The dimensions of the proposed filter are as follows: a = 13mm,w1 = 1mm, b = 3.17mm, c = 2mm, p1 = 1.6mm, w2 = 0.5mm, w3 = 2.3mm, p2 = 2mm and

w= 1 mm, which is the width of 50 microstrip lineThe feed-line using microstrip line type gives the convenience to connect with other circuitsin systems. The two loops located on mid-layer and top-layer provide two transmission paths toRF signal. The proposed filter generates two separated passbands by using two loops resonatingdifferent frequency, mid-layer loop for higher passband, and top-layer loop for lower passband. Inaddition, each dual-mode loop generates own passband and two attenuation poles at respectivestopbands, which keeps good selectivity of the proposed filter. In order to reduce the couplingbetween the two perturbation patches and the mutual effects of the two passbands, the two patchesare located at different corners of top-layer loop and mid-layer loop.

5. SIMULATED RESULTS

The simulation is accomplished by using EM simulator tool (ADS). Figure 5 shows the simulatedresults of the proposed dual-mode dual-band bandpass filter centered at 1.8 GHz and 2.4 GHz, the

minimum insertion losses of the two passbands are0.01dB and0.0001 dB, respectively. Fourattenuation poles at 1.72, 1.89, 2.26, and 2.53 GHz are realized. Quality factor for first and secondbands are 56 and 35, respectively. Which can greatly improve the selectivity of the proposed dual-mode dual-band bandpass filter. The return losses of the two bands are better than25 dB and15 dB, respectively.

6. CONCLUSION

In this paper, a dual-band bandpass filter using loop structure is presented with the advantagesof low loss, narrow bandwidth. The two dual-mode loop resonators on different layers generaterespective passband, and changing the operating frequency of each of them does not vary that ofthe other. Four attenuation poles in the stopbands are realized to improve the selectivity of thefilter and isolation between the two passbands. Morever, this new structure has size reduction of

about 30% against the loop filter at the same center frequency. By this new design, the traditionaldual band bandpass filters performance is improved.

REFERENCES

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3. Dai, X.-W., C.-H. Liang, B. Wu, and J.-W. Fan, Novel dual-band bandpass filter design usingmicrostrip open-loop resonators,Journal of Electromagnetic Waves and Applications, Vol. 22,No. 23, 219225, 2008.

4. Wang, J., Y.-X. Guo, B.-Z. Wang, L.-C. Ong, and S. Xiao, High-selectivity dual-band stepped-impedance bandpass filter, Electronics Letters, Vol. 42, No. 9, 538540, 2006.

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5. Sun, S. and L. Zhu, Novel design of dual-band microstrip bandpass filters with good in-between isolation, IEEE APMC, Suozhou, 2005.

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