Compact LPF Using T-shaped Resonator



DOI 10.1515/freq-2012-0043 

 Frequenz 2013; 67(1–2): 17 – 20

H. Sariri*, Z. Rahmani, A. Lalbakhsh and S. Majidifar

Compact LPF Using T-shaped Resonator Abstract: A novel microstrip lowpass filter based on a T-shaped resonator is proposed in this paper that presents wide stopband and compact size. A method of scaling is applied to and studied on the proposed resonator in order to tune its response parameters. To achieve wide stopband, a suppressing cell is designed and connected in series form to the resonator. The resulting filter is then ­fabricated and measured. Simulated and measured results of the proposed filter verify its advantages over those presented in the references in terms of stopband-width and physical size. Keywords: microstrip filter, tapered resonator, wide ­stopband PACS® (2010). 84.40.Az *Corresponding author: H. Sariri: Electrical Engineering Department, Eslamabad-E-Gharb Branch, Islamic Azad University, Eslamabad-E-Gharb, Kermanshah, Iran E-mail: [email protected] Z. Rahmani: Electrical Engineering Department, Razi University, Kermanshah, Iran A. Lalbakhsh: Electrical Engineering Department, Islamic Azad University, Kermanshah, Iran S. Majidifar: Electrical Engineering Department, Kermanshah University of Technology, Kermanshah, Iran

1 Introduction Wide-stopband microstrip lowpass filters are in high demand in modern communication systems in order to suppress undesired harmonics and high frequency noise. Considering this importance, numerous works have concentrated on the design of optimized lowpass filters so far. The Defected Ground Structure (DGS) is used in [1] to improve the transition sharpness and the passband insertion loss but the stopband performance is not satisfactory. In [2], tapered compact microstrip resonant cells are utilized to achieve wide stopband with the cost of large physical size. Using beeline CMRC is another approach that is followed by [3] causing low out-of-band rejection. Symmetrically loaded resonant patches and meander transmission line are adopted in [4] to provide wide stopband,

but it has the drawback of gradual cut-off. The patch resonator proposed in [5], has resulted in high and wide rejection in the stopband, as well as high selectivity. Another approach is to use hairpin resonators [6] that are very compact. Using a variety of defected ground structures (DGS) is also followed by a number of works [7–9]. Filters that are composed of these kinds of resonators have usually good passband performance; however they suffer from large physical size and increased complexity. Cascaded semi-circle patch and semi-circle stepped impedance resonator has been used in [10] to achieve a lowpass filter with wide stopband and sharp roll-off, but the constitutive sections are hard to modify for other applications. In this paper, a single T-shaped resonator is utilized in combination with a suppressing cell to implement a low­ pass filter with wide stopband and sharp roll-off. To evaluate the impact of each part of the proposed resonator on its response, a novel approach is followed by which the effects of the variations of the resonator dimensions on its response are justified by changing the numerical values of the LC elements in the loss-less equivalent circuit. Also, the good agreement between the Electromagnetic (EM) and LC simulation results of the proposed resonator, verifies the validity of the proposed LC model.

2 Design and study of the proposed filter structure The layout and loss-less LC model of the proposed T-­ shaped resonator are depicted in Fig. 1a and b respectively. The dimensions in the layout are: L1 = 5.4 mm, L2 =  7.2 mm, L3 = 0.5 mm, W1 = 0.8 mm, W2 = 0.2 mm, W3 =  0.3 mm, W4 = 0.2 mm, H1 = 2.3 mm and H2 = 3.7 mm. The values of the circuit elements are: Ls1 = 1.79 nH, Ls2 =  1.29 nH, Lt1 = 1.63 nH and C = 0.41 pF. The EM and circuit simulation results of the resonator are in good agreement as shown in Fig. 1c. As it is clear, the resonator presents wide stopband which is caused by only one transmission zero at 3.66 GHz with −44.38 dB attenuation level. This transmission zero is caused by resonance of the LC circuit of the T-shaped stub. One of the features of the proposed resonator is that its cut-off frequency can be modified for different applications. Two different approaches can be followed to achieve this goal. The first one is to scale all of the resonator

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 H. Sariri et al., Compact LPF Using T-shaped Resonator

Fig. 1: Proposed resonator; (a) layout, (b) LC equivalent circuit, (c) EM and circuit simulation results

­ imensions with a constant factor. This method is suitable d to make significant variations to the cut-off frequency. The second approach works based on the fact that some of the resonator dimensions have more significant  effect on its behavior. Thus, by scaling these par­ ticular dimensions – while the other dimensions are still constant – small variations in the cut-off frequency as well as precise tuning of the response parameters can be accomplished. These dimensions are H2, W1 and W4. For both of the mentioned methods, variations of the cut-off frequency and position of the near-band transmission zero, as a function of scale factor are shown in Fig. 2a and b, respectively. Now, to show how the three important dimensions of  the proposed resonator affect its response, simulated results of the resonator – as a function of H2, W1 and W4

Fig. 2: (a) Cut-off frequency as a function of scale factor, (b) position of near-band transmission zero as a function of scale factor

– are shown in Fig. 3a, b and c, respectively. The corresponding changes in the value of the LC elements in the equivalent circuit are presented in Table 1. As it can be seen, by increasing the value of H2 from 3.7 to 5.7 mm, the near-band transmission zero gets smaller significantly because this work increases the value of Ls1 from 1.79 to 3.14 nH. Also, the near-band transmission zero has shifted to lower frequencies as the value of W1 has been increased from 0.8 to 1.6 mm. This is because of the enhanced overall area that has consequently increased the value of the capacitance C from 0.41 to 0.53 pF. The third dimension is W4 that increasing its value from 0.2 to 0.6 mm decreases the value of the inductance Lt1 from 1.63 to 0.74 nH and since this inductance is not involved in the creation of the transmission zero, it only affects the in-band and out-of-band performances.

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H. Sariri et al., Compact LPF Using T-shaped Resonator 

Dimensions (mm)

Lt (nH)

Ls1 (nH)

Ls2 (nH)

C (pF )

H2 = 3.7 (mm) H2 = 4.7 (mm) H2 = 5.7 (mm) W1 = 0.8 (mm) W1 = 1.2 (mm) W1 = 1.6 (mm) W4 = 0.2 (mm) W4 = 0.4 (mm) W4 = 0.6 (mm)

1.63 1.63 1.63 1.63 1.63 1.63 1.63 1.12 0.74

1.79 2.36 3.14 1.79 1.79 1.79 1.79 1.79 1.79

1.29 1.29 1.29 1.29 1.36 1.5 1.29 1.29 1.29

0.41 0.41 0.41 0.41 0.48 0.53 0.41 0.41 0.41

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Table 1: LC changes as a function of resonator dimensions variations

Fig. 4: Proposed suppressing cell; (a) layout, (b) simulated results Fig. 3: (a) Resonator response as a function of H2, (b) resonator response as a function of W1, (c) resonator response as a function of W4

To design a lowpass filter using the proposed resonator, an appropriate suppressing cell is necessary to guarantee high attenuation level in the stopband. Thus, a horizontally symmetrical unit is proposed to play this role. Layout and simulated results of this suppressing cell are shown in Fig. 4a and b respectively. As it can be seen, this structure has one transmission zero at 12 GHz with −44 dB attenuation level.

The proposed resonator and suppressing cell are connected in series to form a wide stopband lowpass filter. This filter is fabricated on RO4003 substrate (dielectric constant = 3.38, thickness = 20 mil). The simulated and measured results are discussed in the next section.

3 Simulation and measurement Simulation was accomplished using EM simulator ADS software and measurement was carried out using a HP8510C network analyzer. The layout, photograph, and

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 H. Sariri et al., Compact LPF Using T-shaped Resonator

onator is then connected to a horizontally symmetrical suppressing cell to achieve a filter with high rejection in the stopband. The designed filter is then fabricated and measured. The measured results show that the proposed filter has wide stopband and sharp roll-off. The mentioned features and the compact size make the designed filter suitable for a variety of modern communication systems applications. Acknowledgments: This research is sponsored in part by EslamAbad-E-Gharb Branch of Islamic Azad University. Received: April 22, 2012.

References

Fig. 5: Proposed filter; (a) layout, (b) photograph, (c) EM-simulated and measured results

the simulated and measured results of the proposed filter are shown in Fig. 5a, b and c respectively. The component with dimensions Lm and Wm is used to tune the value of return loss in the passband. From the results, it can be seen that the proposed filter has a cut-off frequency of 3.09 GHz. The value of insertion loss from DC to 2.95 GHz is less than 0.5 dB and the value of return loss in this range is higher than 10 dB. Width of the transition band from −3 to −20 dB is 0.37 GHz that is quite sharp. The stopband, with a rejection level higher than −20 dB is from 3.46 to 18.63 GHz. This stopband width is equal to 4.91 fc that is improved, compared to the conventional structures of [1], [3], [5–7], and [9]. Physical size of the proposed filter, excluding the feeding lines, is 13.1 mm * 8.2 mm that is smaller than the conventional structures of [2–5] and [7–10].

4 Conclusion In this paper, a T-shaped resonator is proposed. The LC equivalent circuit of the resonator is derived and studied to provide a visual understanding of its structure. This res-

[1] T. Moyra, K. Parui, and S. Das, “Application of a defected ground structure and alternative transmission line for designing a quasi-elliptic lowpass filter and reduction of insertion loss,” Int. Journal of RF and Microw. Computer-Aided Eng., 20 (2010), 682–688. [2] L. Li, Z. F. Li, and Q. F. Wei, “Compact and selective lowpass filter with very wide stopband using tapered compact microstrip resonant cells,” Electronics Letters 45 (2009), 267–268. [3] F. Zhang and C. Li, “Compact UWB microstrip lowpass filter with novel CMRC,” Int. Journal of RF and Microw. Computer-Aided Eng., 17 (2007), 469–472. [4] L. Ge, J. P. Wang, and Y. X. Guo, “Compact microstrip lowpass filter with ultra-wide stopband,” Electronics Letters 46 (2010), 689–691. [5] J. L. Li, S. W. Qu, and Q. Xue, “Compact microstrip lowpass filter with sharp roll-off and wide stopband,” Electronics Letters, 45 (2009), 110–111. [6] S. Luo, L. Zhu, and S. Sun, “Stopband-expanded lowpass filters using microstrip coupled-line hairpin units,” IEEE Microw. and Wireless Comp. Letters, 18 (2008), 506–508. [7] J. Yang and W. Wu, “Compact elliptic-function lowpass filter using defected ground structure,” IEEE Microwave and Wireless Components Letters, 18 (2008), 578–580. [8] J. Park, J. P. Kim, and S. Nam, “Design of a novel harmonicsuppressed microstrip lowpass filter,” IEEE Microwave and wireless components letters, 17 (2007), 424–426. [9] R. Rouhi, C. Ghobadi, J. Nourinia, and S. Pirani, “Compact Elliptic function lowpass filter based on defected ground structure,” IEICE Electron. Express, 7 (2010), 434–439. [10] M. Hayati and A. Sheikhi, “Design of wide stopband lowpass filter with sharp roll-off,” IEICE Electron. Express, 8 (2011), 1348–1353.

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