Zin

Z1, 2θ1 Z2, θ2 Z3, θ3

Fig.1. Geometry of the λ/2 tri section SIR

Tri-Band Bandpass Filter with Transmission Zeros between Each Band Using a Combination of Dual

Step and Tri-Step SIR

Rowdra Ghatak1, Manimala Pal2, Pankaj Sarkar3, A K Aditya2, D R Poddar4 1 ECE Dept., National Institute of Technology Durgapur, West Beangal, INDIA

2 ECE Dept., NSHM Knowledge Campus, Durgapur-Group of Institutions, Durgapur, West Bengal, INDIA 3 ECE Dept., ITER Siksha ‘O’ Anusandhan University, Bhubaneswar, Odisha, INDIA

4 ETCE Dept., Jadavpur University, Kolkata, West Bengal, INDIA

Abstract— A compact tri band bandpass filter (BPF) using a combination of half wavelength tri-section and dual section stepped impedance resonators is presented, which has a triple pass band at 2.45 GHz, 3.5 GHz and 5.5 GHz respectively. The passband bandwidth is suitable for WiMAX and WLAN application. Insertion loss within each of the bands is less than 2 dB. The overall filter dimension is 38 by 36.5 mm.

Keywords- Bandpass filter, tri-band filter, multiband filter, stepped impedance resonator

I. INTRODUCTION

In modern wireless and mobile communication systems, RF/microwave filters are essential components. Compact, high performance radio frequency (RF) and microwave components capable of operating at more than one frequency band play an important role in a multiband wireless communication system. Planar band pass filters (BPF) are particularly popular structures because they can be fabricated using printed circuit technology and are suitable for commercial applications due to their compact size and low cost of integration [1]. There are three broad categories of multiband filter design [2-9]. First one is based upon use of multimode resonators for multi-frequency application like stepped impedance resonators (SIR) which is used in this work [2]. Second deals with assembly of multi frequency resonators in same plane to achieve multiband BPF. Third category represents a class of multiband filter design based upon multi-resonators in conjunction with defected ground structures (DGS). A tri-band bandpass filter (BPF) is implemented by cascading multiband resonators [3] but with overall large dimension. In [4] a new tri-band BPF is designed using the vias that connect the microstrip line to the ground plane and line segments which provide the transmission zeros not only can sharpen the passband skirts, but can enhance the stop band rejection. Moreover pass bands do not have similar matching and selectivity is poor particularly at the lower two bands due to absence of any transmission zeros. In [5] a triband bandpass filter with improved coupling between tri-section SIR are reported. Though transmission zeros between passbands are achieved but the performance especially at the second passband is poor. A cross coupling path is

established by connecting two parallel coupled line sections to the I/O feeding lines that helps in achieving three passband and transmission zeros at each passband [6]. A tri-section SIR is used to design a tri-band filter which has three passbands but poor selectivity with not so well defined transmission zeros [7]. A triband BPF using SIR intercoupled using speudointerdigital configuration to produce transmission zeros between each band is presented in [8]. However, the introduction of additional transmission zeros degrade the pass band performance. In [9] a triband filter design is proposed using stub loaded resonators and DGS based resonators where the second passband is not so well defined with lack of proper selectivity of the passbands. This brings forth some challenges to triband bandpass filter design where apart from achieving three passbands, proper transmission zeros needs to be inserted without affecting the passband performance. This puts a lot of constraints on filter designers.

In this work a novel topology of second order tri-band BPF using a novel configuration of dual step and tri step is proposed. The triple passband center frequencies are located at 2.4 GHz (WLAN), 3.5 GHz (WiMAX) and 5.5 GHz (WLAN). Moreover proper trsnamission zeros ensure good selectivity. In addition, the overall size is also made compact. Rest of the paper is as follows. Section II deals with filter design and parametric study followed by results and discussion in Section III as well as conclusions in Section IV respectively.

II. FILTER DESIGN AND PARAMETRIC STUDY The basic ingredients of the proposed triband band pass

filter, namely the tri-section SIR structure is shown in Fig.1. The other one which is the dual step SIR is well documented in [2]. The SIR is symmetrical and has three different characteristic impedances Z1, Z2, and Z3. For practical application all segments are generally considered of equal

978-1-4577-1099-5/11/$26.00 ©2011 IEEE

Fig.2 Layout of the proposed triband bandpass filter.

Fig. 3 S-parameter plot for parametric study of G1

Fig. 4 S-parameter plot for parametric study of G2

electrical length for ease of design calculation. Using the three different impedance we calculate the impedance ratio K1=Z3/Z2 and K2 =Z2/Z1 respectively. The input impedance of trisection SIR from its center can be written as given in (1) [4].

( )

3212121213

2132

1321

2 tantantantantantan

tantantantantan

θθθθθθ

θθθθθ

KKKK

KK

KjZZin −++

+++−= (1)

The electric lengths for the first three resonant modes of the TSSIR can be derived as given below.

1tan

21

21101 ++

= −

KKKKθ (2)

2

211102

1tan

KKKK ++

= −θ (3)

203πθ = (4)

Since f1, f2 and f3 are specified, the required impedance ratio for the tri-section SIR can be expressed as

BABBK

++=

11 (5)

1

12 1

1K

KK

−+

= (6)

Where ⎟⎟⎠

⎞⎜⎜⎝

⎛=

3

1

1

22

2tan

ff

ff

A π (7)

⎟⎟⎠

⎞⎜⎜⎝

⎛=

3

12

2tan

ff

B π (8)

In this work, the filter is realized on 0.787 mm thick substrate with a dielectric constant 2.2. The center frequency of the three pass band is 2.4 GHz, 3.5 GHz and 5.5 GHz. The corresponding impedance ratio is calculated as K1=0.56 and K2=0.61. Then the electrical length of the tri-section SIR is computed as θ01=38.760, θ02=38.080 and θ03=77.370.

Fig. 2 shows the layout of the proposed tri-band microstrip BPF. From the computed electrical length and impedance of each section the physical dimensions are synthesized. The trisection SIR is bent in hairpin format and the two sections are coupled together along the high impedance section. However if the second resonator is oriented as the first one then the coupling is observed to be poor and the desired bands are not achieved. By flipping the second resonator with respect to the

TABLE 1 DIMENSIONS OF TRI-BAND BANDPASS FILTER Parameters Dimension (mm)

L1 10 L2 9.6 L3 19 L4 2 L5 5 L6 14 L7 7.7 L8 12 L9 10 W1 0.5 W2 1.4 W3 3.4 W4 4 W5 0.4 G1 0.5 G2 0.125 G3 0.25 W6 0.4 W7 2 W8 0.5 W9 0.5

(a)

(b)

Fig. 6 (a) Simulated result of tri-band filter (b) Closer view of the insertion loss within each passband.

Fig. 5 S-parameter plot for parametric study of G3

first one the necessary pass bands are achieved. An observation similar to the hair pin resonator case. The I/O lines are [parallel coupled lines that are in close proximity to one of the high impedance section of the tri-section SIR. The most sensitive parameters that influence the filter passband response drastically are the inter-resonator spacing for coupling between the resonator given as ‘G1’ and the spacing between the coupled lines and high impedance section for I/O feed given by ‘G2’ and ‘G3’. These are put to futher parametric study. Before the parametric study is discussed it is desirable to mention that the transmission zeros are obtained by the judicious placement of the two sets of half wavelength dual step SIR which are computed to achieve transmission zero at the higher end of the third band centered about 5.65 GHz and the transmission zero between the 2.4 GHz and 3.5 GHz band using design guidelines as given in [2]. The other transmission zero

between the second and third band is achieved by extending the outer feed line.

Influence of changing the value of the G1 (separation between the two resonator) on the S11 characteristics is shown in Fig. 3. If G1 is decreased the pass band becomes narrow at

the desired frequency and when increased wide band is achieved at the expense of matching. So, the optimum G1 is found to be 0.5 mm is chosen. The parametric study of G2 is shown in Fig. 4. Here G2=0.125 mm value is selected for the best response of the proposed tri-band filter. In Fig. 5 the parametric study of G3 (the gap between the resonator and the feed I/O coupling) is illustrated. For suitable passband response G3 of 0.25 is selected. For these values of spacing it is observed that a dual mode response within the pass band is setup.

III. RESULTS AND DISCUSSION The final design dimensions for proposed triband filter are tabulated in Table I. The simulated s-parameters of this tri-band filter is shown in Fig. 6. The three passband bandwidth centered at 2.45 GHz, 3.5 GHz and 5.5 GHz are 0.1 GHz, 0.175 GHz and 0.175 GHz respectively. The insertion loss within each band is within 2 dB. Impedance bandwidth of the proposed filter as 4.2% at first band, 4.7% at second band and 3.1% at the third. The overall size of this filter is 38×36.5 mm2. The passbands show that the proposed bandpass filter can be utilized for WLAN and WiMAX as well as Hiper LAN systems.

IV. CONCLUSION

A novel topology of second order tri-band microstrip BPF is proposed to provide three pass bands centered at 2.4 GHz, 3.5 GHz and 5.5 GHz as well as transmission zeros are realized using two half wavelength tri-sections and two dual section

SIRs. The insertion loss within each band remains within 2dB. The overall filter dimension is 38 by 36.5 mm.

V. REFERENCES [1] David M. Pozar, Microwave Engineering, John Wiley and Sons, New

Delhi, 2006. [2] M. Makimoto and S. Yamashita, Microwave resonators and Filters for

Wireless Communication, Springer, 2003. [3] Chi Feng Chen, Ting Huang and Ruey Beei Wu, “ Design of dual and

triple passband filters using alternately cascaded multiband resonators,” IEEE Transactions on Microwave Theory and Technique, vol. 54, no.9, pp.3550-3558, Sept. 2006.

[4] Ching Her Lee, Chung I. G. Hsu and He Kai Jhuang, “ Design of a new Tri-band Microstrip BPF using combined Quarter-wavelength SIRs,” IEEE Microwave and Wireless Components Letter, vol.16, no. 11, pp. 594-596, November 2006

[5] X M Lin and Q X chu, “A novel ttripple band filter with transmission zeros using tri-section SIRs,” IEEE International Conference on

Microwave and Millimeterwave Technology Proceedings, pp.1261-1263, 2008.

[6] Chung IG. Hsu, Ching Her Lee and Yi Huan Hsieh, “Tri-band bandpass filter with sharp passband skirts designed using Tri-section SIRs”, IEEE Microwave and Wireless Components Letter, vol. 18, no.1, pp. 19-21, January 2008.

[7] Q. X Chu and X. M Lin, “ Advanced triple-band bandpass filter using tri-section SIR,” Electronic Letters, vol. 44, no. 4, pp. 295-296, February 2008.

[8] Fu Chang Chen and Zing-Xiu Chu, “ Design of compact Tri-band Bandpass Filters Using Assembled Resonators,” IEEE Transactions on Microwave Theory and Techniques, vol. 37, no. 1, pp.165-171, Jan 2010.

[9] Xin Lai, Chiang Hong Liang, Hao Di and Bian Wu, “ Design of Tri-Band Filter Based on Stub Loaded Resonator and DGS Resonator,” IEEE Microwave and Wireless Components Letter, vol. 20, no. 5, pp. 265-267, May 2010.