A tri band bandpass filter using short stub loaded SIR

544 PIERS Proceedings, Suzhou, China, September 12–16, 2011A Tri-band Bandpass Filter Using Short-stub Loaded SIR Xuehui Guan1, 2, Fang Wen 1 , Wei Fu 1 , Haiwen Liu 1 , Guohui Li 2 , an

544 PIERS Proceedings, Suzhou, China, September 12–16, 2011

A Tri-band Bandpass Filter Using Short-stub Loaded SIR

Xuehui Guan1, 2, Fang Wen 1 , Wei Fu 1 , Haiwen Liu 1 , Guohui Li 2 , and Lu Zhu 1

1 College of Information Engineering, East China Jiaotong University, Nanchang 330013, China

2 Key Laboratory of Specialty Fiber Optics and Optical Access Networks

Shanghai University, Shanghai 200072, China

Abstract— A novel compact triple-passband bandpass filter is presented in this paper A shorted-stub is loaded to a stepped-impedance resonator (SIR) The resonant frequencies are determined by the length of the loaded-stub and the impedance ratio of the SIR The input and output port are coupled to the resonator by utilizing pseudo-interdigital structure A two-pole microstrip triple-passband bandpass filter with central frequencies 1.5 GHz, 2.4 GHz and 5.78 GHz

is designed for GPS and WLAN applications.

1 INTRODUCTION

Due to the rapid development of intelligentized wireless communication systems, compact, high performance and low-cost filters are needed Researches show great interest on bandpass filters (BPFs) with dual- and tri-passband that can work in multiple communication bands Multi-band bandpass filters with different circuit topology have been reported [1–3] In [4], tri-band BPF is presented by insetting two narrow stopband in a broadband BPF In [5], three dissimilar dual-behavior resonators are connected by a transmission line, each dual-dual-behavior resonator creates a passband and two transmission zeros, and triple passband is achieved However, these kinds of filters suffered from large size A tri-band BPF based on cascaded three similar uniform impedance resonators (UIRs) is proposed in [6] In [7], a tri-band BPF is constructed by using three pairs

of open-stub loaded open-loop resonators and the parallel coupled microstrip feed lines In [8], triple passband BPF is realized by two pairs of quarter-wavelength resonators and a pair of half-wavelength resonators In [9], a triple-band bandpass filter is designed using two pairs of stepped impedance resonators, larger resonator resonant at two different frequencies and two passbands are formed In [10], a tri-layer tri-band (TLTB) bandpass filter (BPF) is presented The proposed tri-band BPF is a three-layers structure with two transmission lines on the top layer, open-loop resonators on the middle layer, and a stepped-impedance resonator (SIR) defected-ground structure (DGS) on the bottom layer

In this paper, a new tri-band BPF based on stub-loaded SIRs and pseudo-interdigital feeding structure is proposed Three operating bands are designed for commercial applications, i.e., 1.5 GHz for GPS application, 2.4 GHz and 5.78 GHz for WLAN application Impedance ratio of the SIR and the loaded shorted stub are used to control the operating frequencies of the resonator Pseudo-interdigital feeding structures are employed to acquire the desired external couplings

2 DESIGN PROCEDURE

Proposed tri-band resonator is presented in Fig 1 It is a SIR with a shorted stub loading The proposed resonators are bended into hairpin structure to reduce the circuit size Simulated fre-quency response is shown in Fig 1 Three resonant frequencies are distributed at 1.5 GHz band, 2.4 GHz band, and 5.78 GHz band The first resonant frequency is related to the fundamental mode of shorted stub, the second resonant frequency is related to the SIR, and the third resonant frequency is related to the higher order mode of shorted stub

Proposed resonator that inset in Fig 1 can be analyzed by using even and odd mode theory, as

to the center symmetrical plane Z1, Z2, Z3 and θ1, θ2, θ3 are the characteristic impedance and electrical length of the proposed SIR, respectively When the resonator works at even modes, as shown in Fig 2(a), the symmetry plane can be seemed as open-ended and the resonant condition can be expressed as:

Z in = −jz3tan θ3(1 −

1

k1 tan θ1tan θ2) + 1

k2tan θ2+ 1

k1k2tan θ1 tan θ3(k1k2tan θ1+ k2tan θ2) + k1tan θ1tan θ2− 1 (1)

where K1 and K2 are the impedance ratio, K1 = Z2/Z1, K2 = Z3/Z2, K1K2 = Z3/Z1 Quarter-wavelength resonant mode (Mode-1) and three quarter-Quarter-wavelength resonant mode (Mode-3) will

Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept 12–16, 2011 545

produce the passband in 1.5 GHz and 5.78 GHz, respectively The first passband located at a frequency 1.5 GHz, which is mainly determined by the loaded stub

When the resonator is excited by odd mode, the symmetrical plane can be seemed as shorted circuit, as shown in Fig 2(b) Ignoring the phase step discontinuity impact and open-edge

capaci-tance, the impedance Z in can be derived as:

z in = jZ1Z1tan θ1+ Z2tan θ2

Z1− Z2tan θ1tan θ2 (2)

It will produce the second passband located at a frequency 2.4 GHz

3 RESULTS AND DISCUSSION

According to above-mentioned design procedure, bandpass filters with three controllable passbands can be realized by changing the impedance ratio of the SIR and electrical length of the loaded stub

A two-pole tri-band microstrip bandpass filter with central frequencies of 1.5 GHz, 2.4 GHz band, and 5.78 GHz is designed The schematic of the proposed tri-band BPF is shown in Fig 3 Pseudo-interdigital and feedline coupling structures are used to obtain the desired external coupling [11, 12]

In this paper, the proposed filter is designed on a 0.8 mm-thick substrate with a dielectric

con-stant of 4.5 and a loss tangent of 0.002 The corresponding structural parameters are w1= 1.8 mm,

w2 = 1 mm, w3 = w4 = 0.2 mm, w5 = w6 = 1 mm, L1 = 7.35 mm, L2 = 3 mm, L3 = 3.75 mm,

L4 = 4.95 mm, L5= 7.75 mm, L6 = 0.55 mm Simulated results of the designed filter are illustrated

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

S21

Frequency (GHz)

Figure 1: Simulated frequency response of the

pro-posed resonator.

1

1 , θ

Z

2

2 , θ

Z

3

3 , θ

Z

1

1 , θ

Z

2

2 , θ

Z

Figure 2: Resonant modes of the resonator.

Figure 3: Schematic of the proposed tri-band BPF.

-70 -60 -50 -40 -30 -20 -10 0

S11

Frequenc y (GHz)

S11

S21

Figure 4: Simulated frequency response of the pro-posed filter.

546 PIERS Proceedings, Suzhou, China, September 12–16, 2011

in Fig 4 Obviously, the three resonated frequencies located at 1.5 GHz, 2.4 GHz, 5.78 GHz,

respec-tively The simulated reflection losses in three passband are about −10.6 dB, −15 dB, and −18 dB.

Four transmission zeros are realized at 1.3 GHz, 2.1 GHz, 3.7 GHz, and 6.2 GHz, which improve the selective in the transition band and attenuation in the stopband

4 CONCLUSION

A novel compact two-order tri-band microstrip BPF with using shorted-stub-loaded SIRs and pseudo-interdigital coupling structure has been proposed Three passbands with central frequencies

of 1.5 GHz, 2.4 GHz and 5.78 GHz for commercially applications are achieved The circuit structure has exhibited good passband selectivity owing to the transmission zeros produced by the pseudo-interdigital coupled filter configuration

ACKNOWLEDGMENT

This work is supported by Shanghai Leading Academic Discipline Project and STCSM (S30108 and 08DZ2231100), NSFC of China (No 61061001), Foundation of Jiangxi Educational Committee (GJJ11437), and the Global Research Network Program of National Research Foundation of Korea (No KRF-2009-220-D00074)

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