Compact microstrip square open-loop bandpass filter using open stub
in the given structure. The equivalent capacitances, CP1 and CP2 , represent the pad capacitance effects in ports 1 and 2, respectively. Using the equivalent circuit model, under the consideration of the lossless condition, the equivalent inductance LS , capacitances Cg , pad capacitances, CP1 and CP2 , can be expressed as [8]:
A novel compact stub-coupled microstrip square open-loop bandpass filter (BPF) is proposed. The fork-shaped open stub is internally inserted inside the square loops using open gaps. The BPF has strong electric coupling, because the open sides of the two loops are proximately placed face-to-face in parallel. The rectangular stubs attached to the internal wall of loops connect to magnetic couplings; as a result the mixed coupling generates strong electromagnetic (EM) coupling in the BPF with an adequate input reflection coefficient at a centre frequency of 11.94 GHz. The measured results of the designed BPF exhibited return loss and insertion loss of 224.8 dB and 21.7 dB, respectively.
Proposed BPF: Fig. 1 shows the schematic layout of the proposed BPF on a Teflon substrate with a relative dielectric constant of 2.52 and thickness of 0.54 mm. The open stub with a variable parameter is used to connect two symmetrical square open loops electromagnetically. The designed BPF was optimised to achieve a resonance frequency of 12.0 GHz. The last optimised geometric dimensions were fixed in which W1 ¼ 0.75 mm, W2 ¼ 2.25 mm, W3 ¼ 0.95 mm, W4 ¼ 0.4 mm, W5 ¼ 0.6 mm, L1 ¼ 10.0 mm, L2 ¼ 4.9 mm, L3 ¼ 1.85 mm, L4 ¼ 1.9 mm, d1 ¼ 0.9 mm, d2 ¼ 1.3 mm, b1 ¼ 0.4 mm, b2 ¼ 0.25 mm, g1 ¼ 0.2 mm, and g2 ¼ 0.15 mm. The overall device size of the fabricated filter is (3 × 10) mm2. The proposed square open-loop resonator is able to be miniaturised compared with previous designs [3– 7].
2Zp Zs sin us cos us + sin us (Zs2 cos2 up − Zp2 sin2 up )
Cp1
Cp2 =
(3)
Im(Y21 ) v
(4)
Cg =
where Zs , Zp and us , up are the corresponding characteristic impedances and electrical lengths of the connecting transmission lines, respectively, and v is the angular cutoff frequency. The parameters Y11 and Y21 are input and output imaginary admittances. port 1 with 0.9 mm Ref. specific length
ZPOP
ZPOP
ZnOn
0
ZnOn
–10 ZPOP
ZPOP port 1 with 0.9 mm Ref. specific length
port 1
Cg
LS1
CP1
CP1
LS2 CP2
L1
port 2
–20
CP2
–30 X=Y=1.4 mm X=Y=1.3 mm X=Y=1.2 mm X=Y=1.1 mm X=Y=1.0 mm
–40
L2
–50 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 frequency, GHz
a
b
Fig. 2 Equivalent circuit model for proposed BPF, and simulated S21 responses of BPF with varying lengths, X and Y of 1.0, 1.1, 1.2, 1.3, 1.4 (in mm) a Equivalent circuit model b Simulated S21 responses 0
–10
–20
–30 S11(simulated) S21(simulated) S11(measured) S21(measured)
–40
1
2
3
4
–50 10.5
11.0
a
L1
11.5 12.0 12.5 frequency, GHz
13.0
13.5
14.0
b
g1
d1 W1
(2)
Im(Y11 + Y21 ) v
10.0
L2
(1)
vZ s 1 Zz Zp 1 − cos us cos 2up − + sin us sin 2up 2 Zp Zs = v2 L s
S-parameters, dB
Introduction: In very-small-apparatus terminal (VSAT) communication and radar systems, highly compatible size and reliable BPFs are always needed to reduce cost and enhance system performance. Microstrip BPFs can be miniaturised and are able to be easily mounted on a dielectric substrate using printed circuit technology [1]. Microstrip BPFs are particularly attractive, because they are compact, light weight, conformable to the mounting structure, integrable with solid-state devices and can be easily fabricated with low cost, as well as having low profile, high selectivity and good reliability. Because of these features, they are not only used in military applications, but also in commercial areas. Therefore, some researchers [2– 4] have studied planar loop filters with various bandwidths for different applications. Some research works [5, 6] have proposed microstrip BPFs using resonators with different kinds of tuning stub, but these configurations still occupy a large circuit area, which is not suitable for VSAT communication systems where miniaturisation is an important factor. Therefore, in this Letter, a very compact planar BPF using the properties of open and attached stubs into the open-loop resonators with enhanced coupling effects is introduced. The proposed BPF can be applied in the down link of Ku-band communication systems.
Ls =
S21, dB
R.K. Maharjan, B. Shrestha and N.Y. Kim
b1
1
X
g1
b1 W5 b2
Y
g2
W2
Fig. 3 Photograph of fabricated BPF, and comparison of simulated and measured S-parameter responses
b1
g2
d2 b2
g1 g2
b1
a Fabricated BPF b Simulated and measured S-parameter responses
g2 W4 g2 L4
g1
W3
L5 d1 2
b1 L3
Fig. 1 Schematic design layout of stub-loaded square open-loop BPF
The equivalent circuit for the design structure is shown in Fig. 2a as a simplified model; where the face-to-face open-loop sides at the centre of the structure show electric coupling, and the open-loaded and attached stubs of the loops enhance the magnetic coupling. Therefore, the overall coupling behaves as a strong and concentrated EM coupling and resonates at a frequency of 11.94 GHz. As shown in the Figure, LS1 and LS2 model the overall equivalent inductances on the sides that correspond to ports 1 and 2, respectively. The existing small values of the interdigital capacitances and corresponding equivalent resistances are not included, so as to simplify the equivalent circuit. The equivalent mutual inductors, L1 and L2 of the open-loop structure behave as a virtual transformer and magnetically couple with each other. The overall equivalent gap capacitance, Cg , generates the electric coupling
ELECTRONICS LETTERS 15th March 2012 Vol. 48
Results: In this design analysis, we used a commercially available electromagnetic (EM) simulator of the Sonnet Tool to realise the resonant frequency response. The operating frequency bandwidth of the filter can be varied by tuning the stub lengths, X and Y. Fig. 2b shows simulated variation of insertion loss S21 under different values of X and Y, which assists in improving the resonance characteristics. The BPF was fabricated using photolithographic techniques and a wet etching process, and is depicted in Fig. 3a. The fabricated filter was tested and characterised using an Agilent 8510C vector network analyser (VNA). The proposed BPF was designed at 12.0 GHz. The measured S11 became a little narrower than the simulated one and the measured S21 was decreased by 21.3 dB when compared with simulated results, which are illustrated in Fig. 3b. However, the frequency was down shifted by 60 MHz in the measurement result. This frequency shifting can be attributed to the dielectric loss of the substrate, the limitation of the accuracy of the physical dimensions and the inaccurate orientation of the connections when the connectors are soldered to the device. The effective fractional bandwidth at 10 dB was measured to be 670 MHz at
No. 6
the centre operating frequency. The measurement results show an acceptable (S11) equal to 224.8 dB and insertion loss (S21) is measured to be 2 1.7 dB at resonant frequency. The spurious suppression was measured to be more than 242 dB at frequencies of about 10.5 and 13.35 GHz. Conclusion: A fork-type stub-coupled resonator based BPF has been designed, fabricated and characterised. The design analysis of the proposed filter is based on the strong coupling effects due to the loaded open stub including attached stubs and the asymmetric feed lines. The proposed filter was able to be designed with a reasonably small size and good bandpass response. The design pattern can also be fabricated with MMIC technology for further reduction in size. Acknowledgments: This research was supported by the National Research Foundation of Korea (NRF) and a grant from the Korea government (MEST) (no. 2011-0030819). This work was also supported by a Research Grant of Kwangwoon University in 2012. # The Institution of Engineering and Technology 2012 21 January 2012 doi: 10.1049/el.2012.0143 One or more of the Figures in this Letter are available in colour online.
References 1 Hong, J.S., and Lancaster, M.J.: ‘Microstrip filters for RF/microwave applications’ (Wiley-Interscience, New York, USA) 2 Hong, J.S., and Lancaster, M.J.: ‘Canonical microstrip filter using square open-loop resonators’, Electron. Lett., 1995, 31, (23), pp. 2020– 2022 3 Fan, J.-W., Liang, C.-H., and Dai, X.-W.: ‘Design of cross-coupled dual-band filter with equal length split-ring resonators’, J. Prog. Electromagn. Res., 2007, 75, pp. 285–293 4 Zhang, X.-Y., Xue, Q., and Hu, B.-J.: ‘Novel bandpass filter with size reduction and harmonic suppression’, Microw. Opt. Technol. Lett., 2007, 49, (4), pp. 914– 916 5 Chen, F.-C., Chu, Q.-X., and Tu, Z.-H.: ‘Tri-band bandpass filter using stub loaded resonators’, Electron. Lett., 2008, 44, (12), pp. 747–749 6 Chu, Q.-X., Wu, X., and Tian, X.-K.: ‘Novel UWB bandpass filter using stub-loaded multiple-mode resonator’, IEEE Microw. Wirel. Compon. Lett., 2011, 8, pp. 403– 405 7 Dai, X.W., Liang, C.H., Wu, B., and Fan, J.W.: ‘Novel dual-band bandpass filter design using microstrip open-loop resonators’, J. Electromagn. Waves Appl., 2008, 22, pp. 219– 225 8 Tang, C.W., and Lu, L.P.: ‘Design of triple– passband filter with interdigital resonators’, Electron. Lett., 2008, 44, (25), pp. 1472– 1473
R.K. Maharjan, B. Shrestha and N.Y. Kim (RFIC Lab, Department of Electronic Engineering, Kwangwoon University, 447-1 Wolgye-dong, Nowon-Gu, Seoul 139-701, Republic of Korea) E-mail:
[email protected]
ELECTRONICS LETTERS 15th March 2012 Vol. 48 No. 6
