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Ultra wideband bandpass filter with dual-notched bands using stub-loaded rectangular ring multi-mode resonator
Microelectronics Journal 43 (2012) 257–262
Contents lists available at SciVerse ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Ultra wideband bandpass filter with dual-notched bands using stub-loaded rectangular ring multi-mode resonator Hung-Wei Wu n, Yu-Fu Chen Department of Computer and Communication, Kun Shan University, Taiwan
a r t i c l e i n f o
abstract
Article history: Received 23 August 2011 Received in revised form 3 January 2012 Accepted 4 January 2012 Available online 20 January 2012
This paper presents a new ultra wideband (UWB) bandpass filter (BPF) with dual-notched bands (at 5.2/ 5.7 GHz) using the stub-loaded rectangular ring multi-mode resonator (MMR). The proposed resonator consists of the dual embedded open-circuited stubs for introducing the dual notch bands and connected with a stub-loaded rectangular ring structure for controlling the two transmission zeros (at 3/11 GHz) at both sides of the UWB passband edge. This study mainly provides a simple method to design a UWB bandpass filter with high passband selectivity and dual-notched bands for satisfying the Federal Communications Commission (FCC-defined) indoor UWB specification. Experimental verification is provided and good agreement has been found between simulation and measurement. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Dual notch bands Bandpass filter Ultra-wideband (UWB) Multi-mode resonator (MMR) Embedded open-circuited stubs
1. Introduction Ultra wideband (UWB) bandpass filters (BPFs) with high performance are desirable in UWB wireless communication systems (from 3.1 to 10.6 GHz). Recently, the UWB filters with single or multi-notched bands having the capabilities to avoid the interferences from Wireless Local Area Network systems (WLAN, IEEE 802.11a, 5.15–5.825 GHz) [1]. Besides, in order to meet UWB radiation limits, steep passband selectivity at lower and higher passband edges in the UWB filter characteristics become more important. Therefore, the design of a compact, high passband selectivity and multi-notched bands UWB bandpass filter is a great challenge to filter designers. According to the requirements of the UWB filters, the UWB filters with steep passband selectivity and multi-notched bands are desirable. For this purpose, several methods have been developed [2–8]. Wu et al. proposed the quintuple-mode UWB bandpass filter with sharp roll-off and super-wide upper stopband [2]. Two transmission zeros generated by the asymmetrical stepped-impedance stub are used to improve the passband selectivity greatly. Although the aperture-backed structure can raise the coupling degree of the I/O lines, the fabrication procedure would become complexity. Liu et al. proposed the UWB filter using circular-ring MMR for achieving the sharp-rejection at passband edges [3]. Chu et al. proposed the UWB filter using
n
Corresponding author. E-mail addresses: [email protected], [email protected] (H.-W. Wu).
0026-2692/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2012.01.002
stepped-impedance stub-loaded resonator (SISLR) [4]. The SISLR is found to have the advantage of providing more degrees of freedom to adjust the resonant frequencies, however, the stopband edges present low attenuation to increase the possibility of signal error rating. Weng et al. proposed the UWB filter with single notch band (at 5.8 GHz) [5]. The embedded open-circuited stub is desirable to introduce single narrow-bandwidth notched band for avoiding the interferences from WLAN. Hao et al. proposed the UWB filter with multiple notch bands using the multilayer nonuniform periodical structure [6]. The filters with single-, double- and triple-notch bands are implemented on the liquid crystal polymer (LCP) substrate using thin film fabrication process. Song et al. proposed the UWB filter with multiple notch bands using the asymmetric coupling strip [7]. The filter can provide two main paths for the signals, which makes it possible to generate multiple transmission zeroes. Wei et al. proposed the UWB filter with dual notch bands based on a simplified composite right/left-handed (SCRLH) resonator [8]. The SCRLH is to introduce the dual notch bands (at 5.2/5.7 GHz) over the UWB frequency spectrum. Huang et al. proposed an ultra-wideband filter based on surface-coupled structure [9]. Two different quarter-wavelength lines are arranged on the ground of UWB BPF to generate dual narrow stop bands at 5.25/5.775 GHz. In order to avoid the interference from the WLAN signals, the UWB filter with high passband selectivity and multiple notch bands is required. In this paper, we proposed a new UWB filter using the stub-loaded rectangular ring multi-mode resonator for achieving the high passband selectivity and dual-notched bands simultaneously. The filter consists of a rectangular ring MMR with
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H.-W. Wu, Y.-F. Chen / Microelectronics Journal 43 (2012) 257–262
dual embedded open-circuited stubs for introducing dual notch bands at 5.2/5.7 GHz and connected with a stub-loaded rectangular ring structure for controlling the two transmission zeros to enhance the passband selectivity. Two capacitive-ended interdigital coupled I/O lines provide the stopband of near 15 dB rejection level from 11–15 GHz. The measured results are in good agreement with simulated predictions.
2. Filter design Fig. 1 shows the configuration of the proposed UWB filter. It consists of a rectangular ring multi-mode resonator (MMR) with dual embedded open-circuited stubs for introducing the dual notch bands at 5.2/5.7 GHz and connected with a stub-loaded rectangular ring structure for controlling the two transmission zeros at the passband edges. Two capacitive-ended interdigital coupled I/O lines are possessed of wide stopband performance [10]. The lengths of the I/O lines (l5 and l8) are chosen to be around quarter wavelength at center frequency (f0 ¼6.8 GHz). The stub-loaded rectangular ring structure consists of a rectangular ring (2y1, Z1) and a short transmission line
Fig. 1. Configuration of the proposed UWB filter. (l and W: physical line length/ width, y: electrical length and Z: characteristic impedance).
(y2, Z2) and which is placed at the symmetric plane of the filter. The UWB passband with high selectivity can be well achieved by tuning the dimensions (Z1, y1, Z2 and y2) of the stub-loaded rectangular ring structure. The design procedure is described as follows. At the first step, the wide band characteristic can be understood from the frequency response of the coupled I/O lines [11]. The equivalent transmission line model of the coupled I/O lines can be further converted to the asymmetric coupled line [12], as shown in Fig. 2(a). In order to simplify the investigation, the impedance Z8 and the length of the coupled line are chosen to be 111 O (W5 ¼0.1 mm) and 4.25 mm (quarter-wavelength at 6.8 GHz and y8 ¼ 901), respectively. As referenced in [12], the characteristic modes for an asymmetric coupled line are named as the ‘‘c’’ mode and ‘‘p’’ mode. The image impedance Zi can be derived from [13] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðZ j0e -Z j0o Þ2 -ðZ j0e þ Z j0o Þ2 cos2 y8 Zi ¼ ð1Þ 2sin y8 where index j indicates c- and p-mode and the Z0e and Z0o indicate the characteristic impedance with even- and odd mode for a general coupled line. When Zi is equal to 7jN and Re(Zi) is equal to 0, a bandstop edge is introduced [13]. We then define a normalized bandwidth (NBW) of image impedance by subtracting right bandstop edge and left bandstop edge. It is found that when S1 varies from 0.1, 0.8 to 1.5 mm, corresponding to (Z0e, Z0o) of (168 O, 51 O), (120 O, 92 O) and (108 O, 85 O) for c-mode and (123 O, 37 O), (74 O, 57 O) and (84 O, 66 O) for p-mode, the estimated normalized bandwidth (NBW) of 73%, 24% and 10% can be well achieved, as shown in Fig. 2(b). In a previous work, it is found that the NBW calculated from the image parameter has corresponding relations with the 1.5 dB FBW and 3 dB FBW estimated from the EM simulation [14]. The relation between the NBW and 3 dB FBW are listed in the Table 1. S1 of 0.1 mm is chosen to have the satisfied 3 dB FBW in this study. At the second step, the relations between the passband selectivity and proposed stub-loaded rectangular ring structure would be described as follows. Fig. 3 shows the equivalent transmission line model of the UWB filter. In practical, the embedded open-circuited stubs would not affect the Table 1 Relation between the NBW and 3 dB FBW for the asymmetric coupled lines.
NBW (%) 1.5 dB FBW by EM simulation (%) 3 dB FBW by EM simulation (%) Ratio of (3 dB FBW/NBW)
Fig. 2. (a) Transmission line model and (b) the real part of the image impedance with c and p mode of the asymmetric coupled line under different coupling gap S1.
S1 ¼0.1 mm
S1 ¼0.8 mm
S1 ¼1.5 mm
73 77 113 1.54
24 25 40 1.65
10 13 16.2 1.62
Fig. 3. Equivalent transmission line model of the proposed UWB filter.
H.-W. Wu, Y.-F. Chen / Microelectronics Journal 43 (2012) 257–262
259
characteristics of the transmission zeros, therefore, the parameters of (y6, Z6) and (y7, Z7) can be ignored in the even- and odd-mode analysis shown in Fig. 4. The model can be divided into two sections along the symmetric plane of the filter, an open circuit for even mode and a short circuit for odd mode [15], as shown in Fig. 4(a) and (b). The even mode input admittance can
Fig. 4. Equivalent transmission line model of (a) even mode and (b) odd mode of the UWB filter.
Fig. 6. Simulated frequency response of the proposed filter with (a) Z1 of 80, 95 and 110 O (Z2 ¼50 O and y1 ¼ 1451 are fixed) and (b) Z2 of 30, 50 and 70 O (Z1 ¼ 95 O and y1 ¼ 1451 are fixed).
Fig. 5. Relations between the input admittance Yine and Yino, length ratios of y2/y1 and normalized transmission zero frequencies (fz1 and fz2) for the proposed UWB filter using the full-wave EM simulation tool. (a) fz1/f0 and (b) fz2/f0 with Z1 of 80, 95 and 110 O (Z2 ¼ 50 O and y1 ¼ 1451 are fixed) and (c) fz1/f0 and (d) fz2/f0 with Z2 of 30, 50 and 70 O (Z1 ¼ 95 O and y1 ¼1451 are fixed). f0 is the center frequency (6.8 GHz) of the proposed filter and fz1 and fz2 indicate the 1st/2nd transmission zero at lower/higher passband edge.
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H.-W. Wu, Y.-F. Chen / Microelectronics Journal 43 (2012) 257–262
transmission zeros occurs when S21 ¼0. The S21 parameters can be derived from (2) and (4), and is given by [15]
be calculated as Y ine ¼
Z 5 þZ A tan y8 Z A Z 5 þ jZ 25 tan y8
ð2Þ S21 ¼
where ZA ¼ j
Z3 2Z2 ðZ 1 cot y1 2Z2 tan y2 ÞðZ3 =2Þtanðy9 =2Þð2Z2 þ Z1 cot y1 tan y2 Þ 2 Z23 ð2Z2 þ Z1 cot y1 tan y2 Þ þ 2Z2 ðZ 1 cot y1 2Z2 tan y2 Þtanðy9 =2Þ
y9 ¼ ðy3 þ y4 Þ=2 when Yine ¼0, even mode resonance will occur and the resonance condition is Z5 þ ZA tan y8 ¼ 0
ð3Þ
similarly, the odd mode resonance input admittance can be calculated as Y ino ¼ Z 5
Z 5 ð1=2ÞZ 3 tan y9 tan y8 jðð1=2ÞZ 3 tan y9 þZ 5 tan y8 Þ
ð4Þ
when Yino ¼0, odd mode resonance will occur and the resonance condition is 1 Z5 Z3 tany9 tany8 ¼ 0 2
ð5Þ
Y ino Y 0 -Y ine Y 0 ¼0 ðY 0 þ Y ino ÞðY 0 þ Y ine Þ
ð6Þ
where Y0 is the characteristic admittance. The condition of transmission zero frequencies are obtained when Yino ¼Yine [16,17]. Fig. 5(a)–(d) shows the relations between the input admittance (Yine and Yino), length ratios of y2/y1 and two transmission zero frequencies (fz1 and fz2) normalized by center frequency (f0 ¼6.8 GHz) of the UWB filter under different values of Z1 and Z2. fz1 and fz2 move to lower frequencies as y2/y1 increased from 0.28 to 0.55. When 9fz1 fz29/f0 ( E1.15) at y2/y1 E0.44 (y2 ¼641 and y1 is fixed at 1451) under Z1 of 80, 95 and 110 O, the approximately UWB passband bandwidth can be considered and verified by Yino ¼Yine (Z1 ¼95 O) as marked point A and B shown in Fig. 5(a) and (b). Similarly, when y2/y1 E0.44 under Z2 of 30, 50 and 70 O, the approximately UWB passband bandwidth of around 7.3 GHz can be verified by Yino ¼Yine (Z2 ¼ 50 O) as marked point C and D shown in Fig. 5(c) and (d). The characteristics of the stub-loaded rectangular ring structure can provide the filter with capability to control the passband bandwidth and selectivity over the UWB frequency spectrum. Fig. 6 shows the simulated frequency response of the proposed
Fig. 8. Simulated frequency response of the proposed filter with and without embedded open-circuited stubs.
Fig. 7. (a) Equivalent transmission line model of the dual embedded opencircuited stubs, (b) simulated frequency response of the UWB filter with tuning the physical length l6 and l7 and (c) the width W6 and W7. (W6 ¼W7 in this work).
Fig. 9. (a) Photograph and measured results and (b) group delay of the fabricated UWB filter. (W1 ¼ 0.2, W2 ¼ 1, W3 ¼ 0.5, W5 ¼W6 ¼ W7 ¼0.1, S1 ¼ 0.1, S2 ¼ 0.32, l1 ¼7.1, l2 ¼ 2.9, l3 ¼ 8, l4 ¼ 2, l5 ¼ 4.43, l6 ¼5.3, l7 ¼5.8, and l8 ¼ 3.8. All are in mm.)
H.-W. Wu, Y.-F. Chen / Microelectronics Journal 43 (2012) 257–262
261
Table 2 Comparisons with other proposed UWB filters. (LCP: liquid crystal polymer; PCB: printed circuit board; FBW: 3 dB fractional bandwidth; l0 is the free-space wavelength of the center passband frequency).
Substrate height (mm)/er Return loss (dB) Insertion loss (dB) Notch band 1 Center freq. (GHz) Rejection level (dB) FBW (%) Notch band 2 Center freq. (GHz) Rejection level (dB) FBW (%) 3 dB FBW (%) Circuit size (mm2) (l0 l0)
Ref. [6]
Ref. [7]
Ref. [8]
Ref. [9]
Proposed filter
(LCP) 0.8/3.15 25 o 0.66
(PCB) 0.787/2.2 15 o 0.8
(PCB) 1/2.2 20 o 0.5
(PCB) 0.8/2.55 18 o 0.5
(PCB) 1.27/10.2 20 (max.) o 0.5 (min.)
5.47 25.38 3.1
4.3 20 4.2
5.8 15 7.9
5.3 22 6
5.2 17 6
6.05 29.9 3.6 109 406 (0.5 0.4)
8 20 3.8 110 63.72 (0.53 0.06)
8 15 6.4 118 680 (0.78 0.46)
5.9 23 7 110 84.7 0.23 0.19
5.7 15 4 113 98 (0.35 0.14)
filter with Z1 of 80, 95 and 110 O (where Z2 ¼50 O and y1 ¼ 1451 are fixed) and Z2 of 30, 50 and 70 O (where Z1 ¼95 O and y1 ¼1451 are fixed). Based on the description of Fig. 5, the relation between the impedance Z1 and Z2 and positions of transmission zeros at lower and upper stopbands of the filter can be obtained by Eq. (6). At the third step, the relations between the dual-notched bands properties and proposed dual embedded open-circuited stubs would be described as follows. Fig. 7(a) shows the equivalent transmission line model of the dual embedded opencircuited stubs in the proposed filter. It is found that the dual notch band characteristics can be controlled by the dimensions of the dual embedded open-circuited stubs, where the length of l6 and l7 dominated the notched band frequencies and the width of W6 and W7 dominated the notch bandwidths. In Fig. 7(b) and (c), it is clearly observed that with increasing l6 and l7 (correspond to near quarter wavelength at 5.2 and 5.7 GHz, respectively), the notch bands are shifted to lower frequency and with increasing W6 ( ¼W7), the bandwidth of the dual notch bands slightly increases. In this work, we choose l6 ¼5.3 mm (y6 ¼1071), l7 ¼5.8 mm (y7 ¼1171) and W6 ¼W7 of 0.1 mm (Z6 ¼Z7 ¼ O). The 10 dB notch bandwidth of around 6%/4% at 5.2/5.7 GHz can be well achieved. Fig. 8 shows the simulated frequency response of the proposed filter with and without dual embedded opencircuited stubs. Obviously, the dual-notched bands were produced using the dual embedded open-circuited stubs in the proposed filter structure. The physical lengths of the dual embedded open-circuited stubs are defined as around quarter wavelength at 5.2/5.7 GHz, respectively. The two notched-band bandwidths were controlled by tuning the widths of the dual embedded open-circuited stubs. In addition, the transmission zeros were not affected using the dual embedded open-circuited stubs. There are no interactions generated between the dual embedded open-circuited stubs and the stub-loaded rectangular ring structure.
3. Results The proposed UWB filter is fabricated on the RT/Duroid 6010 substrate with a dielectric constant of 10.2, a thickness of 1.27 mm and loss tangent of 0.0023. The size of the fabricated filter is 15.7 6.3 mm2, approximately 0.35l0 0.14l0, where l0 is the free-space wavelength of the center frequency (6.8 GHz). Measured results of the filter are characterized in an HP 8510C network analyzer. Fig. 9(a) and (b) show the photograph, measured results and group delay of the fabricated UWB filter. The
measured results of the filter have 3 dB fractional bandwidth (FBW) of 113%, maximum return loss ( 20 log 9S119) of around 20 dB, minimum insertion loss ( 20 log 9S219) of around 0.5 dB and two transmission zeros located at 3/11 GHz. The dual notch bands at 5.2/5.7 GHz show around 15 dB rejection with FBW of 6%/4%. The measured group delay is also shown in Fig. 9. Table 2 summarized the comparison of the proposed filter with other reported UWB filters [6–9]. Slightly mismatch between the simulated and measured results might be due to the fabrication errors or the variation of material properties. It has been shown that the UWB filter has good performance, including dual notch bands at 5.2/5.7 GHz, good UWB passband selectivity and wide stopband from 11–15 GHz.
4. Conclusion In this paper, a new UWB filter with high passband selectivity and dual notch bands is presented. The dual embedded opencircuited stubs provide dual notch bands at 5.2/5.7 GHz for avoiding the interference from WLAN signals. The stub-loaded rectangular ring structure provides two tunable transmission zeros for enhancing the passband selectivity. There are no interactions between the dual embedded open-circuited stubs and the stub-loaded rectangular ring structure. Therefore, the passband selectivity and notched bands can be well designed individually. Two capacitive-ended interdigital coupled I/O lines are possessed of wide stopband from 11–15 GHz. Both measured and simulated results are in good agreement. The superior features indicate that the proposed UWB filter has a potential to be utilized in modern ultra wideband wireless communication systems.
Acknowledgment This work was supported by the National Science Council of Taiwan under Contract NSC-100–2628-E-168-001-MY2. References [1] Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems, First Note and Order Federal Communications Commission, ET-Docket (2002) p. 98–153. [2] X.H. Wu, Q.X. Chu, X.K. Tian, X. Quyang, UWB Quintuple-mode, Bandpass filter with sharp roll-off and super-wide upper stopband, IEEE Microwave Wireless Comp. Lett. (2011) 661–663. [3] W. Liu, Z. Ma, C.P. Chen, G. Zheng, A novel ultra wideband bandpass filter using microstrip double-ring resonator, IEEE Trans. MTT (2007) 303–305.
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[4] Q.X. Chu, X.K. Tian, Design of UWB bandpass filter using stepped-impedance stub-loaded resonator, IEEE Microwave Wireless Comp. Lett. (2010) 501–503. [5] M.H. Weng, C.T. Liauh, H.W. Wu, S.R. Vargas, An ultra-wideband bandpass filter with an embedded open-circuited stub structure to improve in-band performance, IEEE Microwave Wireless Comp. Lett. (2009) 146–148. [6] Z.C. Hao, J.S. Hong, Ultra-wideband bandpass filter with multiple notch bands using nonuniform periodical slotted ground structure, IEEE Trans. MTT (2009) 3080–3088. [7] K. Song, Q. Xue, Compact ultra-wideband (UWB) bandpass filter with Multiple notch bands, IEEE Microwave Wireless Comp. Lett. (2010) 447–449. [8] F. Wei, Q.Y. Wu, X.W. Shi, L. Chen, Compact UWB bandpass filter with dual notch bands based on SCRLH resonator, IEEE Microwave Wireless Comp. Lett. (2011) 28–30. [9] J.Q. Huang, Q.X. Chu, C.Y. Liu, Compact UWB filter based on surface-coupled structure with dual notched bands, Progress In Electromagnetics Research, 2010, pp. 311–319.
[10] S. Sun, L. Zhu, Capacitive-ended interdigital coupled lines for UWB bandpass filters with improved out-of-band performances, IEEE Microwave Wireless Comp. Lett. (2006) 440–442. [11] D.M. Pozar, Microwave Engineering, NewYork, 2nd ed., Wiley, 1998. [12] G.M. Yang, R. Jin, C. Vittoria, V.G. Harris, N.X. Sun, Small ultra-wideband (UWB) bandpass filter with notch band, IEEE Microwave Wireless Comp. Lett. (2008) 176–178. [13] K.C. Gupta, R. Garg, I. Bahl, P. Bhartia, Microstrip Lines and Slotlines, 2nd ed., Artech House, 1996. [14] IE3D Simulator, Zeland Software, Inc., 2002. [15] J.S. Hong, Microstrip Filters for RF/Microwave Application, 2nd ed., Wiley, New York, 2011. [16] K. Ma, K.S. Yeo, J. Ma, M.A. Do, An ultra-compact hairpin band pass filter with additional zero points, IEEE Microwave Wireless Comp. Lett. (2007) 262–264. [17] K.S. Chin, Y.C. Chiang, J.T. Kuo, Microstrip open-loop resonator with multispurious suppression, IEEE Microwave Wireless Comp. Lett. (2007) 574–576.
Contents lists available at SciVerse ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Ultra wideband bandpass filter with dual-notched bands using stub-loaded rectangular ring multi-mode resonator Hung-Wei Wu n, Yu-Fu Chen Department of Computer and Communication, Kun Shan University, Taiwan
a r t i c l e i n f o
abstract
Article history: Received 23 August 2011 Received in revised form 3 January 2012 Accepted 4 January 2012 Available online 20 January 2012
This paper presents a new ultra wideband (UWB) bandpass filter (BPF) with dual-notched bands (at 5.2/ 5.7 GHz) using the stub-loaded rectangular ring multi-mode resonator (MMR). The proposed resonator consists of the dual embedded open-circuited stubs for introducing the dual notch bands and connected with a stub-loaded rectangular ring structure for controlling the two transmission zeros (at 3/11 GHz) at both sides of the UWB passband edge. This study mainly provides a simple method to design a UWB bandpass filter with high passband selectivity and dual-notched bands for satisfying the Federal Communications Commission (FCC-defined) indoor UWB specification. Experimental verification is provided and good agreement has been found between simulation and measurement. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Dual notch bands Bandpass filter Ultra-wideband (UWB) Multi-mode resonator (MMR) Embedded open-circuited stubs
1. Introduction Ultra wideband (UWB) bandpass filters (BPFs) with high performance are desirable in UWB wireless communication systems (from 3.1 to 10.6 GHz). Recently, the UWB filters with single or multi-notched bands having the capabilities to avoid the interferences from Wireless Local Area Network systems (WLAN, IEEE 802.11a, 5.15–5.825 GHz) [1]. Besides, in order to meet UWB radiation limits, steep passband selectivity at lower and higher passband edges in the UWB filter characteristics become more important. Therefore, the design of a compact, high passband selectivity and multi-notched bands UWB bandpass filter is a great challenge to filter designers. According to the requirements of the UWB filters, the UWB filters with steep passband selectivity and multi-notched bands are desirable. For this purpose, several methods have been developed [2–8]. Wu et al. proposed the quintuple-mode UWB bandpass filter with sharp roll-off and super-wide upper stopband [2]. Two transmission zeros generated by the asymmetrical stepped-impedance stub are used to improve the passband selectivity greatly. Although the aperture-backed structure can raise the coupling degree of the I/O lines, the fabrication procedure would become complexity. Liu et al. proposed the UWB filter using circular-ring MMR for achieving the sharp-rejection at passband edges [3]. Chu et al. proposed the UWB filter using
n
Corresponding author. E-mail addresses: [email protected], [email protected] (H.-W. Wu).
0026-2692/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2012.01.002
stepped-impedance stub-loaded resonator (SISLR) [4]. The SISLR is found to have the advantage of providing more degrees of freedom to adjust the resonant frequencies, however, the stopband edges present low attenuation to increase the possibility of signal error rating. Weng et al. proposed the UWB filter with single notch band (at 5.8 GHz) [5]. The embedded open-circuited stub is desirable to introduce single narrow-bandwidth notched band for avoiding the interferences from WLAN. Hao et al. proposed the UWB filter with multiple notch bands using the multilayer nonuniform periodical structure [6]. The filters with single-, double- and triple-notch bands are implemented on the liquid crystal polymer (LCP) substrate using thin film fabrication process. Song et al. proposed the UWB filter with multiple notch bands using the asymmetric coupling strip [7]. The filter can provide two main paths for the signals, which makes it possible to generate multiple transmission zeroes. Wei et al. proposed the UWB filter with dual notch bands based on a simplified composite right/left-handed (SCRLH) resonator [8]. The SCRLH is to introduce the dual notch bands (at 5.2/5.7 GHz) over the UWB frequency spectrum. Huang et al. proposed an ultra-wideband filter based on surface-coupled structure [9]. Two different quarter-wavelength lines are arranged on the ground of UWB BPF to generate dual narrow stop bands at 5.25/5.775 GHz. In order to avoid the interference from the WLAN signals, the UWB filter with high passband selectivity and multiple notch bands is required. In this paper, we proposed a new UWB filter using the stub-loaded rectangular ring multi-mode resonator for achieving the high passband selectivity and dual-notched bands simultaneously. The filter consists of a rectangular ring MMR with
258
H.-W. Wu, Y.-F. Chen / Microelectronics Journal 43 (2012) 257–262
dual embedded open-circuited stubs for introducing dual notch bands at 5.2/5.7 GHz and connected with a stub-loaded rectangular ring structure for controlling the two transmission zeros to enhance the passband selectivity. Two capacitive-ended interdigital coupled I/O lines provide the stopband of near 15 dB rejection level from 11–15 GHz. The measured results are in good agreement with simulated predictions.
2. Filter design Fig. 1 shows the configuration of the proposed UWB filter. It consists of a rectangular ring multi-mode resonator (MMR) with dual embedded open-circuited stubs for introducing the dual notch bands at 5.2/5.7 GHz and connected with a stub-loaded rectangular ring structure for controlling the two transmission zeros at the passband edges. Two capacitive-ended interdigital coupled I/O lines are possessed of wide stopband performance [10]. The lengths of the I/O lines (l5 and l8) are chosen to be around quarter wavelength at center frequency (f0 ¼6.8 GHz). The stub-loaded rectangular ring structure consists of a rectangular ring (2y1, Z1) and a short transmission line
Fig. 1. Configuration of the proposed UWB filter. (l and W: physical line length/ width, y: electrical length and Z: characteristic impedance).
(y2, Z2) and which is placed at the symmetric plane of the filter. The UWB passband with high selectivity can be well achieved by tuning the dimensions (Z1, y1, Z2 and y2) of the stub-loaded rectangular ring structure. The design procedure is described as follows. At the first step, the wide band characteristic can be understood from the frequency response of the coupled I/O lines [11]. The equivalent transmission line model of the coupled I/O lines can be further converted to the asymmetric coupled line [12], as shown in Fig. 2(a). In order to simplify the investigation, the impedance Z8 and the length of the coupled line are chosen to be 111 O (W5 ¼0.1 mm) and 4.25 mm (quarter-wavelength at 6.8 GHz and y8 ¼ 901), respectively. As referenced in [12], the characteristic modes for an asymmetric coupled line are named as the ‘‘c’’ mode and ‘‘p’’ mode. The image impedance Zi can be derived from [13] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðZ j0e -Z j0o Þ2 -ðZ j0e þ Z j0o Þ2 cos2 y8 Zi ¼ ð1Þ 2sin y8 where index j indicates c- and p-mode and the Z0e and Z0o indicate the characteristic impedance with even- and odd mode for a general coupled line. When Zi is equal to 7jN and Re(Zi) is equal to 0, a bandstop edge is introduced [13]. We then define a normalized bandwidth (NBW) of image impedance by subtracting right bandstop edge and left bandstop edge. It is found that when S1 varies from 0.1, 0.8 to 1.5 mm, corresponding to (Z0e, Z0o) of (168 O, 51 O), (120 O, 92 O) and (108 O, 85 O) for c-mode and (123 O, 37 O), (74 O, 57 O) and (84 O, 66 O) for p-mode, the estimated normalized bandwidth (NBW) of 73%, 24% and 10% can be well achieved, as shown in Fig. 2(b). In a previous work, it is found that the NBW calculated from the image parameter has corresponding relations with the 1.5 dB FBW and 3 dB FBW estimated from the EM simulation [14]. The relation between the NBW and 3 dB FBW are listed in the Table 1. S1 of 0.1 mm is chosen to have the satisfied 3 dB FBW in this study. At the second step, the relations between the passband selectivity and proposed stub-loaded rectangular ring structure would be described as follows. Fig. 3 shows the equivalent transmission line model of the UWB filter. In practical, the embedded open-circuited stubs would not affect the Table 1 Relation between the NBW and 3 dB FBW for the asymmetric coupled lines.
NBW (%) 1.5 dB FBW by EM simulation (%) 3 dB FBW by EM simulation (%) Ratio of (3 dB FBW/NBW)
Fig. 2. (a) Transmission line model and (b) the real part of the image impedance with c and p mode of the asymmetric coupled line under different coupling gap S1.
S1 ¼0.1 mm
S1 ¼0.8 mm
S1 ¼1.5 mm
73 77 113 1.54
24 25 40 1.65
10 13 16.2 1.62
Fig. 3. Equivalent transmission line model of the proposed UWB filter.
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characteristics of the transmission zeros, therefore, the parameters of (y6, Z6) and (y7, Z7) can be ignored in the even- and odd-mode analysis shown in Fig. 4. The model can be divided into two sections along the symmetric plane of the filter, an open circuit for even mode and a short circuit for odd mode [15], as shown in Fig. 4(a) and (b). The even mode input admittance can
Fig. 4. Equivalent transmission line model of (a) even mode and (b) odd mode of the UWB filter.
Fig. 6. Simulated frequency response of the proposed filter with (a) Z1 of 80, 95 and 110 O (Z2 ¼50 O and y1 ¼ 1451 are fixed) and (b) Z2 of 30, 50 and 70 O (Z1 ¼ 95 O and y1 ¼ 1451 are fixed).
Fig. 5. Relations between the input admittance Yine and Yino, length ratios of y2/y1 and normalized transmission zero frequencies (fz1 and fz2) for the proposed UWB filter using the full-wave EM simulation tool. (a) fz1/f0 and (b) fz2/f0 with Z1 of 80, 95 and 110 O (Z2 ¼ 50 O and y1 ¼ 1451 are fixed) and (c) fz1/f0 and (d) fz2/f0 with Z2 of 30, 50 and 70 O (Z1 ¼ 95 O and y1 ¼1451 are fixed). f0 is the center frequency (6.8 GHz) of the proposed filter and fz1 and fz2 indicate the 1st/2nd transmission zero at lower/higher passband edge.
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transmission zeros occurs when S21 ¼0. The S21 parameters can be derived from (2) and (4), and is given by [15]
be calculated as Y ine ¼
Z 5 þZ A tan y8 Z A Z 5 þ jZ 25 tan y8
ð2Þ S21 ¼
where ZA ¼ j
Z3 2Z2 ðZ 1 cot y1 2Z2 tan y2 ÞðZ3 =2Þtanðy9 =2Þð2Z2 þ Z1 cot y1 tan y2 Þ 2 Z23 ð2Z2 þ Z1 cot y1 tan y2 Þ þ 2Z2 ðZ 1 cot y1 2Z2 tan y2 Þtanðy9 =2Þ
y9 ¼ ðy3 þ y4 Þ=2 when Yine ¼0, even mode resonance will occur and the resonance condition is Z5 þ ZA tan y8 ¼ 0
ð3Þ
similarly, the odd mode resonance input admittance can be calculated as Y ino ¼ Z 5
Z 5 ð1=2ÞZ 3 tan y9 tan y8 jðð1=2ÞZ 3 tan y9 þZ 5 tan y8 Þ
ð4Þ
when Yino ¼0, odd mode resonance will occur and the resonance condition is 1 Z5 Z3 tany9 tany8 ¼ 0 2
ð5Þ
Y ino Y 0 -Y ine Y 0 ¼0 ðY 0 þ Y ino ÞðY 0 þ Y ine Þ
ð6Þ
where Y0 is the characteristic admittance. The condition of transmission zero frequencies are obtained when Yino ¼Yine [16,17]. Fig. 5(a)–(d) shows the relations between the input admittance (Yine and Yino), length ratios of y2/y1 and two transmission zero frequencies (fz1 and fz2) normalized by center frequency (f0 ¼6.8 GHz) of the UWB filter under different values of Z1 and Z2. fz1 and fz2 move to lower frequencies as y2/y1 increased from 0.28 to 0.55. When 9fz1 fz29/f0 ( E1.15) at y2/y1 E0.44 (y2 ¼641 and y1 is fixed at 1451) under Z1 of 80, 95 and 110 O, the approximately UWB passband bandwidth can be considered and verified by Yino ¼Yine (Z1 ¼95 O) as marked point A and B shown in Fig. 5(a) and (b). Similarly, when y2/y1 E0.44 under Z2 of 30, 50 and 70 O, the approximately UWB passband bandwidth of around 7.3 GHz can be verified by Yino ¼Yine (Z2 ¼ 50 O) as marked point C and D shown in Fig. 5(c) and (d). The characteristics of the stub-loaded rectangular ring structure can provide the filter with capability to control the passband bandwidth and selectivity over the UWB frequency spectrum. Fig. 6 shows the simulated frequency response of the proposed
Fig. 8. Simulated frequency response of the proposed filter with and without embedded open-circuited stubs.
Fig. 7. (a) Equivalent transmission line model of the dual embedded opencircuited stubs, (b) simulated frequency response of the UWB filter with tuning the physical length l6 and l7 and (c) the width W6 and W7. (W6 ¼W7 in this work).
Fig. 9. (a) Photograph and measured results and (b) group delay of the fabricated UWB filter. (W1 ¼ 0.2, W2 ¼ 1, W3 ¼ 0.5, W5 ¼W6 ¼ W7 ¼0.1, S1 ¼ 0.1, S2 ¼ 0.32, l1 ¼7.1, l2 ¼ 2.9, l3 ¼ 8, l4 ¼ 2, l5 ¼ 4.43, l6 ¼5.3, l7 ¼5.8, and l8 ¼ 3.8. All are in mm.)
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Table 2 Comparisons with other proposed UWB filters. (LCP: liquid crystal polymer; PCB: printed circuit board; FBW: 3 dB fractional bandwidth; l0 is the free-space wavelength of the center passband frequency).
Substrate height (mm)/er Return loss (dB) Insertion loss (dB) Notch band 1 Center freq. (GHz) Rejection level (dB) FBW (%) Notch band 2 Center freq. (GHz) Rejection level (dB) FBW (%) 3 dB FBW (%) Circuit size (mm2) (l0 l0)
Ref. [6]
Ref. [7]
Ref. [8]
Ref. [9]
Proposed filter
(LCP) 0.8/3.15 25 o 0.66
(PCB) 0.787/2.2 15 o 0.8
(PCB) 1/2.2 20 o 0.5
(PCB) 0.8/2.55 18 o 0.5
(PCB) 1.27/10.2 20 (max.) o 0.5 (min.)
5.47 25.38 3.1
4.3 20 4.2
5.8 15 7.9
5.3 22 6
5.2 17 6
6.05 29.9 3.6 109 406 (0.5 0.4)
8 20 3.8 110 63.72 (0.53 0.06)
8 15 6.4 118 680 (0.78 0.46)
5.9 23 7 110 84.7 0.23 0.19
5.7 15 4 113 98 (0.35 0.14)
filter with Z1 of 80, 95 and 110 O (where Z2 ¼50 O and y1 ¼ 1451 are fixed) and Z2 of 30, 50 and 70 O (where Z1 ¼95 O and y1 ¼1451 are fixed). Based on the description of Fig. 5, the relation between the impedance Z1 and Z2 and positions of transmission zeros at lower and upper stopbands of the filter can be obtained by Eq. (6). At the third step, the relations between the dual-notched bands properties and proposed dual embedded open-circuited stubs would be described as follows. Fig. 7(a) shows the equivalent transmission line model of the dual embedded opencircuited stubs in the proposed filter. It is found that the dual notch band characteristics can be controlled by the dimensions of the dual embedded open-circuited stubs, where the length of l6 and l7 dominated the notched band frequencies and the width of W6 and W7 dominated the notch bandwidths. In Fig. 7(b) and (c), it is clearly observed that with increasing l6 and l7 (correspond to near quarter wavelength at 5.2 and 5.7 GHz, respectively), the notch bands are shifted to lower frequency and with increasing W6 ( ¼W7), the bandwidth of the dual notch bands slightly increases. In this work, we choose l6 ¼5.3 mm (y6 ¼1071), l7 ¼5.8 mm (y7 ¼1171) and W6 ¼W7 of 0.1 mm (Z6 ¼Z7 ¼ O). The 10 dB notch bandwidth of around 6%/4% at 5.2/5.7 GHz can be well achieved. Fig. 8 shows the simulated frequency response of the proposed filter with and without dual embedded opencircuited stubs. Obviously, the dual-notched bands were produced using the dual embedded open-circuited stubs in the proposed filter structure. The physical lengths of the dual embedded open-circuited stubs are defined as around quarter wavelength at 5.2/5.7 GHz, respectively. The two notched-band bandwidths were controlled by tuning the widths of the dual embedded open-circuited stubs. In addition, the transmission zeros were not affected using the dual embedded open-circuited stubs. There are no interactions generated between the dual embedded open-circuited stubs and the stub-loaded rectangular ring structure.
3. Results The proposed UWB filter is fabricated on the RT/Duroid 6010 substrate with a dielectric constant of 10.2, a thickness of 1.27 mm and loss tangent of 0.0023. The size of the fabricated filter is 15.7 6.3 mm2, approximately 0.35l0 0.14l0, where l0 is the free-space wavelength of the center frequency (6.8 GHz). Measured results of the filter are characterized in an HP 8510C network analyzer. Fig. 9(a) and (b) show the photograph, measured results and group delay of the fabricated UWB filter. The
measured results of the filter have 3 dB fractional bandwidth (FBW) of 113%, maximum return loss ( 20 log 9S119) of around 20 dB, minimum insertion loss ( 20 log 9S219) of around 0.5 dB and two transmission zeros located at 3/11 GHz. The dual notch bands at 5.2/5.7 GHz show around 15 dB rejection with FBW of 6%/4%. The measured group delay is also shown in Fig. 9. Table 2 summarized the comparison of the proposed filter with other reported UWB filters [6–9]. Slightly mismatch between the simulated and measured results might be due to the fabrication errors or the variation of material properties. It has been shown that the UWB filter has good performance, including dual notch bands at 5.2/5.7 GHz, good UWB passband selectivity and wide stopband from 11–15 GHz.
4. Conclusion In this paper, a new UWB filter with high passband selectivity and dual notch bands is presented. The dual embedded opencircuited stubs provide dual notch bands at 5.2/5.7 GHz for avoiding the interference from WLAN signals. The stub-loaded rectangular ring structure provides two tunable transmission zeros for enhancing the passband selectivity. There are no interactions between the dual embedded open-circuited stubs and the stub-loaded rectangular ring structure. Therefore, the passband selectivity and notched bands can be well designed individually. Two capacitive-ended interdigital coupled I/O lines are possessed of wide stopband from 11–15 GHz. Both measured and simulated results are in good agreement. The superior features indicate that the proposed UWB filter has a potential to be utilized in modern ultra wideband wireless communication systems.
Acknowledgment This work was supported by the National Science Council of Taiwan under Contract NSC-100–2628-E-168-001-MY2. References [1] Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems, First Note and Order Federal Communications Commission, ET-Docket (2002) p. 98–153. [2] X.H. Wu, Q.X. Chu, X.K. Tian, X. Quyang, UWB Quintuple-mode, Bandpass filter with sharp roll-off and super-wide upper stopband, IEEE Microwave Wireless Comp. Lett. (2011) 661–663. [3] W. Liu, Z. Ma, C.P. Chen, G. Zheng, A novel ultra wideband bandpass filter using microstrip double-ring resonator, IEEE Trans. MTT (2007) 303–305.
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