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Center frequency and bandwidth tunable HTS filter
Physica C 471 (2011) 1224–1227
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Center frequency and bandwidth tunable HTS filter H. Harada a, N. Sekiya a,⇑, S. Kakio a, S. Ohshima b a b
Department of Electrical Engineering, Yamanashi University, Kakio-Sekiya Laboratory, 4-3-11 Takeda, Kofu 400-8511, Japan Yamagata University, 4-3-16 Johnan, Yonezawa 992-8510, Japan
a r t i c l e
i n f o
Article history: Available online 14 May 2011 Keywords: Microwave devices HTS filter Bandwidth Center frequency Tunable Waveguide and trimming
a b s t r a c t We have developed a bandwidth and a center frequency tuning method for use in high-temperature superconducting microstrip filters. Several p-shaped waveguides are placed between the resonators, and the bandwidth is adjusted by changing the switch states of the waveguides. Additional electrical pads are placed open ends of the resonators for tuning the center frequency. Pads are also placed around the input/output coupled-line elements to enable the coupling strength between the coupled-line and resonator to be adjusted, thereby reducing the insertion loss caused by tuning. A prototype three-pole tunable bandpass filter was designed and analyzed using an electromagnetic simulator based on the moment method. The filter was designed at a center frequency of 5.00 GHz and a bandwidth of 150 MHz. The simulated bandwidth and center frequency of the filter were tuned from 150 to 300 MHz and 4.5–5 GHz without degradation of the insertion loss, respectively. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction High temperature superconducting (HTS) microwave filters have low loss and sharp skirt characteristics, making them well suited for the receiving system of mobile telecommunication base stations. HTS filters are also potentially suitable for systems that require wide-range frequency and bandwidth tunability, such as cognitive radio. Thus, interest in tunable bandpass HTS filters, which may be able to meet this requirement, has been increasing [1–11]. Center frequency tuning and bandwidth tuning have been widely researched, and various methods have been devised for increasing the range of tunable filters. However, there have been few reports on both the center frequency and bandwidth tuning of these filters. Both tuning of these filters are difficult to achieve without a consequent increase in the insertion loss due to variations in the external quality factors from the design parameters. We previously developed a method for adjusting the external quality factors by using additional electrical pads around the feed lines for a discrete bandwidth and center frequency tunable microstrip bandpass filter [8,9]. We also developed a method for adjusting the coupling coefficients by using two types of waveguides placed between the resonators for tuning a bandwidth [10,11]. Moreover, we developed a technique for tuning the center frequency of HTS microstrip filters that uses several additional electrical pads [9].
⇑ Corresponding author. Tel.: +81 55 220 8393; fax: +81 55 220 8514. E-mail address: [email protected] (N. Sekiya). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.165
We have now demonstrated a center frequency and a bandwidth tunable HTS filter that does not suffer a significant increase in insertion loss. It use p-shaped waveguides for adjusting the coupling coefficients for bandwidth tuning, additional electrical pads placed open ends of the resonators for adjusting the effective length of the resonators for center frequency tuning and additional electric pads placed around the feed lines for adjusting the external quality factors. 2. Design 2.1. Tuning method and trimming method The configuration of the designed tunable filter is shown in Fig. 1. The filter consists of a three-pole superconducting microstrip combine filter with a center frequency of 5 GHz. The electric pads placed at the open ends of the resonators are used for center frequency tuning. Those placed around the input/output (I/O) coupled-line elements are used for trimming the external quality factors and p-shaped waveguides placed at the end of the spaces between resonators are used for bandwidth tuning. The electrical junctions are assumed to be wire bondings or micro electro mechanical system switches. With our center frequency tuning technique, electric pads are placed at the open ends of the resonators. The lengths of the resonators strongly affect the resonant frequency. As shown in Fig. 1, connecting these pads and resonators in series electrically enables the effective lengths of the resonators to be adjusted, resulting in center frequency tuning.
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H. Harada et al. / Physica C 471 (2011) 1224–1227
Fig. 1. Layout of center frequency and bandwidth tunable filter.
We previously proposed the basic concept of a bandwidth tunable HTS microstrip filter [10,11]. The filter is composed of resonators and p-shaped waveguides placed at the end of the spaces between resonators. The bandwidth is tuned by changing the switch states of the waveguides, which changes the coupling coefficients. The waveguides are connected or isolated: when the waveguides are on-state, the bandwidth is narrow; when they are off-state, the bandwidth is wide. Trimming pads are also placed around the I/O coupled-line elements. Obtaining wide-range tunability with no degradation requires adjustment of the external quality factors so that they do not vary from the design parameters after center frequency and/ or bandwidth tuning. We previously showed that using these pads around the elements is useful for trimming the external quality factors and thereby improving the filter characteristics after center frequency and bandwidth tuning [8,9]. As shown in Fig. 1, connecting these pads and coupled-line elements in series electrically enables the external quality factors to be adjusted, resulting in reduced insertion loss.
Table 1 Filter design parameters. Parameter
Value
Center frequency (GHz) Bandwidth (MHz) Pass-band ripple (dB) Substrate Dielectric constant of substrate Thickness of substrate (mm)
5.00, 4.75, 4.50 150, 300 0.1 MgO 9.9 0.5
2.2. Filter We designed and analyzed a three-pole center frequency and bandwidth tunable HTS microstrip filter by using 2.5-dimensional high-frequency electromagnetic (EM) analysis software (S-NAP Field), which is based on the moment method. We assumed MgO was used for the substrate and that the substrate had a dielectric constant of 9.9 and a thickness of 0.5 mm. The conductivity of the superconducting layer was assumed to be that of a zero-thickness perfect conductor. Table 1 lists the design parameters. The design bandwidth was set to 150 MHz with waveguides in the onstate and 300 MHz in the off-state. Moreover, the design center frequency was set to 5.00 GHz without pad, 4.75 GHz with a pad connected to resonators and 4.50 GHz with two pads connected to ones. Fig. 2 shows the simulated return loss (S11) and insertion loss (S21) before filter tuning at a center frequency of 5.00 GHz and a
Fig. 2. Simulated frequency response of return loss (S11) and insertion loss (S21) before tuning at 5.00 GHz.
bandwidth of 150 MHz. The pads and waveguides had no evident effect and good filter properties of the frequency response were obtained.
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Fig. 3. Simulated frequency response after center frequency tuning (A) without and (B) with trimming.
3. Results First, we discuss the center frequency tuning. Fig. 3A shows the simulated frequency response before and after center frequency tuning without trimming. ‘‘Filter A’’ means the initial state. ‘‘Filter B’’ means a pad and resonators were connected in series and waveguides were in the on-state. ‘‘Filter C’’ means two pads and resonators were connected in series and waveguides were in the on-state. Increasing the number of pads shifted the center frequency about 500 MHz, but it also increased the insertion loss due to the external quality factors varying from the design parameter. The electric pads around the I/O coupled-line elements were used to trim the external quality factors, which mitigated this increase. The simulated frequency response with trimming after center frequency tuning is shown Fig. 3B. ‘‘Filter Btrim’’ means after trimming 2Filter B using only one upper electric pad. ‘‘Filter Ctrim’’ means after trimming Filter C using two upper and one lower electric pads. As
shown in Fig. 3B, use of the pads mitigated the insertion loss, which remained below 0.13 dB across the entire band. Next, we discuss both the center frequency and the bandwidth tuning. Fig. 4A shows the simulated frequency response after center frequency and bandwidth tuning without trimming across the entire band. ‘‘Filter A2’’ means waveguides were in the off-state in Filter A. ‘‘Filter B2’’ means waveguides were in the off-state in Filter B. ‘‘Filter C2’’ means waveguides were in the off-state in Filter C. The bandwidth was tuned from 150 to 300 MHz at 4.75, 5.00 GHz. And the bandwidth was tuned from 150 to 260 MHz at 4.50 GHz. However, the insertion loss in the passband increased due to variations in the external quality factors from the design parameters. Therefore, it is necessary to trim the filter characteristics by adjusting the external quality factors to reduce degradation of the bandpass characteristics after tuning. The simulated frequency response with trimming after center frequency and bandwidth tuning is shown Fig. 4B, and the filter states are
Fig. 4. Simulated frequency response after bandwidth and center frequency tuning (A) without and (B) with trimming.
H. Harada et al. / Physica C 471 (2011) 1224–1227 Table 2 States of tuning and trimming pads.
Filter Filter Filter Filter Filter Filter Filter Filter Filter Filter Filter
A B Btrim C Ctrim A2 A2trim B2 B2trim C2 C2trim
Electric pads for center frequency tuning
Waveguide for bandwidth tuning
Electric pads for trimming (upper, lower)
0 1 1 2 2 0 0 1 1 2 2
ON ON ON ON ON OFF OFF OFF OFF OFF OFF
(0, (0, (1, (0, (2, (0, (4, (0, (5, (0, (5,
0) 0) 0) 0) 1) 0) 1) 0) 2) 0) 3)
1227
means after trimming Filter B2 using five upper and two lower electric pads. ‘‘Filter C2trim’’ means after trimming Filter C2 using five upper and three lower electric pads. Fig. 5 shows magnified view of frequency response of insertion loss after tuning without and with trimming at 4.5 GHz. As shown Fig. 5, the passband ripple was reduced from 3.4 to 0.13 dB. Therefore, use of the pads mitigated the insertion loss, which remained below 0.15 dB across the entire band. 4. Conclusion We designed a center frequency and a bandwidth tunable HTS filter. The filter uses waveguides for tuning the bandwidth, additional electric pads placed open ends of the resonators for tuning center frequency and additional electric pads placed around the feed lines for adjusting the external quality factors. In simulation testing, both center frequency and bandwidth tuning was achieved without degradation of the insertion loss. Acknowledgment This work was supported in part by CASIO Science Promotion Foundation and Research Foundation for the Electrotechnology of Chubu. References
Fig. 5. Simulated frequency response of insertion loss after tuning without and with trimming (magnified view).
summarized in Table 2 ‘‘Filter A2trim’’ means after trimming Filter A2 using four upper and one lower electric pads. ‘‘Filter B2trim’’
[1] A. Trotel, M. Pyee, B. Lavigne, D. Chambonnet, P. Lederer, Appl. Phys. Lett. 68 (1996) 2559. [2] S. Hontsu, T. Sakatani, A. Fujimaki, H. Nishikawa, M. Nakamori, T. Kawai, Jpn. J. Appl. Phys. 40 (2001) 1148. [3] Y. Terashima, H. Fuke, F. Aiga, M. Yamazaki, H. Kayano, R. Kato, Physica C 366 (2002) 183. [4] Eric M. Propher, J. Musolf, B.F. Zuck, S. Jimenez, K.E. Kihlstrom, B.A. Willemsen, IEEE Trans. Appl. Supercond. 15 (2005) 956. [5] S. Pal, C. Stevens, D. Edwards, Supercond. Sci. Technol. 18 (2005) 927. [6] C.Y. Tan, C.K. Ong, Supercond. Sci. Technol. 19 (2006) 212. [7] F. Aita, N. Sekiya, S. Ono, A. Saito, S. Hirano, S. Ohshima, IEICE Trans. Electron. E89-C (2006) 119. [8] N. Sekiya, Y. Nakagawa, A. Saito, S. Ohshima, Physica C 468 (2008) 1958. [9] N. Sekiya, Y. Nakagawa, A. Saito, S. Ohshima, Physica C 469 (2009) 1653. [10] N. Sekiya, H. Harada, Y. Nakagawa, S. Ohshima, IEICE Tech. Rep. SCE2010-7 MW2010-7 (2010-4) 3 (in Japanese). [11] N. Sekiya, H. Harada, Y. Nakagawa, S. Ono, S. Ohshima, Physica C 470 (2010) 1499.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Center frequency and bandwidth tunable HTS filter H. Harada a, N. Sekiya a,⇑, S. Kakio a, S. Ohshima b a b
Department of Electrical Engineering, Yamanashi University, Kakio-Sekiya Laboratory, 4-3-11 Takeda, Kofu 400-8511, Japan Yamagata University, 4-3-16 Johnan, Yonezawa 992-8510, Japan
a r t i c l e
i n f o
Article history: Available online 14 May 2011 Keywords: Microwave devices HTS filter Bandwidth Center frequency Tunable Waveguide and trimming
a b s t r a c t We have developed a bandwidth and a center frequency tuning method for use in high-temperature superconducting microstrip filters. Several p-shaped waveguides are placed between the resonators, and the bandwidth is adjusted by changing the switch states of the waveguides. Additional electrical pads are placed open ends of the resonators for tuning the center frequency. Pads are also placed around the input/output coupled-line elements to enable the coupling strength between the coupled-line and resonator to be adjusted, thereby reducing the insertion loss caused by tuning. A prototype three-pole tunable bandpass filter was designed and analyzed using an electromagnetic simulator based on the moment method. The filter was designed at a center frequency of 5.00 GHz and a bandwidth of 150 MHz. The simulated bandwidth and center frequency of the filter were tuned from 150 to 300 MHz and 4.5–5 GHz without degradation of the insertion loss, respectively. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction High temperature superconducting (HTS) microwave filters have low loss and sharp skirt characteristics, making them well suited for the receiving system of mobile telecommunication base stations. HTS filters are also potentially suitable for systems that require wide-range frequency and bandwidth tunability, such as cognitive radio. Thus, interest in tunable bandpass HTS filters, which may be able to meet this requirement, has been increasing [1–11]. Center frequency tuning and bandwidth tuning have been widely researched, and various methods have been devised for increasing the range of tunable filters. However, there have been few reports on both the center frequency and bandwidth tuning of these filters. Both tuning of these filters are difficult to achieve without a consequent increase in the insertion loss due to variations in the external quality factors from the design parameters. We previously developed a method for adjusting the external quality factors by using additional electrical pads around the feed lines for a discrete bandwidth and center frequency tunable microstrip bandpass filter [8,9]. We also developed a method for adjusting the coupling coefficients by using two types of waveguides placed between the resonators for tuning a bandwidth [10,11]. Moreover, we developed a technique for tuning the center frequency of HTS microstrip filters that uses several additional electrical pads [9].
⇑ Corresponding author. Tel.: +81 55 220 8393; fax: +81 55 220 8514. E-mail address: [email protected] (N. Sekiya). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.165
We have now demonstrated a center frequency and a bandwidth tunable HTS filter that does not suffer a significant increase in insertion loss. It use p-shaped waveguides for adjusting the coupling coefficients for bandwidth tuning, additional electrical pads placed open ends of the resonators for adjusting the effective length of the resonators for center frequency tuning and additional electric pads placed around the feed lines for adjusting the external quality factors. 2. Design 2.1. Tuning method and trimming method The configuration of the designed tunable filter is shown in Fig. 1. The filter consists of a three-pole superconducting microstrip combine filter with a center frequency of 5 GHz. The electric pads placed at the open ends of the resonators are used for center frequency tuning. Those placed around the input/output (I/O) coupled-line elements are used for trimming the external quality factors and p-shaped waveguides placed at the end of the spaces between resonators are used for bandwidth tuning. The electrical junctions are assumed to be wire bondings or micro electro mechanical system switches. With our center frequency tuning technique, electric pads are placed at the open ends of the resonators. The lengths of the resonators strongly affect the resonant frequency. As shown in Fig. 1, connecting these pads and resonators in series electrically enables the effective lengths of the resonators to be adjusted, resulting in center frequency tuning.
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H. Harada et al. / Physica C 471 (2011) 1224–1227
Fig. 1. Layout of center frequency and bandwidth tunable filter.
We previously proposed the basic concept of a bandwidth tunable HTS microstrip filter [10,11]. The filter is composed of resonators and p-shaped waveguides placed at the end of the spaces between resonators. The bandwidth is tuned by changing the switch states of the waveguides, which changes the coupling coefficients. The waveguides are connected or isolated: when the waveguides are on-state, the bandwidth is narrow; when they are off-state, the bandwidth is wide. Trimming pads are also placed around the I/O coupled-line elements. Obtaining wide-range tunability with no degradation requires adjustment of the external quality factors so that they do not vary from the design parameters after center frequency and/ or bandwidth tuning. We previously showed that using these pads around the elements is useful for trimming the external quality factors and thereby improving the filter characteristics after center frequency and bandwidth tuning [8,9]. As shown in Fig. 1, connecting these pads and coupled-line elements in series electrically enables the external quality factors to be adjusted, resulting in reduced insertion loss.
Table 1 Filter design parameters. Parameter
Value
Center frequency (GHz) Bandwidth (MHz) Pass-band ripple (dB) Substrate Dielectric constant of substrate Thickness of substrate (mm)
5.00, 4.75, 4.50 150, 300 0.1 MgO 9.9 0.5
2.2. Filter We designed and analyzed a three-pole center frequency and bandwidth tunable HTS microstrip filter by using 2.5-dimensional high-frequency electromagnetic (EM) analysis software (S-NAP Field), which is based on the moment method. We assumed MgO was used for the substrate and that the substrate had a dielectric constant of 9.9 and a thickness of 0.5 mm. The conductivity of the superconducting layer was assumed to be that of a zero-thickness perfect conductor. Table 1 lists the design parameters. The design bandwidth was set to 150 MHz with waveguides in the onstate and 300 MHz in the off-state. Moreover, the design center frequency was set to 5.00 GHz without pad, 4.75 GHz with a pad connected to resonators and 4.50 GHz with two pads connected to ones. Fig. 2 shows the simulated return loss (S11) and insertion loss (S21) before filter tuning at a center frequency of 5.00 GHz and a
Fig. 2. Simulated frequency response of return loss (S11) and insertion loss (S21) before tuning at 5.00 GHz.
bandwidth of 150 MHz. The pads and waveguides had no evident effect and good filter properties of the frequency response were obtained.
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H. Harada et al. / Physica C 471 (2011) 1224–1227
Fig. 3. Simulated frequency response after center frequency tuning (A) without and (B) with trimming.
3. Results First, we discuss the center frequency tuning. Fig. 3A shows the simulated frequency response before and after center frequency tuning without trimming. ‘‘Filter A’’ means the initial state. ‘‘Filter B’’ means a pad and resonators were connected in series and waveguides were in the on-state. ‘‘Filter C’’ means two pads and resonators were connected in series and waveguides were in the on-state. Increasing the number of pads shifted the center frequency about 500 MHz, but it also increased the insertion loss due to the external quality factors varying from the design parameter. The electric pads around the I/O coupled-line elements were used to trim the external quality factors, which mitigated this increase. The simulated frequency response with trimming after center frequency tuning is shown Fig. 3B. ‘‘Filter Btrim’’ means after trimming 2Filter B using only one upper electric pad. ‘‘Filter Ctrim’’ means after trimming Filter C using two upper and one lower electric pads. As
shown in Fig. 3B, use of the pads mitigated the insertion loss, which remained below 0.13 dB across the entire band. Next, we discuss both the center frequency and the bandwidth tuning. Fig. 4A shows the simulated frequency response after center frequency and bandwidth tuning without trimming across the entire band. ‘‘Filter A2’’ means waveguides were in the off-state in Filter A. ‘‘Filter B2’’ means waveguides were in the off-state in Filter B. ‘‘Filter C2’’ means waveguides were in the off-state in Filter C. The bandwidth was tuned from 150 to 300 MHz at 4.75, 5.00 GHz. And the bandwidth was tuned from 150 to 260 MHz at 4.50 GHz. However, the insertion loss in the passband increased due to variations in the external quality factors from the design parameters. Therefore, it is necessary to trim the filter characteristics by adjusting the external quality factors to reduce degradation of the bandpass characteristics after tuning. The simulated frequency response with trimming after center frequency and bandwidth tuning is shown Fig. 4B, and the filter states are
Fig. 4. Simulated frequency response after bandwidth and center frequency tuning (A) without and (B) with trimming.
H. Harada et al. / Physica C 471 (2011) 1224–1227 Table 2 States of tuning and trimming pads.
Filter Filter Filter Filter Filter Filter Filter Filter Filter Filter Filter
A B Btrim C Ctrim A2 A2trim B2 B2trim C2 C2trim
Electric pads for center frequency tuning
Waveguide for bandwidth tuning
Electric pads for trimming (upper, lower)
0 1 1 2 2 0 0 1 1 2 2
ON ON ON ON ON OFF OFF OFF OFF OFF OFF
(0, (0, (1, (0, (2, (0, (4, (0, (5, (0, (5,
0) 0) 0) 0) 1) 0) 1) 0) 2) 0) 3)
1227
means after trimming Filter B2 using five upper and two lower electric pads. ‘‘Filter C2trim’’ means after trimming Filter C2 using five upper and three lower electric pads. Fig. 5 shows magnified view of frequency response of insertion loss after tuning without and with trimming at 4.5 GHz. As shown Fig. 5, the passband ripple was reduced from 3.4 to 0.13 dB. Therefore, use of the pads mitigated the insertion loss, which remained below 0.15 dB across the entire band. 4. Conclusion We designed a center frequency and a bandwidth tunable HTS filter. The filter uses waveguides for tuning the bandwidth, additional electric pads placed open ends of the resonators for tuning center frequency and additional electric pads placed around the feed lines for adjusting the external quality factors. In simulation testing, both center frequency and bandwidth tuning was achieved without degradation of the insertion loss. Acknowledgment This work was supported in part by CASIO Science Promotion Foundation and Research Foundation for the Electrotechnology of Chubu. References
Fig. 5. Simulated frequency response of insertion loss after tuning without and with trimming (magnified view).
summarized in Table 2 ‘‘Filter A2trim’’ means after trimming Filter A2 using four upper and one lower electric pads. ‘‘Filter B2trim’’
[1] A. Trotel, M. Pyee, B. Lavigne, D. Chambonnet, P. Lederer, Appl. Phys. Lett. 68 (1996) 2559. [2] S. Hontsu, T. Sakatani, A. Fujimaki, H. Nishikawa, M. Nakamori, T. Kawai, Jpn. J. Appl. Phys. 40 (2001) 1148. [3] Y. Terashima, H. Fuke, F. Aiga, M. Yamazaki, H. Kayano, R. Kato, Physica C 366 (2002) 183. [4] Eric M. Propher, J. Musolf, B.F. Zuck, S. Jimenez, K.E. Kihlstrom, B.A. Willemsen, IEEE Trans. Appl. Supercond. 15 (2005) 956. [5] S. Pal, C. Stevens, D. Edwards, Supercond. Sci. Technol. 18 (2005) 927. [6] C.Y. Tan, C.K. Ong, Supercond. Sci. Technol. 19 (2006) 212. [7] F. Aita, N. Sekiya, S. Ono, A. Saito, S. Hirano, S. Ohshima, IEICE Trans. Electron. E89-C (2006) 119. [8] N. Sekiya, Y. Nakagawa, A. Saito, S. Ohshima, Physica C 468 (2008) 1958. [9] N. Sekiya, Y. Nakagawa, A. Saito, S. Ohshima, Physica C 469 (2009) 1653. [10] N. Sekiya, H. Harada, Y. Nakagawa, S. Ohshima, IEICE Tech. Rep. SCE2010-7 MW2010-7 (2010-4) 3 (in Japanese). [11] N. Sekiya, H. Harada, Y. Nakagawa, S. Ono, S. Ohshima, Physica C 470 (2010) 1499.