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Design of a narrowband HTS filter at 7.4-GHz with improved upper-stopband performance
Physica C 483 (2012) 5–7
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Design of a narrowband HTS filter at 7.4-GHz with improved upper-stopband performance Q.R. Li a, X.B. Guo a, X.P. Zhang a, B. Wei a, W. Chen a, Y. Zhang a, C. Feng a, Z.J. Yin a, S.C. Jin b, B.S. Cao a,⇑ a b
Department of Physics, Tsinghua University, Beijing 100084, China Space Star Technology Co., Ltd., Beijing 100086, China
a r t i c l e
i n f o
Article history: Received 14 November 2010 Accepted 11 March 2011 Available online 15 June 2012 Keywords: HTS filter Stepped-impedance resonator Open stub Transmission zero
a b s t r a c t This paper presents a high-temperature superconducting filter at 7.42 GHz with an improved upperstopband performance. U-type stepped-impedance resonators are used to relocate the spurious frequency and realize wide stopband response. The compact resonators also reduce the filter size and push away the housing box resonant frequencies. Open stubs are introduced at the midpoints of the resonators to provide additional transmission zeros, which can improve both the band-edge steepness and the outof-band rejection of the upper stopband. As a result, the spurious response of the filter is relocated to above 18 GHz, and the out-of-band rejection is higher than 70 dB. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction High temperature superconducting (HTS) filters with low insertion loss, steep band-edges, and high out-of-band rejection are of great importance for certain wireless communications systems [1–6]. They can improve the sensitivity and significantly reduce the interference of the receivers. In recent years, there has been an increasing interest to create HTS filters at high frequency range [5,6]. With the increase of frequency, the following issues should be addressed in the filter design procedure. Firstly, the resonant frequencies of the housing box may affect the filter performance. The housing dimensions should be as small as possible to move the box modes far from the frequency band of interest, thus improving the upper stopband performance of the filter. Secondly, the microwave surface resistance of HTS films increases rapidly with the growth of frequency. The conventional method to realize steep band-edges and high out-of-band rejection by increasing the pole number of a filter will result in high insertion loss and large filter size. One effective solution to this problem is to generate finite transmission zeros. By tapping open stubs at the midpoint of a resonator, transmission zeros can be produced at desired frequencies [7–10]. A bandpass filter at L-band with two transmission zeros is built up by using two stub-tapped resonators [7,8]. Open stubs are also used in wideband and ultra-wideband filter design to provide transmission zeros and improve stopband response [9,10]. ⇑ Corresponding author. Tel./fax: +86 10 6279 2473. E-mail address: [email protected] (B.S. Cao). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2011.03.011
In this paper, we present a narrowband HTS filter comprising Utype stepped-impedance resonators (SIRs) tapped with open stubs. Compact U-type SIRs are used to push away the spurious response and to reduce the filter size. Two open stubs are tapped at the midpoint of two SIRs to produce two independent transmission zeros and to improve the band-edge steepness and out-of-band rejection of the upper stopband. The design, fabrication, and experiment of the filter are demonstrated in detail. 2. Filter design procedure 2.1. The basic filter design Fig. 1a shows the layout of the four-pole basic filter with a bandwidth of 160 MHz centered at 7.42 GHz. It is designed according to the general procedures of coupled resonator filter [11]. The required coupling matrix and external quality factor are M12 = 0.02331, M23 = 0.01712, M34 = 0.02331, Qe = 33.059. We use a 0.5-mmthick MgO substrate with a dielectric constant of 9.7 in the design procedure. Sonnet EM software is applied to generate the filter layout and to simulate the filter response. As we use compact U-type resonators in the design process, the dimension of the housing box is 24.0 mm 8.0 mm 4.5 mm with a lowest box resonant frequency fh of 18.5 GHz. Moreover, we use SIR structure to relocate the spurious frequency of the filter. The ratio of the first spurious frequency f1 and the fundamental resonance frequency f0 of the SIRs can be adjusted by changing the impedance ratio of the transmission lines [12]. In our design, the SIRs are optimized to have a 0.28-mm-width high-impedance line and a 0.96-mm-width
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Q.R. Li et al. / Physica C 483 (2012) 5–7
Fig. 1. (a) The layout of the four-pole filter with the U-type SIRs. (b) The simulated wideband response of the filter. The inset shows the magnified passband response.
low-impedance line, resulting in a frequency ratio f1/f0 of 2.6. Therefore, a SIR with the first spurious frequency at 19 GHz, which is high than the housing box mode fh, is realized. Fig. 1b shows the simulated wideband response of the filter. The upper stopband of the filter is up to 18.5 GHz, and the out-of-band rejection is higher than 70 dB. A transmission zero is located at the lower stopband and improves the bandedge steepness, which may be produced by the noadjacent couplings between the resonators. However, the bandedge steepness of the upper stopband is unsatisfied.
Fig. 3. (a) The layout of the filter with tapped stubs. (b) The simulated response of the filter with and without stubs.
The parameters of the stubs are optimized to obtain both a high in-band performance and the desired locations of transmission zeros. The simulated frequency response of the filter after optimization is shown in Fig. 3b. Two transmission zeros appear at the upper stopband, improving both the out-of-band rejection and the bandedge steepness. 3. Fabrication and measurement
We use stub-tapped resonators to generate transmission zeros and improve the bandedge steepness of the upper stopband. The proposed stub is shown in Fig. 2. It includes a folded high-impedance line tapped to the SIR and a low-impedance line at the open end. The tapped stub has little influence on the resonance frequency f0 of the SIR, since a virtual ground exists at the midpoint of the resonator. Moreover, it introduces a transmission zero ftz. By varying the parameters of the stub, the frequency of the transmission zero ftz can be easily tuned. As we can see from Fig. 2, when W changes from 1.40 mm to 1.52 mm, ftz is shifted by 290 MHz, while f0 is less affected. Compared with the stubs in [7–10], the proposed structure is more compact and provides more variable parameters. The layout of the four-pole filter with stub-tapped SIRs is shown in Fig. 3a. We add two stubs at the midpoints of the first and the end resonator to produce two independent transmission zeros.
The HTS filter is fabricated on a 24.0 mm 8.0 mm 0.51 mm MgO substrate with double-sided YBCO films, and then packed it in a metal shield box. The fabrication process is carefully carried out because the filter response at high frequency range is sensitive to fabrication errors. The filter is placed onto a cryogenic platform, which is connected to the cooled finger of a Stirling cooler, and enclosed in a vacuum chamber. The frequency response of the filter is measured by an Agilent 5230C network analyzer at 45 K. Calibration is carried out inside the vacuum chamber before cooling. The effect of the I/O cables and connectors inside the vacuum chamber have been compensated by measurements at both the room temperature and the cryogenic one. The measured response of the filter without tuning is shown in Fig. 4. The filter has a center frequency of 7.42 GHz and a narrow bandwidth of 160 MHz. There are one transmission zero at the lower stopband and two transmission zeros at upper stopband. Their positions are in good agreement with the simulations. The experimental insertion loss is 0.3 dB, the return loss is lower than 15 dB, and the out-of-band rejection is higher than 70 dB. Fig. 5
Fig. 2. The simulated response of the stub-tapped resonator for various values of W.
Fig. 4. The simulated and measured responses of the HTS filter. The inset shows the magnified passband insertion loss.
2.2. Filter designed with stub-tapped resonators
Q.R. Li et al. / Physica C 483 (2012) 5–7
7
frequency range. Good agreements between the measured and simulated results are obtained. Acknowledgement This work was supported by the National Natural Science Foundation of China under Grants 60871004 and 60901002. References
Fig. 5. The wideband response of the HTS filter.
shows the wideband response of the filter. The first spurious passband and the box resonant mode are both located at above 18 GHz.
4. Conclusion A narrowband HTS filter at 7.42 GHz is presented. The U-type SIRs are used to reduce the filter size and relocate the spurious response. Two independent transmission zeros are realized by the proposed stub-tapped SIRs. The measured results of the filter exhibit an excellent performance for an HTS device at such high
[1] R.W. Simon, R.B. Hammond, S.J. Berkowitz, B.A. Willemsen, Proc. IEEE 92 (2004) 1585. [2] G.Y. Zhang, F. Huang, M.J. Lancaster, IEEE Trans. Microwave Theory Tech. 53 (2005) 947. [3] J.S. Hong, E.P. McErlean, B.M. Karyamapudi, IEEE Trans. Microwave Theory Tech. 53 (2005) 1976. [4] C. Li, Q. Zhang, Q. Meng, L. Sun, J. Huang, Y. Wang, X. Zhang, A. He, H. Li, Y. He, S. Luo, Supercond. Sci. Technol. 19 (2006) S398. [5] D.C. Niu, T.W. Huang, H.J. Lee, C.Y. Chang, IEEE Trans. Appl. Supercond. 15 (2005) 1008. [6] L. Gao, J. Guo, Y.H. Wang, T. Yu, Q. Zhang, C.G. Li, X.Q. Zhang, H. Li, J.J. Li, W.X. Li, C.Z. Gu, J.B. Meng, J. Feng, Y.S. He, Chinese Sci. Bull. 54 (2009) 3485. [7] J.R. Lee, J.H. Cho, S.W. Yun, IEEE Microwave Guided Wave Lett. 10 (2000) 526. [8] L. Zhu, W. Menzel, IEEE Microwave Wireless Components Lett. 13 (2003) 16. [9] K.X. Ma, K.C.B. Leong, R.M. Jayasuriya, K.S. Yeo, IEEE Microwave Wireless Components Lett. 19 (2009) 24. [10] Q.X. Chu, X.K. Tian, IEEE Microwave Wireless Components Lett. 20 (2010) 501. [11] J.S. Hong, M.J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, 2001. [12] S.C. Jin, B. Wei, X.P. Zhang, B.S. Cao, X.B. Guo, H.L. Peng, L.M. Gao, B.X. Gao, Micro. Opt. Tech. Lett. 49 (2007) 2097.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Design of a narrowband HTS filter at 7.4-GHz with improved upper-stopband performance Q.R. Li a, X.B. Guo a, X.P. Zhang a, B. Wei a, W. Chen a, Y. Zhang a, C. Feng a, Z.J. Yin a, S.C. Jin b, B.S. Cao a,⇑ a b
Department of Physics, Tsinghua University, Beijing 100084, China Space Star Technology Co., Ltd., Beijing 100086, China
a r t i c l e
i n f o
Article history: Received 14 November 2010 Accepted 11 March 2011 Available online 15 June 2012 Keywords: HTS filter Stepped-impedance resonator Open stub Transmission zero
a b s t r a c t This paper presents a high-temperature superconducting filter at 7.42 GHz with an improved upperstopband performance. U-type stepped-impedance resonators are used to relocate the spurious frequency and realize wide stopband response. The compact resonators also reduce the filter size and push away the housing box resonant frequencies. Open stubs are introduced at the midpoints of the resonators to provide additional transmission zeros, which can improve both the band-edge steepness and the outof-band rejection of the upper stopband. As a result, the spurious response of the filter is relocated to above 18 GHz, and the out-of-band rejection is higher than 70 dB. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction High temperature superconducting (HTS) filters with low insertion loss, steep band-edges, and high out-of-band rejection are of great importance for certain wireless communications systems [1–6]. They can improve the sensitivity and significantly reduce the interference of the receivers. In recent years, there has been an increasing interest to create HTS filters at high frequency range [5,6]. With the increase of frequency, the following issues should be addressed in the filter design procedure. Firstly, the resonant frequencies of the housing box may affect the filter performance. The housing dimensions should be as small as possible to move the box modes far from the frequency band of interest, thus improving the upper stopband performance of the filter. Secondly, the microwave surface resistance of HTS films increases rapidly with the growth of frequency. The conventional method to realize steep band-edges and high out-of-band rejection by increasing the pole number of a filter will result in high insertion loss and large filter size. One effective solution to this problem is to generate finite transmission zeros. By tapping open stubs at the midpoint of a resonator, transmission zeros can be produced at desired frequencies [7–10]. A bandpass filter at L-band with two transmission zeros is built up by using two stub-tapped resonators [7,8]. Open stubs are also used in wideband and ultra-wideband filter design to provide transmission zeros and improve stopband response [9,10]. ⇑ Corresponding author. Tel./fax: +86 10 6279 2473. E-mail address: [email protected] (B.S. Cao). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2011.03.011
In this paper, we present a narrowband HTS filter comprising Utype stepped-impedance resonators (SIRs) tapped with open stubs. Compact U-type SIRs are used to push away the spurious response and to reduce the filter size. Two open stubs are tapped at the midpoint of two SIRs to produce two independent transmission zeros and to improve the band-edge steepness and out-of-band rejection of the upper stopband. The design, fabrication, and experiment of the filter are demonstrated in detail. 2. Filter design procedure 2.1. The basic filter design Fig. 1a shows the layout of the four-pole basic filter with a bandwidth of 160 MHz centered at 7.42 GHz. It is designed according to the general procedures of coupled resonator filter [11]. The required coupling matrix and external quality factor are M12 = 0.02331, M23 = 0.01712, M34 = 0.02331, Qe = 33.059. We use a 0.5-mmthick MgO substrate with a dielectric constant of 9.7 in the design procedure. Sonnet EM software is applied to generate the filter layout and to simulate the filter response. As we use compact U-type resonators in the design process, the dimension of the housing box is 24.0 mm 8.0 mm 4.5 mm with a lowest box resonant frequency fh of 18.5 GHz. Moreover, we use SIR structure to relocate the spurious frequency of the filter. The ratio of the first spurious frequency f1 and the fundamental resonance frequency f0 of the SIRs can be adjusted by changing the impedance ratio of the transmission lines [12]. In our design, the SIRs are optimized to have a 0.28-mm-width high-impedance line and a 0.96-mm-width
6
Q.R. Li et al. / Physica C 483 (2012) 5–7
Fig. 1. (a) The layout of the four-pole filter with the U-type SIRs. (b) The simulated wideband response of the filter. The inset shows the magnified passband response.
low-impedance line, resulting in a frequency ratio f1/f0 of 2.6. Therefore, a SIR with the first spurious frequency at 19 GHz, which is high than the housing box mode fh, is realized. Fig. 1b shows the simulated wideband response of the filter. The upper stopband of the filter is up to 18.5 GHz, and the out-of-band rejection is higher than 70 dB. A transmission zero is located at the lower stopband and improves the bandedge steepness, which may be produced by the noadjacent couplings between the resonators. However, the bandedge steepness of the upper stopband is unsatisfied.
Fig. 3. (a) The layout of the filter with tapped stubs. (b) The simulated response of the filter with and without stubs.
The parameters of the stubs are optimized to obtain both a high in-band performance and the desired locations of transmission zeros. The simulated frequency response of the filter after optimization is shown in Fig. 3b. Two transmission zeros appear at the upper stopband, improving both the out-of-band rejection and the bandedge steepness. 3. Fabrication and measurement
We use stub-tapped resonators to generate transmission zeros and improve the bandedge steepness of the upper stopband. The proposed stub is shown in Fig. 2. It includes a folded high-impedance line tapped to the SIR and a low-impedance line at the open end. The tapped stub has little influence on the resonance frequency f0 of the SIR, since a virtual ground exists at the midpoint of the resonator. Moreover, it introduces a transmission zero ftz. By varying the parameters of the stub, the frequency of the transmission zero ftz can be easily tuned. As we can see from Fig. 2, when W changes from 1.40 mm to 1.52 mm, ftz is shifted by 290 MHz, while f0 is less affected. Compared with the stubs in [7–10], the proposed structure is more compact and provides more variable parameters. The layout of the four-pole filter with stub-tapped SIRs is shown in Fig. 3a. We add two stubs at the midpoints of the first and the end resonator to produce two independent transmission zeros.
The HTS filter is fabricated on a 24.0 mm 8.0 mm 0.51 mm MgO substrate with double-sided YBCO films, and then packed it in a metal shield box. The fabrication process is carefully carried out because the filter response at high frequency range is sensitive to fabrication errors. The filter is placed onto a cryogenic platform, which is connected to the cooled finger of a Stirling cooler, and enclosed in a vacuum chamber. The frequency response of the filter is measured by an Agilent 5230C network analyzer at 45 K. Calibration is carried out inside the vacuum chamber before cooling. The effect of the I/O cables and connectors inside the vacuum chamber have been compensated by measurements at both the room temperature and the cryogenic one. The measured response of the filter without tuning is shown in Fig. 4. The filter has a center frequency of 7.42 GHz and a narrow bandwidth of 160 MHz. There are one transmission zero at the lower stopband and two transmission zeros at upper stopband. Their positions are in good agreement with the simulations. The experimental insertion loss is 0.3 dB, the return loss is lower than 15 dB, and the out-of-band rejection is higher than 70 dB. Fig. 5
Fig. 2. The simulated response of the stub-tapped resonator for various values of W.
Fig. 4. The simulated and measured responses of the HTS filter. The inset shows the magnified passband insertion loss.
2.2. Filter designed with stub-tapped resonators
Q.R. Li et al. / Physica C 483 (2012) 5–7
7
frequency range. Good agreements between the measured and simulated results are obtained. Acknowledgement This work was supported by the National Natural Science Foundation of China under Grants 60871004 and 60901002. References
Fig. 5. The wideband response of the HTS filter.
shows the wideband response of the filter. The first spurious passband and the box resonant mode are both located at above 18 GHz.
4. Conclusion A narrowband HTS filter at 7.42 GHz is presented. The U-type SIRs are used to reduce the filter size and relocate the spurious response. Two independent transmission zeros are realized by the proposed stub-tapped SIRs. The measured results of the filter exhibit an excellent performance for an HTS device at such high
[1] R.W. Simon, R.B. Hammond, S.J. Berkowitz, B.A. Willemsen, Proc. IEEE 92 (2004) 1585. [2] G.Y. Zhang, F. Huang, M.J. Lancaster, IEEE Trans. Microwave Theory Tech. 53 (2005) 947. [3] J.S. Hong, E.P. McErlean, B.M. Karyamapudi, IEEE Trans. Microwave Theory Tech. 53 (2005) 1976. [4] C. Li, Q. Zhang, Q. Meng, L. Sun, J. Huang, Y. Wang, X. Zhang, A. He, H. Li, Y. He, S. Luo, Supercond. Sci. Technol. 19 (2006) S398. [5] D.C. Niu, T.W. Huang, H.J. Lee, C.Y. Chang, IEEE Trans. Appl. Supercond. 15 (2005) 1008. [6] L. Gao, J. Guo, Y.H. Wang, T. Yu, Q. Zhang, C.G. Li, X.Q. Zhang, H. Li, J.J. Li, W.X. Li, C.Z. Gu, J.B. Meng, J. Feng, Y.S. He, Chinese Sci. Bull. 54 (2009) 3485. [7] J.R. Lee, J.H. Cho, S.W. Yun, IEEE Microwave Guided Wave Lett. 10 (2000) 526. [8] L. Zhu, W. Menzel, IEEE Microwave Wireless Components Lett. 13 (2003) 16. [9] K.X. Ma, K.C.B. Leong, R.M. Jayasuriya, K.S. Yeo, IEEE Microwave Wireless Components Lett. 19 (2009) 24. [10] Q.X. Chu, X.K. Tian, IEEE Microwave Wireless Components Lett. 20 (2010) 501. [11] J.S. Hong, M.J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, 2001. [12] S.C. Jin, B. Wei, X.P. Zhang, B.S. Cao, X.B. Guo, H.L. Peng, L.M. Gao, B.X. Gao, Micro. Opt. Tech. Lett. 49 (2007) 2097.