Journal of information and communication convergence engineering 2024; 22(3): 173-180
Published online September 30, 2024
https://doi.org/10.56977/jicce.2024.22.3.173
© Korea Institute of Information and Communication Engineering
Correspondence to : Phally Phan (E-mail: jynhlu73@sch.ac.kr)
Department of Information and Convergence Technology, Soonchunhyang University, Asan-si 31538, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
This study proposed an interdigital band-pass filter based on the microstrip transmission line. When designing a conventional structure for an interdigital filter based on the characteristics of the 5th order transmission line, seven resonators are required. By changing the impedance of the resonator adjacent to the feed line, the proposed interdigital band-pass filter was designed to reduce the number of resonators compared to conventional interdigital band-pass filters. Consequently, the resonator order decreased, and return and insertion losses become comparable to that in case of a conventional interdigital filter design. The proposed band-pass filter was designed with a center frequency of 2.75 GHz and a bandwidth of 1.5 GHz. Furthermore, based on various transmission characteristics such as group delay and coupling coefficient, two band-pass filters were designed, compared, and analyzed.
Keywords Coupling Coefficient, Group Delay, Interdigital Band-Pass Filter, Microstrip Substrate
A band-pass filter is used to control the frequency response by transmitting frequencies within the passband and attenuating frequencies. Therefore, it is an essential component of wireless communication systems. Microwave filters are commonly used in transmitters and receivers that operate at higher frequencies. With the advancement of communication system technology, the required specifications for filters have become more stringent. Consequently, there are increased demands for compact size, minimum insertion and return losses, low cost, and high selectivity [1]. Interdigital is a type of planar transmission line topology commonly used in filter synthesis. It provides a relatively good Q-factor with a simple design method. The interdigital structure, wherein transmission line resonators are arranged adjacent to each other with alternating ends that are either short- or open-circuited, is among the most compact filter designs. Coupling is achieved through fringing fields between adjacent resonators, which are separated by a small distance. Each line element serves as a resonator that contains an impedance-matching function, and interdigital band-pass filters are commonly used for microstrip implementation [2,3,4]. Interdigital band-pass filters exhibit several unique features, including a very compact structure. As the spacing between the resonator elements is relatively large, the fabrication errors can be relatively reduced. The second passband lies at 3 times the midfrequency of the first passband. The filter can be fabricated in structures that are self-supporting, thereby eliminating the need for dielectric material. Consequently, dielectric loss can be eliminated. Further, several orders of magnitude of attenuation in DC and even multiples of the center frequency of the first-pass band can be used to increase the strength of the stopband and cutoff rates. Interdigital filters comprise an array of parallel lines between two ground planes [5,6,7]. This study proposed a 5-order interdigital band-pass filter with a conventional structure with 7 resonators spaced apart from each other. The first and last resonators were ports, with the first and last resonators as the source and load, respectively. Consequently, the 5 resonators formed a 5-pole. We designed another proposed structure of the 5-order interdigital band-pass filter, where the 5 resonators were spaced apart from each other. The first and last resonators could be reduced in size, thus facilitating their function as both a port and resonator with different impedances [8]. The resonator was connected to the feed lines of the input and output ports to form a 5-pole.
The design theory of interdigital filters involves determining the coupling coefficient between resonators during filter design. After obtaining the coupling matrix value, we calculated and designed the coupling coefficient using the group delay across the gap between resonators and the resonator length through an appropriate simulation.
The SynMatrix program specifications presented in Table 1 produced the coupling matrix as follows:
Table 1 . Specifications of the proposed Interdigital filter
Type | Interdigital Filter |
---|---|
Order | 5 |
Center frequency (GHz) | 2.75 |
Start frequency (GHz) | 2 |
Stop frequency (GHz) | 3.5 |
Bandwidth (GHz) | 1.5 |
Return Loss (dB) | -10 |
Insertion Loss (dB) | -1 |
where M_{S1} represents the 1^{st} column coupling value. denotes the normalized coupling coefficient of each resonance.
The fractional bandwidth (FBW) can be obtained using (1), where (f_{1}) represents the lower frequency, (f_{2}) denotes the higher frequency, and (f_{0}) is the center frequency. The external quality factor ((Q_{e})) is calculated based on the coupling between each resonator
Thereafter, we obtained the de-normalized coupling matrix, which was converted from the normalized matrix by using (1), (2), and (3) [9,10].
where K_{ij} represents the classical coupling coefficient. Table 3 presents the transformation into the K-matrix using the Mmatrix from Table 2 and (3).
Table 2 . Values of the Normalized Matrix
M-matrix | Value |
---|---|
M_{S.1} = M_{L.5} | 1.0137 |
M_{1.2} = M_{4.5} | 0.8653 |
M_{2.3} = M_{3.4} | 0.6357 |
Table 3 . Values of the De-Normalized Matrix
K-matrix | Value |
---|---|
K_{S.1} = K_{L.5} | 1.7165 |
K_{1.2} = K_{4.5} | 0.4906 |
K_{2.3} = K_{3.4} | 0.3604 |
Table 4 presents the specification of the Taconic substrate used for band-pass filter simulation and fabrication.
Table 4 . Substrate Specifications
Substrate | Taconic |
---|---|
Dielectric constant (ε_{r}) | 2.97 |
Dielectric loss tangent (tan δ) | 0.0012 |
Dielectric Thickness [mm] | 0.762 |
Copper Thickness [mm] | 0.035 |
The conventional structure was designed with 7 spaced resonators. The first and last resonators were used as ports as shown in Fig. 1; where, K_{ij} is a coupling efficiency and G is a space between two resonators.
Group delay is a critical parameter for evaluating the performance and applicability of filters in high-speed digital systems. It can also be defined as the rate of change of the transmission phase angle with respect to frequency and is used to measure the time delay across the frequency spectrum of a signal.
To minimize the signal distortion, it is preferable to have a minimum and uniform group delay [1].
The External Q_{e} is extracted through the group delay. The result of the τ_{0} = 0.32 ns at 2.75 GHz center frequency was obtained as
τ_{0}: Group Delay result of S_{11} at the center frequency.
ω_{0} = 2πf_{0} represents the center frequency.
The resonator coupling coefficient is a dimensionless value that characterizes the interaction of two resonators. In the resonator filter theory, the coupling coefficient is used. The resonance frequency and coupling factor combined with the external quality factor are the general filter parameters. Only these generalized parameters are required to be optimized to tune the frequency response of the filter [11].
The coupling coefficient is extracted via two peak frequencies in the S_{21} range, as shown in Fig. 4 and 5.
where K_{12} is the coupling of the first and second resonators, f_{1} is the lower peak frequency, and f_{2} is the upper peak frequency.
Fig. 5 shows the S_{21} magnitude results. The coupling coefficient can be obtained using (6).
The 5-order interdigital band-pass filter of the conventional structure with the EM simulation.
Fig. 6 and Table 5 show the information on the 5-order band-pass filter. A symmetrical structure was observed. Thus, the lengths of L_{1} and L_{7}, L_{2} and L_{6}, and L_{3} and L_{5} were the same, and the gap between the resonators was also symmetric.
Table 5 . Values of all design parameters of the Conventional structure
Parameters | Dimensions (mm) |
---|---|
L_{1}, L_{7} | 18 |
L_{2}, L_{6} | 15.8 |
L_{3}, L_{5} | 16.5 |
L_{4} | 16.6 |
W_{1}, W_{2}, W_{3}, W_{4}, W_{5}, W_{6}, W_{7} | 1 |
S_{12}, S_{67} | 0.2 |
S_{23}, S_{56} | 0.4 |
S_{34}, S_{45} | 0.48 |
The simulation result of the conventional structure of the 5-order interdigital band-pass filter.
Fig. 8 shows the simulation results. The S-parameter is shown as a 5-pole. Further, the maximum return loss was −15.28 dB, and the maximum insertion loss was −1.24 dB.
The proposed structure was the 5-order interdigital bandpass filter, wherein five resonators were spaced apart from each other, as in Fig. 5. The first and last resonators were the result of the connection of lines with different impedances. The resonators were connected to the feed lines of the input and output ports and provided the result of a 5-pole.
The external Q_{e} was extracted by the group delay. In the proposed structure, the first and last resonators could function as a port and resonator with different impedances in case of size reduction. To achieve impedance matching, a stepped-impedance transmission line transformer must be created, as shown in Fig. 13.
The results of the group delay is the τ_{0} = 0.74 ns at 2.75 GHz center frequency were
The coupling coefficients were extracted based on two peak frequencies in the S_{21} magnitude.
The 5-order interdigital band-pass filter of the proposed structure was implemented by emplying an EM simulation. We used the parameters of Z_{S} = 111.18 Ω and Z_{L} =50 Ω, which are the characteristic impedances of a single, N = 1, quarter-wave transformer:
Fig. 17 and Table 6 present the information regarding the 5-order band-pass filter. It is a symmetric structure. Thus, the length of L_{1} and L_{5}, L_{2} and L_{4} were the same, and the gap between the resonators was also symmetrical.
Table 6 . Values of all design parameters of the proposed structure
Parameters | Dimensions (mm) |
---|---|
L_{1}, L_{5} | 17.68 |
L_{2}, L_{3}, L_{4} | 16.8 |
W_{1}, W_{2}, W_{3}, W_{4}, W_{5} | 1 |
L_{1_cut}, L_{5_cut} | 8.225 |
W_{1_cut}, W_{5_cut} | 0.65 |
G_{12}, G_{45} | 0.21 |
G_{23}, G_{34} | 0.3 |
structure of the 5-order interdigital band-pass filter. The simulation results indicated a maximum return loss of −13.8 dB and a maximum insertion loss of −0.52 dB.
The fabrication results of the proposed 5-order interdigital band-pass filter structure are shown in Fig. 21. The bandwidth of the fabricated filter ranged as 2.04-3.41 GHz, which was narrower than that of the simulation. Within the bandwidth of the fabricated filter, the minimum insertion loss was −1.36 dB and the maximum return loss was −8.29 dB
The comparisons between the simulation result and the measurement result are presented.
Fig. 21 shows the comparison results between the simulation and the measurement result. The bandwidth of the manufactured band-pass filter ranged as 2.19-3.45 GHz, which was 0.24 GHz lower than the simulation. At 2.35 GHz, the return and insertion losses were −5.87 and −1.47 dB, respectively, which were different from that of the simulation; however, this showed that it functioned as a band-pass filter within the bandwidth.
Fig. 22 shows the results of the comparison between the simulation and measurement results. The bandwidth of the manufactured band-pass filter ranged as 2.04-3.44 GHz, which was 0.1 GHz lower than the simulation. At 2.23 GHz, the return and insertion losses were −8.33 and −1.21 dB, respectively, which were different from that of the simulation; however, it indicated that within the bandwidth, it functioned as a band-pass filter.
The solid line in Fig. 23 is the result of the conventional structure, and the dotted line is the result of the proposed structure.
The two types of filters indicated errors in fabrication and were not the same as the simulation results. However, the filter functioned as a band-pass filter within the targeted bandwidth.
The proposed filter had a bandwidth that was 0.14 GHz wider than the conventional filter. Further, the minimum return and maximum insertion losses were improved by 2.46 and 0.26 dB.
This study designed a conventional structure 5-order interdigital band-pass filter, where 7 resonators were spaced apart. The first and last resonators were ports. The result of this 5-port structure was a maximum return loss of −15.28 dB and an insertion loss of −1.24 dB at 1.5 GHz bandwidth at 2.75 GHz center frequency.
In the proposed structure of a 5-order interdigital bandpass filter, wherein 5 resonators are spaced apart from each other, the sizes of the first and last resonators were reduced such that they could function as both ports and resonators with different impedances. The results of this structure also made the 5-pole yield a maximum return loss of −13.8 dB, and an insertion loss of −0.52 dB, and a bandwidth of 1.5 GHz at the center frequency of 2.75 GHz.
This research was supported by the Ministry of Science and ICT (MSIT), Korea, under the ICT Challenge and Advanced Network of HRD (ICAN) program (IITP-2024-2020-0-01832) supervised by the Institute of Information & Communications Technology Planning & Evaluation (IITP). Additional support was received from the Soonchunhyang University Research Fund.
Phally Phan
received her B.S. degree in telecommunication and electronic engineering from the Royal University of Phnom Penh in 2021. She is pursuing her M.S. degree in ICT convergence from the Soonchunhyang University, Asan, South Korea. She has been working as Research student at the RF and Microwave Components Research Center (RAMREC), Soonchunhyang University, Korea. She is currently working at the Ministry of posts and Telecommunications in Cambodia.
Donghoon Kang
received his B.S. degree in electrical engineering from the Soonchunhyang University, Asan, Korea in 2023. He is pursuing his M. S. degree in electrical communication and system engineering from the Soonchunhyang university since 2023.
Dal Ahn
received his B.S., M.S., and Ph.D. degrees from the Sogang University, Seoul, Korea, in 1984, 1986, and 1990, respectively, all in electronics. From 1990 to 1992, he was with the Mobile Communications Division, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Korea. Since 1992, he has been with the School of Electrical and Electronic Engineering, Soonchunhyang University, Asan, Chungnam, Korea, where he is currently a professor. He is also currently the Chief of the RF and Microwave Component Research Center (RAMREC), Soonchunhyang University. He is also a technical consultant for Tel Wave Inc., Suwon, Korea. His current research interests include the design and application of passive and active components at radio and microwave frequencies, design of the RF front-end module for various handset system using low-temperature co-fired ceramic (LTCC) technology, DGS circuit applications, and circuit modeling using a commercial EM analysis program. He is an Editor for the Journal of Korea Electromagnetic Engineering Society. Prof. Ahn is a senior member of the Korea Electromagnetic Engineering Society (KEES).
Youna Jang
received her M.S. degree in electronics and computer engineering from the Hanyang University, Seoul, Korea in 2014. She obtained her Ph.D. in electrical communication system engineering from Soonchunhyang University, Choongnam, Korea in 2019. She participated as a Designer of Duplexer filter in an internship program in 2015 at Qorvo, Bundang, South Korea. She was a lecturer at the Soonchunhyang University from 2016 to 2023. She is currently a research professor of Radio and Mechatronics Research Center. Her research areas include the design of passive components in the microwave band.
Journal of information and communication convergence engineering 2024; 22(3): 173-180
Published online September 30, 2024 https://doi.org/10.56977/jicce.2024.22.3.173
Copyright © Korea Institute of Information and Communication Engineering.
Phally Phan ^{1*}, Donghoon Kang ^{2}, Dal Ahn ^{3}, and Youna Jang^{4 }
^{1}Department of Information and Convergence Technology, Soonchunhyang University, Asan-si 31538, Republic of Korea
^{2}Department of Information and Convergence Technology, Soonchunhyang University, Asan-si 31538, Republic of Korea
^{3}Department of Electrical Engineering, Soonchunhyang University, Asan-si 31538, Republic of Korea
^{4}Department of Information and Convergence Technology, Soonchunhyang University, Asan-si 31538, Republic of Korea
Correspondence to:Phally Phan (E-mail: jynhlu73@sch.ac.kr)
Department of Information and Convergence Technology, Soonchunhyang University, Asan-si 31538, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
This study proposed an interdigital band-pass filter based on the microstrip transmission line. When designing a conventional structure for an interdigital filter based on the characteristics of the 5th order transmission line, seven resonators are required. By changing the impedance of the resonator adjacent to the feed line, the proposed interdigital band-pass filter was designed to reduce the number of resonators compared to conventional interdigital band-pass filters. Consequently, the resonator order decreased, and return and insertion losses become comparable to that in case of a conventional interdigital filter design. The proposed band-pass filter was designed with a center frequency of 2.75 GHz and a bandwidth of 1.5 GHz. Furthermore, based on various transmission characteristics such as group delay and coupling coefficient, two band-pass filters were designed, compared, and analyzed.
Keywords: Coupling Coefficient, Group Delay, Interdigital Band-Pass Filter, Microstrip Substrate
A band-pass filter is used to control the frequency response by transmitting frequencies within the passband and attenuating frequencies. Therefore, it is an essential component of wireless communication systems. Microwave filters are commonly used in transmitters and receivers that operate at higher frequencies. With the advancement of communication system technology, the required specifications for filters have become more stringent. Consequently, there are increased demands for compact size, minimum insertion and return losses, low cost, and high selectivity [1]. Interdigital is a type of planar transmission line topology commonly used in filter synthesis. It provides a relatively good Q-factor with a simple design method. The interdigital structure, wherein transmission line resonators are arranged adjacent to each other with alternating ends that are either short- or open-circuited, is among the most compact filter designs. Coupling is achieved through fringing fields between adjacent resonators, which are separated by a small distance. Each line element serves as a resonator that contains an impedance-matching function, and interdigital band-pass filters are commonly used for microstrip implementation [2,3,4]. Interdigital band-pass filters exhibit several unique features, including a very compact structure. As the spacing between the resonator elements is relatively large, the fabrication errors can be relatively reduced. The second passband lies at 3 times the midfrequency of the first passband. The filter can be fabricated in structures that are self-supporting, thereby eliminating the need for dielectric material. Consequently, dielectric loss can be eliminated. Further, several orders of magnitude of attenuation in DC and even multiples of the center frequency of the first-pass band can be used to increase the strength of the stopband and cutoff rates. Interdigital filters comprise an array of parallel lines between two ground planes [5,6,7]. This study proposed a 5-order interdigital band-pass filter with a conventional structure with 7 resonators spaced apart from each other. The first and last resonators were ports, with the first and last resonators as the source and load, respectively. Consequently, the 5 resonators formed a 5-pole. We designed another proposed structure of the 5-order interdigital band-pass filter, where the 5 resonators were spaced apart from each other. The first and last resonators could be reduced in size, thus facilitating their function as both a port and resonator with different impedances [8]. The resonator was connected to the feed lines of the input and output ports to form a 5-pole.
The design theory of interdigital filters involves determining the coupling coefficient between resonators during filter design. After obtaining the coupling matrix value, we calculated and designed the coupling coefficient using the group delay across the gap between resonators and the resonator length through an appropriate simulation.
The SynMatrix program specifications presented in Table 1 produced the coupling matrix as follows:
Table 1 . Specifications of the proposed Interdigital filter.
Type | Interdigital Filter |
---|---|
Order | 5 |
Center frequency (GHz) | 2.75 |
Start frequency (GHz) | 2 |
Stop frequency (GHz) | 3.5 |
Bandwidth (GHz) | 1.5 |
Return Loss (dB) | -10 |
Insertion Loss (dB) | -1 |
where M_{S1} represents the 1^{st} column coupling value. denotes the normalized coupling coefficient of each resonance.
The fractional bandwidth (FBW) can be obtained using (1), where (f_{1}) represents the lower frequency, (f_{2}) denotes the higher frequency, and (f_{0}) is the center frequency. The external quality factor ((Q_{e})) is calculated based on the coupling between each resonator
Thereafter, we obtained the de-normalized coupling matrix, which was converted from the normalized matrix by using (1), (2), and (3) [9,10].
where K_{ij} represents the classical coupling coefficient. Table 3 presents the transformation into the K-matrix using the Mmatrix from Table 2 and (3).
Table 2 . Values of the Normalized Matrix.
M-matrix | Value |
---|---|
M_{S.1} = M_{L.5} | 1.0137 |
M_{1.2} = M_{4.5} | 0.8653 |
M_{2.3} = M_{3.4} | 0.6357 |
Table 3 . Values of the De-Normalized Matrix.
K-matrix | Value |
---|---|
K_{S.1} = K_{L.5} | 1.7165 |
K_{1.2} = K_{4.5} | 0.4906 |
K_{2.3} = K_{3.4} | 0.3604 |
Table 4 presents the specification of the Taconic substrate used for band-pass filter simulation and fabrication.
Table 4 . Substrate Specifications.
Substrate | Taconic |
---|---|
Dielectric constant (ε_{r}) | 2.97 |
Dielectric loss tangent (tan δ) | 0.0012 |
Dielectric Thickness [mm] | 0.762 |
Copper Thickness [mm] | 0.035 |
The conventional structure was designed with 7 spaced resonators. The first and last resonators were used as ports as shown in Fig. 1; where, K_{ij} is a coupling efficiency and G is a space between two resonators.
Group delay is a critical parameter for evaluating the performance and applicability of filters in high-speed digital systems. It can also be defined as the rate of change of the transmission phase angle with respect to frequency and is used to measure the time delay across the frequency spectrum of a signal.
To minimize the signal distortion, it is preferable to have a minimum and uniform group delay [1].
The External Q_{e} is extracted through the group delay. The result of the τ_{0} = 0.32 ns at 2.75 GHz center frequency was obtained as
τ_{0}: Group Delay result of S_{11} at the center frequency.
ω_{0} = 2πf_{0} represents the center frequency.
The resonator coupling coefficient is a dimensionless value that characterizes the interaction of two resonators. In the resonator filter theory, the coupling coefficient is used. The resonance frequency and coupling factor combined with the external quality factor are the general filter parameters. Only these generalized parameters are required to be optimized to tune the frequency response of the filter [11].
The coupling coefficient is extracted via two peak frequencies in the S_{21} range, as shown in Fig. 4 and 5.
where K_{12} is the coupling of the first and second resonators, f_{1} is the lower peak frequency, and f_{2} is the upper peak frequency.
Fig. 5 shows the S_{21} magnitude results. The coupling coefficient can be obtained using (6).
The 5-order interdigital band-pass filter of the conventional structure with the EM simulation.
Fig. 6 and Table 5 show the information on the 5-order band-pass filter. A symmetrical structure was observed. Thus, the lengths of L_{1} and L_{7}, L_{2} and L_{6}, and L_{3} and L_{5} were the same, and the gap between the resonators was also symmetric.
Table 5 . Values of all design parameters of the Conventional structure.
Parameters | Dimensions (mm) |
---|---|
L_{1}, L_{7} | 18 |
L_{2}, L_{6} | 15.8 |
L_{3}, L_{5} | 16.5 |
L_{4} | 16.6 |
W_{1}, W_{2}, W_{3}, W_{4}, W_{5}, W_{6}, W_{7} | 1 |
S_{12}, S_{67} | 0.2 |
S_{23}, S_{56} | 0.4 |
S_{34}, S_{45} | 0.48 |
The simulation result of the conventional structure of the 5-order interdigital band-pass filter.
Fig. 8 shows the simulation results. The S-parameter is shown as a 5-pole. Further, the maximum return loss was −15.28 dB, and the maximum insertion loss was −1.24 dB.
The proposed structure was the 5-order interdigital bandpass filter, wherein five resonators were spaced apart from each other, as in Fig. 5. The first and last resonators were the result of the connection of lines with different impedances. The resonators were connected to the feed lines of the input and output ports and provided the result of a 5-pole.
The external Q_{e} was extracted by the group delay. In the proposed structure, the first and last resonators could function as a port and resonator with different impedances in case of size reduction. To achieve impedance matching, a stepped-impedance transmission line transformer must be created, as shown in Fig. 13.
The results of the group delay is the τ_{0} = 0.74 ns at 2.75 GHz center frequency were
The coupling coefficients were extracted based on two peak frequencies in the S_{21} magnitude.
The 5-order interdigital band-pass filter of the proposed structure was implemented by emplying an EM simulation. We used the parameters of Z_{S} = 111.18 Ω and Z_{L} =50 Ω, which are the characteristic impedances of a single, N = 1, quarter-wave transformer:
Fig. 17 and Table 6 present the information regarding the 5-order band-pass filter. It is a symmetric structure. Thus, the length of L_{1} and L_{5}, L_{2} and L_{4} were the same, and the gap between the resonators was also symmetrical.
Table 6 . Values of all design parameters of the proposed structure.
Parameters | Dimensions (mm) |
---|---|
L_{1}, L_{5} | 17.68 |
L_{2}, L_{3}, L_{4} | 16.8 |
W_{1}, W_{2}, W_{3}, W_{4}, W_{5} | 1 |
L_{1_cut}, L_{5_cut} | 8.225 |
W_{1_cut}, W_{5_cut} | 0.65 |
G_{12}, G_{45} | 0.21 |
G_{23}, G_{34} | 0.3 |
structure of the 5-order interdigital band-pass filter. The simulation results indicated a maximum return loss of −13.8 dB and a maximum insertion loss of −0.52 dB.
The fabrication results of the proposed 5-order interdigital band-pass filter structure are shown in Fig. 21. The bandwidth of the fabricated filter ranged as 2.04-3.41 GHz, which was narrower than that of the simulation. Within the bandwidth of the fabricated filter, the minimum insertion loss was −1.36 dB and the maximum return loss was −8.29 dB
The comparisons between the simulation result and the measurement result are presented.
Fig. 21 shows the comparison results between the simulation and the measurement result. The bandwidth of the manufactured band-pass filter ranged as 2.19-3.45 GHz, which was 0.24 GHz lower than the simulation. At 2.35 GHz, the return and insertion losses were −5.87 and −1.47 dB, respectively, which were different from that of the simulation; however, this showed that it functioned as a band-pass filter within the bandwidth.
Fig. 22 shows the results of the comparison between the simulation and measurement results. The bandwidth of the manufactured band-pass filter ranged as 2.04-3.44 GHz, which was 0.1 GHz lower than the simulation. At 2.23 GHz, the return and insertion losses were −8.33 and −1.21 dB, respectively, which were different from that of the simulation; however, it indicated that within the bandwidth, it functioned as a band-pass filter.
The solid line in Fig. 23 is the result of the conventional structure, and the dotted line is the result of the proposed structure.
The two types of filters indicated errors in fabrication and were not the same as the simulation results. However, the filter functioned as a band-pass filter within the targeted bandwidth.
The proposed filter had a bandwidth that was 0.14 GHz wider than the conventional filter. Further, the minimum return and maximum insertion losses were improved by 2.46 and 0.26 dB.
This study designed a conventional structure 5-order interdigital band-pass filter, where 7 resonators were spaced apart. The first and last resonators were ports. The result of this 5-port structure was a maximum return loss of −15.28 dB and an insertion loss of −1.24 dB at 1.5 GHz bandwidth at 2.75 GHz center frequency.
In the proposed structure of a 5-order interdigital bandpass filter, wherein 5 resonators are spaced apart from each other, the sizes of the first and last resonators were reduced such that they could function as both ports and resonators with different impedances. The results of this structure also made the 5-pole yield a maximum return loss of −13.8 dB, and an insertion loss of −0.52 dB, and a bandwidth of 1.5 GHz at the center frequency of 2.75 GHz.
This research was supported by the Ministry of Science and ICT (MSIT), Korea, under the ICT Challenge and Advanced Network of HRD (ICAN) program (IITP-2024-2020-0-01832) supervised by the Institute of Information & Communications Technology Planning & Evaluation (IITP). Additional support was received from the Soonchunhyang University Research Fund.
Table 1 . Specifications of the proposed Interdigital filter.
Type | Interdigital Filter |
---|---|
Order | 5 |
Center frequency (GHz) | 2.75 |
Start frequency (GHz) | 2 |
Stop frequency (GHz) | 3.5 |
Bandwidth (GHz) | 1.5 |
Return Loss (dB) | -10 |
Insertion Loss (dB) | -1 |
Table 2 . Values of the Normalized Matrix.
M-matrix | Value |
---|---|
M_{S.1} = M_{L.5} | 1.0137 |
M_{1.2} = M_{4.5} | 0.8653 |
M_{2.3} = M_{3.4} | 0.6357 |
Table 3 . Values of the De-Normalized Matrix.
K-matrix | Value |
---|---|
K_{S.1} = K_{L.5} | 1.7165 |
K_{1.2} = K_{4.5} | 0.4906 |
K_{2.3} = K_{3.4} | 0.3604 |
Table 4 . Substrate Specifications.
Substrate | Taconic |
---|---|
Dielectric constant (ε_{r}) | 2.97 |
Dielectric loss tangent (tan δ) | 0.0012 |
Dielectric Thickness [mm] | 0.762 |
Copper Thickness [mm] | 0.035 |
Table 5 . Values of all design parameters of the Conventional structure.
Parameters | Dimensions (mm) |
---|---|
L_{1}, L_{7} | 18 |
L_{2}, L_{6} | 15.8 |
L_{3}, L_{5} | 16.5 |
L_{4} | 16.6 |
W_{1}, W_{2}, W_{3}, W_{4}, W_{5}, W_{6}, W_{7} | 1 |
S_{12}, S_{67} | 0.2 |
S_{23}, S_{56} | 0.4 |
S_{34}, S_{45} | 0.48 |
Table 6 . Values of all design parameters of the proposed structure.
Parameters | Dimensions (mm) |
---|---|
L_{1}, L_{5} | 17.68 |
L_{2}, L_{3}, L_{4} | 16.8 |
W_{1}, W_{2}, W_{3}, W_{4}, W_{5} | 1 |
L_{1_cut}, L_{5_cut} | 8.225 |
W_{1_cut}, W_{5_cut} | 0.65 |
G_{12}, G_{45} | 0.21 |
G_{23}, G_{34} | 0.3 |