Tunable dual‐mode microstrip patch resonators and filters
2013; Institution of Engineering and Technology; Volume: 7; Issue: 6 Linguagem: Inglês
10.1049/iet-map.2012.0398
ISSN1751-8733
AutoresHui Tang, Jian‐Xin Chen, Li‐Heng Zhou, Zhi‐Hua Bao,
Tópico(s)Advanced Antenna and Metasurface Technologies
ResumoIET Microwaves, Antennas & PropagationVolume 7, Issue 6 p. 408-414 ArticleFree Access Tunable dual-mode microstrip patch resonators and filters Hui Tang, Corresponding Author Hui Tang [email protected] School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this authorJian-Xin Chen, Jian-Xin Chen School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this authorLi-Heng Zhou, Li-Heng Zhou School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this authorZhi-Hua Bao, Zhi-Hua Bao School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this author Hui Tang, Corresponding Author Hui Tang [email protected] School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this authorJian-Xin Chen, Jian-Xin Chen School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this authorLi-Heng Zhou, Li-Heng Zhou School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this authorZhi-Hua Bao, Zhi-Hua Bao School of Electronics and Information, Nantong University, 9 Seyuan Road, Nan Tong 226019, Jiangsu Province, People's Republic of ChinaSearch for more papers by this author First published: 01 April 2013 https://doi.org/10.1049/iet-map.2012.0398Citations: 17AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract In this study, tunable dual-mode microstrip patch resonators are presented and utilised to design microstrip bandpass filters (BPFs). The novelty of the designs lies in that the performance of frequency agility can be realised by using either the signal line or the return path (the ground) of microwave transmission line. For the proposed resonators, the resonant frequencies of two coupled degenerate modes can be tuned using a square-loop slot with loaded tunable capacitors on the ground plane or on the patch. The locations of the capacitors are determined according to the electric field distributions of the two orthogonal modes. The variations of the modes in spectrum are studied. Finally, two types of demonstration compact tunable BPFs are designed and fabricated, and the simulated and measured results show good agreement. 1 Introduction Tunable or reconfigurable microwave devices are in great demand for intelligent RF applications because of their advantages in improving the capability of wireless systems [1-8]. Microwave bandpass filters (BPFs) as essential microwave components are accordingly required to have adjustable frequency response. In the designs of microstrip BPFs, dual-mode resonators with ring, loop and patch structures are more attractive because of their half-reduced number of resonators and compact filter configurations, compared with single-mode resonators [4-13]. The BPFs designed by using dual-mode patch resonators show higher power-handling capability than those using ring or loop resonators [8-13]. Lots of effort has been done for tunable single-mode patch resonators [3] and for dual-mode ring or loop resonators [4-6]. In [7], a dual-mode reconfigurable filter using a patch resonator provides two switchable passbands by using a set of p-i-n diodes in series configuration to connect or isolate the loaded stubs to the patch. However, continuously tunable dual-mode patch BPFs are necessary because they are more suitable to support multiple wireless communication standards. In this paper, two types of continuously tunable microstrip dual-mode square patch resonators are proposed to design tunable BPFs. A capacitor-loaded square-loop slot on the ground plane under the microstrip patch or a slot on the patch is utilised to realise frequency-agility without enlarging the circuit size. According to the electric field distributions of the two orthogonal modes on the ground or on the patch, the locations for loading capacitors are determined. The results of the two demonstrated tunable BPFs show fractional tuning ranges about 20 and 14%, respectively. 2 Patch resonators A microstrip square patch resonator with side length a on a substrate with dielectric constant εr and thickness h can be analysed using a Wheeler's cavity model. The electromagnetic fields in terms of , are given by Hong and Lancaster [8]. The two fundamental modes ( and ) are a pair of the degenerate modes orthogonal to each other, with identical resonant frequency (1)where εeff is the effective dielectric constant. For the square patch, when , there is (2)where λg is the guided wavelength at f. There are two commonly used feeding structures for designing a filter using square patch resonator, collinear feeding and orthogonal feeding [3, 9, 10]. Both of them are single-mode resonators without perturbation. One obvious difference of the two resonators is that in a collinearly-fed patch resonator only fundamental mode can be excited, while in an orthogonally-fed resonator two degenerate modes and work with physical orthogonality, equal amplitude and 90° phase imbalance similarly to the operating principle of a circularly polarised radiating antenna [10]. Therefore in an orthogonally feeding structure, the two degenerate modes can be electrically coupled with each other by applying perturbation elements to generate a dual-mode resonator. The other difference is that the orthogonally fed resonator shows a two-pole transmission behaviour because of the parasitic coupling between the input and output ports [14, 15]. To achieve dual-mode performance, orthogonal feed lines and two cuts are used to perturb the proposed resonator. Weak coupling feed lines are used in the analysis to study the dual-mode mechanism, as shown in Fig. 1a. The microstrip patch resonator displays two split modes simulated by Ansoft HFSS, and the two degenerate modes function as two coupled resonators in the equivalent circuit which is depicted in Fig. 1b, as described in [8]. The maximum electric fields of the two modes appear in pairs along the diagonal plane A–A′ and B–B′ close to the corresponding corners of the patch, respectively, as shown in Fig. 1c. To simplify description, the mode at frequency f1 on A–A′ plane is defined as Mode A, and the mode at frequency f2 on B–B′ plane is Mode B. The resonant frequencies of the two modes can be calculated using (3)where Li indicates the effective inductance of flowing currents and Ci represents the effective capacitance between the patch and the ground plane. Fig 1Open in figure viewerPowerPoint Orthogonally-fed dual-mode patch resonator and the electric fields of the degenerate modes a Dual-mode patch resonator b Equivalent circuit c Electric field distributions The length lc of the cuts can control the splitting of the two degenerate modes and reduce the circuit size, as shown in Table 1. The coupling coefficient between the two modes is given by (4) Table 1. Frequency of each mode with different length of the cuts (a = 30 mm, la = 20 mm) Length of the cuts lc, mm Frequency of Mode A, f1, GHz Frequency of Mode B, f2, GHz Difference between two modes, MHz Coupling coefficient K 3 2.235 2.205 30 0.013513 6 2.150 2.095 55 0.025908 7 2.105 2.005 100 0.048633 10 1.985 1.755 230 0.122531 13 1.845 1.335 510 0.312711 To obtain good frequency-agile performance of the two modes, the locations for loaded capacitors are identified to be in the regions with maximum electric fields of the two modes, that is, the capacitors are loaded in pairs along the diagonal plane A–A′ and B–B′. Since the ground plane provides a return path for the patch resonator, the controlling capacitors can be located either on ground plane or on the patch. Tunable capacitors on the ground plane A square-loop slot under the patch is etched on the ground plane and used to load tunable capacitors in the return path to realise a frequency-agile patch resonator as similar to [3], as shown in Fig. 2. As desired, the maximum electric fields of each mode on the separated ground plane remain along the diagonal plane A–A′ or B–B′, and are close to the corresponding corners of the slot. Accordingly the tunable capacitors with values CA and CB are therefore added along the two diagonal planes (A–A′ and B–B′ planes) on the square-loop slot, connecting the two parts of the ground. The equivalent circuit for a slotted ground-plane structure with loaded capacitors is given in Fig. 2d. In the equivalent circuit, and correspond to the capacitances resulting from the loaded capacitors, and correspond to the effective capacitances of the proposed patch, and represent the effective inductances. The resonant frequencies of the proposed dual-mode resonator become (5)where , . The etched slot on the ground brings additional capacitance and inductance, resulting in (6) (7)where CP denotes the capacitance between the patch and the ground plane, Cf is the fringe capacitance and Cs corresponds to the effective capacitance of the slot including the fringe capacitance Cfs between the edges of the slot and the patch and the capacitance Css across the slot. It is obvious that the slot makes the two resonant frequencies of the proposed structure become lower without loaded tunable capacitors, according to (6) and (7). Fig 2Open in figure viewerPowerPoint Proposed ground-controlled tunable dual-mode resonator a Top layer b Bottom layer c Electric field distributions on the slotted ground plane d Equivalent circuit and the capacitances of the ground-slotted dual-mode resonator without loaded tunable capacitors The capacitance Cs and the equivalent inductance both increase when the size of the slot grows large. So the size of the slot on the ground plane has significant contribution to the performance of the patch resonator without loading capacitors. The greater the size b is, the more obviously it reduces the circuit size, but when it exceeds the side length a of the patch, the size-reduction effect subsides. This is because the fringe capacitance Cfs of the slot decreases when b obtains greater than a. According to (5), the capacitors added on the slot can shift the resonant frequencies while the configuration of the patch is determined. Therefore Mode A and Mode B can be controlled by tunable capacitors CA and CB, respectively. To validate the proposed idea, a ground-controlled tunable dual-mode resonator is designed on FR-4 substrate. Simulated results using HFSS show that the two modes can be tuned independently. In Fig. 3a, Mode A is shifted from 1.52 to 1.255 GHz as CA increases from 0.5 to 3.5 pF while Mode B is almost fixed at 1.175 GHz because CB = 3.5 pF keeps unchanged. Meanwhile, in Fig. 3b Mode B is shifted from 1.375 to 1.175 GHz as CB increases from 0.5 to 3.5 pF while Mode A is almost fixed at 1.52 GHz because CA = 0.5 pF keeps constant. It is obvious that each mode can be tuned separately by changing the corresponding capacitance value. Fig 3Open in figure viewerPowerPoint Simulated S-parameters of the ground-controlled tunable dual-mode resonator with different loading capacitances a Frequency response against CA b Frequency response against CB Tunable capacitors on the patch The similar control mechanism is used on the patch, as shown in Fig. 4. There is a square-loop slot on the patch for loading tunable capacitors. However, the size of the square-loop slot etched on the patch is limited because of the patch size and the length lc of the cuts. After the slot added, the maximum electric fields of each mode still appear along A–A′ or B–B′, near the corresponding corners of the slot, and accordingly tunable capacitors are still loaded along A–A′ and B–B′, respectively, on the slot. The dual-mode resonator with tunable capacitors on the patch has the same equivalent circuit as the ground-controlled one, with different values of all the capacitances and the inductances, and the resonant frequencies of the two modes can also be expressed by (5)–(7). Each mode can be tuned by changing the corresponding capacitance value, as illustrated in Fig. 5. Fig 4Open in figure viewerPowerPoint Proposed patch-controlled tunable dual-mode resonator a Layout of the resonator b Electric field distributions on the slotted patch Fig 5Open in figure viewerPowerPoint Simulated S-parameters of the patch-controlled dual-mode resonator with different loading capacitances a Frequency response against CA b Frequency response against CB 3 Tunable dual-mode BPFs and the results Based on the above investigation, tunable dual-mode BPFs can be realised by utilising the tunable patch resonators. For the proposed BPFs, the coupled feeds need very narrow slots between the microstrip and patch. In order to decrease fabrication uncertainty and insertion loss, the feed lines of the input and output ports of the tunable BPFs are directly connected to the patch. Here, the width and length of the cuts are used not only to split the two degeneration modes, but also to adjust the impedance matching [16]. Ground-controlled tunable BPF Fig. 6 depicts the ground-controlled tunable BPF using a dual-mode patch resonator with tunable capacitors on the ground plane. In addition to a square patch resonator and a square-loop slot on the ground plane under the patch for loading tunable capacitors, there are two cross slots with length l along the two diagonals on the patch resonator to reduce the circuit size of the proposed tunable BPF [10]. Fig 6Open in figure viewerPowerPoint Ground-controlled tunable dual-mode BPF a Top layer b Bottom layer c Photograph and the implementation circuit of CA and CB The demonstrated ground-controlled tunable dual-mode BPF is designed and fabricated on the FR-4 substrate. In Fig. 6, the dimension w = 1.84 mm is the width of 50 Ω microstrip line for input and output ports. Four identical tunable capacitors are employed to realise CA and CB. The implementation circuit of CA or CB consists of a varactor diode (Var) and a direct current (DC) block in series, and a radio frequency choke (RFC) for applying DC biasing voltage Vbias. The Var JDV2S71E is from Toshiba, a 7 pF lumped capacitor C acts as DC block, and a 5 kΩ resistor is chosen for RFC. The implementation circuit is modelled using software ADS from Agilent to achieve the relationship between the capacitance value CA or CB and the biasing voltage Vbias. Figs. 7a and b illustrate the simulated and measured results of the demonstrated ground-controlled tunable dual-mode BPF, and Fig. 7c shows its measured variation of the centre frequency and the bandwidth effect against the biasing voltage Vbias. All the results are measured by E5071C Network Analyser from Agilent. The capacitances CA and CB are tuned simultaneously (CA = CB) from 2.9 to 0.7 pF approximately as the bias voltage Vbias of the varactor diode varies from 2 to 18 V, and then the centre frequency f0 of the passband changes from 1.163 to 1.395 GHz, showing 232 MHz (about 20%) tuning range and about 62% size reduction (a = 30 mm = 0.19λg at 1.163 GHz). The bandwidth varies from 10 to 15% when the center frequency f0 is adjusted, because the length of cuts lc, which affects the mode-splitting, remains unchanged. It should be noted that when Vbias obtains lower, the resistances of the employed varactor diodes become larger, leading to increase of the insertion loss. In the demonstrated tunable dual-mode BPF, the measured insertion loss is less than 3.5 dB within the tunable frequency range. Fig 7Open in figure viewerPowerPoint Simulated and measured results of the ground-controlled tunable BPF a Simulated (dash dot lines) and measured (solid lines) S11 b Simulated (dash dot lines) and measured (solid lines) S21 c Measured centre frequency and bandwidth with different biasing voltages Patch-controlled tunable BPF The patch-controlled tunable BPF is designed using dual-mode resonator with tunable capacitors on the patch, as shown in Fig. 8. Four additional slots with length l along the two diagonals on the patch resonator are still for reducing the circuit size. A varactor diode JDV2S71E (Var) and a lumped capacitor C = 7 pF as DC block in series, and two 5 kΩ-resistors chosen as RFC for applying DC biasing voltage Vbias, are used for implementing CA or CB, and the DC circuit is grounded through a RFC and a via hole in the centre of the patch. Fig 8Open in figure viewerPowerPoint Patch-controlled tunable dual-mode BPF a Circuit schematic b Photograph and implementation circuits of CA and CB The simulated and measured results of the demonstrated patch-control tunable dual-mode BPF are illustrated in Fig. 9. The capacitances CA and CB are tuned simultaneously from 2.9 to 0.6 pF approximately as Vbias varies from 2 to 22 V, and then the centre frequency f0 of the passband changes from 1.499 to 1.71 GHz, showing 211 MHz (about 14%) tuning range and about 52% size reduction (a = 30 mm = 0.24λg at 1.499 GHz). Fig 9Open in figure viewerPowerPoint Simulated and measured results of the patch-controlled BPF a Simulated (dash dot lines) and measured (solid lines) S11 b Simulated (dash dot lines) and measured (solid lines) S21 c Measured center frequency and bandwidth with different biasing voltages The patch-controlled tunable BPF shows higher centre frequency and narrower tuning range than ground-controlled tunable BPF with the same size, as compared in Table 2. This can be attributed to the size restrictions of the square-loop slot and the diagonal slots. Table 2. Comparison between the demonstrated ground-controlled and patch-controlled tunable BPFs Type of BPF Biasing voltage Vbias, V Centre frequency f0, GHz Tuning range, % Side length A = 30 mm ground-controlled 2 → 18 1.163 → 1.395 20 0.19λg at 1.163 GHz patch-controlled 2 → 22 1.499 → 1.71 14 0.24λg at 1.499 GHz 4 Conclusion In this paper, tunable dual-mode patch resonators and novel compact tunable BPFs using the resonators have been presented. The frequency agility has been realised by loading variable capacitors either on the signal line or on the ground (return path) of the transmission line. Square-loop slots on the ground plane and on the patch are, respectively, used and the loading locations of tunable capacitors are determined in the regions where the electric fields reach maximum value. The frequency-tuning principles of the two proposed resonators have been investigated and simulated, showing the modes can be independently and continuously controlled by the loaded capacitors on the ground or on the patch. Two kinds of demonstrated tunable BPFs have been designed and fabricated with compact size. The ground-controlled tunable BPF shows more compact size, wider tuning range and lower insertion loss, benefiting from the larger slot size on the ground plane, while the single-side structure of the patch-controlled tunable BPF decreases the assembly difficulty for applications. 5 Acknowledgment This work was supported by the Program for New Century Excellent Talents in University (NCET-11–0993), by the Natural Science Foundation of Jiangsu Province (BK2010272), by the Six types of Talents Project of Jiangsu Province (2011-DZXX-014), and by the Nantong Application Research Technology Program (K2010052). 6 References 1Abbosh, A.M.: 'Tunable phase shifter employing variable odd-mode impedance of short-section parallel-coupled microstrip lines', IET Microw. 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