Balanced quad‐band diplexer with wide common‐mode suppression and high differential‐mode isolation
2016; Institution of Engineering and Technology; Volume: 10; Issue: 6 Linguagem: Inglês
10.1049/iet-map.2015.0538
ISSN1751-8733
AutoresChing‐Her Lee, Chung‐I. G. Hsu, S.-W. Wu, Pi‐Hung Wen,
Tópico(s)Radio Frequency Integrated Circuit Design
ResumoIET Microwaves, Antennas & PropagationVolume 10, Issue 6 p. 599-603 Research ArticleFree Access Balanced quad-band diplexer with wide common-mode suppression and high differential-mode isolation Ching-Her Lee, Corresponding Author Ching-Her Lee iecher@cc.ncue.edu.tw Department of Electronic Engineering, Graduate Institute of Communications Engineering, National Changhua University of Education, Changhua, 500 TaiwanSearch for more papers by this authorChung-I Gavin Hsu, Chung-I Gavin Hsu Department of Electrical Engineering, National Yunlin University of Science and Technology, Yunlin, 640 TaiwanSearch for more papers by this authorShang-Xing Wu, Shang-Xing Wu Department of Electronic Engineering, Graduate Institute of Communications Engineering, National Changhua University of Education, Changhua, 500 TaiwanSearch for more papers by this authorPi-Hung Wen, Pi-Hung Wen Department of Electronic Engineering, Graduate Institute of Communications Engineering, National Changhua University of Education, Changhua, 500 TaiwanSearch for more papers by this author Ching-Her Lee, Corresponding Author Ching-Her Lee iecher@cc.ncue.edu.tw Department of Electronic Engineering, Graduate Institute of Communications Engineering, National Changhua University of Education, Changhua, 500 TaiwanSearch for more papers by this authorChung-I Gavin Hsu, Chung-I Gavin Hsu Department of Electrical Engineering, National Yunlin University of Science and Technology, Yunlin, 640 TaiwanSearch for more papers by this authorShang-Xing Wu, Shang-Xing Wu Department of Electronic Engineering, Graduate Institute of Communications Engineering, National Changhua University of Education, Changhua, 500 TaiwanSearch for more papers by this authorPi-Hung Wen, Pi-Hung Wen Department of Electronic Engineering, Graduate Institute of Communications Engineering, National Changhua University of Education, Changhua, 500 TaiwanSearch for more papers by this author First published: 01 April 2016 https://doi.org/10.1049/iet-map.2015.0538Citations: 16AboutSectionsPDF 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 A balanced quad-band diplexer is proposed with wide common-mode (CM) suppression and high differential-mode (DM) isolation in this study. The six-port balanced diplexer was formed by combining two dual-band bandpass filters operating at 1.92/5.25 and 2.45/5.8 GHz through two stepped-impedance resonator (SIR) inserted T-junctions. By carefully designing the section length of the T-junction and the SIR length from its open-end to the joint-point with the T-junction, high isolation between diplexer channels can be attained. A prototype of the proposed balanced quad-band diplexer was implemented and measured with a DM isolation of higher than 51.8 dB and a CM suppression of larger than 47.9 dB in the four DM passbands. Good agreement was observed between the simulated and the measured results. 1 Introduction Compact diplexers/multiplexers with low cost and high isolation are highly demanded in modern multi-band and multi-service wireless communication systems. Most diplexers that have been published in recent years are of single-ended configuration and constructed by combining two bandpass filters (BPFs) having distinct frequencies with an impedance-matching network such as a T-junction [1, 2] or a common dual-mode resonator [3-5]. For diplexers consisting of BPFs and a T-junction, the performance depends highly on the channel filters as well as the T-junction. The latter works as a frequency selector and its two line sections serve as impedance transformers. By carefully adjusting the lengths of the two T-junction arms, the extended input port of each connected filter can become nearly open-circuited at the passband centre frequency of the other to result in high channel isolation. For the diplexers constructed using a dual-mode resonator as the input impedance-matching network, the operating frequencies of the two diplexer channels can be designed very close to each other. In addition, since the common resonator at the input port can provide one more transmission pole for each channel, the number of resonators needed in a diplexer with the same passband responses can be reduced [5]. Compared with diplexers of the single-ended type, research works of balanced-type diplexers are relatively less seen in the literature [6-9]. In [6], the proposed balanced diplexer was deployed in a clever manner such that the feed-line coupled to one BPF will produce a transmission zero by the passband of the other BPF and vice versa, thus resulting in good isolation and high common-mode (CM) suppression. In [7], a balanced diplexer and a balun diplexer are designed using balanced BPFs which are composed of only two stepped-impedance slot-line resonators. In [8], two three-port balun filters (having one single-ended port and one balanced port) were combined through a T-junction to form a high-isolation five-port balun diplexer. In [9], two balun filters were combined to form not only a five-port balun diplexer but also a four-port one, both with high differential-mode (DM) isolation. The balanced diplexers mentioned in the last section are all of dual-band ones, in which each physical channel supports only one operating band. Though several single-ended quad-band diplexers (namely, two passbands for each channel) have been proposed in the literature [10-12], balanced quad-band diplexers have not been seen so far, to the best of the authors’ knowledge. In this paper, we will be devoted to the design of a new balanced quad-band diplexer that is intended for multi-service applications at the wireless local area network and global system of mobile bands. For that purpose, two microstrip balanced dual-band BPFs will first be constructed. Then the balanced quad-band diplexer with good CM suppression and high isolation can be formed by combining the two balanced filters with SIR-inserted T-junctions. 2 Balanced quad-band diplexer design 2.1 Schematic analysis of the balanced quad-band diplexer The schematic diagram of the proposed balanced quad-band diplexer is shown in Fig. 1. It consists of two balanced dual-band BPFs and two SIR-inserted T-junctions. For constructing such a diplexer circuit, the two dual-band BPFs are first designed, with the operating bands separately centred at f1/f3 and f2/f4 (where f1/f3 = 1.92/5.25 and f2/f4 = 2.45/5.8 GHz for this paper). Then the two balanced BPFs are combined through two T-junctions to form the balanced quad-band diplexer. Since the two BPFs may cause loading effect to each other when combined together, it is important that the input port of each channel of the diplexer should exhibit open-circuited characteristic at the operating frequencies of the opposite channel. Fig. 1Open in figure viewerPowerPoint Schematic diagram of the proposed balanced quad-band diplexer To alleviate the mutual loading effect between the two diplexer channels, SIR-inserted T-junctions [13] are adopted in this design, as shown in Fig. 1. To illustrate how the tap-connected SIRs and the T-junctions work, we depict in Fig. 2 the DM half circuit of SIR 1 and part of the T-junction. In DM operation, due to the virtually grounded symmetry plane, the half SIR 1 works as a quarter-wavelength type resonator, with its first two resonant frequencies designed at f1 and f3 to allow for signal transmission of Channel 1. From the resonance properties of terminated transmission lines [14], ZU + ZD = 0 and at f1 and f3, with ZU and ZD being the input impedance looking into either side of SIR 1 at junction A, regardless of where the position of A is. Hence, the parallel impedances of ZU and ZD become infinity at those two frequencies. This provides a through pass to the transmission-line section of the T-junction at the centre frequencies (namely, f1 and f3) of the Channel 1 passbands. Fig. 2Open in figure viewerPowerPoint DM half circuit of SIR 1 and part of the T-junction On the other hand, since a balanced quad-band diplexer is of interest in this design, the input port of Channel 1 should exhibit zero admittance at f2 and f4, the two passband centre frequencies of Channel 2. In other words, we should have ΓC1(f2) = ΓC1(f4) = 1, where ΓC1(f2) and ΓC1(f4) are the reflection coefficients looking into the input port of Channel 1 from junction C. In what follows we will explain how this can be achieved. For simplicity, assume that |ΓB1(f2)| = |ΓB1(f4)| = 1 at the input of BPF 1 (i.e. point B1), which is a reasonable assumption if BPF 1 is well designed and if f2 and f4 are not too close to f1 and f3. In addition, assume that losses in all transmission-line sections are so small that they can be neglected. Owing to the latter assumption, we have |ΓU(f)| = |ΓD(f)| = 1 at all frequencies. With θ1, θ3, and ΓU (or ΓD) adjustable, we can seek two simultaneous objectives: ΓC1(f2) = 1 and ΓC1(f4) = 1. This system of two equations with three unknowns can be solved using a standard root-searching procedure. However, we will adopt a different approach in which the two objectives are attained in two steps [13]. First, let fa be one of the two frequencies f2 and f4 (and fb be the other) such that, after adjusting the position of junction A1 in Fig. 2, ΓU(fa) = −1 or ΓD(fa) = −1. The objective ΓC1(fa) = 1 can then be achieved with θ1(fa) set to be π/2 (or equivalently, with θ1 line length set to be a quarter-wavelength at fa). Second, with θ1 and ΓU fixed at fb, the remaining objective ΓC1(fb) = 1 can be attained by properly adjusting θ3 in a very straightforward manner. The above-outlined approach can also be applied to the design of Channel 2 to construct the desired combining network of the proposed balanced quad-band diplexer. In particular, in the design of Channel 1 for not affecting Channel 2 performance, we have chosen fa = f2, while in that of Channel 2 for not loading Channel 1, fa = f3. 2.2 Design of the balanced dual-band BPFs Fig. 3 shows the configuration of the proposed third-order balanced dual-band BPF working at the 2.45/5.8 GHz bands that is to be used in this diplexer design. The BPF consists of four bi-section microstrip SIRs. For convenience, refer to the outer two SIRs as SIRs A and the inner two as SIRs B as denoted in Fig. 3. Note that SIRs A are placed across the plane of symmetry (POS), whereas SIRs B are not and are instead symmetrically deployed on the two opposite sides of the POS. The purpose for such an arrangement is to prevent the even-mode resonant signal excited in SIRs A from being coupled to SIRs B, as will be explained below. Fig. 3Open in figure viewerPowerPoint Configuration of the proposed balanced dual-band BPF For DM operation, the POS can be replaced with an electrical wall which corresponds to a short-circuited-to-ground boundary condition; for CM operation, the POS can be replaced with a magnetic wall which corresponds to an open-circuited boundary condition. The original complete circuit can then be investigated by analysing its corresponding two simpler half circuits in DM and CM operations. In DM operation, due to the virtually grounded POS, SIRs A can support only the odd-distributed resonant modes because SIRs A are deployed across the POS. On the contrary, all the modes of SIRs B can be used to transmit DM signals because SIRs B are not positioned across the POS. Hence, the first two odd-mode resonant frequencies ( and ) of SIRs A should be designed the same as the first two of SIRs B ( and ). For simplicity, we assume θs1 = θs2 and θs3 = θs4. For the design of the balanced dual-band BPF with its two DM passbands centred at 2.45 and 5.8 GHz (i.e. and GHz), the impedance ratios can be calculated as KA = Z2/Z1 = 1.82 for SIRs A and KB = Z4/Z3 = 0.61 for SIRs B [15]. Moreover, the required electric lengths are θs1 = θs2 = 54° and θs3 = θs4 = 37° at 2.45 GHz. Considering the precision limit of the fabrication process, the widths of the Z2 and Z3 sections are chosen to be 0.4 mm, and thus those of the Z1 and Z4 sections can be estimated to be 1.18 and 1.1 mm, respectively, by using Agilent Advanced Design Software (ADS). With the structural dimensions of SIRs A and B determined, the corresponding resonant frequencies are shown up to 12.5 GHz in Fig. 4. Note that these frequencies are calculated without taking into account the dispersion properties and the discontinuities in the microstrip lines. As can be seen in Fig. 4, the third odd-mode resonant frequency of SIRs A occurs at GHz, which deviates far away enough from (= 9.15 GHz) and (=11.6 GHz) of SIRs B. One can thus expect that the designed balanced dual-band BPF should have a wide upper DM stopband. Fig. 4Open in figure viewerPowerPoint Calculated resonant frequencies for SIRs A and B with dimensions shown in Fig. 3 In CM operation, SIRs A can support only the even-distributed resonant modes with their resonant frequencies denoted by even numbers in the suffixes, whereas SIRs B can support all their resonant modes. As indicated in Fig. 4, the even-numbered resonant frequencies of SIRs A are not close enough to any of the resonant frequencies of SIRs B, implying that CM signals excited in SIRs A will not be efficiently coupled to the other through the inner SIRs B. However, though not through SIRs B, CM transmission can still occur through directly coupling between the two SIRs A. Hence, the gap between these two SIRs should be large enough to provide a satisfactory CM suppression. Figs. 5a and b, respectively, show the simulated DM and CM responses of the 2.45/5.8GHz balanced BPF. The DM transmission exhibits a suppression of more than 36 dB up to 10.8 GHz in the upper stopband. Moreover, the CM suppression is larger than 30 dB in the frequency range of 1–11.7 GHz. Note that three local peaks of the can be observed around 4.3, 8.2, and 12 GHz, which are very close to the calculated (4.125 GHz), (8.25 GHz), and (12.375 GHz), respectively. These CM transmission peaks are due to the direct coupling between the two SIRs A with their even-numbered resonant modes excited. The gap between these two SIRs plays the role of a capacitance that corresponds to a susceptance in proportion to the operating frequency, and a larger susceptance will cause a stronger coupling between the SIRs. This is a reason why the CM transmission around the higher resonant frequency is larger than those around and . Fig. 5Open in figure viewerPowerPoint Simulated results for the proposed dual-band BPF of 2.45/5.25 GHz band a DM response b CM response If SIRs A are tapped by the feeding microstrip lines instead of being coupled by the feeding lines, each SIR A can be regarded as part of the feeding structure. In such a case, CM suppression would be worst around all resonant frequencies of SIRs B. This phenomenon can be observed in [16], in which poor CM suppressions occur around the resonant frequencies of the two inner quarter-wavelength SIRs because the outer SIRs are tap-fed by the feeding transmission lines. 2.3 Design of the balanced quad-band diplexer To design the proposed balanced quad-band diplexer, we need another balanced dual-band BPF which is operated at the 1.92/5.25 GHz bands. Such a filter can easily be obtained by following a similar design procedure introduced in Section 2.2. Once the two balanced dual-band BPFs are designed, they can subsequently be combined through an impedance-matching network to form a balanced quad-band diplexer. In this paper, we follow the analysis and design idea presented in Section 2.1 and use SIR-inserted T-junctions as the combining networks for the balanced diplexer, as shown in Fig. 6. Here, the first two resonant frequencies of the DM half circuit of SIRs 1 (2) are designed at 1.92 and 5.25 GHz (2.45 and 5.8 GHz) to provide a through pass to the two passbands of Channel 1 (2). Isolation between channels can also be attained using the approach illustrated in Section 2.1. Fig. 6Open in figure viewerPowerPoint Circuit layout of the proposed balanced quad-band diplexer 3 Results and discussion The proposed diplexer in this paper was printed on a 0.635 mm thick RT/Duroid 6010 substrate with dielectric constant 10.2 and loss tangent 0.0023 and measured using an Agilent E8361C vector network analyser. To validate the effectiveness of the SIR-inserted T-junctions in achieving quad-band channel isolation, we take Channel 1 as an example to examine the variation of the DM reflection coefficient when looking into this channel from points C and C′ in Fig. 1 [or, equivalently, ΓC1(f) indicated in Fig. 2]. Figs. 7a and b show the reflection coefficients obtained when the T-junctions have no SIRs inserted. It is found that as the length of the θ1 + θ3 section is set to be 18.5 and 22 mm, good isolation for the 2.45 and 5.8 GHz bands, respectively, can be attained since the corresponding reflection coefficients in Figs. 7a and b are very close to 1. However, it is difficult to simultaneously establish good isolation for both 2.45 and 5.8 GHz bands by merely tuning the length of the θ1 + θ3 section. Fig. 7c shows the reflection coefficients obtained using SIR-inserted T-junctions. This figure reveals that, by the approach addressed in Section 2.1, such a circuit configuration can indeed give favourable isolation for both bands of Channel 2. Similar results relevant to Channel 2 with an appropriately inserted SIR can be observed, which for conciseness are omitted here. Fig. 7Open in figure viewerPowerPoint DM reflection coefficients looking into Channel 1 from points C and C ′ in Fig. 1 for different lengths of the θ1 + θ3 section a Section θ1 + θ3 = 18.5 mm, without SIR 1 b Section θ1 + θ3 = 22 mm, without SIR 1 c With appropriate section length of θ1 + θ3 and SIR 1 inserted The simulated and measured S-parameters of the designed quad-band diplexer are shown in Fig. 8. For Channel 1, the measured (simulated) first and second DM passbands are centred at 1.905 (1.92) and 5.255 (5.25) GHz, respectively, with their 3 dB passbands ranging from 1.83 to 1.98 (1.837 to 2.003) and 5.15 to 5.36 (5.14 to 5.36) GHz. The measured (simulated) minimum DM insertion losses are 1.41 (1.38) and 1.58 (1.52) dB for the first and second passbands, respectively (see Fig. 8a). Furthermore, the measured (simulated) minimum CM suppressions in the first and second passbands are 68.4 (78.4) and 50.8 (52) dB, respectively, and the out-of-band CM suppression are larger than 35.8 (35.7) dB (see Fig. 8b). Fig. 8Open in figure viewerPowerPoint Simulated and measured S-parameters of balanced quad-band diplexer a DM response b CM response c DM and CM isolations For Channel 2, the measured (simulated) first and second DM passbands are centred at 2.46 (2.45) and 5.83 (5.81) GHz, respectively, with their 3 dB passbands of 2.38–2.54 (2.38–2.52) and 5.73–5.93 (5.71–5.91) GHz. The measured (simulated) minimum DM insertion losses are 1.65 (1.6) and 1.9 (1.8) dB for the first and second passbands, respectively. Moreover, the measured (simulated) minimum CM suppressions within the first and second DM passbands are 63.4 (62.2) and 46.2 (47.9) dB, respectively, and the out-of-band CM suppression are larger than 39.7 (38.3) dB. The measured (simulated) DM isolations for both channels are over 44.8 (45.7) dB, and those for CM isolations are over 50.5 (46.2) dB in the displayed frequency range of 1–7 GHz (see Fig. 8c). Table 1 summarises the specification and performance of the balanced quad-band diplexer in this paper and the balanced dual-band diplexers in [6-9]. Clearly, the minimum in-band isolations for our balanced diplexer are far better than those for [6-9]. The minimum in-band DM insertion loss and CM suppression for our work are comparable with or even better than those for [6-9]. Especially, the minimum in-band CM rejection levels for the two higher bands of our work are 52 and 47.9 dB revealing that very good CM performance was attained. A photograph of the fabricated balanced quad-band diplexer is shown in the inset of Fig. 8c, which when exclusive of the 50 Ω feeding lines occupies an area of 41 × 85 mm2. Table 1. Comparison between the proposed balanced quad-band diplexer and the dual-band references References Centre frequency, GHz Minimum in-band DM insertion loss, dB Minimum in-band CM suppression, dB Minimum in-band DM isolation, dB Circuit size, mm2 this work 1.92/2.45/5.25/5.81 1.38/1.6/1.52/1.8 78.4/62.2/52/47.9 56.7/57.6/65.4/ 51.8 41 × 85 [6] 2.45/3.6 1.3/1.8 54/57 33/42 36.6 × 75.4 [7] 2.45/3.55 1.95/2.11 54.3/52.2 39.5/44.5 80.58 × 15.06 [8] 1/1.2 2.2/2.35 41.5/42.3 42.1/43.3 45.8 × 128.2 [9] 1.84/2.45 1.42/1.77 40.3/44.1 40.4/38 63 × 19 The effects of the tap-connected SIRs (namely, SIRs 1 and 2) on the balanced diplexer's DM performance have been clearly demonstrated and validated. Before closing this section, it is worthwhile to briefly discuss how they affect the CM performance. For that purpose, the CM half circuit of SIR 1 to be discussed is the same as the one in Fig. 2 except that the POS should be open-circuited. In our design of SIR 1, θa and θb have also been set equal, as what has been done for SIRs A, and the impedance ratio Zb/Za is the same as that for SIRs A. Hence, the resonant frequencies of SIR 1 will be identical to those of SIRs A. That is, both SIR 1 and SIRs A can support the same resonant modes ( and for DM operation and for CM operation). In other words, the presence of SIR 1 will neither improve nor deteriorate the CM performance of BPF 1; consequently, CM suppression solely relies on how the balanced BPF is designed. If further enhancement of CM suppression is desired, the two SIRs A can be loaded at their centres with a stub of different lengths [17]. However, SIRs with a stub loaded at the centre in such a compact circuit may raise unwanted mutual coupling between SIRs as well as between each stub-loaded SIR and its nearby circuit traces, making tuning of the structural parameters difficult. In addition, from the data listed in Table 1, the measured minimum CM suppression data of the proposed balanced quad-band diplexers are already comparable with those of the balanced dual-band ones, both being very high. We hence did not pursue additional strategies for further improving the CM suppression level. 4 Conclusion In this paper, we have proposed a balanced quad-band diplexer with high isolation and CM rejection. Two third-order balanced dual-band filters were first designed using microstrip SIRs. The inner SIRs were properly designed not to resonate around the even-mode resonant frequencies of the outer ones to result in high CM suppression. SIR-inserted T-junctions were used to combine the two dual-band filters to form a balanced quad-band diplexer with high channel isolation. 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