Research Article  Open Access
Jessada Konpang, Natchayathorn Wattikornsirikul, "FourPort DualMode Diplexer with High Signal Isolation", Active and Passive Electronic Components, vol. 2020, Article ID 4032673, 5 pages, 2020. https://doi.org/10.1155/2020/4032673
FourPort DualMode Diplexer with High Signal Isolation
Abstract
An ease of fourport dualmode diplexer with high signal isolation is presented. A compact dualmode diplexer with high signal isolation between the Rx and Tx modules is achievable by only using one resonator filter topology. Two backtoback dualmode diplexers have a 180° phase shift in one branch. The high isolation can be achieved by amplitude and phase cancellation technique. The delayed transmission line can be easily achieved by the phase shifter. The simulated and measured fourport dualmode diplexers are designed at the centre frequency of Rx/Tx at 1.95 GHz and 2.14 GHz, respectively. The measured results of Rx/Tx dualmode diplexer devices are presented with 47.1 dB Rx/Tx isolation. This fourport dualmode diplexer achieves the isolation (S_{32}) of more than 24.1 dB when compared with the conventional threeport dualmode diplexer structure.
1. Introduction
In RF frontends of several communication systems, a diplexer is usually used to discriminate two different signal frequency bands for transmitting (Tx) and receiving (Rx) channels while sharing a single antenna. Diplexer structure is composed of two bandpass filters with different passband frequencies. In multiband communications, filters and diplexer are recently required to design with compact size, light weight, and high signal isolation. Microstrip bandpass filters can be easily mounted on a dielectric substrate and can provide a more flexible design of the circuit layout [1]. To keep small circuit size and light weight, common microstrip resonators are employed for filters and diplexer design. The microstrip openloop resonator filters are highly desirable in the wireless communication systems with compactness and high performance [2]. Many research studies have been performed on compact resonator filters and diplexers such as stepped impedance openloop resonators [3], compact openloop resonator filter structure [4], and microstrip square openloop with steppedimpedance resonator filter and diplexer [5].
Furthermore, it is difficult to apply the high isolation technique for multiband application as the coupling area is constrained by the size of the common resonator and highorder filters. Therefore, it is challenging to design diplexer circuit with high signal isolation and small circuit size. When the signal transmitting power is too high in the diplexer device, the leakage of high signal power from transmitting channels increases. The channel interference between Tx/Rx ports can destroy Rx components in consequence of high transmitting signal. High signal isolation between Tx/Rx ports can be increased by using an ease of structure design; many research papers have been made to increase the signal isolation in diplexers. A lot of efforts to design filters and diplexers have been made on increasing the signal isolation in and out of the band of diplexer [6–11]. To achieve high Tx/Rx isolation signal, the common diplexer designs require highorder filters. As a result of a very complicated filter design and fabrication process, the insertion loss of the filter and diplexer can increase. Moreover, an alternative technique to design diplexer with low cost, high signal isolation, and ease of fabrication process was introduced by using fourport network [12, 13]. The realized microstrip filter prototype with small circuit size can be achieved by this technique. The technique for size reduction and high isolation signal by using a dualmode resonator filter and diplexer is presented in [14, 15].
In this paper, an ease of fourport dualmode diplexer with high signal isolation is presented. The dualmode structure by using Ushaped resonator with open stub enables a compact and ease of design. High signal isolation between the Tx and Rx module is achievable by only using one resonator filter topology. Two backto back threeport dualmode diplexers and a 180° phase shifter are easily employed to construct the proposed device, which are combined to form a fourport dualmode diplexer. The high signal isolation can be achieved by amplitude and phase cancellation technique. In order to cancel the same amplitude signal but different phase in transmitter and receiver, a delayed microstrip transmission line can be used to achieve a 180° phase shift in one branch.
1.1. DualMode Resonator Filter Analysis
The concept of fourport dualmode diplexer is based on a dualmode resonator filter design. The topology of fourport dualmode diplexer is formed of two conventional threeport dualmode diplexers joined backtoback and a 180° phase shifter as illustrated in Figure 1.
To verify the validation of the concept idea and experiment, the microstrip dualmode resonator structure can be introduced as an example. The design of dualmode resonator filter is based on a singlemode openloop resonator [16,17] which focuses only on the oddmode resonance. Actually, an evenmode resonance of the singlemode resonator is present approximately at twice the fundamental resonant frequency, and the even mode is of little use in single band resonator filter synthesis. The even mode will emerge as the first spurious response which degrades the filter performance. On the other hand, the even mode of dualmode filters may also be used as a doubly tuned circuit [18].
For this reason, an openloop filter may be adjusted to act as a doubly tuned filter. Based on the proposed structure in [18,19], the evenmode resonance can be tuned to close the operating frequency band (the odd mode). Therefore, a secondorder response filter can be created by these two poles. The schematic circuit of the dualmode filter is depicted in Figure 2. The opencircuited stub is added in the centre of the Ushaped resonator filter to lower the evenmode resonant frequency. The extended stub shown has no effect on the odd mode [18]. Therefore, the two modes (odd and even modes) can be tuned independently.
The equivalent circuits of even and odd modes at the resonant mode are shown in Figure 3. An opencircuited half wavelengthtype resonator shows the even mode resonator as Figure 3(a) while a shortcircuited quarter wavelengthtype resonator shows the odd mode as in Figure 3(b).
(a)
(b)
The dualmode resonator by using a Ushaped resonator with open stub can be illustrated as an example design. The opencircuited stub can be used to tune the even mode of dualmode performance [19]. The dualmode resonator consists of two sections of the same impedances as illustrated in Figure 2. Dimensions were calculated using the following equation:
The opencircuited stub (Z_{2}) connects to the middle of the resonator (Z_{1}). αZ_{2} represents the evenmode equivalent impedances of the sections with impedance Z_{2}. The electrical length (θ_{2} of the opencircuited stub) may be defined fromwhere θ_{x} (x = 1, 2, 3) corresponds to the electrical length of the section in Figure 1 and c is the speed of light in vacuum.
To demonstrate the proposed dualmode microstrip filters, the resonators are of U shape which is loaded by an openend stub. The filters are designed on a RT/Duroid substrate having a thickness h = 1.27 mm with a relative dielectric constant ε_{r} = 6.15. The filters were simulated by IE3D fullwave EM simulations. The input and output coupledfeed lines are used to couple the signal to the dualmode resonator having a line width (cf) and coupling spacing (). The odd and even modes are referred to as the first two resonating modes. These two modes can have the same or different modal frequencies which depend on the lengths of the open stub. The basic structure of a dualmode microstrip resonator is pictured in Figure 4.
The operational frequencies compared to the first spurious mode by tuning the openstub lengths have been investigated using IE3D fullwave EM simulations. The dualmode resonator is designed to achieve the desired resonant frequencies by fixing the length of the Ushaped resonator (a and c). Evenmode characteristic can be achieved by adjusting the length of opencircuit stub loaded (b). Two input/output microstrip lines with 50 Ω characteristic impedance are used to feed the proposed dualmode resonator with openstub loaded resonator. As can be seen in Figure 5, the openstub loaded length does not affect the S_{21} response at oddmode resonant frequency while the evenmode resonant frequency is flexibly controlled by changing the length of open stub (b). An inherent transmission zero (TZ) can be easily adjusted to optimize the response. The TZ causes an asymmetric response. As the resonator is coupled to the input and output ports with a couplingfeed structure, the first two resonating modes are referred to as the odd and even modes. These two modes can have the same or different modal frequencies depending on the dimensions of the resonator. Moreover, when the two modes split, a finite transmission zero is produced on the high side of the two modes when even mode frequency is higher than the odd mode frequency. The major outcome of this property is realization of filters with an asymmetrical frequency response (the upper stopband of the filter).
1.2. Design of FourPort DualMode Diplexer
The layout structure of the proposed fourport microstrip dualmode diplexer is presented in Figure 6(a). The Tx/Rx filters are interconnected by an appropriately designed matching circuit of Tjunction that has the width of the 50 Ω line. The diplexer geometry is optimized at the Tjunction for better return loss performance in both the channels. It is noted that using port 1 with an impedance of 50 Ω as the feed line has advantages of obtaining better diplexer insertion loss and rejection performances. Here, the lengths of the Tjunction are optimized such that each filter in the diplexer should look like an open circuit to the other filter at its centre frequency.
(a)
(b)
Fourport dualmode diplexer layout is formed of two conventional threeport dualmode diplexers joined backtoback and a 180° phase shifter. The design technique based on two diplexers is joined backtoback to form the fourport diplexer. A 180° phase shifter is added in one of the channel filters between port 2 and port 4. To achieve such a phase shifter, a half wavelength delayed line is adopted. A phase shift of 180°±°2 is achieved across the Tx and Rx bands. The significant change in isolation from a 3port to a 4porttype is that if we allow the antenna impedance to change, we can tune the load impedance (port 4) to compensate for the antenna mismatch and recover the isolation back again. Thus, the effects of a mismatched antenna port are considered and recovered by using the 4port type. The photograph of fourport dualmode prototype is shown in Figure 6(b). The milling machine can be used to fabricate the circuit pattern. The dimensions of the fourport microstrip dualmode diplexer are detailed in Table 1.

Measurements are carried out using an Agilent Vector Network analyzer. The measured and simulated results of the fourport dualmode diplexer are shown in Figure 7(a). The measured inband return loss is better than 25 dB in the first bandpass (1.95 GHz) and 24 dB in the second bandpass (2.14 GHz), respectively. The insertion losses are approximately 1.1/1.16 dB at the two bandpasses. The simulation and measurement results are in good agreement. The comparison of signal isolation, S_{32}, of the isolation of fourport dualmode diplexer and threeport dualmode diplexer between the Rx and Tx bands is shown in Figure 7(b). The measured signal isolation of the conventional threeport dualmode diplexer is 23 dB, and it is 47.1 dB for the fourport dualmode diplexer. The excess losses in the measurements are believed to be due to the SMA connectors and fabrication errors.
(a)
(b)
To compare the size of the proposed fourport dualmode diplexer, a conventional fourport diplexer [13] is simulated by using a singlemode microstrip openloop resonator. The total number of degrees required in a singlemode bandpass filter can be reduced by half for dualmode resonators. High signal isolation between the Tx and Rx modules is achievable by only using one resonator filter topology. Moreover, the fourport microstrip dualmode diplexer still reduces overall signal losses with the same or better isolation compared to the existing stateofthe art diplexers [13].
2. Conclusions
A fourport dualmode diplexer with high signal isolation based on amplitude and phase cancellation technique is presented. A small dualmode bandpass filter with high signal isolation between the Tx and Rx modules is achievable by only using one resonator filter topology. Two backtoback dualmode diplexers have a 180° phase shift in one branch. The high isolation can be achieved by amplitude and phase cancellation technique. The delayed transmission line can be easily achieved by the phase shifter. The fourport microstrip dualmode diplexer can enhance the isolation (S_{32}) to more than 24.1 dB from the conventional threeport diplexer. Finally, the low complexity design and ease of fabrication process are proposed by using a fourport dualmode diplexer which can be used in wireless communications.
Data Availability
The data that support the findings of this study are included in the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors thank the Department of Electronic and Telecommunication Engineering, Faculty of Engineering, Rajamangala University of Technology Phranakhon, and Department of Electronic and Telecommunication Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, for supporting the research successfully.
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Copyright
Copyright © 2020 Jessada Konpang and Natchayathorn Wattikornsirikul. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.