WO2020176054A1

WO2020176054A1 - Dual-band microstrip balun bandpass filter

Info

Publication number: WO2020176054A1
Application number: PCT/TR2020/050119
Authority: WO
Inventors: Ali Kürşad GÖRÜR
Original assignee: Nevşehi̇r Haci Bektaş Veli̇ Üni̇versi̇tesi̇ Rektörlüğü
Priority date: 2019-02-28
Filing date: 2020-02-18
Publication date: 2020-09-03
Also published as: TR201903086A2

Abstract

This invention refers to a dual-band microstrip balun bandpass filter and performs balun and filtering features simultaneously on two frequency bands with close central frequencies of bandpass units and low amplitude and phase differences between the outputs on both bandpass units. This invention also has compact sizes to bring two poles on both bandpass units.

Description

DUAL-BAND MICROSTRIP BALUN BANDPASS FILTER Technological Scope:

This invention concerns the dual-band microstrip balun bandpass filter to be considered under the framework of circuit design for satellite and space communication systems. Recognized Status of Applied Technique:

Baiuns are the circuit elements converting an unbalanced input into two balanced outputs. On the other hand, filters are the circuit elements that allow or refuse the transmission of a specific signal on any intended frequency band. Balun bandpass filter is the combined version of balun and filter on a single circuit. Since this method provide savings in terms of circuit size and cost, it has become popular in literature in recent years.

A major part of current balun bandpass filters operates on a single frequency band whereas the balun bandpass filters operating on two frequency bands are very scarce. For the dual-band balun bandpass filters, the failure to adjust frequency bands close to each other is a major issue. In addition, it is also difficult to receive multiple low amplitude rates and phase differences on both bandpass units. It is observed by various studies in the literature that a single pole is found in bandpass units and the band selectivity is considerably low.

Regarding the recognized status of applied technique, various applications are made to provide savings in terms of circuit size and cost. In order to achieve high circuit performance, it is also intended to contribute to the literature in certain aspects such as resonator types and design methods. In the patent application numbered US4800344 A,“Baiun” is described. This invention concerns the circuit design including a relatively balanced line with similar impedance rates that transmits differential signals on a broad band and weakens balun and common mode signals for interconnection between relatively unbalanced lines.

In the patent application numbered US5697088 A,“Balun transformer” is described. This invention concerns the balun transformer coupled with asymmetric short and open circuit transmission lines.

In the patent application numbered US7176776 Bl,“Multi-layer RF balun and filter” are described. This invention performs filtering and signal balancing operations through the resonators used in the material and on the multiple plane dielectric grounds By means of the resonators used on multiple planar dielectric grounds and materials, this invention performs filtering and signal balancing operations in both ends.

In the patent application numbered US20030042992 Al,“Integrated filter balun” is described. This invention concerns the integrated filter balun circuit with relevant structures developed by the resonators through lumped circuit elements.

In the patent application numbered CN106252805 A,“Hybrid balun filter” is described. This invention concerns the hybrid balun filter developed by combining a metal grounding plate, a metal coat, a coaxial resonator and two dielectric resonators.

In the patent application numbered US20130200959 Al,“Filtering balun with flexible microwave frequency” is described. This invention consists of flexible structures designed in accordance with a specific frequency area. In addition, this invention concerns the varactor diodes used in split ring balun resonators through a balun filter with electronically adjustable frequency area.

In the patent application numbered US7541888 B2, “Dual-band balanced and unbalanced bandpass filter” is described. In the circuit subjected to this invention, the frequency mechanism is analyzed as well as the balun characteristic. This invention concerns the usage of coupled transmission lines for dual-band balun bandpass filters.

In addition to the patents, various articles are found on single and dual-band balun bandpass filters. As the stated works, [C. M. Chen, S. J. Chang, S. M. Wu, Y. T. Hsieh, and C. F. Yang,“Investigation of Compact Baiun-Bandpass Filter Using Folded Open- Loop Ring Resonators and Microstrip Lines,” Mathematical Problems in Engineering, vol. 2014, pp. 1-6.] and [F. Huang, J. Wang and L. Zhu, "A New Approach to Design a Microstrip Dual-Mode Balun Bandpass Filter," in IEEE Microwave and Wireless Components Letters, vol. 26, no. 4, pp. 252-254, April 2016.] present two main methods for the single-band balun bandpass filter designs. A balun filter structure is designed by coupling two split ring resonators per feeder line in [C. M. Chen, S. J. Chang, S. M. Wu, Y. T. Hsieh, and C. F. Yang,“Investigation of Compact Baiun-Bandpass Filter Using Folded Open-Loop Ring Resonators and Microstrip Lines,” Mathematical Problems in Engineering, vol. 2014, pp. 1-6.]. On the other hand, the balun filter specified in [F. Huang, J. Wang and L. Zhu, "A New Approach to Design a Microstrip Dual-Mode Balun Bandpass Filter," in IEEE Microwave and Wireless Components Letters, vol. 26, no. 4, pp. 252-254, April 2016.] depends on the wave pattern on the half-wave transmission line with open circuit ends. Both designs are able to bring balun filter features in a single frequency area. In a study conducted by Hsu et. al. in 2012 for the literature, four poles are found in the first band of dual-band balun filter design. In this study, partially coupled impedance resonators are used with half and quarter wave length [Hsu H. C., C. H. Lee, C. I. G. Hsu and P. H. Wen,“Balanced dual-band BPF with partially coupled bi-section l/2 and l/4 SIRs”, Asia Pacific Microwave Conference Proceedings, Kaohsiung, pp. 244-246, 2012.]. In the dual-band balun design presented by Zhang et. al. in 2015, a multi-layer configuration is used [Zhang G. Q., J. X. Chen, J. Shi, H. Tang, H. Chu and Z. H. Bao,“Design of Multilayer Balun Filter With Independently Controllable Dual Passbands” IEEE Microwave and Wireless Components Letters, vol. 25, no. 1, pp. 10-12, 2015] In this study, two half-load patch resonators are used consecutively in two different planes and the bands are controlled independently. In the studies titled [F. Huang, J. Wang, L. Zhu, Q. Chen and W. Wu, "Dual-Band Microstrip Balun With Flexible Frequency Ratio and High Selectivity," IEEE Microwave and Wireless Components Letters, vol. 27, no. 11, pp. 962-964, Nov. 2017.] and [J. Wang, F. Huang, L. Zhu, C. Cai and W. Wu, "Study of a New Planar- Type Baiun Topology for Application in the Design of Baiun Bandpass Filters," in IEEE Transactions on Microwave Theory and Techniques, vol. 64, no. 9, pp. 2824- 2832, Sept. 2016.], dual-band balun units are designed by means of coupled resonators with side lines and open circuit ends. In the first unit, dual-band is obtained through the short circuit end of a specific port in 4 symmetrical ports. However, both bandpass units fail to reach intended selectivity rates due to the single pole structures [F. Huang, J. Wang, L. Zhu, Q. Chen and W. Wu, "Dual-Band Microstrip Balun With Flexible Frequency Ratio and High Selectivity," IEEE Microwave and Wireless Components Letters, vol. 27, no. 11, pp. 962-964, Nov. 2017.]. In the second unit, 4 resonators are used in total for two bandpass units and high level of selectivity is achieved in accordance with the resonator type [J. Wang, F. Huang, L. Zhu, C. Cai and W. Wu, "Study of a New Planar-Type Balun Topology for Application in the Design of Balun Bandpass Filters," in IEEE Transactions on Microwave Theory and Techniques, vol. 64, no. 9, pp. 2824-2832, Sept. 2016.]. Conventional E-type dual mode resonators are used in the stated design.

In the above-mentioned sections, various circuit designs are analyzed. The circuits analyzed in such applications fail to overcome the problems arising from the microwave circuit design under a single circuit structure. Yet, they provide advantage in certain manners by means of the circuit topologies. Therefore, it would be suggested that a new compact circuit structure with low amplitude and low amplitude difference between two balanced outputs shall play an important role in the communication systems.

In conclusion, a new technology with compact sizes and two poles on both bandpass units is required to overcome the above-mentioned disadvantages, to perform balun and filtering features on two frequency bands and to obtain very low amplitude and phase differences between the outputs on both bandpass units through central frequencies of bandpass units close to each other. Description of the Invention:

In order to meet all requirements arising from the detailed descriptions mentioned above and in following parts, this invention shall be used efficiently for circuit-based satellite and space communication systems.

This invention demonstrates the balun feature and the filtering operation in two frequency bands. In both switching bands, slight amplitude and phase differences are found. The invention has a compact size.

The product subjected to this invention has high selectivity due to the dipole formation on both transmission bands whereas the stated transmission bands are close to each other. Furthermore, the bandwidth rate of both bands is also controlled simultaneously depending on the bandwidth rates of both bands.

The advantages of this method would be realized better by the following chart and the descriptions with reference to this chart.

Description of Figures:

This invention is described with reference to the attached figures. Therefore, the relevant features would be realized and appreciated better. However, it is not intended to restrain this invention with certain modifications. On the contrary, this invention should include all alternatives, modifications and equivalents in the pre-defmed area specified by the attached requirements. It should be recognized that the details are given to describe the preferred adjustments for this invention and to clarify the layout of applied methods together with relevant rules and conceptual features. In the stated figures:

Figure 1 Displays the weak coupling of codirectional discrete ring resonator with interdigital capacitor load

Figure 2a Displays the impact of track length of interdigital capacitor on resonance frequencies

Figure 2b Displays the impact of transmission line length of external split ring resonator on resonance frequencies (l_x=0.8 mm, 1_iVn=6 6 mm) Figure 3 Displays the dual-band microstrip balun bandpass unit subjected to the invention

Figure 4 Displays the impact of distance between identical resonators on (d) frequency response

Figure 5 a Displays the results of dissipative and non-dissipative simulation on the dual-band microstrip balun bandpass filter

Figure 5b Displays the impedance and phase differences between balanced outputs for both bandpass units

As indicated on the attached table, the figures intending to clarify this invention are duly numbered. The stated figures are given in the following section with relevant names.

Description of References:

1 Input port

2. Feeder line for weak coupling of the resonator

3. Output port

4. External split ring resonator

5. Internal split ring resonator with capacitor load

6 Length of external transmission line on external split ring resonator (l_r)

7. Thumb length of interdigital capacitor (l_x)

8 Length of horizontal transmission line on internal split ring resonator (k_?y) 9. Length of vertical transmission line on internal split ring resonator (h_?ci) 10. Thickness of transmission line on resonator (w_r)

11. Change of first resonance frequency depending on the thumb length of interdigital capacitor (l_x)

12. Change of second resonance frequency depending on the thumb length of interdigital capacitor (l_x)

13. Change of first resonance frequency depending on the length of external transmission line on external split ring resonator (l_r)

14. Change of second resonance frequency depending on the length of external transmission line on external split ring resonator (l_r)

15. Unbalanced input port

16. Balanced 2^nd output port

17. Balanced 3 output port

18. External split ring resonator 2

19. External split ring resonator with interdigital capacitor load 2

20. Feeder line of input port

21. Feeder line of output port

22. Curve length of feeder line on output port (In)

23. Length of feeder line on output port (le)

24. Thickness of feeder line on output port (w_f)

25. Thickness of transmission line on resonator 2 (w_r)

26. Distance between codirectional resonators (d)

27. Length of horizontal transmission lines on internal split ring resonator (k^)

28. Thumb length of interdigital capacitor (l^)

29. Length of external transmission line on external split ring resonator (l_r2)

30. Length of vertical transmission line on external split ring resonator (l_di§)

Detailed Definition of the Invention:

The figure indicating the codirectional split ring resonator with interdigital capacitor load and the weak coupling in input/output ports (Figure 1), the impacts of resonator size on frequency responses (Figure 2a and Figure 2b) and the invention components (Figure) are as follows: Input port (1), the feeder line for weak coupling of the resonator (2), output port (3), external split ring resonator (4), internal split ring resonator with interdigital capacitor load (5), length of external transmission line on external split ring resonator (l_r) (6), thumb length of interdigital capacitor (l_x) (7), length of horizontal transmission line on internal split ring resonator (l^) (8), length of vertical transmission line on internal split ring resonator ( _gd) (9), length of transmission line on resonator (w_r) (10), change of first resonance frequency depending on the thumb length of interdigital capacitor (l_x) (11), change of second resonance frequency depending on the thumb length of interdigital capacitor (l_x) (12), change of first resonance frequency depending on the length of external transmission line on external split ring resonator (l_r) (13), change of second resonance frequency depending on the length of external transmission line on external split ring resonator (l_r) (14), unbalanced input port (15), balanced 2^nd output port (16), balanced 3^rd output port (17), external split ring resonator 2 (18), internal split ring resonator with interdigital capacitor load 2 (19), feeder line of input port (20), feeder line of output port (21), curve length of feeder line on output port (In) (22), length of feeder line on output port (In) (23), thickness of feeder line on output port (w_f) (24), thickness of feeder line on output port 2 (w_r) (25), distance between codirectional resonators (d) (26), length of horizontal transmission lines on internal split ring resonator (li_?y2) (27), thumb length of interdigital capacitor (l^) (28), length of external transmission line on external split ring resonator (l_r2) (29) and length of vertical transmission line on external split ring resonator (l_di5) (30).

This invention refers to the dual-band microstrip balun bandpass filter whereas codirectional split ring resonators with interdigital capacitor load are used for the design of recommended balun bandpass filter. A similar resonator has been presented to the literature previously by the inventor [A. K. Gorur,“A novel dual-band bandpass filter using co-directional split ring resonators with closely spaced passbands and wide upper stopband” International Journal of RF and Microwave Computer-Aided Engineering, 2018, vol. 28, no. 4, e21230]. As indicated on Figure 1, the codirectional split ring resonator with interdigital capacitor load and weak coupling on input (1) and output (3) ports consists of external split ring resonator (4) and internal split ring resonator with interdigital capacitor load (5). Furthermore, both resonators are also called as codirectional since the open ends are in same direction. The stated resonator has two resonance frequency areas. Thus, two bandpass units with one transmission pole are obtained from a single resonator. The resonators consist of concentric split ring resonators (4) and internal digital capacitor is placed on the open ends of internal split ring resonator. The reason to place such interdigital capacitors is mainly to check the first resonance frequency independently of the second unit. On the other hand, there is not any disadvantage in terms of circuit size since an additional surface area is not required. The increase and decrease of electrical length on external split ring resonator (4) are also effective in controlling the second resonance frequency rate.

For the frequency control on the weak coupled circuit in Figure 1, the simulations are performed with Rogers 4003C unit with the material thickness of 0.813mm, the dielectric tangent loss of 0.0027 and the relative dielectric constant rate of 3.38. The numerical results are given in Figure 2a and Figure 2b. Here, input (1) and output (3) ports have the approximate impedance rate of 50 ohm whereas the feeder line has also the impedance rate of 50 ohm. The thickness of feeder lines is 8.2 mm. The sizes of weak coupled circuit indicated in Figure 1 are as follows: Length of horizontal transmission line on internal split ring resonator (k_?y) (8)=5.5 mm, length of vertical transmission line on internal split ring resonator (li_? ) (9)=2.3 mm, thumb length of interdigital capacitor (l_x) (7) = 3.0 mm, length of transmission line on the resonator (w_r) (10) = 0.4 mm. The length of transmission line on both internal and external split ring resonators (w_r) (10) correspond to 0.4 mm, which is identical in all zones. The space between internal and external split ring resonators (4) corresponds to 0.3 mm, which is identical in all zones. The thumb length of interdigital capacitor is 0.2 mm. Furthermore, the space between the external split ring resonator (4) and the feeder line (2) providing weak coupling for the resonator is 0.2 mm. Figure 2a and 2b display the impact of change in thumb length of interdigital capacitor (l_x) (7) and the length of external transmission line on external split ring resonator (l_r) (6) on the resonator. As seen from such figures, the frequencies of both resonance units are controlled without a significant change. First resonance is controlled at the approximate range between 1.8 GHz and 2.3 GHz whereas second resonance is controlled at the approximate range between 2.5 GHz and 3.5 GHz. The difference of first resonance frequency is controlled depending on the change in the thumb length of interdigital capacitors (l_x) (7). At this point, it should be considered that any change in the thumb length of interdigital capacitor (l_x) (7) leads to the shift in horizontal transmission line of internal split ring resonator (h_?y) (8). The stated frequency rate determines the frequency allocation for the balun bandpass filter. First, the frequency rate is specified in accordance with the method applied in this study. Subsequently, necessary optimizations are performed to achieve the balun bandpass filter characteristic on this frequency and to finalize coupling.

The structure of dual-band microstrip balun bandpass filter is indicated on Figure 3. As in the resonator analysis, the design is made with Rogers 4003 C unit with the material thickness of 0.813mm, the dielectric tangent loss of 0.0027 and the relative dielectric constant rate of 3.38. The difference between the balaned 2^nd port (16) and the balanced 3 port (17) is fed from different sides to obtain balun features. Thus, the phase difference of 180° is intended between balanced outputs. Since the resonators located on the signal paths towards outputs have identical electric length, it is intended to obtain minimum amplitude difference between outputs. The thicknesses of transmission lines on unbalanced input port (15), balanced 2^nd output port (16) and balanced 3^rd output port (17) are determined at the approximate characteristic impedance rate of 50 ohms. As indicated on Figure 3, an input port feeder line with the thickness of 1.8 mm and the length of 19.4 mm is connected to the unbalanced input port (15). Two resonators are coupled on the top and bottom sides of this feeder line. As both resonators have different resonance frequency rates, two separate bandpass units with a single pole are placed on two balanced outputs. In order to obtain two transmission poles on both bandpass units, it is required to couple codirectional resonators by increasing the transmission poles. Accordingly, the signal is transmitted to two balanced output ports through two codirectional resonators as indicated in Figure 3. The selectivity rates of bandpass units are enhanced by increasing transmission poles. The feeder lines with high impedance are also available on the balanced 2^nd output port (16) and the balanced 3 output port (17) of the designed circuit. By means of such feeder lines, the signal incoming from the unbalanced input port (15) is transmitted to the balanced 2^nd output port (16) and the balanced 3 output port (17). Furthermore, a curved shape is preferred for the feeder lines in order to obtain a clear interruption band on higher frequencies. The sizes of the designed circuit are as follows: The thickness of transmission line in external split ring resonator 2 (w_r) = 0.4 mm, ), the length of horizontal transmission line in internal split ring resonator (f^) (27) = 0.4 mm, the length of external transmission line in external split ring resonator (l_r2) (29) = 7.7 mm, l_di§= 4.5 mm, the thumb length of interdigital capacitor (1^) (28) = 0.8 mm, the curve length of output port feeder line (In) (22) = 3.4 mm, the length of output port feeder line (1b) (23) = 17.2 mm and the thickness of output port feeder line (w_f) = 0.2 mm. In addition, the thicknesses of transmission lines in internal and external split ring resonators 2 (w_r) are correspond to 0.4 mm, which is identical in all areas. The space between internal and external split ring resonators 2 (18) corresponds to 0.3 mm, which is identical in all areas. The thickness of interdigital capacitor is 0.2 mm. Furthermore, the space between external split ring resonator 2 (18) and the transmission lines is 0.2 mm.

The distance between codirectional resonators in the dual-band microstrip balun bandpass filter (d) (26) has an impact on the bandwidth rates of bandpass units. The effect of such change on bandwidth rates is indicated in Figure 4. Accordingly, the bandwidth rates increase simultaneously on both bandpass units in line with the increase in coupling codirectional resonators (in other words, by placing codirectional resonators close to each other).

The sizes of dual-band microstrip balun bandpass filter are 22.4x29.0 mm² whereas the frequency response achieved through simulation is indicated in Figure 5a. The results of dissipative and non-dissipative simulations are given in this figure. All transmission lines are represented by the copper with thickness of 35 pm and conductivity coefficient of 5.8xl0⁷ S/m. As the central frequency rate of bandpass units ranges between 2.25 GHz and 2.5 GHz, the bandpass units are at close frequency areas. The minimum interruption losses found by dissipative and non-dissipative simulations are 4.07 dB and 3.39 dB respectively for the first bandpass unit. The minimum interruption losses are found as 4.59 dB and 3.47 dB respectively for the second bandpass unit. The in-band reflection loss is higher than 12 dB on both bandpass units. As zero transmission rate is obtained between the bandpass units, the isolation level between the bands is considerably high. On the other hand, it is observed that the upper holding band in the top side of bandpass units stands clear up to 7.2 GHz. The balun performance of designed circuit is also high. In this context, the amplitude and phase differences between balanced outputs indicated in Figure 5b are close to the optimum level. Accordingly, the phase differences in balanced outputs are lower than 180°±1° whereas the amplitude differences are lower than 0.07 dB in both bandpass units. Such balance parameters present that the designed circuit is able to serve for any satellite/ space communication system.

Claims

1- As indicated in Figure 3, this invention refers to a dual-band microstrip balun bandpass filter consisting of an unbalanced input port (15), an unbalanced 2^nd output port (16), a balanced 3 output port (17), an external split ring resonator 2 (18), an internal split ring resonator 2 with interdigital capacitor load (19), a feeder line in input port giri§ (20), a feeder line in output port (21), the curve length of feeder line in output port (In) (22), the length of output port feeder line (le) (23), the thickness of output port feeder line (w_f) (24), the thickness of transmission line in resonator 2 (w_r) (25), the distance between codirectional resonators (d) (26), the length of horizontal transmission line in internal and external resonators (k_?y2) (27), the thumb length of interdigital capacitor (1_x2) (28), the length of external transmission line in external split ring resonator (l_r2) (29) and the length of vertical transmission line in external split ring resonator (1^) (30).

2- As described in Requirement 1, this invention refers to the dual-band microstrip balun bandpass filter characterized by transmitting the initial signal to balanced 2^nd output port (16) and balanced 3 output port (17) upon the coupling of codirectional discrete ring resonators on balanced 2^nd output port (16) and balanced 3^rd output port (17) towards the curved feeder lines in output port (21).

3- As described in Requirement 1, this invention refers to the dual-band microstrip balun bandpass filter characterized by obtaining bandpass units with identical frequencies in both outputs due to the identical electrical length towards balanced 2^nd output port (16) and balanced 3 output port (17) and by achieving low amplitude difference in the stated outputs.

4- As described in Requirement 1, this invention refers to the dual-band microstrip balun bandpass filter characterized by obtaining the approximate phase difference of 180°±1° between the outputs by placing balanced 2^nd output port (16) and balanced 3^rd output port (17) in different directions. 5- As described in Requirement 1, this invention refers to the dual-band microstrip balun bandpass filter characterized by obtaining two transmission poles on both bandpass units by two codirectional discrete ring resonators coupled in unbalanced input port (15) through an input port feeder line (20).