Dual-passband filtering balun

Abstract

The invention discloses a dual-passband filtering balun, and relates to the technical field of radio frequency wireless. Comprising a top micro-strip, ground metal I, ground metal II, a bottom micro-strip and a plurality of metal via holes. A first interlayer dielectric is arranged between the top microstrip and the ground metal I, a second interlayer dielectric is arranged between the ground metal I and the ground metal II, and a third interlayer dielectric is arranged between the ground metal II and the bottom microstrip; a plurality of metal via holes penetrate through the top-layer micro-strip, the first interlayer dielectric, the ground metal I, the second interlayer dielectric, the ground metal II, the third interlayer dielectric and the bottom-layer micro-strip from top to bottom; windows with the same size are chiseled in the centers of the ground metal I and the ground metal II, and the top layer microstrip and the bottom layer microstrip are each provided with two single-passband resonant cavities; and adding a coupling resonant cavity below each single-passband resonant cavity to obtain the dual-passband filtering Balun. According to the invention, greater coupling strength can be provided, and greater design freedom can be provided for design.

Description

Technical Field

[0001] The present invention relates to the field of radio frequency wireless technology, and in particular to a dual-passband filtering balun. Background Art

[0002] Balun is the transliteration of balanced-unbalanced conversion (Balun). In radio frequency systems, balun is mainly used for the mutual conversion between single-ended signals and differential signals. Differential signals have a strong ability to suppress common-mode noise. External electromagnetic interference often appears in the form of common-mode noise, and the differential structure can effectively reduce the impact of this common-mode noise, thereby improving the integrity and accuracy of the signal. At the same time, the swing of the differential signal is relatively large, which means that it has a higher noise tolerance and can transmit information more reliably in a noisy environment.

[0003] Most existing microstrip baluns are microstrip marchand structures, which are mainly composed of four quarter-wavelength (λ / 4) microstrip transmission lines. These transmission lines are coupled to each other in a specific way to form a symmetrical four-port network. Among them, one port is used as an unbalanced port to receive or send unbalanced signals; while the other two ports are used as balanced ports to output or input balanced signals with the same amplitude but a phase difference of 180°. Based on the coupled microstrip transmission line structure, a wide-band balanced-unbalanced conversion is achieved.

[0004] In existing RF systems, multiple frequency bands need to be selected, which requires cascading filters in the system, which often leads to greater losses and system overhead, so the balun needs to be combined with the filtering function. However, the inter-line coupling strength between microstrip transmission lines is limited, and the four-port network formed is a planar structure with relatively low design freedom, making it difficult to meet complex functional design requirements. Summary of the invention

[0005] Based on this, the purpose of the present invention is to provide a dual-passband filtering balun that can solve the problems of insufficient coupling strength of planar microstrip lines and insufficient freedom in microstrip line planar design.

[0006] The embodiment of the present invention provides a dual-passband filtering balun, including: a single-passband filtering balun, a coupling resonant cavity R3, a coupling resonant cavity R4, a coupling resonant cavity R3' and a coupling resonant cavity R4', wherein the single-passband filtering balun includes from top to bottom: a top microstrip, a ground metal I, a ground metal II, a bottom microstrip and a plurality of metal vias; A first interlayer dielectric is arranged between the top microstrip and the ground metal I, a second interlayer dielectric is arranged between the ground metal I and the ground metal II, and a third interlayer dielectric is arranged between the ground metal II and the bottom microstrip; a plurality of metal vias penetrate the top microstrip, the first interlayer dielectric, the ground metal I, the second interlayer dielectric, the ground metal II, the third interlayer dielectric and the bottom microstrip from top to bottom; windows of the same size are cut at the center of the ground metal I and the ground metal II, and the windows are used for coupling between the top microstrip and the bottom microstrip; the top microstrip and the bottom microstrip are both provided with two single-passband resonant cavities; A coupling resonant cavity R3 is set on the ground metal I below the single-passband resonant cavity R1 in the top microstrip, and a coupling resonant cavity R4 is set on the ground metal I below the single-passband resonant cavity R2 in the top microstrip; a coupling resonant cavity R3´ is set on the ground metal II above the single-passband resonant cavity R1´ in the bottom microstrip, and a coupling resonant cavity R4´ is set on the ground metal II above the single-passband resonant cavity R2´ in the bottom microstrip, thereby obtaining a dual-passband filtering balun.

[0007] Optionally, the first interlayer dielectric and the third interlayer dielectric are both interlayer dielectric materials RT / duroid 5880 made of polytetrafluoroethylene glass fiber reinforced material, with a thickness of 0.203 mm; The second interlayer dielectric is specifically a prepreg material RO 4450F with a thickness of 0.101 mm.

[0008] Optionally, a first microstrip line is further provided in the top microstrip, and a second microstrip line and a third microstrip line are further provided in the bottom microstrip; the single-passband resonant cavity R1 and the single-passband resonant cavity R2 in the top microstrip are connected to the unbalanced port of the first microstrip line, and the single-passband resonant cavity R1' and the single-passband resonant cavity R2' in the bottom microstrip are respectively connected to the balanced port of the second microstrip line and the balanced port of the third microstrip line, and the width of the three microstrip lines is 0.7 mm.

[0009] Optionally, the radii of the multiple metal vias are all 0.1 mm and are evenly distributed.

[0010] Optionally, the single-passband resonant cavity R1 in the top microstrip, the single-passband resonant cavity R2 in the top microstrip, the single-passband resonant cavity R1´ in the bottom microstrip and the single-passband resonant cavity R2´ in the bottom microstrip are dumbbell-shaped, and the coupled resonant cavity R3, the coupled resonant cavity R4, the coupled resonant cavity R3´ and the coupled resonant cavity R4´ are C-shaped.

[0011] Optionally, the operating frequency band center frequency of the single-passband filtering balun is 2.9 GHz and the bandwidth is 400 MHz.

[0012] Optionally, the center frequency of the first operating frequency band of the dual-passband filtering balun is 2.1 GHz, and the bandwidth is 490 MHz; the center frequency of the second operating frequency band of the dual-passband filtering balun is 3.05 GHz, and the bandwidth is 310 MHz.

[0013] Compared with the prior art, the dual-passband filtering balun provided in the embodiment of the present invention has the following beneficial effects: The present invention proposes a spatial coupling structure, in which a window is provided at the center of ground metal I and ground metal II, so that the top microstrip and the bottom microstrip are coupled through the window. Compared with the traditional planar coupling structure, this structure can obtain a larger coupling coefficient and provide a greater coupling strength without the need for a cascade filter, thereby solving the shortcomings of insufficient coupling strength of planar microstrip lines and insufficient freedom in microstrip line planar design. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A dual-passband filtering balun layout of a dual-passband filtering balun provided in an embodiment, Figure 1 (a) is a top view from a 3D perspective. Figure 1 (b) is a bottom view from a 3D perspective. Figure 1 (c) is a cross-sectional view; Figure 2 A resonant cavity coupling topology diagram of a dual-passband filtering balun provided in one embodiment, Figure 2 (a) is the resonant cavity coupling topology diagram of the single-passband filtering balun. Figure 2 (b) is the resonant cavity coupling topology diagram of the dual-passband filtering balun; Figure 3 FIG. 1 is a coupling matrix fitting result diagram of a dual-passband filtering balun provided in an embodiment, Figure 3 (a) is the dual-passband filter resonant cavity coupling topology diagram and coupling parameter settings. Figure 3 (b) is the S parameter response calculated based on the coupling matrix; Figure 4 A four-port network balun design schematic diagram of a dual-passband filtering balun provided in one embodiment; Figure 5 A single-passband filtering balun layout of a dual-passband filtering balun provided in an embodiment, Figure 5 (a) is the 3D perspective. Figure 5 (b) is a cross-sectional view. Figure 5 (c) is the detailed shape of each metal layer; Figure 6 is a simulation performance diagram of a single-passband filtering balun of a dual-passband filtering balun provided in an embodiment, Figure 6 (a) is the S parameter response. Figure 6 (b) shows the phase difference and amplitude imbalance response; Figure 7 is a dual-passband filtering balun simulation performance diagram of a dual-passband filtering balun provided in an embodiment, Figure 7 (a) is the S parameter response. Figure 7 (b) is the phase difference and amplitude imbalance response of the first passband. Figure 7 (c) shows the phase difference and amplitude imbalance response of the second passband. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0016] In one embodiment, a dual-passband filtering balun is provided, a design idea and layout drawing method of the dual-passband filtering balun are proposed, and electromagnetic simulation function verification is completed. Based on the coupling topology and coupling strength calculated by the coupling matrix, single-passband and dual-passband filtering responses are supported. Moreover, through the balun design method of the four-port network, one port of the symmetrical four-port network is opened, and the transmission functions and reflection coefficients of the other ports are reasonably adjusted to support the balun response. Figure 1 and Figure 5 is a layout of a dual-passband filtering balun and a single-passband filtering balun according to an exemplary embodiment, Figure 1 (a) is a top view of the dual-passband filter balun layout from a 3D perspective. Figure 1 (b) is a 3D bottom view of the dual-passband filter balun layout. Figure 1 (c) is a cross-sectional view of the dual-passband filtering balun layout. Figure 5 (a) is a 3D view of the single-passband filtering balun layout. Figure 5 (b) is a cross-sectional view of the single-passband filtering balun layout. Figure 5 (c) is a detailed diagram of the shape of each metal layer in the single-passband filter balun layout. The following is a detailed introduction to the implementation plan:

[0017] The dual-passband filtering balun includes: a single-passband filtering balun, a coupling resonant cavity R3, a coupling resonant cavity R4, a coupling resonant cavity R3' and a coupling resonant cavity R4'.

[0018] The single-passband filtering balun includes: a top microstrip, a ground metal I, a ground metal II, a bottom microstrip and a plurality of metal vias. A first interlayer dielectric is arranged between the top microstrip and the ground metal I, a second interlayer dielectric is arranged between the ground metal I and the ground metal II, and a third interlayer dielectric is arranged between the ground metal II and the bottom microstrip. A plurality of metal vias penetrate the top microstrip, the first interlayer dielectric, the ground metal I, the second interlayer dielectric, the ground metal II, the third interlayer dielectric and the bottom microstrip from top to bottom. A window of the same size is chiseled at the center of the ground metal I and the ground metal II, and the window is used for coupling between the top microstrip and the bottom microstrip.

[0019] The top microstrip and the bottom microstrip are both provided with two single-passband resonant cavities. A coupling resonant cavity R3 is provided on the ground metal I below the single-passband resonant cavity R1 in the top microstrip, a coupling resonant cavity R4 is provided on the ground metal I below the single-passband resonant cavity R2 in the top microstrip, a coupling resonant cavity R3' is provided on the ground metal II above the single-passband resonant cavity R1' in the bottom microstrip, and a coupling resonant cavity R4' is provided on the ground metal II above the single-passband resonant cavity R2' in the bottom microstrip, thereby obtaining a dual-passband filtering balun.

[0020] The first interlayer dielectric and the third interlayer dielectric are both interlayer dielectric materials RT / duroid 5880 made of polytetrafluoroethylene glass fiber reinforced material, with a thickness of 0.203 mm, used for PCB production. The second interlayer dielectric is specifically a prepreg material RO 4450F, with a thickness of 0.101 mm, used as a laminate bonding layer.

[0021] The single-passband resonant cavity R1 in the top microstrip, the single-passband resonant cavity R2 in the top microstrip, the single-passband resonant cavity R1´ in the bottom microstrip and the single-passband resonant cavity R2´ in the bottom microstrip are dumbbell-shaped, and the coupled resonant cavity R3, the coupled resonant cavity R4, the coupled resonant cavity R3´ and the coupled resonant cavity R4´ are C-shaped.

[0022] The top microstrip also has a first microstrip line, and the bottom microstrip also has a second microstrip line and a third microstrip line. The single-passband resonant cavity R1 and the single-passband resonant cavity R2 in the top microstrip are connected to the unbalanced port of the first microstrip line, and the single-passband resonant cavity R1' and the single-passband resonant cavity R2' in the bottom microstrip are connected to the balanced port of the second microstrip line and the balanced port of the third microstrip line respectively. The width of the first microstrip line, the second microstrip line and the third microstrip line is 0.7 mm, which is used to achieve a transmission line characteristic impedance of 50Ω.

[0023] The radius of the multiple metal vias is 0.1 mm and they are evenly distributed to achieve the same potential of the ground metal I and the ground metal II.

[0024] The size of the window will directly affect the coupling strength. Since the coupling required by the single-passband filter balun and the dual-passband filter balun is different, the window size of the single-passband filter balun can be set to 1.2mm*1.9mm, and the window size of the dual-passband filter balun can be adjusted to 2.4mm*5.9mm.

[0025] The center frequency of the working band of the single-passband filter balun is 2.9GHz, and the bandwidth is 400MHz. The center frequency of the first working band of the dual-passband filter balun is 2.1GHz, and the bandwidth is 490MHz; the center frequency of the second working band of the dual-passband filter balun is 3.05GHz, and the bandwidth is 310MHz.

[0026] Step 1, topology selection based on resonant cavity: To design a filtering balun based on a microstrip resonant cavity, we must first determine the size and parameters of the resonant cavity and the order and passband number of the designed filter; the required main coupling strength and coupling topology of the resonant cavity can be calculated through the coupling matrix; an ordinary second-order filter network requires two resonant cavities, and a dual-passband filter network requires four resonant cavities for coupling, and its coupling topology and coupling strength need to be selected in a specific way.

[0027] Figure 2 The coupled topology of the dual-passband filter response adopted by the present invention is Figure 2 (a) is the resonant cavity coupling topology diagram of the single-passband filtering balun. Figure 2 (b) is the resonant cavity coupling topology diagram of the dual-passband filtering balun. It can be seen that the single-passband resonant cavity R1 and the single-passband resonant cavity R2, the single-passband resonant cavity R1 and the coupled resonant cavity R3, the single-passband resonant cavity R2 and the coupled resonant cavity R4 are strongly coupled, and the single-passband resonant cavity R1´ and the single-passband resonant cavity R2´, the single-passband resonant cavity R1´ and the coupled resonant cavity R3´, the single-passband resonant cavity R2´ and the coupled resonant cavity R4´ are strongly coupled, and the dual-passband filtering response can be obtained. Figure 3 The present invention is calculated by the coupling matrix based on the matlab tool. Figure 3 (a) is the dual-passband filter resonant cavity coupling topology diagram and coupling parameter settings. Figure 3 (b) is the S-parameter response calculated from the coupling matrix. The response generated by the dual-passband filtering topology also takes into account the weak coupling between different resonant cavities and between the resonant cavity and the source load.

[0028] Step 2, balun design method based on four-port network: The desired filtering effect can be obtained through reasonable resonator design and topology selection. The present invention adopts a balun design method based on a four-port network. By performing odd and even mode analysis on any symmetrical four-port network, as long as its reflection coefficient meets the corresponding conditions, the four-port network can realize the function of the balun.

[0029] Figure 4 middle, is the reflection coefficient, is the transmission coefficient, even Represents the even mode, odd represents odd mode. The corresponding conditions mentioned here are specifically Figure 4 Formula (1) and formula (2) in:

[0030] ; ; Satisfying formula (1) can achieve the reverse characteristics of port 2 and port 3, and satisfying formula (2) can achieve the matching characteristics of port 1, that is, the return loss is 0.

[0031] Figure 4 This is the design principle of the four-port network balun adopted by the present invention. The specific content of the design principle is: first, in the odd-mode state, the symmetry plane can be regarded as a virtual ground. When only the half-side circuit response is considered, the resonator is grounded, so it is a quarter-wavelength resonator. When in the even-mode state, the symmetry plane is regarded as an open circuit, so the resonator of the half-side circuit is open circuit, which is a half-wavelength resonator. According to basic electromagnetic microwave knowledge, the operating frequency of the half-wavelength resonator is twice that of the quarter-wavelength resonator. Therefore, for the half-side single-channel response, the passband frequency of the odd mode is half of that of the even mode. Therefore, when the operating frequency is set at the passband frequency of the odd mode, the even mode can be considered to have no propagation and does not pass the signal. Therefore, the even-mode transmission coefficient = 0, and the reflection coefficient = -1.

[0032] In order to satisfy formula (2) to achieve port matching, the propagation coefficient and reflection coefficient of the odd mode must be: ; This value is mainly achieved by changing the feeding position and feeding shape of the 50-ohm port in the resonant cavity. By continuously simulating and adjusting the appropriate position and shape, a good match can be achieved. It can be seen that for any symmetrical four-port network, when one of the ports is open, only the response of the half-side circuit is analyzed through odd-even mode analysis. The odd mode and even mode meet different transmission and reflection conditions respectively, and the differential response of ports 2 and 3 and the input matching of port 1 can be achieved.

[0033] The different transmission and reflection conditions mentioned here are: ; Step 3: Design and layout of single-passband filter balun based on spatial coupling structure: The layout design of the single-passband filter balun is drawn by using a four-port symmetrical layout design; the present invention innovatively proposes a spatial coupling structure, which hollows out a part of the middle ground metal to allow the upper and lower microstrips to couple through the middle window. Compared with the traditional planar coupling structure, this structure can obtain a larger coupling coefficient, and the vertical operation on the multi-layer PCB can bring greater freedom to the design. The single-passband filter balun and dual-passband filter use the ADS (Advanced Design System) and HFSS (High Frequency Structure Simulator) tools to reasonably select the interlayer medium and complete the layout drawing. They have good filtering characteristics and balun unbalanced characteristics, and can be widely used in RF wireless communication scenarios.

[0034] Figure 5 It is a layout of a single-passband filtering balun shown in an embodiment, and only a half-side symmetrical circuit is analyzed, and its response is a second-order filtering response based on two coupled resonant cavities.

[0035] Step 4: Design and layout of dual-passband filter balun based on spatial coupling structure: After verifying the function of the single-passband filtering balun, a dual-passband filtering balun was proposed to further verify the design applicability and degree of freedom of this invention. By adding two additional coupled resonant cavities to achieve dual-passband response, and placing the additional resonant cavity under the original single-passband resonant cavity, the topological structure of dual-passband filtering can be realized. The interlayer dielectric is reasonably selected and the layout drawing is completed through ADS and HFSS tools.

[0036] Figure 1 It is a layout of a dual-passband filtering balun shown according to an exemplary embodiment. Only half of the symmetrical circuit is analyzed. Compared with the filtering response of a single passband, a dual-passband filtering response based on four coupled resonant cavities can be achieved by adding a resonant cavity under each of the original two coupled resonant cavities to produce strong coupling.

[0037] Step 5, electromagnetic simulation verification: According to the above requirements, the layout is reasonably designed and drawn, the electromagnetic simulation of the layout is completed, and the S parameter curve is drawn to obtain the performance of the corresponding balun response. The simulation response of the single-passband filter balun is as follows Figure 6 As shown, Figure 6 (a) is the S parameter response. Figure 6 (b) in the figure is the phase difference and amplitude imbalance response. It can be seen that the single-pass filter balun has a good second-order filter response and excellent amplitude and phase imbalance response. The simulation response of the dual-passband filter balun is shown in Figure 7 As shown, Figure 7 (a) is the S parameter response. Figure 7 (b) is the phase difference and amplitude imbalance response of the first passband. Figure 7 (c) in the figure is the phase difference and amplitude imbalance response of the second passband. It can be seen that the dual-passband filtering balun has good filtering response and excellent amplitude and phase imbalance response.

[0038] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A dual-passband filtering balun, characterized in that: include: A single-passband filtering balun, a coupling resonant cavity R3, a coupling resonant cavity R4, a coupling resonant cavity R3' and a coupling resonant cavity R4', wherein the single-passband filtering balun comprises from top to bottom: a top microstrip, a ground metal I, a ground metal II, a bottom microstrip and a plurality of metal vias; A first interlayer dielectric is arranged between the top microstrip and the ground metal I, a second interlayer dielectric is arranged between the ground metal I and the ground metal II, and a third interlayer dielectric is arranged between the ground metal II and the bottom microstrip; the plurality of metal vias penetrate the top microstrip, the first interlayer dielectric, the ground metal I, the second interlayer dielectric, the ground metal II, the third interlayer dielectric and the bottom microstrip from top to bottom; windows of the same size are chiseled at the center of the ground metal I and the ground metal II, and the windows are used for coupling between the top microstrip and the bottom microstrip; the top microstrip and the bottom microstrip are each provided with two single-passband resonant cavities; The coupling resonant cavity R3 is set on the ground metal I below the single-passband resonant cavity R1 in the top microstrip, and the coupling resonant cavity R4 is set on the ground metal I below the single-passband resonant cavity R2 in the top microstrip; the coupling resonant cavity R3' is set on the ground metal II above the single-passband resonant cavity R1' in the bottom microstrip, and the coupling resonant cavity R4' is set on the ground metal II above the single-passband resonant cavity R2' in the bottom microstrip, so as to obtain a dual-passband filtering balun.

2. A dual-passband filtering balun as claimed in claim 1, characterized in that: The first interlayer dielectric and the third interlayer dielectric are both made of polytetrafluoroethylene glass fiber reinforced material RT / duroid 5880, with a thickness of 0.203 mm; The second interlayer dielectric is specifically a prepreg material RO 4450F, with a thickness of 0.101 mm.

3. A dual-passband filtering balun as claimed in claim 1, characterized in that: A first microstrip line is also provided in the top microstrip, and a second microstrip line and a third microstrip line are also provided in the bottom microstrip; the single-passband resonant cavity R1 and the single-passband resonant cavity R2 in the top microstrip are connected to the unbalanced port of the first microstrip line, and the single-passband resonant cavity R1' and the single-passband resonant cavity R2' in the bottom microstrip are respectively connected to the balanced port of the second microstrip line and the balanced port of the third microstrip line, and the width of the three microstrip lines is 0.7 mm.

4. A dual-passband filtering balun as claimed in claim 1, characterized in that: The radius of the multiple metal vias is 0.1 mm and they are evenly distributed.

5. A dual-passband filtering balun as claimed in claim 1, characterized in that: The single-passband resonant cavity R1 in the top microstrip, the single-passband resonant cavity R2 in the top microstrip, the single-passband resonant cavity R1´ in the bottom microstrip and the single-passband resonant cavity R2´ in the bottom microstrip are dumbbell-shaped, and the coupled resonant cavity R3, the coupled resonant cavity R4, the coupled resonant cavity R3´ and the coupled resonant cavity R4´ are C-shaped.

6. A dual-passband filtering balun as claimed in claim 1, characterized in that: The operating frequency band center frequency of the single-passband filtering balun is 2.9 GHz and the bandwidth is 400 MHz.

7. A dual-passband filtering balun as claimed in claim 1, characterized in that: The center frequency of the first working frequency band of the dual-passband filtering balun is 2.1 GHz, and the bandwidth is 490 MHz; the center frequency of the second working frequency band of the dual-passband filtering balun is 3.05 GHz, and the bandwidth is 310 MHz.

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