A low process sensitivity secondary folded substrate integrated waveguide bandpass filter

Abstract

The application is a secondary folding substrate integrated waveguide band-pass filter with low processing sensitivity, which is a layered structure and sequentially comprises a top layer metal (1), a first layer dielectric substrate (2), an intermediate layer metal (3), a second layer dielectric substrate (4), a third layer dielectric substrate (5), and a bottom layer metal (6) from top to bottom; a signal enters through an input port (S), passes through a first cavity (C1), a second cavity (C2), a third cavity (C3), and a fourth cavity (C4), and finally flows out through an output port (L); the application proposes a coupling structure with low processing sensitivity, improves the robustness of DFSIW to PCB via positioning error and gap etching, and realizes DFSIW filter using PCB process in the millimeter wave band. Moreover, the DFSIW cavity structure is optimized for multilayer PCB process, which significantly improves the Q value, thereby reducing the loss and further improving the out-of-band rejection of the filter.

Description

Technical Field

[0001] This invention relates to the field of microwave communication, and in particular to a miniaturized bandpass filter with low fabrication sensitivity based on a double-folded substrate integrated waveguide resonant cavity. Background Technology

[0002] Substrate integrated waveguides (SIWs) have garnered widespread attention from academia and industry due to their advantages such as low fabrication cost, low loss, high power capacity, and ease of integration. However, the side length of a traditional square SIW resonator is equal to the wavelength, resulting in a large area for SIW filters, which cannot meet the miniaturization requirements of RF front-ends. To address this issue, various solutions have been reported, including the use of higher-order mode resonators, multimode resonators, hybrid coupling structures, ridge waveguide resonators, folded substrate integrated waveguides (FSIWs), and double-folded substrate integrated waveguides (DFSIWs). FSIWs, by folding the SIW in one direction, distribute the area of ​​a single layer across two layers, thus reducing the area by approximately 50%. DFSIWs go further, folding the SIW in two directions, further reducing the area by approximately 75%.

[0003] Existing SIW miniaturization solutions suffer from numerous problems. For example, high-order mode resonators may reduce the quality factor (Q), ridge waveguide resonators may introduce additional losses, the design and optimization of hybrid coupling structures are complex, and interference between multiple modes in multimode resonators is prone to occur. Furthermore, as the frequency increases, the size of miniaturized SIW filters needs to be further reduced, placing higher demands on fabrication precision. Currently, only DFSIW filters based on printed circuit boards (PCBs) in the low-frequency band have been reported. In addition, some researchers have implemented DFISW filters in the millimeter-wave band using low-temperature co-fired ceramics (LTCC) with higher fabrication precision. However, integrating LTCC filters into the RF front-end incurs additional assembly costs and introduces more insertion loss. Currently, there are no reports of implementing DFISW filters in the millimeter-wave band using PCB technology.

[0004] Existing filter miniaturization solutions approach the limits of PCB fabrication precision at high frequencies, easily leading to mismatches between design and actual manufacturing results. Furthermore, traditional DFSIW cavities introduce high Q-value losses, thus increasing filter insertion loss.

[0005] To address the shortcomings of existing technologies, this invention proposes a low-processing-sensitivity coupling structure, improving the robustness of DFSIW to PCB via positioning errors and gap etching, and enabling the implementation of DFSIW filters using PCB processes in the millimeter-wave band. Furthermore, the DFSIW cavity structure is optimized for multi-layer PCB processes, significantly improving the Q value and thus reducing losses. In addition, the coupling structure is transformed into a cross-coupling structure, further enhancing the out-of-band rejection of the filter. Summary of the Invention

[0006] Technical Problem: The purpose of this invention is to propose a low-processing-sensitivity, double-folded substrate integrated waveguide bandpass filter to address the problem that current filter miniaturization schemes are significantly affected by processing errors at higher frequencies, leading to a mismatch between design and actual production results. Furthermore, the filter insertion loss is reduced by optimizing the stacked structure, and a cross-coupling structure adapted to the DFSIW cavity is designed to introduce two transmission zeros outside the band, thereby improving out-of-band suppression.

[0007] Technical Solution: This invention discloses a low-processing-sensitivity, double-folded substrate integrated waveguide bandpass filter. The filter has a layered structure, comprising, from top to bottom, a top layer metal forming the boundary of the upper DFSIW cavity, a first layer dielectric substrate constituting the upper half of the DFSIW cavity dielectric region, an intermediate layer metal that folds the DFSIW cavity and defines the coupling strength, a second layer dielectric substrate constituting the lower half of the DFSIW cavity dielectric region, a third layer dielectric substrate constituting the lower half of the DFSIW cavity dielectric region and bonding adjacent layers, and a bottom layer metal forming the boundary of the lower DFSIW cavity dielectric region. The signal enters through the input port, passes through the first, second, third, and fourth cavities, and finally exits through the output port. The coupling between the first and second cavities is electrical coupling. k 12 The coupling between the second cavity and the third cavity is electrical coupling. k 23 The coupling between the second cavity and the third cavity is electrical coupling. k 34 The coupling between the first cavity and the fourth cavity is a cross coupling. k 14 This creates a transmission zero outside the transmission passband.

[0008] The top layer metal has a first metallized through hole arranged in an Ω shape, consisting of multiple metallized through holes. The bottom layer metal has a fourth metallized through hole arranged in a wave shape, consisting of multiple metallized through holes, forming the cavity boundary of the DFSIW. A single second metallized through hole is provided in the first metallized through hole arranged in an Ω shape. The single second metallized through hole is located between the second cavity and the third cavity. Below the single second metallized through hole, there are three consecutive third metallized through holes, which are located in the middle of the first cavity, the second cavity, the third cavity, and the fourth cavity.

[0009] A U-shaped groove is provided on both sides between the first metallized via arranged in an Ω shape and the fourth metallized via arranged in a wave shape. The U-shaped grooves on both sides are slots that separate the ground and the signal by grounded coplanar waveguides (GCPWs). A first metal strip and a second metal strip are respectively provided in the two U-shaped grooves.

[0010] The first metal strip and the second metal strip are GCPW signal transmission structures. The first metal strip and the second metal strip are connected to the input port and the output port, respectively. Each of the first metal strip and the second metal strip is provided with a pair of first metallized blind holes connecting the top layer metal and the middle layer metal, so as to conduct the signal from the first metal strip and the second metal strip to the middle layer metal.

[0011] The intermediate metal layer includes a fifth, a sixth, a seventh, and an eighth metallized via. The metallized vias of the intermediate metal layer correspond to the metallized vias of the top metal layer, that is, the fifth metallized via corresponds to the first metallized via, the sixth metallized via corresponds to the second metallized via, the seventh metallized via corresponds to the third metallized via, and the eighth metallized via corresponds to the fourth metallized via. The second metallized blind via of the intermediate metal layer corresponds to the first metallized blind via of the top metal layer, thus realizing the transmission of signals from the top metal layer to the intermediate metal layer.

[0012] The upper part of the fifth metallized through hole is provided with two L-shaped slots with symmetrical center lines, and the two L-shaped slots are disconnected; a pair of parallel rectangular slots are provided on both sides of the seventh metallized through hole, and a T-shaped slot is provided at the lower part of the seventh metallized through hole. The T-shaped slot is embedded in the folded groove of the DFSIW cavity to form a cross-coupling structure suitable for the DFSIW structure; J-shaped slots are connected on both sides of the folded groove of the DFSIW cavity.

[0013] The bottom metal includes a ninth, tenth, eleventh, and twelfth metallized via. The metallized vias of the bottom metal correspond to the metallized vias of the top metal, that is, the ninth metallized via corresponds to the first metallized via, the tenth metallized via corresponds to the second metallized via, the eleventh metallized via corresponds to the third metallized via, and the twelfth metallized via corresponds to the fourth metallized via. The bottom metal is a complete metal plane, which constitutes the lower boundary of the DFSIW cavity.

[0014] The following are the definitions and explanations of terms used in this invention:

[0015] Substrate Integrated Waveguide (SIW); Folded Substrate Integrated Waveguide (FSIW); Double-Folded Substrate Integrated Waveguide (DFSIW); Printed Circuit Board (PCB); Quality factor (Q); Low Temperature Co-fired Ceramic (LTCC); Grounded Coplanar Waveguide (GCPW); decibel (dB). Beneficial effects

[0016] 1. Miniaturization: For the first time, DFSIW filters are implemented using PCB technology in the millimeter-wave band, reducing the size by approximately 75% compared to traditional SIW filters, with the overall size controlled to 0.49 mm. λ 0 ×0.49 λ 0 ,in λ 0 The wavelength in vacuum at the center frequency of the filter.

[0017] 2. Excellent robustness: The use of coupling structures (3-6) and (3-8) with low processing sensitivity optimizes the performance of the filter in actual production from the perspectives of insertion loss, return loss and frequency offset.

[0018] 3. Strong frequency selectivity: A cross-coupling structure suitable for DFSIW structure is proposed (3-11), which introduces a transmission zero in each of the upper and lower frequency bands, thereby enhancing frequency selectivity.

[0019] 4. Low insertion loss: A high-Q DFSIW cavity based on a multilayer PCB was designed, which significantly reduces insertion loss compared to traditional multilayer millimeter-wave filters. Attached Figure Description

[0020] Figure 1 An exploded 3D view of a low-process-sensitivity filter structure based on DFSIW; Figure 2 The coupling topology diagram of a low-process-sensitivity filter based on DFSIW is shown. Figure 3 Top view of the top metal layer of a DFSIW-based low-process-sensitivity filter; Figure 4 Top view of the intermediate metal layer of a DFSIW-based low-process-sensitivity filter; Figure 5 Top view of the underlying metal layer of a DFSIW-based low-process-sensitivity filter; Figure 6 The results of sensitivity analysis on 100 filter error samples are as follows. Figure 6 (a) in the figure represents the analysis results of the traditional structure. Figure 6 (b) in the figure represents the analysis results of the structure of the present invention.

[0021] The diagram shows: top metal layer 1, first dielectric substrate layer 2, middle metal layer 3, second dielectric substrate layer 4, third dielectric substrate layer 5, bottom metal layer 6, input port S, output port L, first cavity C1, second cavity C2, third cavity C3, fourth cavity C4, first metallized via 1-1, second metallized via 1-2, third metallized via 1-3, first metallized blind via 1-4, first metal strip 1-5, second metal strip 1-6, U-groove 1-7, fourth metallized via 1- 8; Fifth metallized through hole 3-1, Sixth metallized through hole 3-2, Seventh metallized through hole 3-3, Second metallized blind hole 3-4, Eighth metallized through hole 3-5, Disconnect structure 3-6, L-shaped slot 3-7, Parallel rectangular slot 3-8, T-shaped slot 3-9, DFSIW cavity folded slot 3-10, Cross-coupling structure 3-11, J-shaped slot 3-12; Ninth metallized through hole 6-1, Tenth metallized through hole 6-2, Eleventh metallized through hole 6-3, Twelfth metallized through hole 6-4. Detailed Implementation

[0022] The low-fabrication-sensitivity double-folded substrate integrated waveguide bandpass filter of the present invention is a miniaturized bandpass filter with low fabrication sensitivity based on a double-folded substrate integrated waveguide (DFSIW) cavity. Its center frequency is 26 GHz and its bandwidth is 1.4 GHz. Its structural diagram is shown below. Figure 1 As shown. The filter has a layered structure, consisting of, from top to bottom, the top layer metal 1 (which serves as the upper metal boundary of the DFSIW cavity), the first dielectric substrate 2 (which forms the dielectric region of the upper half of the DFSIW cavity), the middle layer metal 3 (which folds the DFSIW cavity and defines the coupling strength), the second dielectric substrate 4 (which forms part of the dielectric region of the lower half of the DFSIW cavity), the third dielectric substrate 5 (which forms part of the dielectric region of the lower half of the DFSIW cavity and bonds adjacent layers), and the bottom layer metal 6 (which serves as the lower metal boundary of the DFSIW cavity). The signal enters through the input port S, passes through the first cavity C1, the second cavity C2, the third cavity C3, and the fourth cavity C4, and finally flows out through the output port L. The coupling between the first cavity C1 and the second cavity C2 is electrical coupling. k 12 The coupling between the second cavity C2 and the third cavity C3 is electrical coupling. k 23 The coupling between the second cavity C2 and the third cavity C3 is electrical coupling. k 34 The coupling between the first cavity C1 and the fourth cavity C4 is a cross coupling. k 14 This creates a transmission zero outside the transmission passband.

[0023] The top layer metal 1 has a first metallized through hole 1-1 arranged in an Ω shape, consisting of multiple metallized through holes. The bottom layer metal 1 has a fourth metallized through hole 1-8 arranged in a wave shape, consisting of multiple metallized through holes, forming the cavity boundary of the DFSIW. A single second metallized through hole 1-2 is provided in the first metallized through hole 1-1 arranged in an Ω shape. The single second metallized through hole 1-2 is located between the second cavity C2 and the third cavity C3. Below the single second metallized through hole 1-2, there are three consecutive third metallized through holes 1-3, which are located in the middle of the first cavity C1, the second cavity C2, the third cavity C3 and the fourth cavity C4. A U-shaped groove 1-7 is provided on both sides between the first metallized via 1-1 arranged in an Ω shape and the fourth metallized via 1-8 arranged in a wave shape. The U-shaped groove 1-7 on both sides is a slot to separate the ground and the signal in a grounded coplanar waveguide (GCPW). A first metal strip 1-5 and a second metal strip 1-6 are respectively provided in the two U-shaped grooves 1-7. The first metal strip 1-5 and the second metal strip 1-6 are GCPW signal transmission structures, and the first metal strip 1-5 and the second metal strip 1-6 are respectively connected to the input port S and the output port L. A pair of first metallized blind vias 1-4 are provided on the first metal strip 1-5 and the second metal strip 1-6 to connect the top layer metal 1 and the middle layer metal 3, thereby conducting the signal from the first metal strip 1-5 and the second metal strip 1-6 to the middle layer metal 3.

[0024] The intermediate metal layer 3 includes a fifth metallized via 3-1, a sixth metallized via 3-2, a seventh metallized via 3-3, and an eighth metallized via 3-5. The metallized vias of the intermediate metal layer 3 correspond to the metallized vias of the top metal layer 1, that is, the fifth metallized via 3-1 corresponds to the first metallized via 1-1, the sixth metallized via 3-2 corresponds to the second metallized via 1-2, the seventh metallized via 3-3 corresponds to the third metallized via 1-3, and the eighth metallized via 3-5 corresponds to the fourth metallized via 1-8. The second metallized blind via 3-4 of the intermediate metal layer 3 corresponds to the first metallized blind via 1-4 of the top metal layer 1, thus realizing the transmission of signals from the top metal layer 1 to the intermediate metal layer 3. The upper part of the fifth metallized through hole 3-1 is provided with two L-shaped slots 3-7 with symmetrical center lines, and the two L-shaped slots 3-7 are connected by a disconnect structure 3-6; a pair of parallel rectangular slots 3-8 are provided on both sides of the seventh metallized through hole 3-3, and a T-shaped slot 3-9 is provided at the lower part of the seventh metallized through hole 3-3. The T-shaped slot 3-9 is embedded in the DFSIW cavity folding groove 3-10 to form a cross-coupling structure 3-11 suitable for the DFSIW structure; J-shaped slots 3-12 are connected on both sides of the DFSIW cavity folding groove 3-10.

[0025] The straight sections of a pair of L-shaped slots 3-7, the DFSIW cavity folding slots 3-10, and a pair of J-shaped slots 3-12 constitute the folding structure of the DFSIW. Through secondary folding, the DFSIW cavity is reduced in size by approximately 75% compared to a substrate-integrated waveguide (SIW) cavity. The bent portion of the J-shaped slot 3-12 separates the second metallized blind via 3-4, which transmits signals, from the ground metal. A break structure 3-6 is provided between the pair of L-shaped slots 3-7. A pair of parallel rectangular slots 3-8 are provided between the first cavity C1 and the second cavity C2, and between the third cavity C3 and the fourth cavity C4. These two structures represent the low-processing-sensitivity coupling structures proposed in this invention. The break structure 3-6 affects the coupling between the second cavity C2 and the third cavity C3. k 23 A pair of parallel rectangular slots 3-8 affect the coupling between the first cavity C1 and the second cavity C2. k 12 Coupling between the third cavity C3 and the fourth cavity C4 k 34 After adding these two structures k 12 , k 23 and k 34 The positional sensitivity of the fifth metallized via 3-1, the sixth metallized via 3-2, the seventh metallized via 3-3, and the eighth metallized via 3-5 decreases, thereby improving the filter's robustness to manufacturing errors. A T-shaped slot 3-9 is embedded within the folded groove 3-10 of the DFSIW cavity, thus forming a cross-coupling structure 3-11, creating cross-coupling between the first cavity C1 and the fourth cavity C4. k 14 This introduces out-of-band transmission zeros.

[0026] The bottom metal 6 includes a ninth metallized through-hole 6-1, a tenth metallized through-hole 6-2, an eleventh metallized through-hole 6-3, and a twelfth metallized through-hole 6-4. The metallized through-holes of the bottom metal 6 correspond to the metallized through-holes of the top metal 1, that is, the ninth metallized through-hole 6-1 corresponds to the first metallized through-hole 1-1, the tenth metallized through-hole 6-2 corresponds to the second metallized through-hole 1-2, the eleventh metallized through-hole 6-3 corresponds to the third metallized through-hole 1-3, and the twelfth metallized through-hole 6-4 corresponds to the fourth metallized through-hole 1-8. The bottom metal 6 is a complete metal plane, which constitutes the lower boundary of the DFSIW cavity.

[0027] While folding the DFSIW cavity reduces its area by approximately 75% compared to the SIW cavity, it lowers the quality factor Q, leading to increased insertion loss. To address this issue, the traditional DFSIW stack-up structure based on a 4-layer PCB process was optimized. For example... Figure 1As shown, the second dielectric substrate 4 and the third dielectric substrate 5 are not metallized, allowing these two dielectric layers to be directly connected, forming an equivalent dielectric with lower loss and greater thickness compared to the second dielectric substrate 4. Intrinsic simulation results show that, compared to a traditional DFSIW cavity, this structure can improve the Q value by approximately 42%, thereby reducing the filter's insertion loss.

[0028] Using the above structure, sensitivity analysis of the coupling parameters of the filter proposed in this invention was performed. Considering the errors in hole positioning (±0.05 mm) and gap etching (±0.03 mm) during PCB manufacturing, the via positions and rectangular slots associated with internal coupling were set as random perturbations, and 100 samples were analyzed. The filter proposed in this invention exhibits significantly better performance than traditional coupling structures in terms of maximum in-band return loss, maximum in-band insertion loss, lower frequency band transmission zero-point offset, and upper frequency band transmission zero-point offset, with values ​​of -10.8 dB, -2.8 dB, 2.66%, and 1.59%, respectively.

Claims

1. A low-processing-sensitivity, double-folded substrate integrated waveguide bandpass filter, characterized in that, The filter has a layered structure, consisting of, from top to bottom, the top layer metal (1) which forms the upper metal boundary of the DFSIW cavity, the first dielectric substrate (2) which forms the dielectric region of the upper half of the DFSIW cavity, the middle layer metal (3) which folds the DFSIW cavity and defines the coupling strength, the second dielectric substrate (4) which forms the dielectric region of the lower half of the DFSIW cavity, the third dielectric substrate (5) which forms the dielectric region of the lower half of the DFSIW cavity and bonds adjacent layers, and the bottom layer metal (6) which forms the lower metal boundary of the DFSIW cavity. The signal enters through the input port (S), passes through the first cavity (C1), the second cavity (C2), the third cavity (C3), and the fourth cavity (C4), and finally flows out through the output port (L). The coupling between the first cavity (C1) and the second cavity (C2) is electrical coupling. k 12 The coupling between the second cavity (C2) and the third cavity (C3) is electrical coupling. k 23 The coupling between the second cavity (C2) and the third cavity (C3) is electrical coupling. k 34 The coupling between the first cavity (C1) and the fourth cavity (C4) is a cross coupling. k 14 This creates a transmission zero outside the transmission passband.

2. The low-processing-sensitivity double-folded substrate integrated waveguide bandpass filter according to claim 1, characterized in that, The top layer metal (1) has a first metallized through hole (1-1) arranged in an Ω shape, consisting of multiple metallized through holes. The bottom layer metal (1) has a fourth metallized through hole (1-8) arranged in a wave shape, consisting of multiple metallized through holes, forming the cavity boundary of the DFSIW. A single second metallized through hole (1-2) is provided in the first metallized through hole (1-1) arranged in an Ω shape. The single second metallized through hole (1-2) is located between the second cavity (C2) and the third cavity (C3). Below the single second metallized through hole (1-2) are three consecutive third metallized through holes (1-3). The three consecutive third metallized through holes (1-3) are located in the middle of the first cavity (C1), the second cavity (C2), the third cavity (C3) and the fourth cavity (C4).

3. The low-processing-sensitivity double-folded substrate integrated waveguide bandpass filter according to claim 2, characterized in that, A U-shaped groove (1-7) is provided on both sides between the first metallized via (1-1) arranged in an Ω shape and the fourth metallized via (1-8) arranged in a wave shape. The U-shaped groove (1-7) on both sides is a slot to separate the ground and the signal by the grounded coplanar waveguide (GCPW). A first metal strip (1-5) and a second metal strip (1-6) are provided in the two U-shaped grooves (1-7).

4. The low-processing-sensitivity double-folded substrate integrated waveguide bandpass filter according to claim 3, characterized in that, The first metal strip (1-5) and the second metal strip (1-6) are GCPW signal transmission structures. The first metal strip (1-5) and the second metal strip (1-6) are connected to the input port (S) and the output port (L) respectively. A pair of first metallized blind holes (1-4) connecting the top metal (1) and the middle metal (3) are provided on the first metal strip (1-5) and the second metal strip (1-6) respectively, so as to conduct the signal from the first metal strip (1-5) and the second metal strip (1-6) to the middle metal (3).

5. A low-processing-sensitivity double-folded substrate integrated waveguide bandpass filter according to claim 1, characterized in that, The intermediate metal layer (3) includes a fifth metallized via (3-1), a sixth metallized via (3-2), a seventh metallized via (3-3), and an eighth metallized via (3-5). The metallized vias of the intermediate metal layer (3) correspond to the metallized vias of the top metal layer (1), that is, the fifth metallized via (3-1) corresponds to the first metallized via (1-1), the sixth metallized via (3-2) corresponds to the second metallized via (1-2), the seventh metallized via (3-3) corresponds to the third metallized via (1-3), and the eighth metallized via (3-5) corresponds to the fourth metallized via (1-8). The second metallized blind via (3-4) of the intermediate metal layer (3) corresponds to the first metallized blind via (1-4) of the top metal layer (1), thus realizing the transmission of signals from the top metal layer (1) to the intermediate metal layer (3).

6. A low-processing-sensitivity double-folded substrate integrated waveguide bandpass filter according to claim 5, characterized in that, The upper part of the fifth metallized through hole (3-1) is provided with two L-shaped slots (3-7) with symmetrical center lines, and the two L-shaped slots (3-7) are disconnected (3-6); a pair of parallel rectangular slots (3-8) are provided on both sides of the seventh metallized through hole (3-3), and a T-shaped slot (3-9) is provided at the lower part of the seventh metallized through hole (3-3). The T-shaped slot (3-9) is embedded in the DFSIW cavity folding groove (3-10) to form a cross-coupling structure (3-11) suitable for the DFSIW structure; J-shaped slots (3-12) are connected on both sides of the DFSIW cavity folding groove (3-10). According to claim 5, a low-processing-sensitivity secondary folded substrate integrated waveguide bandpass filter is characterized in that: two L-shaped slots (3-7) with symmetrical center lines are provided in the upper part of the fifth metallized via (3-1), and the two L-shaped slots (3-7) are disconnected (3-6); a pair of parallel rectangular slots (3-8) are provided on both sides of the seventh metallized via (3-3); a T-shaped slot (3-9) is provided in the lower part of the seventh metallized via (3-3), and the T-shaped slot (3-9) is embedded in the DFSIW cavity folding slot (3-10) to form a cross-coupling structure (3-11) suitable for the DFSIW structure; and J-shaped slots (3-12) are connected on both sides of the DFSIW cavity folding slot (3-10).

7. A low-processing-sensitivity double-folded substrate integrated waveguide bandpass filter according to claim 5, characterized in that, The bottom metal (6) includes a ninth metallized through-hole (6-1), a tenth metallized through-hole (6-2), an eleventh metallized through-hole (6-3), and a twelfth metallized through-hole (6-4). The metallized through-holes of the bottom metal (6) correspond to the metallized through-holes of the top metal (1), that is, the ninth metallized through-hole (6-1) corresponds to the first metallized through-hole (1-1), the tenth metallized through-hole (6-2) corresponds to the second metallized through-hole (1-2), the eleventh metallized through-hole (6-3) corresponds to the third metallized through-hole (1-3), and the twelfth metallized through-hole (6-4) corresponds to the fourth metallized through-hole (1-8). The bottom metal (6) is a complete metal plane, which constitutes the lower boundary of the DFSIW cavity.

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