Crossing structure of transmission lines

Abstract

A crossing structure of transmission lines according to the present invention is constructed such that, within a region where a coplanar transmission line (CPW1) formed of a conductor of a first layer and a microstrip transmission line (MSL1) formed of a conductor of a second layer below the first layer intersect, a ground conductor (50) is disposed in a third layer immediately below the microstrip transmission line (MSL1) in a region where a ground conductor (11) forming the coplanar transmission line (CPW1) is absent in a layer above the microstrip transmission line (MSL1).

Description

Crossing structure of transmission lines

[0001] This invention relates to a structure in which a coplanar transmission line and a microstrip transmission line are intersected in a three-dimensional manner.

[0002] In the design of (Radio Frequency) transmission lines using multilayer wiring within an MMIC (Monolithic Microwave Integrated Circuit), there are cases where it is necessary to cross multiple types of RF transmission lines. When crossing a coplanar transmission line CPW1 composed of conductors in the top layer (first layer) and a microstrip transmission line MSL1 composed of conductors in the second layer, a configuration like that shown in Figure 13 is generally adopted (Non-Patent Literature 1). In Figure 13, to make the crossing structure easier to understand, the conductors of the first layer and vias (described later) are shown with solid lines, and the conductors of the second layer are shown in color. Cross-sectional views of the crossing structure in Figure 13 are shown in Figures 14 and 15.

[0003] The coplanar transmission line CPW1 consists of a signal line 10 and ground conductors 11 formed along the signal line 10 on both sides of the signal line 10. A bridge conductor 20 is formed on the same layer as the microstrip transmission line MSL1 to connect the two ground conductors 11. The ground conductors 11 and the bridge conductor 20 are connected by vias 30. Figure 14 shows the structure of the part containing the ground conductors 11 and the microstrip transmission line MSL1, but the structure of the part containing the signal line 10 and the microstrip transmission line MSL1 is the same as in Figure 14. In Figures 14 and 15, 40 is a dielectric.

[0004] In the crossing structure shown in FIGS. 13 to 15, in a region where there is no ground conductor 11 above the microstrip transmission line MSL1, the electric field mode of the microstrip transmission line MSL1 changes significantly, and there is a problem that radiation loss and reflection loss increase. FIG. 16 shows the results of simulating the transmission loss of the microstrip transmission line MSL1 when it intersects with the coplanar transmission line CPW1 and when it does not intersect. 100 in FIG. 16 indicates the transmission loss when the microstrip transmission line MSL1 does not intersect with the coplanar transmission line CPW1, and 101 indicates the transmission loss when they intersect. According to FIG. 16, it can be seen that the loss of the microstrip transmission line MSL1 increases due to the intersection with the coplanar transmission line CPW1.

[0005] Leo G.Maloratsky, “Reviewing the basics of microstrip lines”, Microwaves and Rf, March 2000, pp.79 - 88

[0006] The present invention is made to solve the above problems, and an object thereof is to provide a crossing structure of a transmission line capable of reducing the transmission loss of a microstrip transmission line in a region where the microstrip transmission line intersects with a coplanar transmission line.

[0007] The crossing structure of the transmission line of the present invention is characterized in that, in a region where a coplanar transmission line composed of a conductor in the first layer intersects with a microstrip transmission line composed of a conductor in the second layer below the first layer, a second ground conductor is arranged in the third layer directly below the microstrip transmission line in a region where there is no first ground conductor constituting the coplanar transmission line above the microstrip transmission line.

[0008] In this invention, in the region where the coplanar transmission line and the microstrip transmission line intersect, in the region where the first ground conductor constituting the coplanar transmission line is not present in the upper layer of the microstrip transmission line, a second ground conductor is placed in the third layer directly below the microstrip transmission line. As a result, in this invention, it is possible to mitigate large disturbances in the electric field modes of the microstrip transmission line in the region where the microstrip transmission line intersects with the coplanar transmission line, thereby reducing the transmission loss of the microstrip transmission line.

[0009] Figure 1 is a perspective view of the cross structure of a transmission line according to the first embodiment of the present invention. Figure 2 is a cross-sectional view of the cross structure of a transmission line according to the first embodiment of the present invention. Figure 3 is a cross-sectional view of the cross structure of a transmission line according to the first embodiment of the present invention. Figure 4 is a cross-sectional view of the cross structure of a transmission line according to the first embodiment of the present invention. Figure 5 is a cross-sectional view of the cross structure of a transmission line according to the first embodiment of the present invention. Figure 6 is a diagram showing the results of a simulation of the transmission loss of a microstrip transmission line according to the first embodiment of the present invention. Figure 7 is a diagram showing the simulation results of the transmission loss of a microstrip transmission line when the width of the ground conductor is changed in the second embodiment of the present invention. Figure 8 is a perspective view of the cross structure of a transmission line according to the second embodiment of the present invention. Figure 9 is a cross-sectional view of the cross structure of a transmission line according to the second embodiment of the present invention. Figure 10 is a perspective view of the cross structure of a transmission line according to the third embodiment of the present invention. Figure 11 is a cross-sectional view of the cross structure of a transmission line according to the third embodiment of the present invention. Figure 12 is a diagram showing the results of a simulation of the transmission loss of a microstrip transmission line according to the third embodiment of the present invention. Figure 13 is a perspective view of a conventional transmission line crossing structure. Figure 14 is a cross-sectional view of a conventional transmission line crossing structure. Figure 15 is a cross-sectional view of a conventional transmission line crossing structure. Figure 16 is a diagram showing the results of a simulation of the transmission loss of a microstrip transmission line when it crosses a coplanar transmission line and when it does not.

[0010] [First Embodiment] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a perspective view of the crossing structure of a transmission line according to the first embodiment of the present invention. In this embodiment, in the region where the microstrip transmission line MSL1 intersects with the coplanar transmission line CPW1, a ground conductor 50 composed of a third layer of conductors is placed below the microstrip transmission line MSL1, as shown in Figure 1. In Figure 1, to make the crossing structure easier to understand, the uppermost layer (first layer) conductor and vias described later are shown with solid lines, the second layer conductor is shown in color, and the third layer conductor is shown with dashed lines. Figures 2 to 5 show cross-sectional views of the crossing structure of Figure 1.

[0011] As in conventional systems, the coplanar transmission line CPW1 consists of a signal line 10 and a ground conductor 11. To connect the two ground conductors 11, a bridge conductor 20 is formed in the same layer as the microstrip transmission line MSL1. The conductors in the first, second, and third layers are insulated by a dielectric 40. The ground conductor 11 and the bridge conductor 20 are connected by vias 30 made of conductors formed in the dielectric 40. The bridge conductor 20 and the ground conductor 50 are connected by vias 60 made of conductors formed in the dielectric 40. Thus, the ground conductor 11 and the ground conductor 50 are connected via vias 30, 60 and the bridge conductor 20.

[0012] In this embodiment, in the region where the microstrip transmission line MSL1 intersects with the coplanar transmission line CPW1, and in the region where there is no ground conductor 11 above the microstrip transmission line MSL1, the ground conductor 50 is placed directly below the microstrip transmission line MSL1. This makes it possible to mitigate large disturbances in the electric field modes of the microstrip transmission line MSL1 in the region where the microstrip transmission line MSL1 intersects with the coplanar transmission line CPW1, thereby reducing the transmission loss of the microstrip transmission line MSL1.

[0013] As shown in Figure 3, in the region where the ground conductor 11 is located above the microstrip transmission line MSL1, the ground conductor 50 is not placed directly below the microstrip transmission line MSL1. The reason for not placing the ground conductor 50 is that if the ground conductor 11 is above the microstrip transmission line MSL1 and the ground conductor 50 is also placed below the microstrip transmission line MSL1, the characteristic impedance of the microstrip transmission line MSL1 will change. Therefore, the ground conductor 50 is placed at a position away from directly below the microstrip transmission line MSL1.

[0014] Figure 6 shows the results of a simulation of the transmission loss of the microstrip transmission line MSL1 in this embodiment. Figure 6 shows the transmission loss when it intersects with the coplanar transmission line CPW1 in the conventional cross structure shown in Figures 13 to 15, and figure 6 shows the transmission loss of the microstrip transmission line MSL1 in this embodiment. In this embodiment, it is possible to maintain the electric field mode of the microstrip transmission line MSL1 between the microstrip transmission line MSL1 and the ground conductor 50 in the intersection region of the coplanar transmission line CPW1 and the microstrip transmission line MSL1, and it can be seen that a large loss improvement effect can be obtained, especially on the high frequency side.

[0015] [Second Embodiment] In the first embodiment, the width of the ground conductor 50, which is positioned directly below the microstrip transmission line MSL1, in the direction perpendicular to the signal propagation direction of the microstrip transmission line MSL1 (left-right direction in Figure 2), was set to be the same as the width of the microstrip transmission line MSL1. However, considering the spread of electric field modes between the microstrip transmission line MSL1 and the ground conductor 50, it is desirable to widen the ground conductor 50. On the other hand, if the width of the ground conductor 50 is made too wide, it will have a significant effect on the frequency characteristics of the coplanar transmission line CPW1. For this reason, it is necessary to design the width of the ground conductor 50 to an appropriate value.

[0016] Figure 7 shows the simulation results of the transmission loss of the microstrip transmission line MSL1 when the width of the ground conductor 50 is changed in this embodiment. Figure 7 shows the transmission loss when the width of the ground conductor 50 is the same as the width of the microstrip transmission line MSL1, as in the first embodiment, and Figure 7 shows the transmission loss when the width of the ground conductor 50 is twice the width of the microstrip transmission line MSL1. Figure 7 shows the transmission loss when the width of the ground conductor 50 is three times the width of the microstrip transmission line MSL1, and Figure 7 shows the transmission loss when the width of the ground conductor 50 is four times the width of the microstrip transmission line MSL1.

[0017] When the width of the ground conductor 50 is increased from one to two times the width of the microstrip transmission line MSL1, the transmission loss of the microstrip transmission line MSL1 is greatly improved. However, it can be confirmed that even when the width is increased to three times or more, the improvement in transmission loss is small. Therefore, considering the effect on the frequency characteristics of the coplanar transmission line CPW1, it is preferable to set the width of the ground conductor 50 to twice the width of the microstrip transmission line MSL1, and this can be said to be one of the critical values ​​for the structure of the present invention.

[0018] Figure 8 shows a perspective view of the cross structure when the width of the ground conductor 50 is set to twice the width of the microstrip transmission line MSL1, and Figure 9 shows a cross-sectional view of the area containing the signal line 10, the microstrip transmission line MSL1, and the ground conductor 50. The cross-sectional structure of the other parts is as shown in Figures 3 to 5.

[0019] [Third Embodiment] Figure 10 is a perspective view of the crossing structure of a transmission line according to the third embodiment of the present invention. In the first and second embodiments, in the region where the microstrip transmission line MSL1 intersects with the coplanar transmission line CPW1, in the region where there is no ground conductor 11 above the microstrip transmission line MSL1, the signal propagating through the microstrip transmission line MSL1 propagates in an electric field mode (first electric field mode) formed between it and the ground conductor 50. On the other hand, in the region where there is a ground conductor 11 above the microstrip transmission line MSL1, the signal propagating through the microstrip transmission line MSL1 propagates in an electric field mode (second electric field mode) formed between it and the ground conductors 11 and 50.

[0020] In this embodiment, at the boundary between the region without a ground conductor 11 and the region with a ground conductor 11 in the upper layer of the microstrip transmission line MSL1, interlayer ground vias 70 connecting the ground conductor 11 and the ground conductor 50 are arranged on both sides of the microstrip transmission line MSL1. This enables smooth mode conversion between the first electric field mode and the second electric field mode, and reduces the loss of the microstrip transmission line MSL1.

[0021] As shown in Figure 11, the interlayer ground via 70 consists of a ground conductor 700 formed in the same layer as the microstrip transmission line MSL1 and the bridge conductor 20, a via 701 connecting the ground conductor 11 and the ground conductor 700, and a via 702 connecting the ground conductor 700 and the ground conductor 50.

[0022] Figure 12 shows the results of a simulation of the transmission loss of the microstrip transmission line MSL1 in this embodiment. Figure 12 shows the transmission loss of the first embodiment (102) and the transmission loss of this embodiment (106). Compared to the first embodiment, which does not have interlayer ground vias 70, it can be seen that this embodiment provides a loss improvement effect on the high-frequency side. In this embodiment, the width of the ground conductor 50 is the same as the width of the microstrip transmission line MSL1, similar to the first embodiment, but as explained in the second embodiment, the width of the ground conductor 50 may be set to twice the width of the microstrip transmission line MSL1.

[0023] Some or all of the above examples may also be described as follows, but are not limited to the following:

[0024] (Note 1) The transmission line crossing structure of the present invention is such that, in the region where a coplanar transmission line consisting of a first layer of conductors and a microstrip transmission line consisting of a second layer of conductors below the first layer intersect, in the region where there is no first ground conductor constituting the coplanar transmission line in the upper layer of the microstrip transmission line, a second ground conductor is placed in the third layer directly below the microstrip transmission line.

[0025] (Note 2) In the transmission line crossing structure described in Note 1, the width of the second ground conductor positioned directly below the microstrip transmission line in a direction perpendicular to the signal propagation direction of the microstrip transmission line is twice the width of the microstrip transmission line.

[0026] (Note 3) The transmission line crossing structure described in Note 1 or 2 is such that, in the region where the coplanar transmission line and the microstrip transmission line intersect, interlayer ground vias connecting the first ground conductor and the second ground conductor are arranged on both sides of the microstrip transmission line at the boundary between the region where the first ground conductor is not present and the region where the first ground conductor is present in the upper layer of the microstrip transmission line.

[0027] (Note 4) The transmission line crossing structure described in Note 1 is such that, in the region where the coplanar transmission line and the microstrip transmission line intersect, the second ground conductor is positioned away from directly below the microstrip transmission line in the region where the first ground conductor is located above the microstrip transmission line.

[0028] CPW1... Coplanar transmission line, MSL1... Microstrip transmission line, 10... Signal line, 11, 50, 700... Ground conductor, 20... Bridge conductor, 30, 60, 701, 702... Via, 40... Dielectric, 70... Interlayer ground via.

Claims

1. A transmission line crossing structure characterized in that, in the region where a coplanar transmission line consisting of a first layer of conductors and a microstrip transmission line consisting of a second layer of conductors below the first layer intersect, in the region where there is no first ground conductor constituting the coplanar transmission line in the upper layer of the microstrip transmission line, a second ground conductor is placed in the third layer directly below the microstrip transmission line.

2. The transmission line crossing structure according to claim 1, characterized in that the width of the second ground conductor, which is positioned directly below the microstrip transmission line, in a direction perpendicular to the signal propagation direction of the microstrip transmission line is twice the width of the microstrip transmission line.

3. A transmission line crossing structure according to claim 1 or 2, characterized in that, in the region where the coplanar transmission line and the microstrip transmission line intersect, interlayer ground vias connecting the first ground conductor and the second ground conductor are arranged on both sides of the microstrip transmission line at the boundary between the region where the first ground conductor is not present and the region where the first ground conductor is present in the upper layer of the microstrip transmission line.

4. The transmission line crossing structure according to claim 1, characterized in that, in the region where the coplanar transmission line and the microstrip transmission line intersect, the second ground conductor is positioned away from directly below the microstrip transmission line in the region where the first ground conductor is located on the upper layer of the microstrip transmission line.

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