GB2617085A

GB2617085A - An impedance matching circuit

Info

Publication number: GB2617085A
Application number: GB2204377.2A
Authority: GB
Inventors: Ware John
Original assignee: Leonardo UK Ltd
Current assignee: Leonardo UK Ltd
Priority date: 2022-03-28
Filing date: 2022-03-28
Publication date: 2023-10-04
Also published as: GB202204377D0

Abstract

An impedance matching circuit to provide impedance matching between a radio frequency source and an output load. The impedance matching circuit 20 comprises a planar transmission line having a conductor track and a ground plane 23, wherein the spacing between the conductor track and the ground plane varies along the length of the planar transmission line to provide variation in the impedance of the planar transmission line along its length. The ground plane may comprise two or more conductive layers 23a,b,c,d having different separations from the conductor track; these conductive layers connected by a via 24. The impedance matching circuit may be implemented in a multi-stage radio frequency signal splitter, having two arms that connect between the output ports through a junction to connect the output ports to input port; and multiple isolating resistors 106 spaced apart from the junction and one another by substantially a quarter-wavelength to provide two or more stages. The stages have different impedances due to the different spacing between conductor and ground plane.

Description

An Impedance Matching Circuit The present invention relates to an impedance matching circuit and in one respect to an improved multi-stage Wilkinson power divider or combiner circuit.

A Wilkinson power divider (or splitter) circuit splits an RF input signal into two equal phase, equal amplitude output signals. The circuit can operate reciprocally to instead function as a combiner.

When operating as a divider, the circuit comprises an input port which is connected, through a junction, to two output ports. An isolation resistor is connected between the two output ports, spaced 214 after the junction. The isolation resistor minimises transmission of signals between the two output ports.

To expand the operational bandwidth of the circuit, it can be adapted to include multiple isolation resistors across the two outputs, each spaced 214 from one another. These are commonly referred to as multi-stage (or multi-section) Wilkinson dividers.

An example of a prior art multi-stage Wilkinson RF power divider is illustrated in Figs lA and 1B Figures lA and 1B illustrates a circuit board 1 Printed on a first side lA of the circuit board is a metallised pattern defining signal tracks 2 of the power divider circuit. As can be seen in Fig 1B, a second opposite side 113 of the circuit board 1 is metallised to provide a ground plane 3 -2 -The signal tracks define an input transmission line 4 and two output transmission lines 5, 6 The input transmission line 4 is connected to the two output transmission lines through junction 7.

Connected between the two output transmission lines are three resistors 8A, 8B, 8C, First resistor 8A is spaced along the output transmission lines from the junction by a distance 2/4, where Xis the wavelength at highest frequency of intended operation. The second resistor 8B is spaced from the first resistor by a further 214 and the third resistor 7C by a further 2/4 from the second resistor 8C.

To provide a gradual transformation from the impedance at the junction 7 to the impedance Zo (often 500) at the output ports 5,6, the stages are stepped in impedance.

This is achieved by broadening the width of the transmission line at each stage from the junction 7 to the final resistor 8C, as can be seen in Fig IA to provide impedances Zi. Z2 and Z where Zo < Zi,< < Z3.

A problem with this design is that the narrower track widths of the earlier stages can limit the power handling capability of the splitter. Avoiding this problem by selecting a wider track width on the earlier stages can result in track widths at the later stages of the splitter that are comparable to the X/4 length of the section, which significantly impairs the performance of the splitter.

According to a first aspect of the invention there is provided an impedance matching circuit assembly to provide impedance matching between a radio frequency source and an output load; the impedance matching circuit comprising: an input port for connection to the RF signal source; an output port for connection to the output load, -3 -a planar transmission line comprising: a conductor track that extends between and interconnects the input port and the output port; and a ground plane that extends alongside the conductor track and is separated from it by a dielectric material comprising one or more dielectric layers characterised in that the spacing between the conductor track and the ground plane varies along the length of the planar transmission line to provide variation in the impedance of the planar transmission line along its length, including at least one intermediate impedance between the input port and the output port that differs from the impedance of the planar transmission line at the input port and output port.

By varying the spacing between the conductor track and ground plane along the transmission line, it is possible to reduce, if not eliminate, the need to vary the width of the conductor track between the input and output ports. This allows for selection of a conductor track width along the length of the transmission line that meets the power handling requirements of the circuit without the need to widen the conductor track to an extent that its transmission performance is impaired.

Although initially conceived as an improvement to traditional multistage Wilkinson power dividers and combiners, the invention has broader utility as an impedance matching circuit used to match (or at least more closely match) the impedance an RF signal source with an output load. Such a circuit is likely to be of benefit where it is desired to increase the power handling capacity of a circuit and where the track width of the input is limited, e.g. by constraints imposed by the input connector or other clashing geometry on the board. Note: a multistage Wilkinson splitter/combiner can be considered an example of this broader circuit type, each branch of the splitter being a separate impedance matching circuit linked with resistors and connected together at one end. -4 -

The dielectric material may have a thickness that varies about the length of the planar transmission line to provide the chat-wing spacing between the conductor track and ground plane. For example, one of the sides of the dielectric material may define a stepped or sloped face on which the ground plane is provided.

Alternatively, the conductor track maybe provided (e.g. printed) on a dielectric material, and in which the ground plane comprises a first electrically conductive layer provided within the dielectric material with a first, relatively small, separation from the conductor track, and a second electrically conductive layer provided within or on the dielectric material with a second, relatively large, separation from the conductor track and wherein the first and second electrically conductive layers are electrically interconnected. The first and second layers are favourably electrically interconnected by a via extending through the dielectric material.

In one arrangement the dielectric material may comprise a stack of dielectric layers and the ground plane comprised from multiple, electrically interconnected, conductive portions provided at interfaces between different layers of the stack.

For example, the dielectric material may comprise a first and second dielectric layers, the conductor track provided on a first surface of the first dielectric layer, the first electrically conductive layer provided at an interface between the first and second dielectric layers, the second electrically conductive layer is provided at a second surface of the second dielectric layer that faces away from the first surface.

This concept may be expanded to more than two dielectric layers and conductive layers, for example, the ground plane may comprise a third electrically conductive layer provided within the dielectric material at a third distance from the conductor track, the third distance being between the first and second distances; the third electrically conductive layer being electrically interconnected to both the first and second electrically conductive layers by one or more vias extending through the dielectric material -5 -In an implementation of this, the dielectric material may comprise a first, second and third dielectric layers; the conductor track is provided on a first surface of the first dielectric layer, the first electrically conductive layer is provided at an interface between the first and second dielectric layers, the second electrically conductive layer is provided at an interface between the second and third dielectric layers and the third conductive layer at a second surface of the third dielectric layer that faces away from the first surface The transmission line may be implemented using, for example, a microstrip, grounded co-planer waveguide, or stripline type circuity.

In another aspect of the invention there is provided a multi-stage radio frequency signal splitter circuit assembly comprising the impedance matching circuit assembly, said impedance matching circuit assembly further comprising a second output port, the conductor track defining an input portion that connects to the input port, two arms that connect between the respective output ports and the input portion through a junction to connect the output ports to the input port, multiple isolating resistors connected between the two arms, each of the multiple isolating resistors being spaced apart from the junction and/or one another by substantially 214 to provide two or more stages; and wherein the spacing between the conductor track and the ground plane varies along the two arms to provide the planar transmission line with different impedances at the different stages.

In another, interrelated, aspect of the invention, there is provided a multi-stage radio frequency signal combiner circuit assembly comprising the impedance matching circuit assembly, said impedance matching circuit assembly further comprising a second input port the conductor track defining an output portion that connects to the output port, two arms that connect between the respective input ports and the output portion through a junction to connect the input ports to the output port; multiple isolating resistors connected between the two arms, each of the multiple isolating resistors being spaced apart from the junction and/or one another by substantially 2./4 to provide two or more stages; and wherein the spacing between the conductor track and the ground plane -6 -varies along the two arms to provide the planar transmission line with different impedances at the different stages The invention will now be described by way of example with reference to the following Figures in which: Figure lA is a plan view of a printed circuit board patterned to provide a multi-stage Wilkinson splitter of conventional design; Figure 1B is a simplified schematic section view of the prior art printed circuit board of Fig 1A; Figure 2A is a plan view of a multilayer printed circuit board patterned to provide an improved multi-stage Wilkinson splitter; Figure 2B is a simplified schematic section view of the printed circuit board of Fig 2A; Figure 2C is a perspective view of the printed circuit board of Figs 2A and 2B shown without the dielectric material to reveal the structure of the ground plane.

Figure 3A is a plan view of a multilayer printed circuit board patterned to provide an improved multi-stage Wilkinson splitter of a variant design; Figure 3B is a simplified schematic section view of the printed circuit board of Fig 3A; and Figure 3C is a perspective view of the printed circuit board of Figs 3A.

Figures 2A, 2B and 2C illustrate a multilayer printed circuit board assembly 20 implementing a multi-stage Wilkinson power divider circuit 100. -7 -

The multi-layer printed circuit board 20 comprises, in this example, four layers of dielectric material 21A, 21B, 21C, 21D -e.g. a glass epoxy composite, and five conductive layers 22, 23A, 23B, 23C, 23D, e.g. of copper. Note that the Figs do not show the board 20 to scale; for example, the thickness of the conductive layers are exaggerated.

A first of the conductive layers 22 printed on a first side 20A of the circuit board assembly 20, is patterned to define signal lines 101. The other conductive layers 23A, 23B, 23C, 23D provide a multi-layered ground plane 110 The circuit 100 comprises a first port 102A that functions as an input, and a second and third ports 102B 102C that function as outputs. A planar transmission line, provided by the combination of the signal lines 101 and the ground plane 110, interconnect the input port 102A to the two outputs 102B, 102C to carry radio frequency signals therebetween. The planar transmission line comprises an input portion 103 that extends from the first port 100A to junction 104; a first branch 105A that extends from the junction 104 to the second port 102B; and second branch 105B which extends from the junction 104 to the third port 102C.

Connected between the two branches 105 are three isolation resistors 106A, 106B, 106C. A first resistor 106A is spaced along the branches from the junction 104 by a distance k/4, where X, is the wavelength of the highest frequency of intended operation.

The second resistor 106B is spaced from the first resistor 106A by a further k/4 and the third resistor 106C by a further ?A from the second resistor 106B. Note that in each case the 2/4 separation refers to the electrical path length rather than the straight line distance.

The resistors 106 sub-divide the branches 105 into three intermediate stages between the input and outputs: stage one between the junction 104 and first resistor 106A; stage two between the first and second resistors 106A 106B; and stage three between the second and third resistors 106B 106C -8 -To provide grounding for coaxial cables to connect to each of the ports, ground pads 107 are provided on either side of the transmission line about each port 102A, 102B, 102C.

To graduate the impedance of the transmission line from the junction 104 to the impedance (Zo) at the second and the third ports 101B 102C (often 501)), each stage is provided with a different impedance, Zi, Z2 Z3 where Zo < Zi < Z2 < Z3, achieved through the arrangement of the multi-layer ground plane 110 Each of conductive layers 23A, 23B, 23C of the ground plane 110 lies at an interface between a different adjacent pair of dielectric layers 21A-21D. In this way each of said layers is spaced from the conductive layer 22, and thus the signal track 101, by a different distance. Conductive layer 23D, which is furthermost from conductive layer 22, is provided on a surface of dielectric layer 23D that provides a second, opposite facing, side 20B of the circuit board assembly 20 to that of first side 20A.

All of the conductive layers 23A-23D of the ground plane 110 are electrically interconnected by vias 24 that extend through layers 23B-23D.

The conductive layers 23A, 23B, 23C are patterned to provide a varying separation distance (>) between the signal line 101 and the ground plane 110 at each stage. The bottom most layer 23D may extend across the whole of side 20B saving the step of patterning face 20B.

Distance > is taken in a direction orthogonal to face 20A, and thus also the direction of travel of RF signals along the transmission line.

The conductive layers 23A-23D are patterned so that the furthest most conductive layer 23D directly faces the branch lines 105A 105B in stage 1; the next furthermost conductive layer 23C directly faces the branch lines 105A 105B in stage 2; conductive layer 23B directly faces the branch lines 105A 105B in stage 3; and that conductive -9 -layer 23A directly faces the signal lines 101 at the input 102A and outputs 102B and 102C.

As the impedance of the transmission line is altered through the varying spacing between the signal lines 101 and ground plane 110, the widths of branches 105A 105B may remain substantially the same between junction 7 and the output ports 102B 102C.

This may require the dielectric layers 21 within the assembly to be of different thicknesses, rather than all the same as represented in Figs 2B and 2C.

Nevertheless, multiple layer PCBs are typically constructed from dielectric layers that are of standardised thicknesses. To simplify and reduce the cost of manufacturing of the afore described device, it is preferable to use these standardised thicknesses. A problem with this is that it will result in spacings between the conductive layers 23A-23D that differ from that needed to provide the desired impedance change between each stage This is compensated for by varying the width w of the branch lines 105A 105B at each stage. As such the difference in impedance of the transmission line between each stage is a function of change in both track width and spacing between the ground plane and signal line. The branch line has a width w1 in stage 1, w2 in stage 2 and w3 in stage 3 where w 1 < w2 < w3. Notably, however, the variation in signal line width needed is significantly smaller compared with that required to provide the same impedance

graduation in the prior art of Figs IA and 1B.

It will also be appreciated that the number of dielectric layers 21 and ground plane conductive layers 23 may differ from the four illustrated depending on the number of stages desired. Further, depending on the smoothness of impedance graduation needed, and the freedom to alter the width of the signal line, it may be acceptable for the same spacing x to be used for multiple adjacent stages, where so, a reduced number of dielectric and conductive layers may be used.

-10 -The vi as 24 need not be vertically inclined to provide the desired electrical performance but it is preferred as it makes the device easier to manufacture. The embodiments illustrated in Figs 2B and 2C show a column of two vias under stage 3 and a column of three vias under the output stage. This arrangement is not essential as it comprises more vias that needed to provide the necessary electrical connections between the layers portions of the ground plane 110.

Figures 3A, 3B and 3C illustrates a variant design of multi-stage Wilkinson power divider circuit 100' having two, independent, differences to that of the earlier embodiment.

A first difference is the profile of the portion of ground plane 110' at the input stage comprises a projection 111 that extends directly underneath the T-junction 104' to improve the impedance transition at the T-junction 104'. This reduces reflection of signal back towards the input 102A' . The projection 111 narrows to a point, in this example it has 'V' shape profile. The projection 111 has reflectional symmetry about notional line A-A which is the same line about which the junction 104 is reflectionally symmetric.

A second difference is that the ground plane 110' is stepped so as to provide three rather than four different spacings with the signal lines, with there being no difference in spacing between stages 2 and 3. Rather the difference in the impedance values Zi, Z2 at respective stages 3 and 2 are achieved solely through the different track widths w of the branch lines 105A' 105B' between stage 2 and stage 3. The difference in impedance Z3 and Z2 between stages 1 and 2 is achieved through both change in separation between the ground plane and signal lines and track width. As such the track width wl, w2, w3, of respective stages 1, 2,3 wl > w2 < w3. Notwithstanding, the variation in signal line width needed between the junction and the output stages is still significantly smaller compared with that required to provide the same impedance profile using the design of prior art Figs IA and 1B.

It will be appreciated that the values of Z1 and Z2 may not be the same as any of the values of Z1, Z2, Z3 of the earlier embodiment.

The variable height ground plane may be implemented using a multi-layered PCB structure as described in the earlier embodiment. Alternatively, it may be implemented by some other means, for example the dielectric material may have a thickness that varies about the length of the planar transmission line to provide the changing spacing between the conductor track and ground plane. For example, one of the sides of the dielectric material may define a stepped face on which the ground plane is provided.

Although described with reference to a three port divider, the invention can equally be applied to multi stage power divider circuits comprising more than three ports The invention may be used in asymmetrical power dividers that provide unequal splitting. In such circuits, each branch requires stages with different impedances. To implement this the assembly may comprise additional dielectric layers and conductive layers to provide additional conductive layers for the ground plane. The layers are patterned such that a number of the layers are used to define the spacings x for the stages for one of the branches and the other layers are used to define the spacings x for the stages of the other branch.

It will be appreciated that the invention is equally applicable where the circuit is used in reverse as a multistage power combiner circuit.

The invention may equally be applied to the manufacture of cascaded splitters and combiners.

Claims

-12 -Claims 1 An impedance matching circuit assembly to provide impedance matching between a radio frequency source and an output load; the impedance matching circuit comprising: an input port for connection to the RF signal source; an output port for connection to the output load, a planar transmission line comprising: a conductor track that extends between and interconnects the input port and the output port; and a ground plane that extends alongside the conductor track and is separated from it by a dielectric material comprising one or more dielectric layers characterised in that the spacing between the conductor track and the ground plane varies along the length of the planar transmission line to provide variation in the impedance of the planar transmission line along its length, including at least one intermediate impedance between the input port and the output port that differs from the impedance of the planar transmission line at the input port and output port.
2 An impedance matching circuit assembly according to claim 1 wherein the conductor track is provided on a first surface of the dielectric material, and in which the ground plane comprises a first electrically conductive layer provided within the dielectric material with a first, relatively small, separation from the conductor track; and a second electrically conductive layer provided within or on the dielectric material with a second, relatively large, separation from the conductor track; -13 -and wherein the first and second electrically conductive layers are electrically interconnected.
An impedance matching circuit assembly according to claim 2 wherein the first and second electrically conductive layers are electrically interconnected by a via extending through the one or more dielectric layers.
4 An impedance matching circuit assembly according to claim 2 or 3 wherein the dielectric material comprises a first and second dielectric layers, the conductor track is provided on a first surface of the first dielectric layer, the first electrically conductive layer is provided at an interface between the first and second dielectric layers and the second electrically conductive layer is provided at a second surface of the second dielectric layer that faces away from the first surface.
An impedance matching circuit assembly according to any claim 2-4 in which the ground plane comprises a third electrically conductive layer provided within the dielectric material at a third distance from the conductor track, the third distance being between the first and second distances; the third electrically conductive layer being electrically interconnected to both the first and second electrically conductive layers by one or more vias extending through one or more dielectric material.
6 An impedance matching circuit assembly according to claim 1-3 wherein the dielectric material comprises a first, second and third dielectric layers; the conductor track is provided on a first surface of the first dielectric layer, the first electrically conductive layer is provided at an interface between the first and second dielectric layers, the second electrically conductive layer is provided at an interface between the second and third dielectric layers and the third conductive layer at a second surface of the third dielectric layer that faces away from the first surface -14 - 7 A multi-stage radio frequency signal splitter circuit assembly comprising the impedance matching circuit assembly of any previous claim, said impedance matching circuit assembly further comprising a second output port, the conductor track defining an input portion that connects to the input port, two arms that connect between the respective output ports and the input portion through a junction to connect the output ports to the input port; multiple isolating resistors connected between the two arms, each of the multiple isolating resistors being spaced apart from the junction and/or one another by substantially 1/4 to provide two or more stages; and wherein the spacing between the conductor track and the ground plane varies along the two arms to provide the planar transmission line with different impedances at the different stages 8 A multi-stage radio frequency signal combiner circuit assembly comprising the impedance matching circuit assembly of any previous claim, said impedance matching circuit assembly further comprising a second input port the conductor track defining an output portion that connects to the output port, two arms that connect between the respective input ports and the output portion through a junction to connect the input ports to the output port; multiple isolating resistors connected between the two arms, each of the multiple isolating resistors being spaced apart from the junction and/or one another by substantially 1/4 to provide two or more stages; and wherein the spacing between the conductor track and the ground plane varies along the two arms to provide the planar transmission line with different impedances at the different stages