WO2020095092A1

WO2020095092A1 - Electromagnetic waveguide and microwave-frequency circuit

Info

Publication number: WO2020095092A1
Application number: PCT/IB2018/058712
Authority: WO
Inventors: Vincent ARBET-ENGELS; Patrick Favre
Original assignee: Aesa Sa
Priority date: 2018-11-06
Filing date: 2018-11-06
Publication date: 2020-05-14

Abstract

A novel printed circuit board waveguide structure based on optimized straight lines and connected at will by 3D multi-angle vertical transition. The complete system is having a well-controlled constant characteristic impedance Z0. The straight line works in a special mode in between coplanar waveguide and pure microstrip mode but without the coplanar intrinsic limitations. The vertical transitions defined by a LC structure approximating a vertical transmission line. The invention also relates to a microwave circuits and particularly switch matrices based on the inventive waveguide.

Description

Electromagnetic Waveguide and Microwave-frequency Circuit

Field of the invention

[0001] The present invention concerns a microwave printed circuit with constant-impedance structures including, for example, straight lines and bends. A wide-bandwidth switch matrix is a special non- exclusive embodiment.

Description of related art

[0002] Several known techniques are used to create microwave transmission lines on printed circuit boards. Those include several well- known variants such as: microstrip guides that include a strip conductor above a— virtually infinite— ground plane, the stripline transmission lines sandwiched between two ground planes, coplanar waveguides that have a central linear track adjacent to two ground conductors in the same plane, and other arrangements. Several known techniques allow to dimension transmission lines having a desired characteristics impedance, for example, and to fabricate them in microwave printed circuits. Waveguides are often characterized by means of the scattering matrix, or "S-parameters" and of parameters like insertion loss, input return loss, voltage standing wave ratio, reflection coefficients, and others that are often derivable from the S parameters. [0003] One shortcoming of all known printed transmission lines is that their characteristics are easily computable only for straight lines. As soon as bends are introduced, as it is unavoidable in most real use cases, the transmission parameters suffer from discontinuities. Seen from the point of view of the characteristic impedance, every bend is a discontinuity that creates local variation of the characteristics impedance Z₀ of the line, generating transfer function variations and multiple wave reflection that occur at multiple frequencies. [0004] A known technique to obviate these limitations consists in avoiding sharp bends and curve the path of the transmission line in an arc of radius much larger than the size related to the operational frequency. Another known technique, which consumes a smaller area of substrate, is the addition of a miter in the outer corner of the bend, to recover the desired impedance and reduce reflections. Nevertheless, the performance of mitered bends depends critically from the miter characteristics.

Moreover, designing such a bent and mitered waveguide requires advanced simulation codes, and the optimization of such structures is extremely time consuming. The optimization needs to be replicated for each bend angle and, despite all efforts, does not restore the characteristics of an ideal straight waveguide.

[0005] The discontinuities at the bends fatally introduce high frequency parasitic transmission modes that generate, by interference, deep notches in Bode diagrams. Mitered bends tend to behave poorly at frequencies very far from the nominal one.

[0006] In all cases, whenever a waveguide change in direction and/or a component is inserted, a special redesign must be done that takes up space on the circuit board and limits severely the resulting circuit performances.

In contrast to electronic realizations at lower frequency, routing a printed circuit for the microwave range is essentially a manual and lengthy process.

[0007] Although the above limitations are most severe in high accuracy measurement equipment, often needed in laboratories or for certification purposes, it is to be expected that, with the continuous increase of clock speeds, also logic circuit must eventually cope with the same problems. There is therefore a need of new techniques for realizing high-frequency waveguides on printed circuits that overcome these shortcomings of the known art.

[0008] Another shortcoming of conventional printed microwave circuits is the difficulty of creating complex circuits on multilayer printed circuit boards. When a waveguide must transition from a layer to another, it is known to insert a vertical transition structure that approximates a coaxial cable, like illustrated in Figure 8 that shows a transition between two coplanar waveguides, each comprising a central conductor 51 , 52

surrounded by ground planes 36, 37. The conductors 51 , 52 are connected by a central signal via 70 and the ground planes are connected by a crown of ground vias 60 that lie on a cylindrical crown around the signal via 70.

[0009] The central and ground vias are dimensioned to match the characteristics impedance Z₀ of the straight coaxial waveguides. The optimization of this structure to mimic a vertical coax transition implies a high proximity between the central and ground vias. Thus, by design the line structure becomes coplanar, with all its intrinsic limitations.

[0010] Switch matrixes are an important component of many high- frequency electronic devices including, automatic test equipment for cable testing. Such instruments generally have a computer-controlled instrument that has a limited number of test ports, for example a network analyzer, and a switch matrix that selectively connects the available test ports with the desired terminals of the device under test. In testing cables at frequency above some GHz, it is conventional to use electromechanical relays connected by coaxial cables. Relays suitable for such high frequencies are mechanically complex, bulky and expensive. Their switching time is slow, their form factor is large, and their lifetime is limited.

[0011] Another important use case of the present invention is that of beam-forming antennas, built around an array of multiple phase-shifted individual antennas. These devices use a switching matrix to introduce appropriate phase shifts in the individual antennas, to form and steer the radiation pattern of the array.

Brief summary of the invention

[0012] According to the invention, these aims are achieved by means of the object of the appended claims, particularly by an electromagnetic waveguide, comprising of a plurality of interconnected segments, each segment being a track on a layer of a multilayer circuit board, comprising: a first segment on a first layer of the circuit board; a second segment on a second layer of the circuit board different from the first layer; a transition between the first segment and the second segment comprising: a vertical conductor connecting the first segment to the second segment; wherein the multilayer circuit board comprises one intermediate layer or more than one intermediate layers sandwiched between the first layer and the second layer and said intermediate layer or intermediate layers have each an intermediate equipotential conductor close to the vertical conductor and separated from the vertical conductor by a nonconductive gap.

[0013] Preferably, the multilayer circuit board has a pair of intermediate layers connected to a ground plane. The intermediate layers are stacked vertically and their spacing is optimized, for example by simulation.

[0014] Several advantageous optional features are described, such as: the choice of the width of the nonconductive gap is such that the transition has essentially the common characteristic impedance; the shape of the gap being that of a circular crown with constant width; equipotential conductors in the first and second layers adjacent to the first, respectively second segment and separated by an optimized distance, and the presence of an angle d between the segments, resulting in a 3D direction change.

[0015] Another aspect of the invention concerns a microwave-frequency switch matrix comprising of a plurality input ports (P1 -P4), a plurality of output ports (Q1 -Q8), electromagnetic waveguides according to any one of the preceding claims connectable by a plurality of switches, such that the switches can be actuated for establishing a constant-impedance line joining a selected input port and a selected output port, preferably wherein the switches have provisions to terminate automatically the isolated ports by a resistor having a value that is perfectly matched to the characteristic impedance Z₀ of the line, to avoid open stubs. The switches may be solid- state switches, MEMS switches, or radiofrequency relays, and are preferably adjacent to transitions of the electromagnetic waveguides. Brief Description of the Drawings

[0016] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: Figure 1 illustrates schematically an electromagnetic waveguide with a 3D change of direction according to one aspect of the invention, and Figure 2 is a cut of such a device.

Figure 3 is an equivalent circuit showing the components of the characteristic impedance of the vertical transition in the waveguide of figures 1 and 2.

Figure 4 shows a possible structure for a switch with termination of the isolated ports.

Figure 5 plots the return loss of an inventive waveguide for diverse geometries. Figure 6 shows an example of switch matrix according to an aspect of the invention.

Figure 7 illustrates schematically the structure of an automatic cable tester.

Figure 8 is an example of multilayer waveguide of the prior art. Detailed Description of possible embodiments of the Invention

[0017] An important aspect of the present invention lies in the creation of a waveguide on a printed circuit board that can be routed at will, yet does not contain bends, in the sense that the track segments are straight but the direction of the waveguide changes as the routing of the circuit requires, thanks to 3D structures that connect together one straight segment in a layer and another straight segment in another layer. The inventors have found a manner of fabricating and dimensioning these 3D transitions that is manufacturable, easily scalable, economizes space on the PCB, and provides excellent results such as: lack of destructive wave reflections, no dips or peaks in the transfer function, and a transfer function without upper frequency limit, up to the limits of the dielectric material.

[0018] Figure 1 shows a 3D change of direction according to the invention on a four-layer printed circuit board 40. Figure 2 is a cut of the structure of figure 1 along the A-A plane. The drawings are not to scale: the thickness of the conductive layers 35, 36, 37, for example, has been chosen arbitrarily for visibility and may not correspond to that of a concrete embodiment.

[0019] The invention requires a plurality of layers, the number of layer in the invention is not limited. As to the materials, the printed circuit can be realized on any material suitable for the desired band of frequencies, for example the "RT/Duroid®" laminates produced by the Rogers corporation.

[0020] As seen in figures 1 and 2, the waveguide of the invention comprised a straight segment 51 on the top layer 31 and another straight segment 52 on the bottom layer 32. Each of the straight segments 51 and 52 is surrounded by a grounded conductor 36, respectively 37 on the same layer, and faces a ground plane 35 on another layer. The characteristic impedance of the straight segments 51, 52 is determined by the

geometrical parameters w, h that denote the width w of the straight segment, and the thickness h of the dielectric between the layers g is preferably greater than h to avoid coplanar waveguide modes, and by design has no influence on characteristic impedance of the line.

[0021] The design presents several features of the coplanar waveguides but, it is dimensioned such that it can be modelized as a microstrip.

Preferably, the thickness h is chosen small enough that, in the dominant transmission mode, the E field concentrates between the segment and the underlying ground plane; the behavior of the waveguide is then

appreciably the same as that of a microstrip. Given that the space between the segments 50, 51 and the ground planes 35 is filled with dielectric (typically e_r ~ 4.2), this condition is enhanced. The coplanar grounded conductors 36, 37 have negligible effect on the line characteristics, but they are useful to reduce crosstalk and connect all the ground planes in the circuit by the conductive vias 60.

[0022] The invention avoids the limitations inherent to the coplanar waveguides, in particular the parasitic radiation that is often associated with this geometry. In fact the small value of the thickness w and the dielectric constant imply that the electro-magnetic field, and the Poynting vector are essentially confined in the dielectric between the conductor 51 and the intermediate ground plane 35 immediately below. At the same time, the lateral ground planes provide isolation between adjacent waveguides. This result is achieved by choosing well defined dielectric layer thickness, line width, and the distance to the conductive claddings. Often, the best results are obtained by using the lowest thickness compatible with the material and process used.

[0023] The straight segments are joined together by a vertical conductor 70 that traverses the intermediate ground planes 35 and, at each passage, is separated from the ground planes 35 by a nonconductive gap 73.

[0024] Figure 3 illustrates how the vertical transition between the two straight segments 51 and 52 can be approximated by a simplified

equivalent circuit with concentrated capacitors and inductors: the electric field will be prevalently in gaps between the conductive planes 35 and the central conductor 70, and these regions can be approximated by capacitors, while the central conductor, can be regarded as a series of inductors. The equivalent circuit 101 is a LC ladder, and his characteristic impedance that is given

where L’ denotes the inductance of each segment of the central via, and respectively, C’ , the capacitance of each central via to ground plane junction. [0025] By approaching the ground planes 35 to the central conductor 70 the designers increases the value of the capacitances. Similarly, the inductive component L' can be modified by changing the dimensions of the central via 70. The inductance could be modified also by providing a central hole in the via 70, or modifying the material of the conductors, for example by adding a gold coating.

[0026] Although it would be possible to dimension the transition to a desired impedance by trial and errors or using approximate formulas to extract the values of L’ and C’, the electromagnetic behavior of the 3D transition of figures 1,2 can be simulated by known computer codes, and the simulation can be used to determine a value for the separation s that optimizes the impedance match. Figure 5 shows an example of this method: the values of the input return loss IRL =—20 loo !S, are computed for different values of the separation s and the optimum one is chosen by comparison. In the example illustrated, the value s=0.35 mm was found to give the best values of the IRL and was selected as optimal.

[0027] Thus, this embodiment of the invention has a vertical structure that provides a matched coupling between two straight waveguides but, in contrast with most known techniques, does not imitate the structure of a coaxial cable, and is defined by a ladder network of capacitances C and inductances L' of figure 3 rather than by distributed impedances elements. This design represents a nth-order LC filter as illustrated in figure 3.

[0028] An interesting aspect of the invention is that, when one increases the number of intermediate layers, and with that the number of steps in the circuit of figure 3, the vertical structures approximate a distributed line, and their performance approaches that of an ideal waveguide. Thus, a higher routing density, that requires more layers as a rule, improves the performances of the circuit rather than the contrary.

[0029] In contrast with other design techniques, the optimal value of s is valid for all values of the angle d (visible in figure 1) between the straight segments and can be used for all the 3D transitions in a project, insofar as the laminate, the dimensions g and w and the diameter d of the vertical conductor remain the same. The inventors have verified experimentally that a waveguide with three segments and two right-angle transitions (6=90°) is indistinguishable from another with three segments of the same length and aligned transitions (6=0°).

[0030] The position of the conductive vias 60 is not critical. We have seen that the impedance Z₀ of the vertical transition is defined by C and L', mainly, and the influences of the vias 60 is not significant. Preferably the vias 60 are placed in a lattice arrangement in all the regions of the PCB where they do not interfere with the routing of the waveguides. The ground conductors 35, 36, 37 can be tied to any suitable low-impedance node and are essentially equipotential at the frequencies of interest.

[0031] A special application of the waveguide of the invention is that of a switch matrix. Such devices are used in several applications, and

particularly in automatic test equipment for testing communication cables, whose structure is illustrated schematically in figure 7. A cable under test 150 may comprise a plurality of transmission lines that must meet certain requirements in attenuation, reflection, cross-talk and so on, across a broad spectrum of frequencies. These parameters are acquired by a vector network analyzer 140 controlled by a host system 160. The conductors of the cable are connected to a switch matrix through a special interface, and the host computer 160 connects automatically selected conductors to the input channels of the VNA 140, according to the measurement needs. In known devices, the switch matrix is built with electromechanical RF relays and coaxial cables and represents a significant fraction of the total cost.

[0032] Figure 6 shows a miniaturized commutation matrix that includes a plurality of input ports P1 -P4 and a plurality of output ports Q1 -Q8 that can be selectively connected by setting the switches 105 appropriately. The actuation of the switches can be done directly by the host system or through a logic decoder. The 3D transitions of the invention are used whenever the direction of a waveguide needs a change, or when a waveguide must pass in another layer, such that the connections between input and output ports are always matched in impedance.

[0033] A known problem of radiofrequency switched circuits is that of open stubs, that arises whenever a portion of a waveguide is isolated from the main path and left open. This is alleviated in the invention by choosing switches that are equipped with internal resistors of known value (Z0) to terminate automatically the isolated ports. This is illustrated in the example of figure 4: the switch 105 has a common port 110 that can be put in contact with either of the switched ports 112 by a logic circuit 109. The isolated port that is not in contact with the common port is automatically terminated by a resistor 108.

[0034] The switch matrix of the invention may have the switches arranged in rows connected to one port on one side, and to a termination resistor on the opposite side. Any switch on a row can be commutated to route the signal through a waveguide running across the row direction to another row of switches leading to any selected port. In the example shown, the input port P2 is in communication with the output port Q5, and the port P4 with the port Q7. The resulting transmission lines are matched, and all the isolated sections are terminated on both sides, without open stubs. Advantageously, as the switches connect transmission lines that are at right angles and on different levels the 3D transitions are placed adjacent to the switches.

[0035] The topology illustrated in figure 6 is provided as an example only; the invention can be applied also to simpler arrangements, with a lower number of ports and switches, that have a reduced number of combinations, as well as more complex devices with densely populated matrixes. The applications of the switch matrix are not limited to automatic test equipment either.

[0036] Several RF switches can be used in the frame of the invention including, but not limited to, solid-state devices such as GaAs HEMT transistors, SiGe MODFETs, or any other suitable solid-state technology, and MEMS actuators.

Reference numbers

[0037] s width of the nonconductive gap

d diameter of the vertical conductor

w strip width

g side gap

h dielectric height

d angle

30 intermediate layer

31 top layer

32 bottom layer

35 intermediate ground plane

36 top ground plane

37 bottom ground plane

40 electromagnetic waveguide

42 dielectric

50 waveguide segment

51 first segment

52 second segment

60 ground via

70 vertical signal via

73 nonconductive gap

103 equivalent circuit

105 switch

106 commutation means

108 termination resistor

109 logic circuit

110 common port

112 switched port

130 switch matrix vector network analyser interface

cable under test

Claims

1. Electromagnetic waveguide, comprising of a plurality of interconnected segments, each segment being a track on a layer of a multilayer circuit board (40), comprising: a first segment (51) on a first layer (31) of the circuit board; a second segment (52) on a second layer (32) of the circuit board different from the first layer (31); a transition between the first segment and the second segment comprising: a vertical conductor (70) connecting the first segment (51) to the second segment (52); wherein the multilayer circuit board comprises one intermediate layer (30) or more than one intermediate layers (30) sandwiched between the first layer (31) and the second layer (32) and said intermediate layer or intermediate layers have each an intermediate equipotential conductor (35) close to the vertical conductor (79) and separated from the vertical conductor by a nonconductive gap (73) with a width (s).

2. The electromagnetic waveguide of the preceding claim, wherein the segments have a common characteristic impedance and the width of the nonconductive gap is such that the transition has essentially the common characteristic impedance.

3. The electromagnetic waveguide of any one of the preceding claims, wherein the intermediate equipotential conductor or conductors (35) surround completely the vertical conductor, the shape of the gap being that of a circular crown with constant width (s).

4. The electromagnetic waveguide of any one of the preceding claims, wherein the first layer (31) has a first equipotential conductor (36) adjacent to the first segment (51) and the second layer (32) has a second equipotential conductor (37) adjacent to the second segment (52).

5. The electromagnetic waveguide of the preceding claim, wherein the first segment (51) and the second segment (52) are separated from the first, respectively second equipotential conductors (36, 37) by a distance (g) greater than the width (s) of the nonconductive gap (73).

6. The electromagnetic waveguide of any one of the preceding claims, in which the second segment is angled with respect to the first segment, resulting in a 3D direction change.

7. A microwave-frequency switch matrix comprising of a plurality input ports (P1-P4), a plurality of output ports (Q1-Q8),

electromagnetic waveguides according to any one of the preceding claims connectable by a plurality of switches, such that the switches can be actuated for establishing a constant-impedance line joining a selected input port and a selected output port.

8. The microwave-frequency switch matrix of the preceding claim, wherein the switches have a common port, a plurality of switched ports, one or more termination resistors, and a circuit arranged to

selectively connect one or more switched ports to the common port, to isolate one or more switched ports from the common port, and to

terminate the switched ports that are isolated from the common port with a termination resistor.

9. The microwave-frequency switch matrix of any one of the claims 7 to 8, wherein the switches include solid-state switches, MEMS switches, or radiofrequency relays.

10. The microwave-frequency switch matrix of any one of the claims 7 to 9, wherein the switches are adjacent to transitions of the electromagnetic waveguides.