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Title:
PRINTED CIRCUIT BOARD BASED HIGH FREQUENCY RF COAXIAL TRANSMISSION LINE USING BURIED VIAS AND METHOD OF FABRICATION THEREOF
Document Type and Number:
WIPO Patent Application WO/2019/211833
Kind Code:
A1
Abstract:
A novel and useful printed circuit board based coaxial transmission line structure (or coaxial via) and a method of fabrication thereof. The coaxial transmission line incorporates a plated through hole signal via passing through all N layers of the PCB. A plurality of buried ground vias electrically coupled to ground are arranged concentrically around the signal via to form a ground shield. The ground vias are fabricated from layer 2 through layer N-l, i.e. the second layer to the next to last layer of the PCB. The signal via in conjunction with the plurality of ground vias form the coaxial transmission line structure. Multiple concentric rings of buried ground vias may be fabricated to provide a high level of shielding and low RF signal leakage.

Inventors:
BARON AMOS
Application Number:
IL2019/050467
Publication Date:
November 07, 2019
Filing Date:
April 25, 2019
Export Citation:
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Assignee:
ARBE ROBOTICS LTD. (HaHashmonaim 107, 21 Tel Aviv, 6713321, IL)
International Classes:
H01P5/02; H05K1/02; H05K3/42
Attorney, Agent or Firm:
ZARETSKY, Howard (P.O.B. 1291, 03 Hadera, 3811103, IL)
Download PDF:
Claims:
CLAIMS

1. A printed circuit board (PCB) assembly, comprising:

a plurality of insulating layers stacked vertically one upon the other, including a top layer, a bottom layer and one or more inner layers;

an electrically conductive plated through hole signal via passing through said top layer, said bottom layer, and said one or more inner layers;

a plurality of buried ground vias electrically coupled to ground and passing only through said one or more inner layers, and arranged in a concentric ring formation in parallel with and surrounding said signal via thereby effectively forming a coaxial shield for said signal via; and

wherein a combination of said signal via and said concentric ring of plurality of ground vias effectively forms a coaxial transmission line through the printed circuit board assembly.

2. The printed circuit board assembly according to claim 1, wherein said ring formation comprises a single ring of ground vias spaced around said signal via.

3. The printed circuit board assembly according to claim 1, wherein said ring formation comprises an inner ring of ground vias spaced around said signal via and an outer ring of ground vias staggeredly spaced around said inner ring of ground vias.

4. The printed circuit board assembly according to claim 1, wherein said ring formation of ground vias are electrically connected to one or more ground planes.

5. The printed circuit board assembly according to claim 1, further comprising an antenna fabricated on said top layer and electrically coupled to said signal via. 6 The printed circuit board assembly according to claim 1, further comprising a coplanar waveguide fabricated on said bottom layer for feeding said signal via.

7. A method of fabricating a printed circuit board (PCB) assembly, the method comprising:

providing a multilayer printed circuit board having a plurality of layers stacked vertically one upon the other, including a top layer, a bottom layer and one or more inner layers;

forming at least one electrically conductive plated through hole signal via extending through said top layer, said bottom layer, and said one or more inner layers, said signal hole electrically connecting a first trace on said top layer to a second trace on said bottom layer;

forming a plurality of buried ground vias electrically coupled to ground that pass only through said one or more inner layers, said ground vias arranged in a concentric ring formation in parallel with and surrounding said signal via thereby effectively forming a coaxial shield for said signal via; and

wherein a combination of said signal via and said concentric ring of plurality of ground vias effectively forms a coaxial transmission line through the printed circuit board assembly.

8. The method according to claim 7, wherein said ring formation comprises a single ring of ground vias spaced around said signal via.

9. The method according to claim 7, wherein said ring formation comprises an inner ring of ground vias spaced around said signal via and an outer ring of ground vias staggeredly spaced around said inner ring of ground vias.

10. The method according to claim 7, wherein said ring formation of ground vias are electrically connected to one or more ground planes.

11. The method according to claim 7, further comprising fabricating an antenna on said top layer electrically coupled to said signal via. 12. The method according to claim 7, further comprising fabricating a coplanar waveguide on said bottom layer for feeding said signal via.

13. An automotive radar sensor, comprising:

a printed circuit board (PCB) assembly having:

a plurality of insulating layers, including a top layer, a bottom layer and one or more inner layers;

one or more electrically conductive plated through hole signal vias, each signal via passing through said top layer, said bottom layer, and said one or more inner layers;

a plurality of buried ground vias electrically coupled to ground passing only through said one or more inner layers, and arranged in a concentric ring formation in parallel with and surrounding each signal via thereby effectively forming a coaxial shield for each signal via;

wherein a combination of a signal via and a concentric ring of ground vias effectively forms a coaxial transmission line through the printed circuit board assembly;

one or more transmitting antennas fabricated on the top layer of said PCB assembly; one or more receiving antennas fabricated on the top layer of said PCB assembly; and a transceiver located on the bottom layer of said PCB assembly, said transceiver coupled to said one or more transmitting antennas and said one or more receiving antennas through respective coaxial transmission lines, said transceiver operative to generate and supply transmitting signals to said one or more transmitting antennas and receive signals of a wave reflected back to said one or more receiving antennas. 14. The automotive radar sensor according to claim 13, wherein said ring formation comprises a single ring of vias connected to ground and spaced around said signal via.

15. The automotive radar sensor according to claim 13, wherein said ring formation comprises an inner ring of vias connected to ground and spaced around said signal via and an outer ring of vias connected to ground staggeredly spaced around said inner ring of ground vias.

16. The automotive radar sensor according to claim 13, wherein said ring formation of ground vias are electrically connected to one or more ground planes.

17. The automotive radar sensor according to claim 13, further comprising an antenna fabricated on said top layer and electrically coupled to said signal via. 18. The automotive radar sensor according to claim 13, further comprising a coplanar waveguide fabricated on said bottom layer for feeding said signal via.

19. The automotive radar sensor according to claim 13, wherein said coplanar coaxial transmission line is configured to operate in the 77-81 GHz range.

Description:
PRINTED CIRCUIT BOARD BASED HIGH FREQUENCY RF COAXIAL TRANSMISSION LINE USING BURIED VIAS AND METHOD OF FABRICATION THEREOF

FIELD OF THE DISCLOSURE

The subject matter disclosed herein relates to the field of electrical transmission lines and more particularly relates to a print circuit board (PCB) based radio frequency (RF) coaxial line transmission line structure suitable for high frequencies that utilizes buried ground vias.

BACKGROUND OF THE INVENTION

Coaxial cable, or coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket. Coaxial cable differs from other shielded cables because the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a transmission line.

Coaxial cable is used as a transmission line for radio frequency signals, e.g., as feedlines connecting radio transmitters and receivers with their antennas. One advantage of coaxial over other types of radio transmission lines is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. Coaxial cable also provides protection of the signal from external electromagnetic interference.

Normally, the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor. The advantage of coaxial design is that electric and magnetic fields are restricted to the dielectric with little leakage outside the shield. Conversely, electric and magnetic fields outside the cable are largely kept from interfering with signals inside the cable. Signal leakage is the passage of electromagnetic fields through the shield of a cable and occurs in both directions. The gaps or holes in the shield, allow some of the electromagnetic field to penetrate to the other side.

Printed circuit boards (PCBs) are used ubiquitously for a wide range of applications. A typical printed circuit board has a top and bottom surface, on which components may be affixed, such as by soldering. Between the top and bottom surfaces are a plurality of layers used to carry signals from one point on the PCB to another point. Each layer is comprised of an insulating material with conductive traces disposed thereon.

Note that PCBs can be single sided (one copper layer), double-sided (two copper layers on both sides of a single substrate layer), or multilayer (outer and inner layers of copper, alternating with layers of substrate). Multilayer PCBs allow for much higher component density, because circuit traces on the inner layers would otherwise take up surface space between components.

PCBs are employed as a foundation for the mounting of various electronic components making up a circuit or system. Among the characteristics needed for good printed circuit board operation is the ability to electrically connect the various components efficiently and reliably. This is typically done through the use of etched patterns of metal conductors on the surface of the printed circuit boards.

Complex electronic systems today have increased the requirements for high packaging densities and high-speed wide bandwidth operation. Often, transmission lines and coplanar waveguide structures must be fabricated on PCBs to handle signal transition times below one nanosecond. PCB layout is now very critical to maintaining signal integrity, such as preserving signal edges and reducing distortion due to reflections and crosstalk. Proper impedance control and impedance termination are required in order to achieve desired signal fidelity.

Typical printed circuit boards are not suitable for such high frequencies due to excessive radiation from the signal lines. At frequencies of tens of gigahertz these signal traces radiate more energy than they transmit. Thus, there is a need for printed circuit boards which are compatible with such high frequencies.

Signals can begin on one layer to move to another layer through the use of vias. A vertical interconnect access or via is an electrical connection between layers in a physical electronic circuit that goes through the plane of one or more adjacent layers. Vias are conductive pathways that connect signals on various layers together. If the layers of the PCB are defined as being horizontal planes, these vias are typically vertical conductive pathways. In some embodiments, the vias pass through the entirety of the PCB. In other embodiments, known as blind vias, the via is accessible on either the top or bottom surface, but does not pass through the entirety of the PCB. In another embodiment, known as buried vias, the via may connect signals on two interior layers and not be accessible on either outer surface.

Vias have been used successfully for many years to carry signals between layers on printed circuit boards. However, as the frequencies of the signals on these PCBs continue to increase, vias have some limitations. For example, the high frequency signal may emit electromagnetic radiation to the interior layers through which it is passing. Furthermore, the impedance of the via may be different than the impedance of the signal traveling on a particular layer. This difference in impedance may cause undesirable reflections, affecting system performance.

In many applications, however, is it critical to have PCBs which display high isolation and low cross talk. In addition, it may be critical to have high frequency RF transmission having high dynamic range.

Therefore, it is desirable to have an improved PCB that is compatible with high frequencies in the tens of gigahertz. In addition, the PCB should be capable of high isolation and low crosstalk as well as high frequency RF transmission with high dynamic range. SUMMARY OF THE INVENTION

The present invention is a printed circuit board based coaxial transmission line structure (or coaxial via) and a method of fabrication thereof. The coaxial transmission line incorporates a plated through hole signal via passing through all N layers of the PCB. A plurality of buried ground vias electrically coupled to ground are arranged concentrically around the signal via to form a ground shield. The ground vias are fabricated from layer 2 through layer N-l, i.e. the second layer to the next to last layer of the PCB. The signal via in conjunction with the plurality of ground vias form the coaxial transmission line structure. Multiple concentric rings of buried ground vias may be fabricated to provide a high level of shielding and low RF signal leakage.

The printed circuit board of the present invention is particularly suited for use in high speed wide bandwidth systems. In one embodiment, the conductive traces fabricated on the PCB have a desired characteristic impedance and low distortion characteristics for high-speed signal transmission lines in the 77-81 GHz range. For example, the coaxial transmission line structure is suitable for use in an automotive radar sensor for autonomous vehicle applications.

This, additional, and/or other aspects and/or advantages of the embodiments of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the embodiments of the present invention.

There is thus provided in accordance with the invention, a printed circuit board (PCB) assembly, comprising a plurality of insulating layers stacked vertically one upon the other, including a top layer, a bottom layer and one or more inner layers, an electrically conductive plated through hole signal via passing through the top layer, the bottom layer, and the one or more inner layers, a plurality of buried ground vias electrically coupled to ground and passing only through the one or more inner layers, and arranged in a concentric ring formation in parallel with and surrounding the signal via thereby effectively forming a coaxial shield for the signal via, and wherein a combination of the signal via and the concentric ring of plurality of ground vias effectively forms a coaxial transmission line through the printed circuit board assembly.

There is also provided in accordance with the invention, a method of fabricating a printed circuit board (PCB) assembly, the method comprising providing a multilayer printed circuit board having a plurality of layers stacked vertically one upon the other, including a top layer, a bottom layer and one or more inner layers, forming at least one electrically conductive plated through hole signal via extending through the top layer, the bottom layer, and the one or more inner layers, the signal hole electrically connecting a first trace on the top layer to a second trace on the bottom layer, forming a plurality of buried ground vias electrically coupled to ground that pass only through the one or more inner layers, the ground vias arranged in a concentric ring formation in parallel with and surrounding the signal via thereby effectively forming a coaxial shield for the signal via, and wherein a combination of the signal via and the concentric ring of plurality of ground vias effectively forms a coaxial transmission line through the printed circuit board assembly.

There is further provided in accordance with the invention, an automotive radar sensor, comprising a printed circuit board (PCB) assembly having a plurality of insulating layers, including a top layer, a bottom layer and one or more inner layers, one or more electrically conductive plated through hole signal vias, each signal via passing through the top layer, the bottom layer, and the one or more inner layers, a plurality of buried ground vias electrically coupled to ground passing only through the one or more inner layers, and arranged in a concentric ring formation in parallel with and surrounding each signal via thereby effectively forming a coaxial shield for each signal via, wherein a combination of a signal via and a concentric ring of ground vias effectively forms a coaxial transmission line through the printed circuit board assembly, one or more transmitting antennas fabricated on the top layer of the PCB assembly, one or more receiving antennas fabricated on the top layer of the PCB assembly, and a transceiver located on the bottom layer of the PCB assembly, the transceiver coupled to the one or more transmitting antennas and the one or more receiving antennas through respective coaxial transmission lines, the transceiver operative to generate and supply transmitting signals to the one or more transmitting antennas and receive signals of a wave reflected back to the one or more receiving antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in further detail in the following exemplary embodiments and with reference to the figures, where identical or similar elements may be partly indicated by the same or similar reference numerals, and the features of various exemplary embodiments being combinable. The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

Fig. 1 is a diagram illustrating a cross section of an example signal via on a printed circuit board;

Fig. 2 is a diagram illustrating a perspective view of an example coaxial transmission line structure in accordance with the present invention;

Fig. 3 is a diagram illustrating a cross sectional view of an example coaxial transmission line structure in accordance with the present invention;

Fig. 4 is a diagram illustrating a top down view of an example coaxial transmission line structure in accordance with the present invention;

Fig. 5 A is a diagram illustrating a top layer view of an example coplanar waveguide feedline in accordance with the present invention;

Fig. 5B is a diagram illustrating a cross-sectional view of an example coplanar waveguide feedline in accordance with the present invention;

Fig. 6 is a diagram illustrating a bottom layer view of an example antenna in accordance with the present invention;

Fig. 7 is a diagram illustrating an all layer view of an example coaxial transmission line structure in accordance with the present invention;

Fig 8 is a flow diagram illustrating an example method of fabricating the coaxial via on a printed circuit board in accordance with the invention; and Fig. 9 is a high-level block diagram illustrating an example MIMO FMCW radar in accordance with the present invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be understood by those skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying figures. Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely illustrative of the invention that may be embodied in various forms. In addition, each of the examples given in connection with the various embodiments of the invention which are intended to be illustrative, and not restrictive.

The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings.

The figures constitute a part of this specification and include illustrative embodiments of the present invention and illustrate various objects and features thereof. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. In addition, any measurements, specifications and the like shown in the figures are intended to be illustrative, and not restrictive. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method. Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system.

Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The phrases“in one embodiment,”“in an example embodiment,” and“in some embodiments” as used herein do not necessarily refer to the same embodiment(s), though it may. Furthermore, the phrases“in another embodiment,”“in an alternative embodiment,” and“in some other embodiments” as used herein do not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments of the invention may be readily combined, without departing from the scope or spirit of the invention.

In addition, as used herein, the term“or” is an inclusive“or” operator, and is equivalent to the term“and/or,” unless the context clearly dictates otherwise. The term“based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of“a,”“an,” and“the” include plural references. The meaning of“in” includes“in” and“on.”

A diagram illustrating a cross section of an example signal via on a printed circuit board is shown in Figure 1. The printed circuit board (PCB) 50 comprises a plurality of layers (i.e. substrates) 52 labeled Layer 1 through Layer N. An integrated circuit (IC) 59 operating at high RF frequencies such as around 80 GHz is located on one side of the PCB. The RF IC 59 is electrically connected to a coplanar waveguide (i.e. planar transmission line) 54 that forms a feedline to a patch antenna 58 located on the opposite side of the PCB. A signal via 56 connects the patch antenna 58 to the coplanar waveguide feedline 54. Note that the coplanar waveguide does not necessarily have a ground plane under it.

Note that in printed circuit board design, a via consists of two pads in corresponding positions on different layers of the board, that are electrically connected by a hole through the board. The hole is made conductive by electroplating, or is lined with a tube or a rivet. High- density multilayer PCBs may have several kinds of vias namely (1) plated through hole that extends through all layers of the PCB; (2) blind vias that are exposed only on one side of the board; and (3) buried vias that connect only internal layers without being exposed on either the top or bottom surfaces.

A via consists of (1) a barrel which is the conductive tube filling the drilled hole; (2) a pad which connects each end of the barrel to the component, plane or trace; and (3) an antipad which is a clearance hole between the barrel and metal layer to which it is not connected.

The basic steps of making a PCB include: (1) making the substrate material; (2) stacking the substrate material in layers; (3) drilling and plating the vias; and (4) copper trace patterning using photolithography and etching. Through holes can be drilled using mechanical or laser drills. Depth controlled drilling techniques using lasers, for example, allow for more varied types of vias. PCB manufacturing typically starts with a so-called core, a basic double- sided PCB. Layers beyond the first two are stacked from this basic building block. Each type of via is made by drilling at each stacking stage.

In many applications, such as MIMO radar (described in more detail infra), it is necessary to place a plurality of patch antennas is very close proximity to each other. This leaves little PCB space for feedlines from and to transmitter and receiver circuitry. Thus, limited access to and extremely high density of the printed antennas on one surface of the PCB forces feeding the antennas from the opposite side of the PCB.

Utilizing a signal via such as shown in Figure 1 to couple the patch antennas to the RF circuitry is problematic. At high RF frequencies such as in the 77-81 GHz range, however, such a structure does not work. The problem is that the signal via structure is very lossy as the RF signal radiates from the plated hole.

The present invention provides a solution to this problem by providing a coaxial transmission line structure within the PCB. A diagram illustrating a perspective view of an example coaxial transmission line structure in accordance with the present invention is shown in Figure 2. The coaxial transmission line structure (also referred to as a coaxial via) is constructed from an electrically conductive plated through hole 208 forming signal via 212 passing through all N layers of the PCB surrounded by one or more concentric rings of buried ground vias 206. The buried ground vias extend only from layers 2 through N-l, i.e. from the second to the next to last layers. In this example, the PCB consists of N layers (three or more) including a top layer 202, bottom layer 204 and one or more inner layers 203. The one or more concentric ground vias 206 are electrically connected to ground (e.g., ground planes) and surround the signal via through the thickness of the PCB thus acting as an RF shield to both (1) prevent radiation and leakage out of the signal via as well as (2) prevent interference from reaching the signal via.

In one embodiment, for example purposes only, the top layer includes a coplanar waveguide (i.e. planar transmission line) consisting of (1) the signal line 210 electrically connected to the signal via 212, and (2) ground trench or channel 211 formed around the signal line and electrically coupled to ground potential. The transmission line does not include a ground plane in this case. The bottom layer includes a patch antenna 200 (partially shown) electrically connected to the signal via 212.

In one embodiment, the multilayer (three or more) printed circuit board comprises a plurality of planar layers made of a dielectric material suitable for the application frequencies and formed one on top of the other having signal conductor lines made of conductive material formed at spaced positions therein and ground lines made of conductive material formed at spaced positions in parallel with and in spaced relation around associated ones of the conductor lines to form coaxial transmission lines therewith. Note that preferably, all layers of the PCB are fabricated of the same material. In addition, the top and bottom layers carry high frequency signals while the inner layers carry lower frequency signals.

The ground vias are grouped concentrically (symmetrically or asymmetrically) around each signal via conductor. An inner ring (i.e. a first ring) is formed around the signal via. Preferably, the ground vias are spaced as close as possible to each other. Due to the limitation in spacing the ground vias, some RF signal leakage may occur between the ground via columns. To further reduce or eliminate the RF leakage, an additional ring (i.e. outer ring or second ring) of ground vias can be placed around the first ring. Note that although additional rings (i.e. third, forth, etc.) can be added, they law of diminishing returns applies. In one embodiment, it has been found that for 79 GHz frequency range, utilizing an inner ring and an outer ring of ground vias provides adequate shielding. Note that the ground vias in the outer ring are preferably staggered and located behind the openings in the inner ring thereby providing shielding for RF radiation that leaks through the openings between the ground vias in the inner ring.

A diagram illustrating a cross sectional view of an example coaxial transmission line structure in accordance with the present invention is shown in Figure 3. The coaxial transmission line structure, general referenced 60, comprises N (e.g., N=8) layers 70 labeled Layer 1 through Layer N. A through hole 64 through all N layers is plated with metal (e.g., copper) forming a conductive cylinder 65 of the signal via. A plurality of holes 66, 68 are drilled and their walls plated with metal forming conductive cylinders 67, 69 of the ground vias. Ground vias are arranged in a concentric ring around the signal via as shown in Figure 2. In an example embodiment, a feedline 72 connects to the signal via on the top layer (i.e. Layer 1) and to a patch antenna 74 on the bottom layer (i.e. Layer N). Ground channel 62 is part of the planar transmission line feeding the signal via.

A diagram illustrating a top down view of an example coaxial transmission line structure in accordance with the present invention is shown in Figure 4. As described supra, the holes 82 for the ground vias 83 pass through only the inner layers and are preferably evenly placed to surround the signal via 85 formed by hole 84 passing through all layers. Note that dashed ring 86 represents the limit of the antipad for the signal via. The placement of the ground vias form at least an inner ring 92 of evenly spaced ground vias. The tightness of the spacing of the ground vias is limited its antipad space defined by dashed circle 88. Preferably the maximum number of ground vias permitted by the PCB manufacturing process is placed in a concentric ring formation 92 around the signal via. To further reduce signal leakage, a second outer concentric ring 90 of ground vias is included. Preferably, the ground vias in the outer ring are centered in the opening between the ground vias in the inner ring. This serves to provide shielding of any RF that leaks between the ground vias in the inner ring.

Note that preferably, the ground vias pass through all inner layers (i.e. other than the first and last layer). This is to maximize the shielding effect of the ground vias. Although the ground vias pay pass through fewer than all the inner layers this will reduce the shielding effect and increase the RF leakage. Thus, it is preferably to maximize the number of layers the ground vias pass through, i.e. all inner layers.

Diagrams illustrating a top layer and cross-sectional views of an example coplanar waveguide feedline in accordance with the present invention is shown in Figures 5 A and 5B. The planar transmission feedline, generally referenced 100, comprises a signal conductor 110 coupled to signal via 116 formed out of plated through hole 112, insulator 108 around the conductor 110, ground plane 106, ground trench or channel 104 and surrounding ground plane 102. Note that the feedline components are fabricated on the top layer of the PCB. Note that the use of the ground channel or trench functions to improve isolation.

A diagram illustrating a bottom layer view of an example antenna in accordance with the present invention is shown in Figure 6. As described supra, the signal vias 126, formed from plated through holes 124 and antipad boundary 128, feed a patch antenna 122 fabricated on the bottom side of the PCB 121.

A diagram illustrating an all layer view of an example coaxial transmission line structure in accordance with the present invention is shown in Figure 7. To aid in illustrating the principles of the present invention, all N layers of the PCB are shown superimposed on one another. The coaxial transmission line, generally referenced 130, comprises signal via 136 formed by plated through hole 134 having antipad limits 140, inner and outer concentric rings of buried ground vias 142 formed by plated holes 140 having antipad limits 144 through all inner layers. These ground vias form the shield for the coaxial transmission line. The top layer comprises the planar transmission line including RF signal conductor 138. Note that the surrounding ground channel shown in Figures 2 and 5 are not shown here for clarity sake but would otherwise be present. The bottom layer comprises patch antennas 146 connected to the planar transmission line via the coaxial transmission line (i.e. signal via and associated plurality of ground vias).

A flow diagram illustrating an example method of fabricating the coaxial via on a printed circuit board in accordance with the invention is shown in Figure 8. First, the N layers (or substrates) of the PCB are fabricated (step 240). The buried ground vias that form the shield of the coaxial transmission line are drilled through layers 2 through N-l (step 242). The buried ground vias are then metal plated (step 244) and electrically connected to ground (step 246).

A through hole is then drilled for the signal via through all layers 1 through N (step 248) and the hole is then plated through to form the signal via (step 250). In one embodiment, the coplanar waveguide feedline is fabricated on the top layer and electrically connected to the signal via (step 252). A patch antenna is then fabricated on the bottom layer and electrically connected to the signal via (step 254).

As an example, consider an eight-layer PCB (i.e. N=8) with a signal via passing through all eight layers and multiple buried ground vias forming inner and outer concentric rings surrounding the signal via. The PCB layers comprise a very low loss dielectric constant (åX) material suitable for millimeter wave frequencies such as Astra® MT77 material manufactured by Isola Group, Chandler, Arizona, United States of America. The bottom side of the PCB comprises patch antennas. The microstrip antennas are adapted to work between 78-79 GHz.

As described supra, due to density of the antennas, they must be fed using vias. The RF signals from the receiver/transmit circuits are fed to the signal vias connected to the patch antennas on the top layer by way of coplanar waveguides (i.e. planar transmission lines). To minimize losses, the antennas are fed via plated through holes passing through layers 1 through 8. In one embodiment, the hole diameter of the signal via 84 is approximately 0.2 mm. Via pads are only located on layers 1 and 8 to minimize losses. Note that standard vias include pads on all eight layers. In one embodiment, the hole diameter of the ground vias 82 is approximately 0.15 mm. The coax structure (i.e. shielding) includes the buried vias extending from layer 2 through layer 6. The impedance of the resulting coaxial transmission line through the PCB is 50 Ohms.

The diameter of the signal via antipad 86 (Figure 4) is approximately 0.8 mm. In one embodiment, the diameter of the inner concentric ring 92 of ground vias in this example is approximately 1.1 mm while the diameter of the outer concentric ring 90 of ground vias in this example is approximately 1.58 mm.

A high-level block diagram illustrating an example MIMO FMCW radar in accordance with the present invention is shown in Figure 9. Considering the use of radar for automotive applications, vehicle manufacturers can currently make use of four frequency bands at 24 GHz and 77 GHz with different bandwidths. While the 24 GHz ISM band has a maximum bandwidth of 250 MHz, the 76-81 GHz ultrawideband (UWB) offers up to 5 GHz. A band with up to 4 GHz bandwidth lies between the frequencies of 77 to 81 GHz. It is currently in use for numerous applications. Note that other allocated frequencies for this application include 122 GHz and 244 GHz with a bandwidth of only 1 GHz. Since the signal bandwidth determines the range resolution, having sufficient bandwidth is important in radar applications.

Frequency modulated continuous wave (FMCW) radars are radars in which frequency modulation is used. The theory of operation of FMCW radar is that a continuous wave with an increasing frequency is transmitted. Such a wave is referred to as a chirp. A transmitted wave after being reflected by an object is received by a receiver.

The radar transceiver sensor, generally referenced 30, comprises a plurality of transmit circuits 38, a plurality of receive circuits 32, 40, local oscillator (LO) 34, ramp or chirp generator 44, e.g., direct digital synthesizer (DDS), and signal processing block 36. In operation, the radar transceiver sensor typically communicates with and may be controlled by a host 46. Each transmit block comprises a mixer 45, power amplifier 43, and antenna 41. Each receive block 32, 40 comprises an antenna 31, low noise amplifier (LNA) 33, mixer 35, intermediate frequency (IF) block 37, and analog to digital converter (ADC) 39. In one embodiment, the radar sensor 30 comprises a separate detection wideband receiver 40 dedicated to listening. The sensor uses this receiver to detect the presence of in band interfering signals transmitted by nearby radar sensors. The processing block uses knowledge of the detected interfering signals to formulate a response (if any) to mitigate and avoid any mutual interference.

Note that in one embodiment, the transmit and receive antennas 31, 41 comprise patch antennas such as shown in Figure 6. They are fed through signal vias to planar transmission lines on the opposite side of the PCB as shown in Figure 5. Inner and outer concentric rings of ground vias are formed around the signal vias as shown in Figure 2, described in detail supra.

Signal processing block 36 may comprise as any suitable electronic device capable of processing, receiving, or transmitting data or instructions. For example, the processing units may include one or more of: a microprocessor, a central processing unit (CPU), an application- specific integrated circuit (ASIC), field programmable gate array (FPGA), a digital signal processor (DSP), graphical processing unit (GPU), or combinations of such devices. As described herein, the terms“sign processor” or“processor” are meant to encompass a single processor or processing unit, multiple processors, multiple processing units, or other suitably configured computing element or elements.

For example, the processor may comprise one or more general purpose CPU cores and optionally one or more special purpose cores (e.g., DSP core, floating point, etc.). The one or more general purpose cores execute general purpose opcodes while the special purpose cores execute functions specific to their purpose.

Attached or embedded memory comprises dynamic random access memory (DRAM) or extended data out (EDO) memory, or other types of memory such as ROM, static RAM, flash, and non-volatile static random access memory (NVSRAM), removable memory, bubble memory, etc., or combinations of any of the above. The memory stores electronic data that can be used by the device. For example, a memory can store electrical data or content such as, for example, radar related data, audio and video files, documents and applications, device settings and user preferences, timing and control signals or data for the various modules, data structures or databases, and so on. The memory can be configured as any type of memory.

Transmitted and received signals are mixed (i.e. multiplied) to generate the signal to be processed by the signal processing unit 36. The multiplication process generates two signals: one with a phase equal to the difference of the multiplied signals, and the other one with a phase equal to the sum of the phases. The sum signal is filtered out and the difference signal is processed by the signal processing unit. The signal processing unit performs all the required processing of the received digital signals and controls the transmitted signal as well. Several functions performed by the signal processing block include determining range, velocity (i.e. Doppler), elevation, azimuth performing interference detection, mitigation and avoidance, performing simultaneous locating and mapping (SLAM), etc.

Note that FMCW radar offers many advantages compared to the other types of radars. These include (1) the ability to measure small ranges with high accuracy; (2) the ability to simultaneously measure the target range and its relative velocity; (3) signal processing can be performed at relatively low frequency ranges, considerably simplifying the realization of the processing circuit; (4) functioning well in various types of weather and atmospheric conditions such as rain, snow, humidity, fog and dusty conditions; (5) FMCW modulation is compatible with solid-state transmitters, and moreover represents the best use of output power available from these devices; and (6) having low weight and energy consumption due to the absence of high circuit voltages.

When using radar signals in automotive applications, it is desired to simultaneously determine the speed and distance of multiple objects within a single measurement cycle. Ordinary pulse radar cannot easily handle such a task since based on the timing offset between transmit and receive signals within a cycle, only the distance can be determined. If speed is also to be determined, a frequency modulated signal is used, e.g., a linear frequency modulated continuous wave (FMCW) signal. A pulse Doppler radar is also capable of measuring Doppler offsets directly. The frequency offset between transmit and receive signals is also known as the beat frequency. The beat frequency has a Doppler frequency component fo and a delay component fr. The Doppler component contains information about the velocity, and the delay component contains information about the range. With two unknowns of range and velocity, two beat frequency measurements are needed to determine the desired parameters. Immediately after the first signal, a second signal with a linearly modified frequency is incorporated into the measurement.

Determination of both parameters within a single measurement cycle is possible with FM chirp sequences. Since a single chirp is very short compared with the total measurement cycle, each beat frequency is determined primarily by the delay component fr. In this manner, the range can be ascertained directly after each chirp. Determining the phase shift between several successive chirps within a sequence permits the Doppler frequency to be determined using a Fourier transformation, making it possible to calculate the speed of vehicles. Note that the speed resolution improves as the length of the measurement cycle is increased.

Multiple input multiple output (MIMO) radar is a type of radar which uses multiple TX and RX antennas to transmit and receive signals. Each transmitting antenna in the array independently radiates a waveform signal which is different than the signals radiated from the other antennae. Alternatively, the signals may be identical but transmitted at non overlapping times. The reflected signals belonging to each transmitter antenna can be easily separated in the receiver antennas since either (1) orthogonal waveforms are used in the transmission, or (2) because they are received at non-overlapping times. A virtual array is created that contains information from each transmitting antenna to each receive antenna. Thus, if we have M number of transmit antennas and K number of receive antennas, we will have M K independent transmit and receive antenna pairs in the virtual array by using only M+K number of physical antennas. This characteristic of MIMO radar systems results in several advantages such as increased spatial resolution, increased antenna aperture, and higher sensitivity to detect slowly moving objects.

As stated supra, signals transmitted from different TX antennas are orthogonal. Orthogonality of the transmitted waveforms can be obtained by using time division multiplexing (TDM), frequency division multiplexing, or spatial coding. In the examples and description presented herein, TDM is used which allows only a single transmitter to transmit at each time.

Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as“associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermediary components. Likewise, any two components so associated can also be viewed as being “operably connected,” or“operably coupled,” to each other to achieve the desired functionality.

Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms“a”, “an” and“the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The use of introductory phrases such as“at least one” and“one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles“a” or“an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases“one or more” or“at least one” and indefinite articles such as“a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as“first,”“second,” etc. are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. As numerous modifications and changes will readily occur to those skilled in the art, it is intended that the invention not be limited to the limited number of embodiments described herein. Accordingly, it will be appreciated that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.