Patent Description:
Radio frequency (RF) and electromagnetic circuits may be manufactured using conventional printed circuit board (PCB) processes. Conventional PCB manufacturing processes may include lamination, electroplating, masking, etching, and other complex process steps, and may require multiple steps, expensive and/or hazardous materials, multiple iterations, extensive labor, etc., all leading to higher cost and slower turnaround time. Additionally, conventional PCB manufacturing processes have limited ability to allow for small feature sizes, such as transmission line (e.g., stripline) dimensions, and dimensions of dielectric materials between conductors (e.g., dielectric thickness, inter-via spacing, etc.), thereby limiting the range of highest frequency signals that may be supported by such circuits.

<CIT> discloses a waveguide system for distributing high-bandwidth signals in a multilayer circuit carrier. The waveguide system comprises at least one coplanar waveguide and one or more ground wires. The coplanar waveguide is disposed with the ground wires associated therewith between at least two insulating layers of the circuit carrier. The surface of the two insulating layers oriented away from the plane of the waveguide has electrically conductive layers. Electrically conductive plated through-holes extend along the waveguide substantially perpendicular to the plane of the waveguide. The ground wires, the electrically conductive layers, and the plated through-holes are electrically connected to ground potential. The waveguide system serves particularly for the three-dimensional distribution of high-bandwidth signals.

<CIT> discloses a multi-layer substrate comprising a first dielectric layer, a coplanar waveguide line formed on a first surface of the first dielectric layer, the coplanar waveguide line including a signal conductor and a pair of ground conductor layers positioned at opposite sides of the signal conductor, separately from the signal conductor, and a second dielectric layer formed to cover the coplanar waveguide line and the first dielectric layer and having an opening positioned at least on the signal conductor of the coplanar waveguide line. A thickness of the first dielectric layer is smaller than the value of c/{<NUM>f(ε<NUM> - <NUM>)<NUM>/<NUM>}, where c is velocity of light, f is a frequency of a signal propagating in the signal line, and ε<NUM> is a dielectric constant of the first dielectric layer.

<CIT> discloses a high-frequency wiring board that includes first coplanar lines and second coplanar lines formed on a different layer than the first coplanar lines; the first coplanar lines and second coplanar lines being connected at the line ends of each. The first coplanar lines are provided with a first signal line and a first planar ground pattern formed on the same wiring layer as the first signal line. The second coplanar lines are provided with second signal line formed on a wiring layer that differs from that of the first signal line, a second planar ground pattern formed on the same wiring layer as the second signal line, and a first ground pattern formed on the same wiring layer as the first coplanar lines. The end of the first planar ground pattern and the end of the first ground pattern are connected and thus unified. In this high-frequency wiring board, the second planar ground pattern is separated from the connection portion at the end of the first planar ground pattern in the direction in which the second coplanar lines extend from the vicinity of the connection portion of the first signal line and the second signal line.

<CIT> discloses an electrical cell that enables experimental measurements of dielectric properties of an electrochromic material in the radio-frequency range of the electromagnetic spectrum. In an example embodiment, the electrical cell includes a layer of the electrochromic material under test that is sandwiched between a conducting base plane and a microstrip line. The conducting base plane and the micro strip line are electrically connected to a coplanar waveguide configured for application of superimposed DC-bias and RF-probe signals using a conventional probe station and a vector network analyzer. The S-parameters of the electrical cell measured in this manner can then be used, e.g., to obtain the complex dielectric constant of the electrochromic material under test as a func-tion of frequency.

<CIT> discloses a multilayer printed-circuit board that includes at least one inner-layer signal line, first and second ground layers between which the inner-layer signal line is sandwiched via a frame member made of an insulating material in a thickness direction of the multilayer printed-circuit board, and metallic wall members which are provided on inner walls of slits formed in the frame member and extending along the inner-layer signal line. The first and second ground layers and the metallic wall members shield the inner-layer signal line.

<NPL> discloses that: the fabrication of microvias and trenches for substrate integrated waveguides (SIWs) in low temperature co-fired ceramic (LTCC) technology is studied using a commercial laser PCB prototyping system. It is shown that by careful control of the laser beam around the edge of the via, clean holes can be produced. Vias with diameter of <NUM> and trenches with width of <NUM> are demonstrated. The effects of laser parameters in making vias and trenches are investigated. The process can be utilized to demonstrate LTCC SIWs operating in the <NUM>-<NUM> range with low loss. It is confirmed that rows of via-holes can achieve the same performance as solid sidewalls.

Aspects and embodiments described herein provide simplified circuit structures, and manufacturing methods thereof, for conveyance of electrical signals, especially radio frequency signals, within a circuit. Various embodiments of circuits in accord with those described herein may be constructed of, e.g., laminate or dielectric substrates, and may have circuit features, signal layers, ground planes, or other circuit structures therebetween. Further, various signal conductors and circuit structures may be fabricated more simply and with smaller feature sizes than conventional techniques. Such circuit structures are suitable for higher frequency operation into the millimeter wave range, as well as conventional microwave ranges. Circuits, structures, and fabrication methods described herein use subtractive and additive manufacturing technology to achieve smaller sizes and higher frequency operation.

There is provided, according to one aspect of the present disclosure, a radio frequency circuit according to claim <NUM>. The radio frequency circuit further may include an electrical conductor disposed through a hole in at least one of the first dielectric substrate and the second dielectric substrate, the electrical conductor being in electrical contact with the transmission line. The radio frequency circuit further may include an electrical component in electrical contact with the electrical conductor and configured to send or receive an electromagnetic signal to or from the transmission line via the electrical conductor, with the electrical component being at least one of a terminal, a connector, a cable, and an electromagnetic radiator. The transmission line may produce <NUM> decibels or less of insertion loss per inch at <NUM>. The pair of reference conductors may be formed from the conductive cladding disposed upon the first substrate.

There is also providing, according to another aspect of the disclosure, a method of fabricating an electromagnetic circuit according to claim <NUM>. The method further may include drilling a hole in at least one of the first dielectric substrate and the second dielectric substrate to provide access to a portion of the transmission line; and providing an electrical conductor disposed through the hole, the electrical conductor arranged to be in electrical contact with the transmission line. The method further may include electrically coupling the electrical conductor to an electrical component, the electrical conductor being thereby configured to convey a signal between the transmission line and the electrical component, the electrical component being at least one of a terminal, a connector, a cable, and an electromagnetic radiator. The method further may include conveying to the transmission line an electromagnetic signal having a frequency in the range of <NUM> to <NUM>.

Still other aspects, examples, and advantages are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to "an embodiment," "some embodiments," "an alternate embodiment," "various embodiments," "one embodiment" or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. Various aspects and embodiments described herein may include means for performing any of the described methods or functions.

The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the disclosure. In the figures, each identical or nearly identical component that is illustrated in various figures may be represented by a like numeral. In the figures:.

Aspects and examples described herein provide signal conductors (e.g., transmission lines, signal traces, strip lines) with reference surfaces and conductors within various circuits for the containment and conveyance of millimeter wave signals. The transmission line structures described herein efficiently distribute signal currents while maintaining characteristic impedance and minimizing signal loss along the transmission line. The transmission line structures described herein are suitable for various circuit board manufacturing, including radio frequency circuit embodiments, and advantageously apply subtractive and additive manufacturing techniques. Such techniques may provide structures capable of conveyance and containment of radio frequency signals in microwave and millimeter wave ranges, for example from <NUM> to <NUM>, and up to <NUM> or more.

Conventional transmission line architectures rely heavily on a center conductor, producing a significant concentration of radio frequency current on the center conductor as a result. The resulting current concentration, combined with a frequency dependent skin effect, produces insertion loss that increases exponentially with respect to frequency. As a result, conventional transmission lines produce significant loss, and begin to defeat their purpose of conveying radio frequency energy when used at higher frequencies, e.g., into the millimeter wave ranges. Waveguides have also been considered as a possible conventional approach, but also exhibit high current density, e.g., on the E-plane walls.

Transmission line architectures in accord with structures and methods described herein overcome the above limiting factors by distributing the radio frequency current over a larger surface area, such that the signal loss mechanism is overcome while accommodating intrinsic skin effects. In various embodiments, transmission line architectures described herein retain the characteristics of transverse electromagnetic (TEM) wave propagation, with linear dispersion, low insertion loss, and a fixed characteristic impedance. The geometry uses multiple conducting surfaces, and a configuration that orients the electric field orthogonal to the conducting surfaces. Transmission lines and methods in accord with those described herein distribute signal currents over three principal conductors in combination with ground planes (e.g., horizontal) and other Faraday boundaries (e.g., vertical).

In some embodiments, a transmission line (e.g., conductor) may be formed on a dielectric substrate by machining away (e.g., milling) a portion of cladding (e.g., electroplate copper) from a surface of the substrate.

In some embodiments, a wire conductor may convey a signal "vertically" between layers (e.g., to/from a transmission line) within a circuit, and may be used to feed a signal to or from various other layers or circuit components, such as a waveguide, a radiator (e.g., an antenna), a connector, or other circuit structures. Such a "vertical" inter-layer signal feed may be formed by machining a hole in one or more dielectric substrates, applying solder to one or more conductor surfaces, inserting a segment of wire (e.g., copper wire) into the hole, and reflowing the solder to mechanically and electrically secure a connection.

In some embodiments, a continuous conducting structure may be formed in one or more dielectric substrates by machining a trench and filling the trench with a conductor, such as a conductive ink applied using <NUM>-D printing techniques, to form an electromagnetic boundary. Such an electromagnetic boundary may enforce boundary conditions of an electromagnetic signal, e.g., to control or limit modes of a signal and/or characteristic impedance, or may provide isolation to confine signals to a region of an electromagnetic circuit, e.g., a Faraday boundary to prevent a signal at one region of the circuit from affecting another region of the circuit, e.g., shielding.

Manufacturing processes described herein may be particularly suitable for fabrication of circuit structures having physically small features capable of supporting electromagnetic signals in the range of <NUM> to <NUM> or more, for example, and up to <NUM> or more, using suitable subtractive (e.g., machining, milling, drilling, cutting, stamping) and additive (e.g., filling, flowing, <NUM>-D printing) manufacturing equipment. Electromagnetic circuit structures in accord with systems and methods described herein may be particularly suitable for application in <NUM> to <NUM> systems, including millimeter wave communications, sensing, ranging, etc. Aspects and embodiments described may also be suitable for lower frequency ranges, such as in the S-band (<NUM> - <NUM>), X-band (<NUM> - <NUM>), or others.

It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. The use herein of "including," "comprising," "having," "containing," "involving," and variations thereof is meant to encompass the items listed thereafter as well as additional items. References to "or" may be construed as inclusive so that any terms described using "or" may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, end, side, vertical and horizontal, and the like, are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation.

The term "radio frequency" as used herein is not intended to be limited to a particular frequency, range of frequencies, band, spectrum, etc., unless explicitly stated and/or specifically indicated by context. Similarly, the terms "radio frequency signal" and "electromagnetic signal" are used interchangeably and may refer to a signal of various suitable frequency for the propagation of information-carrying signals, for any particular implementation. Such radio frequency signals may generally be bound at the low end by frequencies in the kilohertz (kHz) range, and bound at the high end by frequencies of up to hundreds of gigahertz (GHz), and explicitly includes signals in the microwave or millimeter wave ranges. Generally, systems and methods in accord with those described herein may be suitable for handling non-ionizing radiation, at frequencies below those conventionally handled in the field of optics, e.g., of lower frequency than, e.g., infrared signals.

Various embodiments of radio frequency circuits may be designed with dimensions selected and/or nominally manufactured to operate at various frequencies. The selection of appropriate dimensions may be had from general electromagnetic principles and are not presented in detail herein.

The methods and apparatuses described herein may support smaller arrangements and dimensions than conventional processes are capable. Conventional circuit boards may be limited to frequencies below about <NUM>. The methods and apparatuses described herein may allow or accommodate the manufacture of electromagnetic circuits of smaller dimensions, suitable for radio frequency circuits intended to be operated at higher frequencies, using safer and less complex manufacturing, at lower cost.

Electromagnetic circuits and methods of manufacture in accord with those described herein include various additive and subtractive manufacturing techniques to produce electromagnetic circuits and components capable of handling higher frequencies, with lower profiles, and at reduced costs, cycle times, and design risks, than conventional circuits and methods. Examples of techniques include machining (e.g., milling) of conductive material from a surface of a substrate to form transmission lines (e.g., signal conductors, striplines) or apertures, which may be of significantly smaller dimensions than allowed by conventional PCB processes, machining of one or more substrates to form a trench, using <NUM>-dimensional printing techniques to deposit printed conductive inks into the trench to form a continuous electric barrier (e.g., a Faraday wall) (as opposed to a series of ground vias with minimum spacing therebetween), "vertical launch" signal paths formed by machining (such as milling, drilling, or punching) a hole through a portion of substrate and in which a conductor (such as a wire segment) is placed and/or conductive ink is printed, to make electrical contact to a transmission line disposed on a surface of the substrate (or an opposing substrate), and using <NUM>-dimensional printing techniques to deposit printed resistive inks to form resistive components.

Any of the above example techniques and/or others (e.g., soldering and/or solder reflow), may be combined to make various electromagnetic components and/or circuits. Aspects and examples of such techniques are described and illustrated herein with respect to a radio frequency transmission line to contain and convey an electromagnetic signal along a layer of an electromagnetic circuit in one dimension and, optionally, vertically through to other layers of the circuit in another dimension. The techniques described herein may be used to form various electromagnetic components, connectors, circuits, assemblies, and systems.

<FIG> illustrates an example of an electromagnetic circuit structure <NUM> that may be a portion of a larger electromagnetic circuit. The circuit structure <NUM> includes a pair of dielectric substrates <NUM> bonded together and having a transmission line <NUM> enclosed between them. The transmission line <NUM> is an electrical conductor configured to convey electromagnetic signals within the circuit, e.g., within the circuit structure <NUM>, and may be formed by machining away a cladding, such as electroplated copper, from a surface of either of the substrates <NUM>. Signals conveyed by the transmission line <NUM> may be defined by electric and magnetic fields, with reference to further conductor(s). For example, the transmission line <NUM> is circumscribed by reference conductors <NUM>, which are co-planar conductors, providing a secondary or "return path" electrical conductor for signals conveyed by the transmission line <NUM>. In some examples, the term "transmission line" may include the combination of the transmission line <NUM> (a first conductor) and its associated reference conductors <NUM> (a second conductor). In various examples, the reference conductors <NUM> may be coupled to a ground reference. The transmission line <NUM> exhibits a characteristic impedance for signals of various frequencies, and the characteristic impedance may depend upon a size of the transmission line <NUM> (e.g., height and width), a size of the gap(s) between the transmission line <NUM> and the reference conductors <NUM>, and material characteristics of the substrates <NUM> and the gap(s). In various embodiments, gaps between the transmission line <NUM> and the reference conductors <NUM> may be filled with a bonding material, e.g., and adhesive used to bond the substrates <NUM> together.

The transmission line <NUM> may also have an electrical connection, such as by solder, to a "vertical launch" conductor <NUM>, which may be disposed within a machined hole in the substrate 110b. Accordingly, the conductor <NUM> and the transmission line <NUM> may form an electrically continuous signal conveyance, and each may convey and provide signals beyond the extent of the portion shown in <FIG>. In some examples, the conductor <NUM> may be a segment of wire, such as a copper wire. In various examples, the conductor <NUM> may be any of various forms, such as solid, hollow, rigid, flexible, straight, coil, spiral, etc. Additional details of at least one example of a vertical launch and its manufacture are disclosed in <CIT>.

In some embodiments, a ground plane <NUM> may be provided and may be formed of a conductive cladding disposed upon a "bottom" surface of the substrate 110a. An additional ground plane <NUM> may be provided upon a "top" surface of the substrate 110b. For example, the ground plane <NUM> may be formed by a conductive cladding disposed upon the substrate 110b. A portion of the conductive cladding may be removed by machining (e.g., milling) to provide a ground plane with an appropriate physical dimension, shape, or extent, e.g., to be suitable to act as the ground plane <NUM>.

Conventional PCB fabrication techniques may incorporate ground planes as reference conductors for a signal trace or strip line and, accordingly, may impose requirements on a distance between the ground planes and to the signal trace therebetween, e.g., to establish a characteristic impedance for signals conveyed by the signal trace. By contrast, the transmission line <NUM> is provided with reference conductors <NUM>, such that the ground planes <NUM>, <NUM> may have little to no impact on the characteristic impedance, may instead serve as isolation or shielding from other circuit components, and may be further away than conventionally required.

Accordingly, the transmission line <NUM>, and the gap(s) between the transmission line <NUM> and the reference conductors <NUM>, may be particularly small, such as down to <NUM> mils (. <NUM> inches) or less, e.g., to accommodate millimeter wave signals. Accordingly, an impedance of the transmission line <NUM> may be less affected by the presence of the ground planes <NUM>, <NUM>, which may allow the thickness of the substrate <NUM> to be designed or selected based upon alternate concerns (e.g., strength, rigidity, etc.).

Additionally, electromagnetic signal(s) conveyed by the transmission line <NUM> are conveyed by the combination of the transmission line <NUM>, the reference conductors <NUM>, and the gap therebetween, such that current density is distributed among the conductors (at least the transmission line <NUM> and the reference conductors <NUM>), resulting in lower current densities than conventional transmission lines using PCB fabrication techniques. Accordingly, signal losses per distance of transmission may be reduced as compared to conventional PCB transmission line structures and manufacturing techniques.

The circuit structure <NUM> also includes a Faraday wall <NUM> that is a conductor providing shielding or isolation as an electromagnetic boundary "vertically" through the substrates <NUM>. The Faraday wall <NUM> may be formed by machining a trench through the substrates <NUM> down to the ground plane <NUM> and filling the trench with a conductive material, such as a conductive ink applied with additive manufacturing techniques, e.g., <NUM>-D printing. The conductive ink, when set, may form a substantially electrically continuous conductor. As shown, the trench in which the Faraday wall <NUM> is formed does not pierce or go through the ground plane <NUM>. The Faraday wall <NUM> may therefore be in electrical contact with the ground plane <NUM>. Additionally, a "top" of the Faraday wall <NUM> may be in electrical contact with the ground plane <NUM>, which may be accomplished by slight over-filling of the machined trench to ensure contact between the conductive ink and the ground plane <NUM> and/or by application of solder, for example. Additional details of at least one example of a Faraday wall and its manufacture are disclosed in <CIT>.

As illustrated in <FIG>, the ground plane <NUM>, ground plane <NUM>, and Faraday wall <NUM> together form a substantially electrically continuous conductor that provides an isolation boundary for signal(s) conveyed by the transmission line <NUM> and its associated reference conductors <NUM>. In some embodiments, dimensional placement of the ground planes <NUM>, <NUM> and the Faraday wall <NUM> may be selected to control or limit a propagating mode of a signal conveyed by the transmission line <NUM> and/or to establish a characteristic impedance for signal(s) conveyed by the transmission line <NUM>. In certain embodiments, the ground planes <NUM>, <NUM> and the Faraday wall <NUM> may be positioned such that only a transverse electromagnetic (TEM) signal mode may propagate along the transmission line <NUM>. In other embodiments, the Faraday wall <NUM> may be positioned to isolate one portion of a circuit from another portion of a circuit without enforcing a particular propagating mode and/or without contributing to an impedance for any particular signal(s).

As stated above, the structure <NUM> is merely an example and portion of a structure in which an electromagnetic circuit may be provided. Further extent of the substrates shown may accommodate various circuit components, and additional substrates having additional layers to accommodate additional circuit components may be provided in various embodiments. Typically, a portion of a circuit may be disposed on a particular layer, and may include ground planes above and/or below, and other portions of a total circuit (or system) may exist at different regions of the same layer or on other layers.

<FIG> shows a portion structure 100a of the circuit structure <NUM> at one stage of manufacture, in accord with aspects and embodiments of the systems and methods described herein. The portion structure 100a includes the substrate 110a which may be provided with conductive (e.g., copper) cladding on various surfaces. In this example, the substrate 110a has a conductive cladding <NUM> on one surface that serves as the conductive material from which the transmission line <NUM> and the reference conductors <NUM> are formed. Also in this example, the substrate 110a has a conductive cladding on an opposing surface to serve as the ground plane <NUM>. The transmission line <NUM> may be formed by machining away at least a portion <NUM> of the cladding <NUM>, thereby leaving a portion of conductive material to serve as the transmission line <NUM>, distinct from the remainder of the cladding <NUM>.

<FIG> shows another portion structure 100b of the circuit structure <NUM> at another stage of manufacture. For the portion structure 100b, the substrate 110b is aligned with the substrate 110a, to be bonded together. In some examples, a temporary bonding or affixing may be applied and a permanent bonding step may be applied at a later time, such as a bonding that may require heat or baking to cure or to secure a permanency of the bonding. Various examples may have a hole in the substrate 110b positioned to align with a portion of the transmission line <NUM>, e.g., to accommodate the conductor <NUM> illustrated in <FIG>. In various embodiments, a "top" surface of the substrate 110b may include a conductive cladding, which may be used to provide a ground plane, if desired. As with the cladding <NUM>, portions of any cladding on the substrate 110b may be machined away to form various other structures, components, or a ground plane having a desired shape or extent.

With reference to <FIG>, the conductive cladding <NUM> from which the transmission line <NUM> is formed (e.g., by machining away the portion <NUM>) may equivalently be associated with the substrate 110b, e.g., on a "bottom" side with respect to <FIG>, instead of being associated with the substrate 110a. In other words, the conductive material from which the transmission line <NUM> is provided may be a conductive cladding associated with either of the substrates <NUM>. Further, the transmission line <NUM> and/or the reference conductors <NUM> may be provided from differing materials and/or through other means in various embodiments.

<FIG> shows another portion structure 100c of the circuit structure <NUM> at another stage of manufacture. For the portion structure 100c, a trench <NUM> is milled through the substrates <NUM>. In this example, the trench <NUM> is milled through the substrates <NUM> and a portion of the cladding <NUM>, down to the conductive cladding that forms the ground plane <NUM>. Machining away material to form the trench <NUM> may also form the reference conductors <NUM>, e.g., by separating a further portion of the cladding <NUM>, leaving a portion of conductive material to serve as the reference conductors <NUM>, distinct from the remainder of the cladding <NUM>. In various embodiments, the trench <NUM> is milled down to the ground plane <NUM> without piercing the ground plane <NUM>. In some embodiments, the intact ground plane <NUM> may provide structural support to portions of the structure 100c while the trench is empty.

<FIG> shows another portion structure 100d of the circuit structure <NUM> at another stage of manufacture. In the portion structure 100d, the trench <NUM> is filled with a conductive fill <NUM> to form the Faraday wall <NUM>. The conductive fill <NUM> may make electrical contact with the ground plane <NUM> to form a substantially electrically continuous ground boundary. As described above with respect to <FIG>, a further ground plane <NUM> may be included, and to which the conductive fill <NUM> may be electrically connected by physical contact and/or by further application of solder at positions along the Faraday wall <NUM>, to electrically join with the ground plane <NUM>. In some embodiments, the intact ground plane <NUM> and the cured (e.g., cooled, solidified) conductive fill <NUM> may provide structural support to the structure 100d, e.g., in place of the material(s) that was machined away to form the trench <NUM>.

<FIG> illustrates one example of a set of dimensions for the transmission line <NUM> and the gap separating the transmission line <NUM> from the reference conductors <NUM>. In this example, a <NUM> mil wide portion of cladding is machined away to provide a <NUM> mil wide transmission line <NUM>. Accordingly, the edges of the reference conductors <NUM> are <NUM> mils away from each other, leaving a <NUM> mil gap between the transmission line <NUM> and each of the reference conductors <NUM> (e.g., the width of the machined away portion of cladding).

Dimensional information shown in the figures is for illustrative purposes and is representative of some dimensions that may be desirable or suitable for certain applications, and may be illustrative of some dimensions achievable with the methods described herein. In various embodiments, dimensions may be significantly smaller, or may be larger, depending upon the capabilities of the subtractive and additive equipment used in production, and depending upon the design and application of a particular circuit.

<FIG> illustrates a method <NUM> of fabricating a millimeter wave transmission line in accord with aspects and examples described herein. The method <NUM> includes machining away a conductive material (e.g., cladding) (block <NUM>) to form a transmission line, and the machined away area forms a gap. The reference conductors are formed by machining away further conductive material (e.g., cladding) (block <NUM>) that is co-planar with the transmission line. Machining away conductive material to form co-planar reference conductors (as in block <NUM>) may be part of machining a trench through one or more substrates that simultaneously machines away a cladding on one of the substrates. Various conductive material may be provided (block <NUM>) to form one or more electrical barriers, that may isolate the transmission line <NUM> and reference conductors <NUM> from other portions of a circuit or system. For example, filling a trench with a conductive material may form an electrical boundary, and/or including cladding on one or more substrates may form an electrical boundary, e.g., as a ground plane.

Further advantages of system and methods described herein may be realized. For example, conventional PCB manufacturing may impose limitations on circuit feature sizes, such as the width of transmission lines, thus limiting the highest frequencies for which conventionally made electromagnetic circuits may be suitable. Further, substrate thicknesses impact characteristic impedance (e.g., due to the distance to ground planes disposed upon opposing surfaces) in relation to width of the traces. Accordingly, wider traces required by conventional PCB processes cause selection of thicker substrates (to maintain a particular characteristic impedance), thus limiting how thin the circuit can be manufactured. For example, general recommendations under conventional PCB manufacturing include total thicknesses of about <NUM> mil (. <NUM> inches). By comparison, electromagnetic circuits in accord with aspects and embodiments described, using additive manufacturing techniques, can result in circuit boards having a low profile down to a thickness of about <NUM> mil or less, with signal line traces having widths of about <NUM> mil, or <NUM> mil, or less, and interconnect geometries substantially flush with a surface of the board.

Ground vias conventionally provide electrical connectivity between ground planes (e.g., on opposing surfaces of substrates) and provide some isolation of signals on the traces from other traces that may be nearby. The conventional ground vias are drilled holes of about <NUM> mil diameter or greater, and are required to be a minimum distance apart to maintain structural integrity of the board. Accordingly, ground vias are leaky structures, exhibiting loss of electromagnetic signal, especially at higher frequencies. As various applications require support for higher frequency signals, the minimum spacing between ground vias act like large openings through which relatively small wavelengths of electromagnetic energy may escape.

By comparison, electromagnetic circuits and methods in accord with aspects and embodiments described herein, which use additive manufacturing techniques, allow for electrically continuous Faraday boundaries, which may further be electrically coupled to ground planes. Accordingly, an electrically continuous structure is provided and disposed vertically through one or more substrates, (e.g., between opposing surfaces of the substrate) to form "Faraday walls" that confine electromagnetic fields. In various embodiments, such Faraday walls may electrically couple two or more ground planes. Further in various embodiments, such Faraday walls may confine and isolate electromagnetic fields from neighboring circuit components. In some embodiments, such Faraday walls may enforce a boundary condition to limit electromagnetic signals to be locally transverse electric-magnetic (TEM) fields, e.g., limiting signal propagation to a TEM mode.

In various embodiments, various subtractive (machining, milling, drilling), additive (printing, filling), and adherent (bonding) steps may be carried out, in various orders, with soldering and reflow operations as necessary, to form an electromagnetic circuit having one or any number of substrate layers, which may include one or more Faraday boundaries as described herein.

A generalized method for making any of various electromagnetic circuits includes milling a conductive material disposed on a substrate to form circuit features. The method may include printing (or depositing, e.g., via <NUM>-D printing, additive manufacturing techniques) additional circuit features, such as resistors formed of resistive ink, for example. The method may include depositing solder on any feature, as necessary. The method may also include milling (or drilling) through substrate material (and/or conductive materials) to form openings, such as voids or trenches, and includes depositing or printing (e.g., via <NUM>-D printing, additive manufacturing techniques) conductive material (such as conductive ink or a wire conductor) into the voids / trenches, for example to form Faraday walls or vertical signal launches (e.g., copper). Any of these steps may be done in different orders, repeated, or omitted as necessary for a given circuit design. In some embodiments, multiple substrates may be involved in the manufacture of an electromagnetic circuit, and the method includes bonding further substrates as necessary, further milling and filling operations, and further soldering and/or reflow operations.

Having described several aspects of at least one embodiment and a method for manufacturing an electromagnetic circuit, the above descriptions may be employed to produce various electromagnetic circuits with an overall thickness of <NUM> mils (. <NUM> inches, <NUM> microns) or less, and may include transmission lines, such as the traces as narrow as <NUM> mils (<NUM> microns), <NUM> mils (<NUM> microns), or even as narrow as <NUM> mils (<NUM> microns), depending upon the tolerances and accuracy of various milling and additive manufacturing equipment used. Accordingly, electromagnetic circuits in accord with those described herein may be suitable for microwave and millimeter wave applications, including S-Band, X-Band, K-Bands, and higher frequencies, with various embodiments capable of accommodating frequencies over <NUM> and up to <NUM> or higher. Some embodiments may be suitable for frequency ranges up to <NUM> or more.

Additionally, electromagnetic circuits in accord with those described herein may have a low enough profile (e.g., thickness of <NUM> mils or less), with accordant light weight, to be suitable for outer space applications, including folding structures to be deployed by unfolding when positioned in outer space.

Further, electromagnetic circuits manufactured in accord with methods described herein accommodate less expensive and faster prototyping, without the necessity for caustic chemicals, masking, etching, electroplating, etc. Simple substrates with pre-plated conductive material disposed on one or both surfaces (sides) may form the core starting material, and all elements of an electromagnetic circuit may be formed by milling (subtractive, drilling), filling (additive, printing of conductive and/or resistive inks), and bonding one or more substrates. Simple solder reflow operations and insertion of simple conductors (e.g., copper wire) are accommodated by methods and systems described herein.

Claim 1:
A radio frequency circuit (<NUM>), comprising:
a first dielectric substrate (110a) having a first surface;
a second dielectric substrate (110b) having a second surface, the first and second dielectric substrates being positioned relative to one another such that the second surface faces the first surface;
a transmission line (<NUM>) formed from conductive cladding (<NUM>) disposed upon the first surface, the transmission line being at least partially encapsulated between the first dielectric substrate and the second dielectric substrate;
a pair of reference conductors (<NUM>), each of the pair of reference conductors positioned adjacent to and co-planar with the transmission line and spaced such that there is a gap between each of the pair of reference conductors and the transmission line, each of the pair of reference conductors disposed upon the first surface or the second surface and being at least partially encapsulated between the first dielectric substrate and the second dielectric substrate;
a first ground plane (<NUM>) provided on a third surface of the first dielectric substrate, the third surface being an opposing and substantially parallel surface to the first surface, and a second ground plane (<NUM>) provided on a fourth surface of the second dielectric substrate, the fourth surface being an opposing and substantially parallel surface to the second surface, each of the first ground plane and the second ground plane being substantially parallel to the other and to the co-planar arrangement of the transmission line and the pair of reference conductors; and
a trench (<NUM>) formed through the first dielectric substrate and the second dielectric substrate, the trench extending between and being substantially perpendicular to the first ground plane and the second ground plane, wherein the trench does not pierce the first ground plane, wherein a conductive material (<NUM>) is filled in the trench, and wherein the conductive material is arranged to be in electrical contact with each of the first ground plane and the second ground plane.