Abstract:
A substrate ( 12 ) having two high frequency components ( 28, 30, 34, 36, 48 ) positioned on substrates ( 12 ) typically used for lower frequency devices. A coplanar strip transmission line ( 38, 40, 46, 52, 58 ), providing for transmission of high frequency signals, comprises first, second and third parallel, spaced conductive traces ( 62, 64, 66 ) positioned on a surface ( 11 ) of the substrate ( 12 ), wherein the substrate ( 12 ) defines a first slot ( 68, 72, 76 ) extending from the first surface ( 11 ) into the substrate ( 12 ) and between the first and second parallel, spaced conductive traces ( 64, 62 ), and a second slot ( 70, 74, 78 ) extending from the first surface ( 11 ) into the substrate ( 12 ) and between the first and third parallel, spaced conductive traces ( 64, 66 ). Optionally, an antenna ( 42, 44, 54, 56 ) is coupled to the coplanar strip transmission line ( 46, 58 ) and comprises first and second antenna traces ( 41, 43 ), the substrate ( 12 ) defining a third slot ( 45 ) therebetween.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to conduction of high frequency signals and more particularly to coplanar strip transmission lines for transmission of high frequency signals on substrates typically used for lower frequency devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    Circuits used in many electronic devices, for example, cellular phones and radios, produce, receive, or function with high frequency signals as well as low frequency signals. Integration of high and low frequency circuits typically involve the use of hybrid substrates, with low frequency devices formed on FR4, for example, and high frequency devices formed on RT/Duroid©, for example. Both the low and high frequency signals may be transmitted across a substrate or printed circuit board by metal traces; however, while low frequency signals may be transmitted along a single metal trace, the high frequency signal is typically transmitted by multiple metal traces which form a waveguide structure, such as a microstrip or coplanar trace. The coplanar trace is one in which two or more metal traces are formed on the same surface, thereby guiding an electromagnetic signal between them. These metal traces typically transmit the high frequency signal between circuits such as amplifiers, oscillators, and mixers positioned on a printed circuit board. 
         [0003]    Coplanar circuit structures conventionally include coplanar waveguide structures and slotline structures. A coplanar waveguide structure has one or more spaced longitudinal coplanar strip signal conductors positioned between and separated from two longitudinal coplanar ground conductors by respective gap widths, wherein the ground conductors are typically much wider than the gaps. A slotline structure has two spaced longitudinal coplanar conductors having a gap therebetween, wherein the gap is typically much smaller than the lateral width of the conductors. 
         [0004]    The metal traces of a coplanar strip transmission line conventionally are formed on a dielectric material, such as a printed circuit board. The high frequency signal exists as an electromagnetic field in the gap between the metal traces. The gap includes the dielectric material as well as air between and above the metal traces. The existence of the electric field in the dielectric material results in undesirable losses in signal strength. This is exacerbated by the electric field naturally concentrating in the higher dielectric constant material over the lower dielectric air. 
         [0005]    This loss in signal strength may be reduced by forming the circuitry (both low and high frequency) on a high frequency substrate. However, substrates typically used for high frequency signals are much more costly than substrates typically used for low frequency signals. For circuit board applications, the loss is reduced by using high frequency substrates such as RT/duroid® from Rogers Corp, instead of traditional circuit board material, such as FR4. 
         [0006]    Another known approach to reduce this loss in signal strength is to form a high frequency substrate material, e.g., RT/duroid®, on or over a low frequency substrate, e.g., an FR4 material. High frequency circuitry would be formed on the high frequency substrate material and the low frequency circuitry would be formed on the low frequency substrate. However, while this may be less costly than using an entire substrate of high frequency substrate material, it is still a complicated and costly process. 
         [0007]    Accordingly, it is desirable to provide a coplanar strip transmission line for transmission of high frequency signals on substrates typically used for lower frequency devices. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    A substrate provides for transmission of high frequency signals between devices formed on a material typically used for lower frequency devices. The substrate comprises a material having a first surface and defining first and second spaced, parallel slots extending from the first surface into the substrate. A first conductive trace is positioned on the first surface and between the first and second, spaced, parallel slots, and a second conductive trace positioned on the first surface and adjacent the first spaced, parallel slot and on a side of the first slot opposed to the first conductive trace. A third conductive trace positioned on the first surface and adjacent the second spaced, parallel slot and on a side of the second slot opposed to the first conductive trace. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
           [0010]      FIG. 1  is a partial block diagram and partial schematic top view of circuitry including an exemplary embodiment; 
           [0011]      FIG. 2  is a partial side view of an initial step in the formation of an exemplary embodiment; 
           [0012]      FIG. 3  is a partial side view of a first exemplary embodiment; 
           [0013]      FIG. 4  is a partial side view of a second exemplary embodiment; 
           [0014]      FIG. 5  is a partial side view of a third exemplary embodiment; 
           [0015]      FIG. 6  is a partial side view of a fourth exemplary embodiment; 
           [0016]      FIG. 7  is a graph comparing the dB loss of the exemplary embodiments with a known structure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
         [0018]    High frequency devices, for example, microwave and millimeter wave modules, are fabricated using existing low cost methods for fabricating lower frequency applications on low cost substrates/printed circuit boards. Standard circuit board manufacturing techniques with minimal post-processing steps enhance performance at a lower cost. Cut outs, which may also be called slots or gaps, in the substrate/printed circuit board are positioned between metal traces carrying a high frequency signal in the range of 2 to 100 gigahertz (GHz). 
         [0019]    Referring to  FIG. 1 , a block diagram of an exemplary embodiment of an RF transmitter/receiver  10  is formed on a substrate  12 . The substrate  12  preferably comprises a printed circuit board made of RF4 (flame resistant 4) material, but may comprise any material, such as epoxy resin, that comprises a lossy material. RF4 material is a composite of resin epoxy reinforced with a woven fiberglass mat and is more economical, absorbs less moisture, has great strength and stiffness and is highly flame resistant. For these reasons, RF4 material is widely used for printed circuit boards for low frequency devices. RF4 material previously has been thought to have an upper frequency limit of around 10.0 GHz. 
         [0020]    Electronic circuitry (or components), which may comprise, for example, a receiver, a transmitter, or as shown, a transceiver, includes baseband circuits  14 , a filter  22 , a detector  28 , a mixer  30 , a local oscillator  34 , an amplifier  36 , a low noise amplifier  48 , and antennas  42 ,  44 ,  54 ,  56 . Baseband circuits  14  includes, e.g., a microprocessor (not shown) and has inputs traces  16  positioned to receive low frequency input signals  18  from “outside” of the substrate  12 . The term “trace” is well known in the industry and is meant to be a conductive line formed on the substrate  12 . The filter  22  is coupled by traces  24  and  26  between the baseband circuits  14  and the detector  28 , respectively. The baseband circuits  14  are further coupled to the mixer  30  by a trace  32 . The mixer  30  is coupled between the local oscillator  34  and the amplifier  36  by coplanar strip transmission lines  38  and  40 , respectively. The amplifier  36  is coupled to tapered slot antennas  42  and  44  by the coplanar strip transmission line  46 . The low noise amplifier  48  is coupled to the detector  28  by a coplanar strip transmission line  52  and to antennas  54  and  56  by coplanar strip transmission lines  58 . The antenna elements pairs  42 ,  44  and  54 ,  56  form two element antenna arrays. The elements  44  and  56  are connected by a transmission line that has a phase shift (nominally 180 degrees or half wavelength) such that the desired phase matching between the elements of the antenna pair is achieved. 
         [0021]      FIG. 2  is a partial side view of the coplanar strip transmission lines  38 ,  40 ,  46 ,  52 , and  58 , taken along the line  2 - 2 , for example, of  FIG. 1 . The fabrication of the coplanar strip transmission lines  38 ,  40 ,  46 ,  52 , and  58  involve the creation of three adjacent traces  62 ,  64 , and  66  formed on the substrate  12 . These traces  62 ,  64 , and  66  may be formed on a first surface  13  (or side) of the substrate  12  by selectively introducing or removing various materials. The patterns that define such traces may be created by lithographic processes. For example, a layer of photoresist material is applied onto a layer overlying the substrate. A photomask (containing clear and opaque areas) is used to selectively expose this photoresist material by a form of radiation, such as ultraviolet light, electrons, or x-rays. Either the photoresist material exposed to the radiation, or that not exposed to the radiation, is removed by the application of a developer. An etch may then be applied to the layer not protected by the remaining resist, and when the resist is removed, the layer overlying the substrate is patterned. Alternatively, an additive process could also be used, e.g., building a structure using the photoresist as a template. Yet another method of forming the traces  62 ,  64 , and  66  may be by ink jet printing. The traces  62 ,  64  and  66  are spatially positioned on the substrate  12  wherein the width, or distance between the traces  62  and  64 , and between traces  64  and  66 , preferably is in the range of 25 to 500 microns. 
         [0022]    Referring to  FIG. 2 , slots  68  and  70  are formed between traces  62  and  64 , and traces  64  and  66 , respectively. The slots  68  and  70  may be created by mechanical drilling, laser burning, or any method of forming a slot in the substrate  12  known in the industry. Alternatively, the  68  and  70  may be formed prior to the traces  62 ,  64 , and  66  being formed. While the slots  68  and  70  extend through the substrate  12 , a second embodiment is shown in  FIG. 3  wherein the slots  72  and  74  extend only partially through the substrate  12 . A third embodiment of  FIG. 4  shows the slots  76  and  78  being tapered so the lower portion  80  at surface  11  of the substrate is larger in area than the area of the upper portion  82  at the surface  13  of the substrate. Mechanical support may be added by leaving small connectors (not shown) of the substrate  12  material extending across the slots  68 ,  70 ,  72 ,  74 ,  76 ,  80 . The connectors would preferably be small enough to not affect a signal being transmitted through the slots  68 ,  70 ,  72 ,  74 ,  76 ,  80 . 
         [0023]    A fourth embodiment ( FIG. 5 ) comprises slots  68  and  70  as in  FIG. 2 , but the walls  84  and  86  of the slot  68  are coated with a metal  88  and  90 , respectively. Likewise, the walls  92  and  94  of the slot  70  are coated with a metal  96  and  98 , respectively. The existence of the metal  88  and  90  increase the capacitance between the two slots  68  and  70 , therefore allowing for the slots  68  and  70  to have a greater distance therebetween. 
         [0024]      FIG. 6  shows a fifth embodiment that includes only two conductive traces  62  and  64 , taken along the line  6 - 6  of  FIG. 1  with the slot  68  formed therebetween. This two trace embodiment may also comprise the slot extending part way through the substrate  12  as shown in the embodiment of  FIG. 3 , a tapered slot as shown in the embodiment of  FIG. 4 , or have metal formed on its sides as shown in  FIG. 5 . 
         [0025]    Referring again to  FIG. 1 , the antennas  42 ,  44 ,  54 , and  56  each comprise (specifically in the case of antenna  42 ) a first antenna trace  41  coupled to the conductive trace  62  and a second antenna trace  43  coupled to the conductive trace  64 . A slot  45  is formed between the first antenna trace  41  and the second antenna trace  43 , and is in line with, or is extended from, the slot  68 , to the edge (in this particular exemplary embodiment) of the substrate  12 . 
         [0026]      FIG. 7  is a simulated graphical representation illustrating the magnitude in dB versus the frequency in GHz of a high frequency signal  102  without the slots in the substrate  12  and a high frequency signal  104  with the slots in the substrate. It may be noted that at 60 GHz, the dB loss of the signal  102  is 8 dB per inch, while the dB loss of the signal  104  transmitted in accordance with the exemplary embodiments is only 4 dB per inch. 
         [0027]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.