Abstract:
A microstrip transmission line structure is disclosed wherein a transmission element is attached to a substrate in a way such that the element is suspended above the substrate separated by a predetermined distance from a ground plane. In one embodiment, the transmission element is supported by a plurality of support legs that are disposed in a way such that the transmission element is suspended above the substrate. In another embodiment, a portion of the substrate is removed in a way such that, when the transmission element is disposed on the substrate, the line is separated from the ground plane by a predetermined distance.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to high frequency electronic circuit boards and, more particularly, to low-loss transmission lines in such circuit boards.  
       BACKGROUND OF THE INVENTION  
       [0002]     Efficient transmission of broadband radio frequency (RF) signals is essential to meet the demanding requirements of high-speed networking systems. Individual circuit elements, such as 50 Ohm impedance microstrip transmission lines, which are very well known in the art, must be carefully designed to minimize signal losses. The importance of minimizing these losses is increasing as such systems operate at higher frequencies over longer distances. Various well-known transmission lines, such as the aforementioned microstrip transmission lines, have been used to carry signals on multilayered circuit boards for relatively low frequency applications, such as at or below 40 GHz.  
         [0003]     However, while transmission lines of this type provide a high-quality signal path at such frequencies, they are limited in their usefulness at higher frequencies, such as at or above 70 GHz. Specifically, as frequencies rise to ≧70 GHz, signal attenuation for a given traditionally-designed transmission line length increases significantly and, accordingly, the received signal strength at a signal&#39;s destination is significantly reduced. Thus, traditional microstrip transmission lines are inadequate for use at such high frequencies.  
       SUMMARY OF THE INVENTION  
       [0004]     The present inventor has realized that the aforementioned RF signal loss at higher frequencies is caused to a large extent by the electromagnetic field in the dielectric substrate underlying the transmission line that results as a signal propagates along the transmission line. At higher frequencies, the degree of substrate interaction per unit length increases, resulting in the aforementioned weak signal at the intended destination of the signal.  
         [0005]     Therefore, the present inventor has invented a transmission line structure that essentially solves this problem. In accordance with one embodiment of the present invention, a transmission element is connected to a substrate in a way such that the transmission element is suspended above the ground plane and separated by a predetermined distance from that plane. In one embodiment, the transmission line comprises a plurality of support legs that are disposed in a way such that the transmission line is suspended above the substrate. In another embodiment, a portion of the substrate is removed in a way such that, when the transmission line is disposed on the substrate, the line is separated from the substrate by a predetermined distance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]      FIG. 1  shows an illustrative prior art transmission line disposed on a substrate;  
         [0007]      FIG. 2  an illustrative transmission line suspended above a substrate in accordance with the principles of a first embodiment of the present invention;  
         [0008]      FIG. 3  shows an illustrative transmission line suspended above a substrate in accordance with the principles of a second embodiment of the present invention;  
         [0009]      FIGS. 4A and 4B  show how a portion of a substrate can be removed in order to create a separation distance between the substrate and the transmission line;  
         [0010]      FIG. 5  shows an illustrative graph illustrating how the insertion loss of a signal is greatly reduced in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]      FIG. 1  shows an illustrative prior art transmission line structure  100  having substrate  102  that is, illustratively, a multi-layered circuit board having a dielectric layer  105  and a metallized ground plane layer  104 . The metallized layer  104  is, illustratively, a thin layer of copper material. The dielectric layer  105  could be for example, a layer of silicon dioxide material. Transmission line  101  is, illustratively, connected to ground plane  104  by lead  107  connected to via  106 . Vias such as via  106  are well known in the art and are, illustratively, metallized holes through the dielectric layer  105  that provide a conducting electrical path to ground layer  104 .  
         [0012]     While the illustrative transmission line of  FIG. 1  is useful as a path for routing RF signals across circuit boards in many applications, problems result as the frequency of the RF signal rises. In particular, as the frequency of the signal increases, that signal is more easily attenuated by the well-known electromagnetic field that is present within the dielectric layer  105  of substrate  102 . This attenuation is often referred to as dielectric attenuation or dielectric loss. The result of such dielectric loss is that as a relatively high frequency signal, such as an illustrative signal at or above 60 GHz, travels along the transmission line  101 , it becomes significantly attenuated over a relatively short distance of travel relative to lower frequency signals, such as signals at or below, for example, 20 GHz. With the advent of systems relying on frequencies near and above 70 GHz, this dielectric loss becomes extremely problematic.  
         [0013]      FIG. 2  shows one illustrative embodiment of a transmission line structure  200  in accordance with the principles of the present invention whereby the aforementioned dielectric signal loss is reduced or substantially eliminated. Specifically,  FIG. 2  shows an illustrative transmission line  201  that is physically suspended above substrate  202  which is, illustratively, a metallized layer functioning as an electrical ground for transmission line  201 . Transmission line  201  is also referred to herein interchangeably as a transmission element. One skilled in the art will recognize that substrate  202  may be, for example, a layer of gold, copper, aluminum, or another electrically conducting material suitable for use as a ground plane. Support elements  203 , here illustratively bent support arms, are attached to both the transmission line and the substrate and function to both support the transmission line above the ground substrate  202  as well as, illustratively, to electrically connect the transmission line to that substrate. Once again, support arms  203  may be, illustratively, manufactured from an electrically conducting material such as the aforementioned gold, copper or aluminum or any other electrically conducting material. One skilled in the art will recognize that other materials, such as plastic may be used to support the transmission element. Support arms  203  have length L and height H and are spaced a distance D from each other. One skilled in the art will recognize that L, D and H can be selected to produce a desired electrical property of transmission element  201 , such as the impedance of the transmission line. For example, if the line width W is selected as 1.08 mm, the length L of the support arms is selected as 3.01 mm, the height H is selected as 250 micrometers, and the support arms are separated by 4 mm from each other, transmission line  201  will illustratively have approximately a 50 Ohm impedance, which is desirable in a number of applications. Other dimensions may be selected to produce a variety of desirable transmission line impedances. The transmission line structure  200  of  FIG. 2  substantially reduces the signal attenuation of a high-frequency RF signal propagating along transmission line  201 . This reduction is the result of separating the transmission line from the substrate and, accordingly, reducing the exposure of the propagating signal to any electromagnetic field present in the substrate.  
         [0014]      FIG. 3  shows another embodiment of a transmission line structure  300  in accordance with the principles of the present invention whereby, similar to the embodiment of  FIG. 2 , support arms  303  support transmission line  301  so that the transmission line is suspended above the substrate  302 . However, unlike in the embodiment of  FIG. 2 , the support arms  303  of  FIG. 3  alternate along the length of the transmission line  301 . Such an arrangement of support arms  303  is advantageous in that approximately half of the number of support arms is required as compared to the embodiment of  FIG. 2 . Accordingly, manufacturing costs may be reduced since fewer arms need to be produced and fewer connections between the arms and the underlying substrate need to be made. Once again, one skilled in the art will recognize and fully appreciate how to tune the transmission line of  FIG. 3  (e.g., to achieve a specific electrical property, such as line impedance) by adjusting the height H and width W of the transmission line  301  as well as the spacing D between and length L of the support arms.  
         [0015]      FIGS. 4A and 4B  illustrate yet another embodiment in accordance with the principles of the present invention whereby, instead of raising the transmission line above the substrate, a portion of the substrate itself is removed in order to provide a separation distance between the substrate and the transmission line  401 . Referring to  FIG. 4A , substrate  402  has, for example, metallized top layer  405 , dielectric layer  408  and metallized ground layer  407 . Illustrative vias  406  function to connect the top metallized layer to the ground layer  407  and, accordingly, act to electrically connect the transmission line  401  to layer  407 . One skilled in the art will recognize that such electrical connection of the transmission line to the ground layer may or may not be required or desired to achieve the desired transmission properties of the transmission line structure. Once again, transmission line  401  and layers  405  and  407  are, illustratively, manufactured from aluminum, copper or gold. Dielectric layer  408  is, illustratively, manufactured from silicon dioxide or any other well-known material suitable for such a use, such as, for example, a soft substrate like the Ro3003 substrate manufactured by Rogers Corporation.  
         [0016]     Transmission line  401  is attached to support arms  403  that are connected to contact areas  404  of metallized top layer  405 , as is illustratively shown in  FIG. 4B . Unlike in the embodiments of  FIGS. 2 and 3 , the support arms are simpler in design in that they are not bent at the ends, as depicted by illustrative support arms  203  in  FIG. 2  and illustrative support arms  303  in  FIG. 3 . Instead, support arms  403  are, illustratively, straight and of a length L that permits the end of each support arm to be lowered in direction  409  and connected to one of the aforementioned contact areas  404 . Such straight support arms are advantageous in that they are potentially less costly to manufacture than a transmission line structure such as that shown in  FIG. 2  or  3 . As described above, by selecting an appropriate height H, width W, length L and separation distance D, one skilled in the art will be readily able to tune the transmission characteristics (e.g., the impedance) of line  401  of  FIGS. 4A and 4B .  
         [0017]      FIG. 5  shows an illustrative graph of the attenuation of a signal traveling across a transmission line of 124 mm in length versus the frequency of the signal. Plot line  501  represents, for example, the attenuation of a signal traveling across the transmission line of  FIG. 1  where a line of width 605 micrometers is disposed on a low-loss dielectric substrate 250 micrometers above the ground plane where the dielectric constant is 3 and the dissipation factor is 0.0013. Plot line  502  represents, on the other hand, the attenuation of a signal traveling across the transmission line of  FIG. 3 . Specifically, plot line  502  represents a signal traveling across a 1087 micrometer wide transmission line on a substrate having a dielectric constant of 1. Similar to  FIG. 3 , the transmission line represented by plot  502  is supported at 250 micrometers above the ground plane by 31 supports spaced 4 mm apart. One skilled in the art will readily appreciate that many different values for the aforementioned design characteristics of the transmission line and supporting substrate can be used to achieve a transmission line structure having desirable functional characteristics.  
         [0018]     Referring to the plot lines  501  and  502  in  FIG. 5 , the benefits of a transmission line supported above a substrate in accordance with the principles of the present invention are clear. Specifically, for high frequency applications, here approximately 70 GHz to approximately 85 GHz, the attenuation experienced by the signal traveling across the transmission line having the above specifications is greatly reduced. For signals throughout that relatively broad frequency band, the attenuation is much lower than that experienced by a traditional transmission line. For example, at approximately 75-80 GHz, the signal traveling across a traditional line is attenuated by 4.0 to 4.5 dB relative to the same signal traveling across a transmission line in accordance with the principles of the present invention.  
         [0019]     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are within its spirit and scope. Furthermore, all examples and conditional language recited herein are intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting aspects and embodiments of the invention, as well as specific examples thereof, are intended to encompass functional equivalents thereof.