Abstract:
A method, apparatus, article of manufacture, and a memory structure for selectively phase shifting one or more frequency components of a signal is described. The apparatus comprises a conductor, a ground plane, a dielectric disposed between the ground plane and the conductor, wherein the dielectric is characterized by a non-homogeneous dielectric constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 60/242,003, entitled “GRADED-DIELECTRIC STRUCTURES FOR PHASE-SHIFTING HIGH SPEED SIGNALS WITHIN MICROSTRIP STRUCTURES,” by Joseph T. DiBene II, filed Oct. 19, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to dielectric structures for selectively phase-shifting the frequency components of a signal along the path of a microstrip transmission line used within a printed circuit board (PCB) in order to mitigate the effects of dispersion as the signal propagates down the transmission path.  
           [0004]    2. Description of the Related Art  
           [0005]    Very high-speed digital and analog signals which are transmitted across microstrip structures, whether in PCBs or in IC packages begin to exhibit dispersion as the signals approach very high frequencies. In addition to dispersion, attenuation of the signals is also present as well as other distortion effects due to parasitics within and around the transmission paths. To combat the effects of dispersion, a mechanism is described herein which illustrates a method for phase shifting the frequency components of the signal as the signals propagate down the transmission paths in a manner so as to maintain the spatial alignment of all the frequency components.  
         SUMMARY OF THE INVENTION  
         [0006]    To address the requirements described above, the present invention discloses a method, apparatus and article of manufacture for selectively phase shifting one or more frequency components of a signal. The apparatus comprises a conductor, a ground plane, a dielectric disposed between the ground plane and the conductor, wherein the dielectric is characterized by a non-homogeneous dielectric constant. The method comprises the steps of disposing a first dielectric having a dielectric constant ε 1  on a ground plane, disposing a second dielectric having a second dielectric constant ε 2  lower than the first dielectric constant ε 1  on the first dielectric, and disposing a third dielectric having a third dielectric constant ε 3  lower than the second dielectric constant ε2, wherein the first dielectric has a first void, the second dielectric has a second void smaller than the first void and disposed over the first void.  
           [0007]    The present invention provides a structure for a microstrip transmission line that compensates for the dispersive characteristics of such lines. This dispersive effect causes the higher frequency components of the transmitted wave to arrive later than the lower frequency components resulting in signal distortion at the receiving end of the line. By re-aligning or phase shifting the high frequency components of the transmitted signal the original shape of the pulse may be somewhat restored and pulse-width increased thus resulting in better signal fidelity.  
           [0008]    The method described herein phase-shifts the components by grading the dielectric structure of the microstrip. As the signal propagates down the transmission path, the higher frequency components of the wave have fields that tend to focus charge density closer to the microstrip and the ground beneath it. By grading the dielectric in a fashion where the dielectric constant gets lower as one gets nearer to the microstrip the high frequency components begin to move at a greater velocity than in the outer dielectric region. This has the effect of compensating for the increase in the effective dielectric constant shift in a normal microstrip that tends to increase the effective dielectric constant as the signal frequencies or harmonics increase. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0010]    [0010]FIG. 1A shows a side view of a conventional microstrip transmission line being fed with a source excitation voltage and terminated with a resistor;  
         [0011]    [0011]FIG. 1B shows a section view of a conventional microstrip transmission line;  
         [0012]    [0012]FIG. 2 shows how the effective dielectric constant of a conventional microstrip transmission line (prior art) varies with frequency;  
         [0013]    [0013]FIG. 3A shows microstrip transmission with a graded index dielectric.  
         [0014]    [0014]FIG. 3B shows a section view of the microstrip transmission with a graded index dielectric together with a graph of the desired dielectric constant of the material as a function of the distance from the center of the line;  
         [0015]    [0015]FIG. 4A illustrates how in a conventional microstrip transmission line the electric fields tend to bind themselves closer to the trace as the frequency of the signal increases;  
         [0016]    [0016]FIG. 4B illustrates how in a graded index microstrip transmission line the electric fields bend away from the microstrip trace as the frequency of the signal increases thus compensating for the field crowding effects around the trace;  
         [0017]    [0017]FIG. 5A shows a plan view of a printed circuit board with a matrix of graded index dielectric sites which provide random routing of microstrip transmission line structures;  
         [0018]    [0018]FIG. 5B shows a section view of the microstrip transmission with matrix graded index dielectric sites together with a graph of the desired dielectric constant of the material;  
         [0019]    [0019]FIG. 5C is a diagram showing a microscrip transmission and a graded dielectric that follows the path of the transmission line; and  
         [0020]    [0020]FIG. 6 shows a section view of a stepwise graded index structure together with a graph of the desired dielectric constant of the composite structure. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
         [0022]    [0022]FIG. 1A is a diagram showing a microstrip transmission line structure  10  constructed of a narrow metal foil strip  22  overlaid on a homogenous dielectric material  21  which is in contact with a generally continuous ground plane  20 .  
         [0023]    [0023]FIG. 1B is a diagram showing the proceeding described structure in section view. Typically, such structures are fabricated using conventional printed circuit board techniques which are well know in the art.  
         [0024]    Referring back to FIG. 1A, one purpose of a microstrip transmission line structure is to present a predictable characteristic impedance to a signal  23  that is injected into the line and to faithfully transmit that signal  23  to a remote end where it is generally terminated with a suitable termination load such as a resistor  25 . However, if the signal  23  has appreciable frequency components (as shown in pulse  24 ), as the signal  23  propagates down the microstrip transmission line structure, the signal  23  may become distorted, for example, to the shape shown in pulse  26 .  
         [0025]    Because a microstrip transmission line structure has inherent dispersive qualities the effective dielectric constant, ε reff , of the dielectric material  21  will exhibit a behavior similar to that shown in FIG. 2. Note that an inflection point  27  occurs at some high frequency where the effective dielectric constant begins to asymptotically approach the dielectric constant of the medium, ε r .  
         [0026]    This dispersive behavior is different from “anomalous” or normal dispersion in that this is not an intrinsic property of the medium. Rather, this dispersion is a function of the non-homogenous dielectric medium in which the signal is propagating, specifically, the dielectric  21  which supports the transmission line and the air above the structure. In order for the electromagnetic wave to satisfy the boundary conditions at the dielectric-air interface, the phase constant of the signal in the direction of the propagation must lie between the phase constants of the air region and the dielectric region respectively. If this were not so, the signal would not propagate and would either be at or below cut-off for a given wave guide mode.  
         [0027]    Referring again to FIG. 2, the effective dielectric constant begins to depart from a flat condition and transitions to the aforementioned point of inflection  27  where the microstrip exhibits the dispersive behavior resulting in signal degradation  26  at the remote end of the microstrip transmission line. This signal degradation can be explained by noting that the velocity of an electric wave is:  
         v   =     c       ɛ   reff           ,                         
 
         [0028]    where c is the velocity of light and ε reff  is the effective dielectric constant  
         [0029]    Thus, as the effective dielectric constant increases with frequency it slows down the higher frequency components of the signal which cause them to arrive late at the far end of microstrip line and distorting the waveform at that point.  
         [0030]    Thus, the objective is to either push this transition out further in frequency or to flatten out the curve over a wider frequency range and, thus, mitigate the dispersive effect.  
         [0031]    [0031]FIG. 3A illustrates a microstrip transmission line structure  11  constructed in a manner similar to that shown in FIG. 1A, except that the narrow metal foil strip  32  is overlaid on a graded index dielectric material  31  which is in contact with a generally continuous ground plane  20 .  
         [0032]    [0032]FIG. 3B is a section view of the structure shown in FIG. 3A. Compared to the structure shown in FIG. 1A, the dielectric constant of the dielectric material  31  is not homogenous as was the case illustrated in FIGS. 1A and 1B. As illustrated by curve  33  the dielectric is graded with the dielectric constant of the material increasing as a distance, X, from the center of the microstrip structure, X 0 , out to the edge of the printed circuit structure where the dielectric constant is that of air. In one embodiment, the dielectric is graded in an elliptical fashion such that a slice in the X-Y plane has a dielectric constant generally characterized by the relationship ε≈{square root}{square root over (aX 2 +bY 2 )}, where a and b are constants.  
         [0033]    Since this structure is graded only in the X and Y directions, this structure is only effective for signals propagating down the Z direction. Multiple parallel microstrip transmission paths may be constructed by a repeated pattern in the X direction of FIG. 3B.  
         [0034]    [0034]FIGS. 4A and 4B illustrate how the operation of the graded index differs from a conventional microstrip structure. In a conventional microstrip structure  10  with a homogenous dielectric (illustrated in FIG. 4A), as the frequency of the signal increases, the fields tend to bind themselves closer to the trace and within the dielectric itself. As this occurs, the electric fields angle closer towards the center of the structure. However, in the graded index structure, (illustrated in FIG. 4B), the fields are forced to bend towards the direction away from the microstrip trace  32  thus compensating for the field crowding effects around the trace  32  itself. This has the effect of compensating for the dispersive nature of the microstrip  32  and forcing the signal to maintain its shape as it propagates down the transmission path. As an example, if a trapezoidal pulse,  34  in FIG. 3A, is transmitted down a graded microstrip structure  32 , the higher frequency components (3 rd , 5 th , etc. harmonics) will propagate at slightly faster rates that the first harmonic because the electromagnetic waves for the higher frequency components are bound closer to the microstrip  32  where the lowest dielectric constant exists in the structure (see, e.g. FIG. 3A). Thus the overall effect is that of a quasi-TEM (transverse electromagnetic) wave essentially bending backwards as the wavefront propagates down the transmission path resulting in a less distorted signal  36  at the far end of transmission line  11  in FIG. 3A. Note that the signal  26  will still undergo attenuation due to other effects.  
         [0035]    It is more common to propagate signals in a microstrip structure down random paths in the X-Z plane rather than in one direction. FIG. 5A illustrates a plan view  12  of a printed circuit board structure where by a matrix of bowl-shaped graded index dielectric sites  41  are located in the projected path of a microstrip transmission line  40 . The sites  41  act upon the microstrip transmission line structure in much the same manner as that described earlier in FIGS. 3B and 4B. Note that microstrip structures must necessarily be aligned with the sites  41  as shown in FIG. 5A in order to effectively compensate for the dispersive nature of the line. Although small portions of the line will not have the full effect of the graded index the average effect will prevail.  
         [0036]    [0036]FIG. 5B illustrates a section view A-A of the printed circuit board structure  12  shown in FIG. 5A. Microstrip transmission line structure  40  is shown centered over dielectric sites  41  which are located over ground plane  42 . Graphical curve  43  illustrates how the relative dielectric constant, ε r  varies from the center of the dielectric site  41  in a circular manner. It will be recognized that a multiplicity of microstrip structures can be routed on the same planar structure.  
         [0037]    [0037]FIG. 5C illustrates another embodiment of the invention in which the graded dielectric  60  follows the path of the transmission line itself. In this embodiment, slices of the dielectric in the plane perpendicular to the microstrip  40  all exhibit the same dielectric grade characteristic.  
         [0038]    [0038]FIG. 6 presents a section view  13  of a step wise graded index structure. This structure is similar to that shown in FIG. 3A, but illustrates a laminated dielectric structure which may be easier to produce than the structure illustrated in FIG. 3A. In the construction of this embodiment of the structure  13 , laminate dielectric  53  with a relative dielectric constant ε r53  is initially joined to ground plane  56 . Dielectric  53  comprises a void  55 . This void  55  is preferably air however it may be any material in which the dielectric constant is less than laminate dielectric  53 . Next, a second laminate dielectric  52  is joined to laminate dielectric  53  and, possibly, material in void  55  (if such material in the void  55  is utilized). Laminate dielectric  52  preferably has a relative dielectric constant ε r53  which is less than ε r52 . Again, laminate dielectric  52  has void  54  but which is smaller in width than void  55 . Void  54  again may be air or any other material in which the relative dielectric constant is less than ε r52 . Finally, a continuous dielectric laminate  51  is joined to dielectric laminate  52  and, possibly, material in void  54 . Dielectric laminate  51  preferably has a dielectric constant ε r51  that is less than ε r52 . Microstrip transmission line  50  is than applied over dielectric  51  in a conventional manner. The continuous characteristic of dielectric  51  assures support of microstrip line  50  in the event voids  54  and  55  are air. Graphical curve  57  illustrates how the effective dielectric constant of the composite structure  13  varies in a step wise manner with the relative dielectric constant, ε r , varying from a maximum at the periphery of microstrip  50  to a minimum at the center of microstrip  50 . It should be noted that many dielectric structures may be constructed to provide the characteristic of graphical curve  57  and that they need not be exactly as described above. In fact, it is not necessarily the case that structure  13  be contained to three dielectric layers as shown. Similar effects can be obtained with two layers or more than three layers.  
         [0039]    Note that structure  13  can be applied to a linear dielectric structure like that shown in FIGS. 3A and 3B. However, it may also be applied to matrix structures as shown in FIGS. 5A and 5B where the geometry of the site may be round or square or other geometrical configurations.  
       Conclusion  
       [0040]    This concludes the description of the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.