Patent Publication Number: US-6670856-B1

Title: Tunable broadside coupled transmission lines for electromagnetic waves

Description:
FIELD OF THE INVENTION 
     This invention relates to coupled transmission lines for electromagnetic waves and, in particular to coupled transmission lines that are tunable and/or tolerant of manufacturing variations. 
     BACKGROUND OF THE INVENTION 
     Coupled transmission lines are used to make a variety of high frequency electromagnetic wave devices (RF and microwave) including frequency selective filters, signal splitters, and combiners, and delay lines. A typical coupled transmission line comprises a pair of elongated conductive strips separated by an intervening layer of dielectric material. FIG. 1, which is prior art, illustrates a transverse cross section of a common form of a coupled transmission line  10  wherein a pair of conductive strips  11 A and  11 B are separated by a gap of spacing G on a dielectric substrate  12  having a dielectric constant E. This form of the transmission line where the strips  11 A and  11 B overlap across at least a portion of the gap is referred to as a broadside coupled line. 
     FIG. 2, also conventional, illustrates an alternative coupled transmission line  20  wherein the conductive strips  11 A and  11 B are partially offset across the gap G. The amount overlap between  11 A and  11 B is designated L. This type of transmission line is called an off-set broadside coupled transmission line. 
     In each form, the degree of coupling C between the two strips  11 A and  11 B is a key parameter in the function of the transmission lines and devices using them. C is inversely proportional to the gap spacing G, and jointly proportional to the square root of the dielectric constant E and the overlap L. 
     In the fabrication of coupled transmission lines, it is difficult to control with desired precision the degree of coupling C. Common methods for manufacturing broadside coupled lines include thin film and thick film circuit technology, laminated printed circuit board technology, low temperature cofired ceramic (LTCC) technology and high temperature cofired ceramic (HTCC) technology. In these technologies, the degree of coupling is affected by manufacturing variations in dielectric constant, conductor width, conductor-to-conductor misalignment and dielectric thickness. Accordingly, there is a need for a broadside coupled transmission line structure that is tolerant of normal manufacturing variation and/or can be readily tuned. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, coupled transmission lines are fabricated in forms that are tolerant of manufacturing variation or can be readily tuned. In accordance with a first embodiment, the transmission line is formed with alternating overlapping edges for enhanced manufacturing tolerance. In a second embodiment, the line is provided one or more overlapping adjustment regions to permit tuning. A third embodiment has both alternating overlapping edges and one or more tuning regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings: 
     FIGS. 1 and 2 are cross sectional illustrations of conventional coupled transmission lines. 
     FIG. 3 illustrates a first embodiment of a fault tolerant transmission line formed with alternating overlapping edges; 
     FIG. 4 shows a transmission line formed with an overlapping adjustment region to permit coupling adjustment; 
     FIG. 5 illustrates a transmission line formed with both alternating overlapping edges and an overlapping adjustment region; 
     FIG. 6 shows a transmission line with overlapping upper and lower strip adjustment regions; and 
     FIGS. 7A,  7 B,  8 A and  8 B are graphical illustrations that show the characteristics of an exemplary embodiment. 
     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 show conventional coupled transmission lines described in the Background of the Invention. 
     FIG. 3 is a top view of an exemplary coupled transmission line  30  comprising strips  31 A and  31 B having alternating overlapping edges for enhanced manufacturing tolerance. The line  30  comprises a longitudinally extending first portion  33 A where the “top” edge of strip  31 A overlaps the “bottom” edge of underlying strip  31 B. Line  30  further comprises a second longitudinally extending portion  33 C where the “bottom” edge of strip  31 A overlaps the “top” edge of underlying strip  31 B. For each strip, the overlap edge thus alternates. Advantageously the length of portion  33 A is approximately the same as the length of portion  33 C. There is also a third transition portion  33 C of line  30  where the edge overlaps switch sides. In the embodiment of FIG. 3, the line  30  is essentially directed in a 180° U-turn in the neighborhood of the transition portion  33 C. 
     The advantage of this structure is tolerance of alignment errors in the fabrication of strips  31 A and  31 B on a dielectric substrate  32 . It is relatively easy to control the configuration of all portions of strip  31 A because all portions are on the same surface of the substrate  32 . Similarly, while strip  31 B is on a different surface of the substrate  32 , all regions of  31 B are on the same surface, permitting easy shape control. The conventional problem, however, is precisely aligning strip  31 A on one major surface with strip  31 B on the other major surface. The structure of FIG. 3 minimizes this problem because misalignment that reduces coupling in one portion, e.g. portion  33 A, will increase coupling in the other portion, e.g. portion  33 C. For small misalignments, the transition portion  33 C is essentially self-compensating, as misalignment will only displace the location of the crossover region, not the degree of coupling. The circuit shown, in FIG. 3 can be fabricated, for example, using the DuPont LTCC System 951 described in the DuPont Material Data Sheet entitled “951 Low-Temperature Cofire Dielectric Tape.” 
     FIG. 4 is a top view of an alternative transmission line structure  40  provided with a readily accessible tuning adjustment region  41 . Here one edge of strip  31 A partially overlaps strip  31 B (one edge of  31 B). Adjustment region  41  of strip  31 A extends transversely to overlap a larger region of strip  31 B. Region  41  provides a region of limited longitudinal extent which can be trimmed to decrease coupling between  31 A and  31 B until a desired value is reached. Region  41  can have a longitudinal length up to 20% of the length of strip  31 A for ease of trimming. Lengths greater than 20% are not preferred because they tend to deteriorate the line impedance and produce unwanted signal reflections. The region  41  may be trimmed, for example, by etching, abrading, or laser trimming. Region  41  can be formed in the center, on the ends or anywhere along the coupled area of the lines  31 A or  31 B. It can be on either or both lines. There can be one region  41  or a multiplicity of such regions along the length of the line. And a single region  41  can be used to change the coupling coefficient of a plurality of adjacent lines on the same or different layers. While such trimming may affect the impedance of the transmission line, we will demonstrate by example below that it does not substantially deteriorate electrical performance. 
     FIG. 5 is a top view of a coupled transmission line  50  designed for both manufacturing tolerance and provided with a trimming region  41 . Here regions of alternating edge overlap  33 A and  33 C extend in the same direction and transition region  33 B provides an area of full overlap for trimming. 
     The invention may now be more clearly understood by consideration of the following specific example. 
     Example 
     The FIG. 6 structure is similar to that shown in FIG. 1 except that it includes overlapping trimming regions  41 A and  41 B on strips  11 A and  11 B, respectively. Such structures are suitable for use as  3 dB couplers. The FIG. 6 structure and a similar structure without regions  41 A and  41 B were fabricated for comparison tests. The two structures were then tested for degree of coupling and return loss. 
     The circuits were fabricated using a process similar to the aforementioned DuPont 951 process. Each tape is a mixture of organic binder and glass. When fired the tape formed the ceramic substrate for the circuit. Individual circuits were formed on a large wafer and then singulated after processing. Prior to firing, holes or vias were punched in the tape. The holes correspond to the location of electrical connections between the coupled lines and the connections out of the package. After punching, the vias were filled with silver conductor ink, which formed electrical connections between layers. Printing was accomplished using a squeegee printer and a metal stencil. After printing, the solvents in the material were dried at 70° C. for 30 minutes. Electrically conductive interconnections were then made by screen printing silver conductor ink. All conductor prints were dried. 
     After the via holes were filled and conductive traces were printed and dried, the separate tape layers were aligned, stacked, and tacked together using a high temperature (200° C.), 3 mm diameter tool. The stacked tapes were then laminated at 3000-4000 PSI at 70° C. After lamination, the assembly was heated to ˜400° C. to burn off the organic materials in the tape layers. After burn-off, the assembly was heated to 850° C. to sinter the glass. After the assembly was removed from the furnace and cooled, the circuit formed a solid ceramic mass. Individual circuits were separated from the wafer by dicing. 
     FIGS. 7A and 7B show the return loss with frequency for the FIG.  6  and FIG. 1 structures. FIGS. 8A and 8B illustrate coupling variation with frequency for the FIG. 6 structure and the FIG. 1 structure, respectively. As can be seen, the coupling variation and return loss are only slightly modified by the presence of the trimming regions. 
     It is understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.