LOW IMPEDANCE COPLANAR MICROWAVE TRANSMISSION LINE

Abstract: A small, very low impedance coplanar waveguide microwave transmission line. A composite coplanar waveguide transmission line section composed of a plurality of interleaved fingers (14, 16, 21, 23) is coupled between input and output transmission line sections. Alternate ones of the interleaved fingers are connected to a main conductor (12) of the input and output transmission line sections and to a metal ground plane layer (11). The composite transmission line section may be connected either in cascade with the input and output transmission line sections or in a shunt arrangement. Composite characteristic impedances of less than two ohms are realized.

Description: LOW IMPEDANCE COPLANAR MICROWAVE TRANSMISSION LINE BACKGROUND OF THE INVENTION The invention pertains to coplanar transmission ** lines having a small size and very low impedance utilized in microwave integrated circuits. Coplanar waveguide transmission lines have found increased application in microwave circuits. The basic configuration of such a coplanar waveguide transmission line is shown in Figure 1. This basic configuration was suggested by Chang P. Wen in 1969 in an article "Coplanar Waveguide: A Surface Strip Transmission Line Suitable for Nonreciprocal Gyromag- netic Device Application", IEEE Transactions on Microwave Theory and Techniques, vol. MTT-17, No. 2, Dec. 1969, p. 1087, ff. This transmission line is fabricated ' entirely upon one surface of a dielectric substrate 10 and includes a center strip 12 of width disposed on the upper surface of a dielectric substrate 10 within a channel 13 of width W 2 cut in a ground plane 11. The impedance of this type of coplanar waveguide transmission line is determined by the ratio of the strip width W-, to the channel width 2 and the die¬ lectric constant of the material of the substrate. For a dielectric substrate formed of l 2 Og having a relative dielectric constant of approximately 10, the lowest realizable transmission line impedance is . about 25 Ω. This corresponds to a ratio W-,/W 2 of ap¬ proximately 0.95. However, much lower impedances are desirable in some instances for implementation of " • particular microwave circuits. Lower impedances have hitherfore not been achievable in coplanar waveguide transmission lines since unrealizably small gap sizes would be required to reduce the impedance much below 25 Ω. . . OMPI SUMMARY OF THE INVENTION In accordance with this invention, a coplanar waveguide transmission line is constructed by arrang¬ ing a plurality of separate coplanar waveguide trans¬ mission lines of reduced size parallel to one another with a fixed ratio of W,/W 2 . In one preferred embodi¬ ment, fingers extend perpendicularly from the sides of a main center conductor. These fingers are inter¬ leaved with grounded fingers, the ends of which adjacent the main center conductor of the transmission line are interconnected through bond wires. This arrangement forms a low impedance transmission line in shunt with the main transmission line. In another preferred embodiment, a composite line of interleaved fingers is arranged in cascade with the main line. In this arrangement, the main line is flared and longitudinal channels formed therein. Internal fingers are disposed longitudinally in each of these channels. The internal fingers are grounded at each end through bond wires. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective/cross-sectional view of a prior art coplanar waveguide; Figure 2 is a top view of a shunt-covered low impedance coplanar microwave transmission line accord¬ ing to a first embodiment of the invention; Figure 3 is a top view of a series-connected low impedance coplanar microwave transmission line accord¬ ing to a second embodiment of the invention; Figure 4 is a cross-sectional view of the device shown in figure 2; and Figure 5 is a plot of the characteristic im¬ pedance of a portion of a coplanar microwave transmis¬ sion line of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the top view of Figure 2, a first embodiment of a coplanar waveguide having a reduced impedance of the invention will be described. In this embodiment, a center or main conductor 12 of a primary transmission line is formed on the surface of a dielectric substrate in a channel 13 surrounded by a ground plane 11, similar to the prior art arrangement. In a center section 17 of the transmission line, a number of fingers 14 extend orthogonally from both sides, in a symmetric fashion, from the main conduc¬ tor 12 of the transmission line. In Figure 2, only 3 such fingers 14 on each side of the main conductor 12 are shown. However, it may be preferred to use a larger number of such fingers depending upon the transmission line impedance desired. The fingers 14 are formed within channels 15 formed in the ground plane 11. Within the channels 15 also are formed ground fingers 16 which are inter¬ leaved with the fingers 14. The ends of the ground fingers 16 nearest the main conductor 12 are shorted together and connected to ground by bond wires 18 and 19 while the opposite ends are connected directly to the main part of the ground plane 11. Figure 4 shows an enlarged cross-sectional view taken through the fingers 14 and 16 of the arrangement shown in Figure 2. Figure 4 also shows the electric and magnetic field patterns which are produced when signals are applied to the transmission line. Because of the symmetrical pattern of the electric and mag¬ netic fields, the computation of the impedance of the transmission line can be reduced to a simple, single cell field problem of a known solution so long as all of the gaps between the fingers 14 and 16 in the channel 15 have a fixed width (here indicated by S) and all of the conductors, both the fingers 14 and the fingers 16, have another fixed width (here indicated as W). In this analysis, each individual conductive finger 14 is considered to be a simple transmission line of impedance Zw. This impedance Z£__> is somewhat higher than would be observed if the neighboring fingers 14 were removed. The impedance of the com¬ posite line Z observed at the main conductor 12 is simply Zc = Zs/N where N is the number of the fingers 14. From the above-mentioned paper by Wen, it is known that the impedance of a coplanar waveguide varies directly as the ratio W-,/W 2 . Thus, it can be readily be appreciated that by proportionally reducing the dimensions W and S while making N large, very low impedances can be realized. The lower limit on the transmission line impedance is determined only by the smallest achieveable size of the gap S. As an example, a transmission line of the type of Figure 2 was constructed. In this arrangement, a substrate having a relative dielectric constant ε = 10 was utilized. Using the dimensions W = 0.002 in. and S = 0.001 in., W/S = 2.0 for a val¬ ue Z of 62.7Ω. Using 20 internal fingers on one side of a main conductor, an impedance of 3.14Ω with a total composite line width of only 0.118 in. could be realized. Arranging two such lines symmetrically as shown in Figure 2, the composite line impedance pro¬ duced was 1.57Ω. A second embodiment of a transmission line of the invention is shown in the top view of Figure 3. In this embodiment, the main conductor 12 is flared and divided into a plurality of parallel transmission paths 21 of reduced width by longitudinally-extending channels 22. Within the channels 22 are disposed longitudinally-extending internal fingers 23. Both ends of all of the internal fingers 23 are intercon¬ nected and connected to ground through bond wires 18 O__H and 19. The bond wires 18 and 19 maintain the ground potential on the isolated internal ground fingers 23. The behavior of the embodiment of Figure 3 is otherwise substantially the same as that shown in Figure 2, the only primary difference being that the arrangement of Figure 3 is a cascade arrangement while that of Figure 2 is a shunt arrangement. The computa¬ tion of the composite characteristic impedance of the transmission line can be carried out in the same as for the embodiment of Figure 2.

Claims: CLAIMS 1. A microwave transmission line including a coplanar waveguide transmission line comprising a dielectric substrate (10), a ground plane metal layer (11) formed on one surface of said substrate, and a main conductor (12) disposed within a chan¬ nel (13) formed in said ground plane metal layer; characterized by: a low impedance composite transmission line section coupled between input and output sections of said waveguide transmission line, said low impedance composite transmission line section comprising a plurality of first conductive fingers (14, 21) con¬ nected to said main conductor of said input section and of said output section and a plurality of second conductive fingers (16, 23) interleaved with said first conductive fingers, said second conductive fingers being grounded. 2. The microwave transmission line of claim 1 wherein said first conductive fingers have first ends connected directly to said " main conductor of said input section and of said output section and extend orthogonally to said main conductor (Fig. 2). 3. The microwave transmission line of claim 2 wherein said first conductive fingers extend from opposite sides of said main conductor in a symmetrical arrangement in two sets of fingers. 4. The microwave transmission line of any one of claims 1-3 wherein said second conductive fingers extend directly from said ground plane metal layer at ends thereof away from said main conductor and ends of said second conductive fingers adjacent said main conductor are interconnected by bond wires (18, 19) to each other and to said ground plane metal layer. O PI 5. The microwave transmission line of claim 1 wherein said first conductive fingers extend parallel to said main conductor- of said input section and of said output section and are connected at one end to said main conductor of said input section and at the other end thereof to said main conductor of said output section, said first fingers being separated by channels, said second conductive fingers being dispos¬ ed in said channels and extending parallel to said first conductive fingers, and further comprising bond wires for connecting each end of each of said second conductive fingers to like ends of ones of said second conductive fingers adjacent thereto and to said ground plane metal layer (Fig. 3). 6. The microwave transmission line of any one of claims 1-6 wherein all of said first conductive fingers have the same widths and wherein the width of gaps formed between each of said first and second conductive fingers are all the same.

Inventors: STEGENS RONALD E, STEGENS, RONALD E.
Applicants: COMMUNICATIONS SATELLITE CORP, COMMUNICATIONS SATELLITE CORPORATION
Application year: 1983
Main classification: H01P3
IPC codes: H01P3