Transmission Line Structure

This invention relates generally to transmission line structures, and particularly to a transmission line structure formed on a substrate.

Transmission line structures, such as resonators or filters, can be formed on dielectric substrates. For example, conventional stripline or microstrip resonators typically utilize a substrate which can be a ceramic or another dielectric material. For microstrip construction a metallized runner is formed on one side of the substrate with a ground plane on the other side. The stripline configuration utilizes two such structures with ground planes on the outside and the runner therebetween. A number of factors, including the dielectric constant of the substrate, the thickness of the substrate, and the length and width of the runner determine the resonant frequency of the resonator. Practical resonators and filters for high frequency, such as above 1 GHz, can be readily constructed utilizing such techniques with resonator lengths under 2 cm. It can be difficult to provide filters for lower (e.g. VHF) frequency ranges in a similar size using a similar manufacturing process. Various techniques, such as widening the capacitive end of the line or using a serpentine layout to increase the line length having been utilized to reduce the physical length required for resonance. These approaches require increased surface area of the substrate, and generally degrade the quality of the resonator.

Another approach, to resonate a transmission line filter at a lower frequency, involves the use of a lumped capacitive element, such as a chip capacitor which is connected to the transmission line structure. A disadvantage of this approach is that it is not readily tuneable. When a lumped capacitive element is not utilized, the frequency of a resonator can be readily trimmed by removing portions of the ground plane element disposed opposite to the capacitive end of the line. When a substantial portion of the capacitance is supplied by a lumped element, the removal of the ground plane has minimal effect on the capacitance, and hence, the resonant frequency of the resonator.

This stripline structure includes a substrate having a portion with a differing electromagnetic characteristic for providing a desired operating characteristic. One aspect of the invention, the transmission line structure includes a dielectric substrate having opposed sides. A transmission line is disposed on one of the sides, while an opposed conductor is disposed to the other sides. The dielectric substrate has a portion of differing electromagnetic characteristics and the transmission line is partially disposed on said area of differing electromagnetic characteristics.

FIG. 1 is a top plan view of a transmission line structure in accordance with the present invention.

FIG. 2 is a cross-sectional view taken on line 2--2 of FIG. 1

FIG. 3 is a cross-sectional view of a stripline structure in accordance with the present invention

FIG. 4 is a top plan view of another embodiment of a transmission line structure in accordance with the present invention.

FIG. 5 is a cross-sectional view taken on line 5--5 of FIG. 4.

Referring now by characters of reference to the drawings and first to to FIGS. 1, and 2, it will be understood that transmission line structure, comprising a microstrip filter 10, includes a substrate 11 having a conductive ground plane 12 disposed of one of its sides and a conductive line 13 disposed on the opposed other side. The ground plane 12 provides an opposed conductor to the conductive line 13. The substrate 11 includes an area 14 of reduced thickness, with the line 13 extending at least into this area. At the area 14, the line 13 is more closely spaced to the ground plane 12; thereby providing increased capacitance and decreased inductance per unit length. The conductive line 13 is shown connected to the ground plane 12 by edge metallization 18, as is conventional in a quarter wavelength resonant line. If desired, one or more tap connections can be provided to conductive line 13.

Since ceramic substrates are typically manufactured from multiple layers of a ceramic tape, the forming of an area of reduced thickness 14 can be accomplished by punching a hole in one of the layers of a "green" (unfired) tape prior to the assembling and firing of the substrate, thereby providing a cavity in the substrate 11. If desired, the ground plane side could include the cavity. In the preferred embodiment the cavity is provided on the transmission line side of the substrate 11.

FIG. 3 illustrates a transmission line structure 20 that is constructed as a stripline rather than as a microstrip. Two microstrip structures 10 are utilized to form the stripline filter or resonator 20. In this preferred embodiment both include the reduced substrate thickness areas to provide increased capacitance to the ground planes 12 at the open ends of conductive lines 13. Such assembly techniques for stripline filters is well known in the art.

While the embodiments of FIGS. 1-3 provide a varying electromagnetic characteristic by disposing a portion of the line 13 in closer proximity to the ground plane 12, other characteristics could also be changed. As is illustrated in FIGS. 4 and 5, rather than utilizing the cavity or pocket in the ceramic substrate 11, one or more plugs of ceramic material having a differing dielectric constant or other characteristics, can be inserted while the ceramic substrate 11 is still in the green unfired state. The transmission line runner 13 extends over an area 31 having the differing dielectric constant; thereby producing a different capacitance than that provided by other portions of the dielectric substrate 11.

At the grounded end of the transmission line 13, the inductance is determined by the distance between the runner and the ground plane. If an area 32 of material having a different permeability, such as a ceramic ferromagnetic material, is included in the substrate, the inductance can be increased.

By varying one or more parameters such as substrate thickness, or the dielectric constant or permeability of a portion of the substrate it is possible to construct a resonator or filter that utilizes less substrate surface area for a given frequency of operation. Structures can be constructed for lower frequency operation than was previously possible in a given size. As this approach does not use lumped capacitances, the structure can be readily trimmed in frequency by removing portions of the ground plane.