Abstract:
A microstrip line type resonator includes a rectangular parallelapiped dielectric plate, and a conductive microstrip line disposed on a top surface of the dielectric plate. The microstrip line has opposite ends respectively spaced from front and rear edges of the dielectric plate. The resonator further includes a first grounding layer disposed on an entirety of the bottom surface of the dielectric plate, and a second grounding layer disposed an entirety of the front, rear and side surfaces of the dielectric plate. The first grounding layer is electrically connected to the second grounding layer at the edges of the bottom surface of the dielectric plate. The microstrip line type resonator is a 1/2 wavelength resonator.

Description:
REFERENCE TO A RELATED APPLICATION 
     This application claims the right of priority under 35 U.S.C. 119 of Japanese Utility Model Application Serial No. 94321/89, filed on Aug. 14th, 1989, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a microstrip line type resonator, and more particularly, to a microstrip line type resonator structure including a rectangular parallelepiped dielectric plate, a ground conductor layer formed on the bottom, front, rear, and two side surfaces of the plate, and a microstrip line formed on the top surface of the plate. 
     2. Brief Description of the Related Art 
     Recently, high frequency microwave band communication technology has played a significant role in mobile communication systems, for example, in the recently developed cellular telephone systems. 
     In this technology, since communication systems require several hundreds of frequency channels in the approximately 800 MHz frequency band, there has been a need for a small filter, having a high quality factor (high-Q) and a low parasitic capacitance, which is suitable for mass production. 
     To realize such a filter, a small, high-Q resonator, which is an essential part of the filter, is required. One example of a conventional resonator is disclosed in a U.S. Pat. No. 4,266,206 issued on May 5, 1981. 
     FIG. 1 illustrates an example of the conventional microstrip line type resonator disclosed in the above mentioned U.S. patent. As shown in FIG. 1, the resonator 10 includes a rectangular dielectric substrate 12 which may be made of ceramic, such as alumina, having a thickness on the order of 0.03 inch. Further, the resonator 10 includes a ground plane conductor 14 on the bottom surface of the substrate 12, a microstrip line 16 on the top surface of the substrate 12, an apron portion 18 provided on the top surface connected to the microstrip line 16 at one end 22 of the microstrip line 16, and a conducting bridge 20 connecting the microstrip line 16 and the ground plane conductor 14 via the apron 18. Dielectric material of the substrate 12 is exposed at a portion of the top surface and two side surface 13a and 13b thereof. 
     Generally, this kind of resonator is designed by approximation calculations, and further, is adjusted by trimming based on repeated trial and error of actual measurements of several characteristics of the resonator, such as the quality factor, resonance frequency, and the like. 
     Publications have been directed to such approximation, some thesis or books have, for example, Wheeler, H. A., &#34;Transmission Line Properties of a Strip on a Dielectric Sheet on a Plane&#34;, IEEE Trans. Microwave Theory Tech., Vol. MTT-25, August 1977, pp. 631-647, or Wheeler, H. A., &#34;Transmission Line Properties of a Stripline Between Parallel Planes&#34;, IEEE Trans. Microwave Theory Tech., Vol. MTT-26, November 1978, pp. 866-876. 
     However, since these approximations are based on an ideal model such as that illustrated in FIG. 2 which includes a dielectric plate 28 having infinite area, a grounding conductor 26 provided on the entire bottom surface of the dielectric plate 28, and a microstrip line 24 having an infinite length on the top surface of the dielectric plate 28, the final adjustments must still be conducted to a complete the resonator, which has a limited area, to account for the differences between the ideal resonator model and the actual resonator structure. 
     Further, to enhance the approximation, the conventional resonators should have similar structure to that of the ideal resonator. On the other hand, since the resonator should be compact for use in high frequency filters featured in mobile telecommunication systems. Further, the electromagnetic fields between the microstrip line and the ground conductor, such as those illustrated as arrow lines (electric field) and broken lines (magnetic field) in FIG. 2, are distrubed by the limited area of the dielectric plate. 
     In other words, there must be such disturbance of the electromagnetic field in the actual resonator which has at least an exposed dielectric plane at the end of the dielectric plate, for example, the side surface 13a and 13b shown in FIG. 1. Further, the disturbed electromagnetic fields reduced the quality factor of the resonator. 
     Therefore, even though it has long been a need for a compact microstrip line type resonator having a high quality factor, the conventional microstrip line type resonator can not realize the desired (High-Q) resonator characteristic. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a relatively small microstrip line type resonator having a high quality factor. 
     Another object of the present invention is to provide a microstrip line type resonator which does not result in as much estimation calculation errors. 
     Still another object of the present invention to provide a microstrip line type resonator which is suitable for mass production. 
     To realize the above mentioned objects, there is provided a microstrip line type resonator which includes a rectangular parallelepiped dielectric plate having a first grounding conductor on the bottom surface thereof, a second grounding conductor on the front, rear, and two side surfaces of the dielectric plate which is connected to the first ground conductor, and a microstrip line conductor provided on the top surface of the dielectric plate. The length of the microstrip line conductor can be selected from one of 1/2 or 1/4 wave lengths of the frequency of the signal which is applied to the resonator. 
     Because of the above mentioned structure, the disturbance of the electromagnetic filed is reduced efficiently by the second ground conductor on the front, rear, and two side surfaces and it becomes possible to provide a compact, high-Q microstrip line type resonator. 
     Explained hereunder several detailed examples of those resonators and detailed electromagnetic characteristic thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention may be more completely understood from the following detailed description of the preferred embodiments of the invention with reference to the accompanying drawings in which: 
     FIG. 1 illustrates a conventional strip line type resonator disclosed in U.S. Pat. No. 4,266,206; 
     FIG. 2 is a partial sectional view of an ideal microstrip line type resonator; 
     FIG. 3(a) illustrates a first example of an ideal microstrip line type resonator of the present invention forming a 1/2 wave length resonator; 
     FIG. 3(b) illustrates an actual second example of the 1/2 resonator of the present invention having an input/output terminal; 
     FIG. 3(c) illustrates a third example of the microstrip line type resonator of the present invention forming a 1/4 wave length resonator; 
     FIG. 4 is a graph for explaining improved quality factor characteristics of the 1/2 wave length resonators of the present invention; 
     FIG. 5 illustrates an example of a dielectric filter mainly comprised of the resonators of the present invention; and 
     FIG. 6 illustrates a partial magnified view of a modification of the resonator used in FIG. 5 as a tri-plate type dielectric resonator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     (First Embodiment) 
     As shown in FIG. 3(a), a first embodiment of the present invention includes a rectangular parallelpiped dielectric plate 42 which is made of a ceramic having a dielectric constant of approximately 77.2, a first grounding layer 38 provided by a plating on the entire bottom surface of the plate 42, a second grounding layer provided by a plating on the front surface 34, rear surface 40, and two side surfaces 32a, 32b of the plate 42. 
     The first grounding layer and the second grounding layer are connected at the each edges of the bottom surface of the dielectric plate 42 to effect a single combined grounding portion of the resonator 30. Therefore, the dielectric material (such as ceramic) of the dielectric plate 42 is exposed only at the top surface of the plate 42. 
     The length of the microstrip line 36 is a 1/2 wave length of a signal which is applied thereto. Thus, the microstrip line resonator 30 resonates at a frequency having a 1/2 wave length which is the same as the length of the microstrip line 36. The resonator 30 is an example of an ideal model of the present invention for estimation calculation, omitting the influence of an input/output terminal, which forms a 1/2 wave length resonator. According to the inventors&#39; estimation, there does not result as much calculation errors between the ideal model disclosed in FIG. 2 and this embodiment. 
     (Second Embodiment) 
     An actual example of the 1/2 wave length resonator having such the input/output terminal 34c is illustrated in FIG. 3(b). In FIG. 3(b), the same reference numerals denote the same or equivalent elements as illustrated in FIG. 3(a). 
     In FIG. 3(b), a part of the second grounding layer 34 on the front surface of the dielectric plate 42 is separated into two guide portions 34a and 34b for guiding the input/output terminal 34c to define slits 35a and 35b. The input/output terminal 34c is provided by a plating on the front surface and is connected to an outer circuit 34d (partially omitted) which applies an input signal to the resonator 44 and also receives an output signal from the resonator 44. Further, the input/output terminal 34c is connected to one end of a microstrip line 36&#39; at an edge of the top surface of the dielectric plate 42. 
     As mentioned above, even though the entire bottom surface of the dielectric plate 42 is covered by a first grounding layer 38, the input/output terminal 34c and the outer circuit 34d are electrically separated from the first grounding layer 38 by a small slot (not shown; located on a reverse side of a connecting point between the input/output terminal 34c and the outer circuit 34d) on the front surface which is adjacent to an edge of the bottom surface to prevent a short circuit. 
     Since the first and second grounding portions, the microstrip line 36&#39;, and the input/output terminal 34c can be made by plating of a conductive material, such as silver, it is relatively easy to manufacture the resonator 44 by mass-production. 
     (Third Embodiment) 
     FIG. 3(c) illustrates a third embodiment of the present invention which forms a 1/4 wave length resonator 46. In FIG. 3(c), the same reference numerals denotes the same or equivalent elements as illustrated in FIG. 3(a) or in FIG. 3(b). 
     The microstrip line type resonator 46 disclosed in FIG. 3(c) is a 1/4 wave length resonator. Therefore, the length of a microstrip line 36&#34; on the dielectric plate 42 is a 1/4 wave length of a signal which is applied to an input/output terminal 34c to resonate at a frequency having a 1/4 wave length which is the same as the length of the microstrip line 36&#34;. 
     The main difference between the 1/4 wave length resonator 46 disclosed in FIG. 3(c) and the 1/2 wave length resonator 44 disclosed in FIG. 3(b) is that one end of the microstrip line 36&#34; of the 1/4 resonator 46 is connected to the second grounding layer 40 at the rear surface of the resonator 46. 
     (Analysis) 
     As described in &#34;BACKGROUND OF THE INVENTION&#34;, analysis of a microstrip line resonator is very difficult and there exists only few estimation calculation methods. Therefore, the present invention can be evaluated only by experimental results. 
     FIG. 4 illustrates an example of such results describing advantages of a microstrip line resonator of the present invention. 
     In this experiment, we tested three 1/2 wave length resonators A, B, and C each having a structure similar to the resonator disclosed in FIG. 3(a). Each of the resonators had a width (W1) of 5.0 mm, length (L) of 24.0 mm, and height (H) of 1.5 mm. The thickness (t) of each of grounding portions and the microstrip lines was approximately 10 microns. Further, as for two of the resonators, we put another dielectric plate on the top surface thereof to cover the microstrip line resulting in a tri-plate type structure. The tri-plate structure itself is a conventional structure as disclosed, for example, in FIG. 1 of the above mentioned U.S. Pat. No. 4,266,206. 
     In detail, resonator A is a microstrip line type resonator of the present invention having the second grounding layer on the front, rear, and two side surfaces without an above mentioned other dielectric plate. Resonator B is a conventional tri-plate type resonator without the second grounding layer on the two the side surfaces. Resonator C is the tri-plate type resonator having the second grounding layer on the front, rear, and two side surfaces. 
     We put an end of a testing cable (15 cm) at one end of the microstrip line to apply/receive a test signal and measured the resonance frequency and quality factor of each resonator while changing the width (W2) of the microstrip line. 
     According to the results of our experiment, at a point that W2 is 1.0 mm, each of tested resonators had maximum quality factor. In detail, resonator A resonated at a frequency of 1.037 GHz and had a quality factor of 344.8. Resonator B resonated at a frequency of 1.100 GHz and had a quality factor of 500.0. Resonator C resonated at a frequecy of 1.076 GHz and had a quality factor of 692.0. 
     According to an estimation calculation, a conventional microstrip line type resonator having the same size as the tested resonators would have a quality factor of approximately less than 100. 
     Therefore, it can be realized that the microstrip line type resonator of the present invention can provide approximately a three times higher quality factor than that of the conventional microstrip line type resonator. Further, comparing with two of characteristics of resonator B and C, it can be realized that a part of the second grounding layer such as 32a and 32b shown in FIG. 3(a) significantly improve the quality factor even in the tri-plate type structure. 
     According to other experiments conducted by the inventors, a tendency of those characteristics may be similar to that of a coaxial resonator rather than that of microstrip line type resonator. We assumed that the second grounding layer on the front, rear, and two side surfaces reduced the disturbance of the electromagnetic field between the microstrip line and the grounding portion and this is why the characteristic of the microstrip line resonator of the present invention is similar to that of the coaxial resonator whose inner conductor is surrounded by an outer conductor. 
     These results have not been discovered by using a conventional resonator lacking the second grounding layer on two of the side surfaces. 
     (Fourth Embodiment) 
     Explained hereunder is an application of the present invention with reference to FIG. 5 and FIG. 6. 
     As shown in FIG. 5, the microstrip line type resonator of the present invention can be used to form a dielectric filter utilized in high frequency band communication technology. 
     In FIG. 5, a hybrid dielectric filter 47 is mainly made up of two microstrip line type resonators 68a, 68b and one coaxial resonator 74. Those resonators are mounted on a dielectric plate 48 whose front surface 50, rear surface 60, two side surfaces 52a, 52b, and a part of the top surface 62 are metalized by plating for grounding. 
     Further, the resonators 74, 68a, and 68b are connected to input/output leads 72a, 72b, and 72c made by plating respectively. Further, the microstrip line type resonator 68b is coupled to an input terminal (through hole) 52a via a coupling capacitor 70d and the resonator 68b is also coupled to the microstrip line type resonator 68a via a coupling capacitor 70c and the resonator 68a is coupled to a coaxial resonator 74 via a coupling capacitor 70b and the coaxial resonator 74 is further coupled to a output terminal (through hole) 64 via a coupling capacitor 70a. 
     Since the microstrip line type resonators 68a, 68b have high quality factor, it is possible to form a high quality factor dielectric filter suitable for high frequency band communication. 
     (Sixth Embodiment) 
     FIG. 6 illustrates an improvement of the dielectric filter disclosed in FIG. 5. As described in the explanation of our experiment, it is possible to increase the quality factor of the resonators by means of a tri-plate structure. In this embodiment, we put another dielectric plate 78 which is entirely covered with a conductive material, such as silver plating, except at a bottom surface 80, on the microstrip line type resonator 68a. Of course, the other microstrip line type resonator 68b can have the same structure. 
     According to this structure, it is possible to increase entire quality factor of the filter 47.