Multilayer filter

[0002] This invention relates to a multilayer filter having characteristics of a band pass filter for use in mobile communication equipment such as a portable cellular telephone and the like.

[0003] A typical conventional multilayer filter comprises a plurality of strip-line resonators in the form of a multilayer body which is generally formed from dielectric and conductive layers which are stacked up by a sheeting or screen printing method before being sintered. In order to reduce the size of the multilayer filter using the strip-line resonators, the resonance frequency is lowered by providing capacitors connected in parallel in the multilayer body to obtain target filter characteristics.

[0004] In such a multilayer filter as formed with the strip-line resonators, however, current is concentrated on the edge portion of the strip-line conductive layer and the Q-factor is degraded, which poses a problem in that good filter characteristics are unobtainable.

[0005] It has been proposed by JP-A 9-35936 to use through-hole electrodes as inductance elements for solving the foregoing problems.

[0006] The multilayer filter disclosed in the aforesaid Japanese Patent Publication is seemingly intended to set the ratio W/d of the diameter d of a through-hole to the width W of a multilayer body is set at about 13. With an arrangement like this, however, the Q-factor would never be improved because the resistance value grows larger, though a large inductance value can be secured.

[0007] An object of the present invention is to provide a multilayer filter using through-holes as inductance elements, in which the multilayer filter is small in size and capable of improving the Q-value further.

[0008] According to the present invention, a multilayer filter comprises a multilayer body formed by stacking and sintering dielectric and conductive layers; input-output terminal electrodes overlaid in both respective edge faces of the multilayer body; ground electrodes overlaid on both respective sides of the multilayer body; inductance elements in a form of a plurality of through-hole electrodes formed in the multilayer body; paralleled capacitors connected to the inductance elements formed in the multilayer body; and in that one end of each inductance element is electrically coupled to the input-output terminal electrode, the other end is connected to the conductive layer as a sealed electrode; and the ratio W/d of the diameter d of the through-hole electrode to width W between the ground electrodes on both edge faces of the multilayer body is set at not less than 1.6 and not greater than 11.4.

[0009] The multilayer filter according to the present invention is thus of quasi-coaxial type, that is, provided with the sealed electrodes in both respective sides of a body of generally cubic shape, and the through-hole electrodes as inductance elements. Moreover, not lower than about 70% of the maximum value is made obtainable as the Q-factor by setting the ratio of the diameter d of the through-hole to the width W of the multilayer body at the range of 1.6 to 11.4.

[0010] Further, in a multilayer filter, an impedance-matching capacitor is provided between the input-output terminal electrode and the inductance element.

[0011] FIG. 1A is a perspective view of a multilayer filter embodying the present invention;

[0012] FIG. 1B is a sectional view taken on line E—E of FIG. 1A;

[0013] FIG. 2 is a layer structural diagram of the multilayer filter of FIGS. 1A and 1B;

[0014] FIG. 3A is a diagram illustrating the diameter d of a through-hole and width W between both sides of a multilayer body;

[0015] FIG. 3B is an equivalent circuit diagram in the multilayer filter;

[0016] FIG. 4 is a diagram showing the relation between the ratio W/d of the diameter d of the through-hole electrode to side-to-side width W and the Q-factor in the multilayer filter; and

[0017] FIG. 5 is a comparative diagram between transmission characteristics when the present invention is applied to a multilayer filter whose central frequency is 1.9 Disc and those of a conventional multilayer filter using strip-line resonators.

[0018] FIG. 1A is a perspective view of a multilayer filter embodying the present invention; FIG. 1B, a sectional view taken on line E—E of FIG. 1A; FIG. 2, a layer-to-layer structural diagram; FIG. 3A, a diagram illustrating the diameter d of a through-hole and width W between both sides of a multilayer body 1; and FIG. 3B, an equivalent circuit diagram in the multilayer filter.

[0019] In FIGS. 1A and 1B, reference numeral 1 denotes a multilayer body comprising a ceramic dielectric layer 2 and a conductive layer which will be described hereinafter. Input-output terminal electrodes 3 and 4 are overlaid in both respective edge faces of the multilayer body 1, and ground electrodes 5 and 5 are overlaid on both respective sides of the multilayer body 1.

[0020] Reference numerals 6 and 7 denote impedance-matching capacitor electrodes each connected to the input-output terminal electrodes 3 and 4 facing capacitor electrodes 8 and 9 via the dielectric layer so as to form impedance-matching capacitors Ci1 and Ci2.

[0021] Reference numerals 10 and 11 denote capacitor electrodes each connected to the capacitor electrodes 8 and 9 via through-hole electrodes 12 and 13 and by placing a capacitor electrode 14 between the capacitor electrodes 8 and 10 and between the capacitor electrodes 9 and 11 via the dielectric layer, a resonator-to-resonator coupling capacitor Cm of FIG. 3B is formed.

[0022] The capacitor electrodes 10 and 11 are placed opposite to a sealed electrode 15 via the dielectric layer whereby to form capacitors Cr1 and Cr2 for resonators each connected to inductance elements L1 and L2 in parallel.

[0023] Reference numerals 16 and 17 denote through-hole electrodes for use as the inductance elements L1 and L2 for resonators as shown in FIG. 3B. One ends of the through-hole electrodes 16 and 17 is connected to the capacitor electrodes 10 and 11 via the through-hole electrodes 19 and 20 passing through the sealed electrode 15. Further, the other ends of the through-hole electrodes 16 and 17 are connected to a sealed electrode 21 which is formed as a conductive layer during the laminating process. The sealed electrodes 21 and 15 are each connected to the ground electrodes 5 and 5 on both sides of the multilayer body 1.

[0024] FIG. 2 shows a layer structure when the multilayer body 1 is produced by a sheeting method (the multilayer filter according to the present invention may also be produced by a printing method). As shown in FIG. 2, the capacitor electrodes, the sealed electrodes and the through-hole electrodes 6-21 are those formed by printing on the surfaces of green sheets 2a-2kas ceramic dielectrics or filled in through-holes. The multiple green sheets 2a-2kprovided with the capacitor electrodes, the sealed electrodes and the through-hole electrodes are stacked up, pressure-welded, cut into individual chips and calcined whereby to form the multilayer body 1. Then the input-output terminal electrodes 3 and 4 and the ground electrodes 5 and 5 are fitted to the edge faces and sides of the multilayer body 1 by baking and plating, respectively.

[0025] FIG. 4 shows the relation between the ratio W/d of the diameter d (see FIG. 3A) of the through-hole electrodes 16 and 17 to side-to-side width W and the Q-factor in the multilayer filter which comprises vertical quasi-coaxial resonators and is formed with the ground electrodes 5 and 5 on the respective sides of the aforementioned multilayer body 1. In the vertical quasi-coaxial structure, the maximum value is established when the above ratio W/d is about 3.4. A point a on the curve of FIG. 4 represents the ratio (≈13) in the multilayer filter described in the aforementioned patent publication, which is about 65% of the maximum value in terms of the Q-factor. In order to secure a Q-factor not lower than 70% of the maximum value, the ratio W/d above is set at not less than 1.6 and not greater than 11.4 and in order to secure a Q-factor not lower than 80% of the maximum value, the ratio W/d above is preferably set at not less than 1.8 and not greater than 8.2 according to the present invention. In order to secure a Q-factor not lower than 90% of the maximum value further, the ratio W/d above is more preferably set at not less than 2.2 and not greater than 6.2 according to the present invention.

[0026] FIG. 5 is a comparative diagram between transmission characteristics when the present invention is applied to a multilayer filter whose central frequency is 1.9 GHz and those of the conventional multilayer filter using strip-line resonators. In this case, the ratio W/d is set to 3.4. As shown in FIG. 5, improvement in the Q-factor is seen to be accomplished according to the present invention.

[0027] According to the present invention, a small-sized multilayer filter offering a high Q-f actor is made obtainable by employing the through-hole electrodes for forming the inductance elements, setting the ratio W/d of the diameter d of the through-hole to the width W between the ground electrodes on the respective both edge faces of the multilayer body at not less than 1.6 and not greater than 11.4, and providing the built-in capacitors in parallel to the inductance elements.