Ceramic filter

Abstract: For use as a filter, a cuboidal dielectric ceramic body includes a plurality of axially substantially parallel holes (22,23,24,25) extending between opposed side surfaces (1a,1b) of the body; a first conductive layer (3) on one of the opposed side surfaces; a second conductive layer (3) on an interior wall of each hole; and an array of substantially parallel slits (41,42,43,44,45) in one of the opposed side surfaces, a slit being located between a pair of adjacent holes, wherein a slit (41,45) towards the edge of the array is deeper than a slit (42,44) towards the centre of the array.

Description: Field of the Invention The present invention relates to a dielectric ceramic body which is adapted to be used for a ceramic bandpass filter. More particularly, the present invention pertains to a dielectric ceramic body of a substantially cuboidal configuration having a conductive or short-circuited surface which is coated with a layer of a conductive material and an open surface which is opposite to the conductive surface and is not coated with a conductive material, one of the conductive and open surfaces being formed with a plurality of spaced apart slits which determine coupling properties in the body. Description of the Prior Art Conventionally, a ceramic filter has been structured to include a plurality of cylidrical ceramic bodies which are arranged in series and electrically connected to provide a transmitter resonator and a receiver resonator, respectively. This type of ceramic filter is disclosed for example by the United States patent 4,255, 729 issued to A. Fukasawa et. al. on March 10, 1981. The known type of ceramic filter is found disadvantageous in that it requires a lot of labors in manufacture. The United States patent 3,505,618 issued to F. B. McKee on April 7, 1970 discloses a filter which is made of a dielectric body of substantially cuboidal configuration. The body is formed with a plurality of through holes which are arranged in series and extend from one surface of the body to the opposite surface. The body is covered by a layer of a conductive material such as silver. The holes have surfaces which are covered by a conductive material and conductive terminals may be inserted into selected holes to provide an input and an output. The patent proposes to provide the body with polystyrene, however, the body may be made of a dielectric ceramic material as disclosed by the United States patent 4,464,640 issued to T. Nishikawa et. al. on August 7, 1984. In the filter structure proposed by Nishikawa et. al., the ceramic body is covered by a conductive material except a surface where one ends of the holes are opened. The surface which is not covered by the conductive material may be referred as the open surface and the surface which is covered by the conductive material and opposite to the open surface may be referred as the short-circuited surface. In order to provide a desired coupling between each two adjacent holes, a slit is formed between the holes. As shown in the United States patent 4,431,977 issued to R. L. Sokola et. al. on February 14, 1984, the ceramic body may be of an elongated configuration and a plurality of holes may be formed and arranged in series in the axial direction of the body. In the structure as shown in the patent to Sokola et. al., a slit is formed between each two adjacent pair of holes. This type of ceramic filters are useful in that it can be readily assembled and less expensively manufactured. The number of the holes may be appropriately determined to obtain a desired function. In this type of ceramic filter, a desired resonating frequency is obtained through an adjustment of the volume of the dielectric material between the conductive layers on the surfaces of each two adjacent holes or the conductive layer on the surface of each hole and the outer surface of the ceramic body. For this purpose, the thickness of the ceramic body or the distance between the open surface and the short-circuited surface is varied, to determine the lengths of the holes. For example, at an axial end portion, the thickness of the ceramic body may be reduced to provide a shortened hole length or a piece of dielectric material may be added to an appropriate portion of the ceramic body. The conventional method for obtaining a desired resonating frequency is inconvenient; the number of parts and the labour involved in manufacture are increased. US-A-4431977 proposes covering even the surface opposite to the short-circuited surface except the area around each hole to provide a desired coupling, but this structure is not satisfactory, either. GB-A-2163606 discloses a dielectric block comprising holes and slits between the holes. EP-A-0038996 discloses a dielectric filter with holes and slits. It is suggested that the depths of the slits may vary. SUMMARY OF THE INVENTION According to the present invention, a cuboidal dielectric ceramic body includes a plurality of axially substantially parallel holes extending between opposed side surfaces of the body; a first conductive layer on one of the opposed side surfaces; a second conductive layer on an interior wall of each hole; and an array of substantially parallel slits in one of the opposed side surfaces, a slit being located between each pair of adjacent holes, in which the depth ratios of each outermost slit to the respective adjacent outermost but one slit and (if there are 5 or more slits) of each outermost but one slit to the respective adjacent outermost but two slits are each from 1:0.65 to 1:0.95, and (if there are 6 or more slits) the depth ratio of any other pair of adjacent slits is from 1:0.8 to 1:1, a relatively deep slit of the pair being the outer slit. The fact that the slits increase in depth towards the ends of the body provides a desirable versatility in obtained a desired coupling property at each hole of the ceramic filter. DESCRIPTION OF THE INVENTION In general, the ceramic body for the filter is required to have a stronger coupling at the hole located at an axially outer portion than at the hole located at an axially inner portion of the body. It is preferred that the slit depth change is substantially symmetrical with respect to the axial centre of the ceramic body. The bottoms of a number of the slits may be on a substantially parabolic or annular curve. The ceramic body may have a decreased thickness at each end for the convenience of attaching a terminal. If there are three slits, the outermost slits are deeper than the central slit. If there are four slits, the two central slits may have substantially the same depth, the two outer slits being deeper. If there are five slits, the depths of the slits increase towards each end of the array of slits. Preferably, the bottoms of the slits are located along an eliptical or parabolic curve. If there are more than five slits, at least three from each end increase in depth towards the edge of the body; the remaining slits may have substantially the same depths, or the depths of the remaining slits may increase gradually towards each end. The above and other objects and features of the present invention will become apparent from the following descriptions of preferred embodiments taking reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view (partially cut-away in section) of a dielectric ceramic body in accordance with one embodiment of the present invention; Figure 2 is a side view similar to Figure 1 but showing a modified form; Figures 3 and 4 are side views showing further modifications; Figure 5 is a side view showing a dielectric ceramic body having slits in the short-circuited surface; Figure 6 is a side view showing a modification of the ceramic body shown in Figure 5; Figures 7 and 8 are side views showing further modifications; Figures 9 and 10 are diagrammatical illustrations of ceramic bodies having three slits; Figures 11 and 12 are diagrammatical illustrations of ceramic bodies having four slits; Figures 13 and 14 are diagrammatical illustrations of ceramic bodies having more than six slits; Figure 15 is a perspective view of a ceramic body showing a manner of adjusting the resonant frequency of each resonator of the filter; Figures 16, 17 and 18 are fragmentary plan views showing manners of adjusting the resonant frequency of each resonator of the filter; Figure 19 is a fragmentary view taken along the line A₁ - A₁ in Figure 18 to show the manner of adjusting the resonant frequency; Figure 20 is a fragmentary plan view showing a further manner of adjusting the resonant frequency; Figure 21 is a sectional view taken along the line A₂-A₂ in Figure 20; Figures 22 and 23 are fragmentary plan view showing further different manners of adjusting the resonant frequency; Figure 24 is a diagram showing the result of resonant frequency adjustment in accordance with the present invention; Figure 25 is a diagram showing the relationship between the spacings of the holes and the depths of the slits; and, Figure 26 is a diagram showing the effect of the conductive coating in the slit. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, particularly to Figure 1 together with Figure 15, there is shown a dielectric ceramic body 1 of a substantially cuboidal configuration having four side surfaces 1a, 1b, 1c and 1d and two end surfaces 1e and 1f. The ceramic body 1 is formed with six through holes 21, 22, 23, 24, 25 and 26 which extend from the top or side surface 1a to the bottom or side surface 1c and arranged in series in the axial direction of the body as shown by an arrow X in Figure 15. The ceramic body 1 is further formed at the top surface 1a with slits 41, 42, 43, 44 and 45 which are located between respective pairs of the holes 21, 22, 23, 24, 25 and 26. Each of the slits extends in the transverse direction or the direction shown by an arrow Y in Figure 15. The ceramic body is covered at the side surfaces 1b, 1c and 1d and the end surfaces 1e and 1f with an electrically conductive material such as silver which thus provides a conductive layer 3. Each of the holes 21, 22, 23, 24, 25 and 26 is also coated by a layer 3 of a conductive material such as silver. The side or top surface 1a is left uncoated so that the surface 1a provides an open surface. The side or bottom surface 1c provides a short-circuited surface. The holes 21, 22, 23, 24, 25 and 26 respectively provide resonating stages Q₁, Q₂, Q₃, Q₄, Q₅ and Q₆ of a ceramic filter which is obtained from the ceramic body 1. In Figure 1, it will be noted that the slits 41 and 45 which are located at the axially outermost positions have the same depth B₁ whereas the slits 42 and 44 which are second from the axial end have the same depth B₂. The axially central slit 43 has a depth B₃ which is smaller than the depths B₁ and B₂. Most preferably, the depth B₂ for the slits 42 and 44 and the depth B₃ for the slit 43 are determined in relation to the depth B₁ for the axially outermost slits 41 and 45 by the formula B₂ = k₂ x B₁ B₃ = k₃ x B₂ where k₂ and k₃ are proportional coefficients having values between 0.65 and 0.95, and between 0.80 and 1.00, respectively. Figure 2 shows another example of the ceramic body 1. In this example, the slits 41, 42, 43, 44 and 45 have walls which are coated with a layer 3 of a conductive material such as silver. In this example, the depths of the slits 41, 42, 43, 44 and 45 are determined in the same manner as in the previous example. The configuration of the ceramic body 1 in the examples shown in Figures 1 and 2 is such that the thickness of the body 1 is reduced at portions axially outside the outermost slits 41 and 45 by forming stepped portions on the top surface 1a. Figure 3 shows an example in which the ceramic body 1 is different from the ceramic body 1 of the example shown in Figure 1 in that the body 1 in Figure 3 has stepped portions on the bottom surface 1c to provide the portions of reduced thickness. Similarly, the ceramic body 1 shown in Figure 4 is different from the body 1 shown in Figure 2 in that the body 1 in Figure 4 has stepped portions on the bottom surface 1c to provide the portions of reduced thickness. In the examples of Figures 3 and 4, the depths of the slits 41, 42, 43, 44 and 45 are determined in the same manner as in the examples of Figures 1 and 2. Figures 5 through 8 show examples which correspond respectively to the examples shown in Figures 1 through 4. The examples in Figures 5 through 8 are different from the examples in Figures 1 through 4 in that the slits 41, 42, 43, 44, 45 and 46 are formed not in the top surface 1a but in the bottom surface 1c. In these examples, the depths of the slits 41, 42, 43, 44, 45 and 46 are determined as in the examples in Figures 1 through 4. Referring to Figure 9, there is shown a dielectric ceramic body 11 which has four through holes 21, 22, 23, and 24 arranged in series in the axial direction of the body and extending from the top surface 11a to the bottom surface 11c. As in the previous embodiments, the body 11 is coated with a layer of a conductive material on the external surfaces except the top surface 11a. Further, the inside wall surfaces of the holes 21 through 24 are also coated with a layer of a conductive material. In this embodiment, the ceramic body 11 has three slits 41, 42 and 43 which are located respectively between the holes 21, 22, 23 and 24. The slits 41 and 43 have substantially the same depth B₁ and the slit 42 has a depth B₂ which is smaller than the depth B₁. Preferably, the depth B₂ is determined in accordance with the formula B₂ = k x B₁ where k is a constant having a value between 0.65 and 0.95. Figure 10 shows a further example of the dielectric ceramic body 11 which is substantially identical with the body 11 shown in Figure 9. In the example shown in Figure 10, the slits 41, 42 and 43 are formed in the short-circuited surface 11c. The depths of the slits 41, 42 and 43 are determined in the same manner as in the example shown in Figure 9. Figure 11 shows a dielectric ceramic body 31 having a top surface 31a and a bottom surface 31c. Through holes 21, 22, 23, 24 and 25 are formed to extend from the top surface 31a to the bottom surface 31c and arranged in series in the axial direction of the body 31. The outer surface of the ceramic body 31 is coated with a layer of an electrically conductive material. Further, the inside walls of the holes 21, 22, 23, 24 and 25 are also coated with a layer of a conductive material. Four slits 41, 42, 43 and 44 are formed in the top surface 31a and located respectively between adjacent pairs of the holes 21, 22, 23, 24 and 25. The axially outer slits 41 and 44 have substantially the same depth B₁ and the inner slits 42 and 43 have substantially the same depth B₂ which is smaller than the depths B₁ of the outer slits 41 and 44. Preferably, the depth B₂ is determined in accordance with the formula B₂ = k x B₁ where k is a constant having a value between 0.65 and 0.95. Figure 12 shows another example of the ceramic body 31 which is different from the ceramic body in Figure 11 in that the slits 41 through 44 are formed in the conductively coated or short-circuited surface 31c. The depths of the slits 41 through 44 are determined substantially in the same manner as in the example of Figure 11. In the examples shown in Figures 9 through 12, the slits have depths which are so determined that the bottom portions of the slits are arranged substantially along an elliptical curve. Figure 13 shows a further embodiment of the present invention in which the ceramic body 51 has top surface 51a and a bottom surface 51c and formed with holes extending from the top surface 51a to the bottom surface 51c. In this example, the number of the holes is N so that the holes are designated from one axial end of the body by the reference characters b₁, b₂, b₃ ...b N-2 , b N-1 and b N . The holes have inside walls which are coated with layers of a conductive material. The ceramic body 51 is formed at the top surface 51a with slits s₁, s₂, s₃ ....s N-3 , s N-2 and s N-1 which are located between respective adjacent pairs of holes. The slits S₁, s₂, s₃ ... s N-3 , s N-2 and s N-1 respectively have depths B₁, B₂, B₃ ... B N-3 , B N-2 and B N-1 which are determined in accordance with the formula B m (or B N-m ) = k m x B m-1 where m is a largest integer which does not exceed N/2 and k m is a proportional coefficient which has a value between 0.65 and 0.95 in case where m is 2 and N-2 and a value between 0.8 and 1.0 in case where m is 3 to N-3. Most preferably, the value 0.8 is adopted to determine the depth of the slit s₃ or s N-3 . Then, at least three slits from the axially outer end have depths which change so that the bottom portions of the slits are laid substantially along a parabolic curve. Figure 14 shows a further example which is similar to the example shown in Figure 13 but has slits s₁ through s N-1 formed in the short-circuited surface 51c. The depths of the slits s₁ through s N-1 are determined as in the example shown in Figure 13. Where the surface having the slits is formed with stepped portions, the depth of the slit is measured from the highest part of the surface adjacent to the slit. The manner of determining the depths of the slits is based on the assumption that the holes have the same diameter and arranged with the same spacings. In case where the spacings between respective adjacent pairs of holes are not uniform, the slit depths must be modified. In case where the hole spacing is decreased with the same hole diameter, the coupling becomes stronger. For example, where the spacing between the holes b₁ and b₂ and the spacing between the holes b N-1 and b N are smaller than the other spacings in the embodiment of Figure 13, the coupling at the end portions will become stronger if the depths of the slits s₁ and s N-1 are unchanged. Therefore, the slit depths must be modified in accordance with the relationship as shown in Figure 25. As an example, as shown in Figure 25, where the hole spacing is decreased to 83 % of the standard spacing, the slit depth should be decreased to 66 % of the standard depth which is the depth of the slit required for obtaining the desired coupling with the standard hole spacing. In case where the slits are coated with conductive layers, slits can be of smaller depths for obtaining the same coupling property. Figure 26 shows the relationship between the relative value of the inter-stage coupling and the depth of the slit with and without the conductive coating. In Figure 26, the depth of the slit is designated as a ratio of the depth d and the thickness 1 of the ceramic body. Where the slit has the conductive coating, the slit depth may be modified in accordance with the relationship as shown in Figure 26. For example, in order to obtain the relative coupling value of 70 %, the salt depth must be 33 % if the slit does not have a conductive coating, but the slit depth can be as small as 8 % where the slit has a conductive coating. Referring to Figure 15, there is shown regions which are used for an adjustment of the resonating frequency. In the example shown in Figure 15, the shadowed areas which are encircled by transverse tangential lines y₁ of the holes are the adjustment regions a. As shown in Figures 16 and 17, the coupling electric field E is strongest in the axial direction X of the ceramic body along which the holes are arranged and the field is decreased toward the transverse direction Y. In the intermediate resonating stage Q₂ to Q₅, the inter-stage coupling is produced at the opposite sides of the hole. Thus, the adjustment region a is defined by the transverse tangential lines y₁ drawn at the opposite sides of the hole. At the end coupling stages Q₁ and Q₆, the inter-stage coupling is produced only at one side of the hole. Thus, the adjustment region is defined by a single transverse tangential line y₁ of the hole drawn at the axially inner side of the ceramic body. It is understood that the coupling electric field is curved in the transverse direction as shown by lines a₁ in Figures 16 and 17, however, in actual practice, the adjustment region can conveniently be defined by the straight lines y₁. In order to adjust the resonating frequency, the dielectric material in the adjustment region a is appropriately removed for example by forming a chamfered configuration c as shown in Figures 18 and 19. Alternatively, the dielectric material may be removed by forming a groove g as shown in Figures 20 and 21. As another example, circular recesses r may be formed in the adjustment region a as shown in Figure 22. For the outermost hole 21 or 26, the chamfered portion may be formed as shown in Figure 23 at the axially outer side of the hole. Referring to Figure 24, it will be noted that the resonating frequency is shifted from the value f₁ to the value f₂ by the removal of the dielectric material as described above. It should however be noted that if the material is removed along the whole periphery of the hole, there will be fluctuations in the value of decrement. According to the manner of the adjustment described above, such fluctuations can be avoided. The invention has thus been shown and described with reference to specific embodiments, however, it should be noted that the invention is in no way limited to the details of the illustrated structures but changes and modifications may be mad without departing from the scope of the appended claims.

Claims: A cuboidal dielectric ceramic body (1) including a plurality of axially substantially parallel holes (21,22,23,24,25,26) extending between opposed side surfaces (1a,1c) of the body; a first conductive layer (3) on one of the opposed side surfaces; a second conductive layer (3) on an interior wall of each hole; and an array of substantially parallel slits (41,42,43,44,45) in one of the opposed side surfaces (1a), a slit being located between each pair of adjacent holes, characterised in that the depth ratios of each outermost slit to the respective adjacent outermost but one slit and (if there are 5 or more slits) of each outermost but one slit to the respective adjacent outermost but two slit are each from 1:0.65 to 1:0.95, and (if there are 6 or more slits) the depth ratio of any other pair of adjacent slits is from 1:0.8 to 1:1, a relatively deep slit of the pair being the outer slit. A dielectric ceramic body in accordance with claim 1, in which the bottoms of a number of the slits lie on a substantially parabolic curve. A dielectric ceramic body in accordance with claim 1, in which the bottoms of the slits lie on a substantially annular curve. A dielectric ceramic body in accordance with any of claims 1 to 3, in which the slits are on the same side surface as the first conductive layer. A dielectric ceramic body in accordance with any of claims 1 to 3, in which the slits are on the side surface opposed to that carrying the first conductive layer. A dielectric ceramic body in accordance with any of claims 1 to 5, in which the slits and their depths are arranged symmetrically about the centre of the array, and the axes of the holes are disposed on a straight line. A dielectric ceramic body in accordance with any of claims 1 to 6, having 5 slits. A dielectric ceramic body in accordance with any of claims 1 to 6, having 3 slits. A dielectric ceramic body in accordance with any of claims 1 to 8, in which each slit is coated with a layer of a conductive material. A dielectric ceramic body in accordance with any of claims 1 to 8, in which some, but not all of the slits are coated with a layer of a conductive material. A dielectric ceramic body in accordance with claim 10, in which a coated slit is of smaller depth than a non-coated slit. A dielectric ceramic body in accordance with any of claims 1 to 11, in which the holes are uniformly spaced. A dielectric ceramic body in accordance with any of claims 1 to 11, in which the spacing of two adjacent holes flanking a relatively deep slit is less than between another two adjacent holes flanking a relatively shallow slit. A dielectric ceramic body in accordance with any of claims 1 to 13, in which dielectric material, at the side surface opposed to the slits, is cut-away except in an area defined by a pair of lines through respective holes, not being outermost holes, transverse to a line joining the holes. A dielectric ceramic body in accordance with any of claims 1 to 13, in which dielectric material, at the side surface opposed to the slits, is cut-away in areas outside lines through the outermost holes and transverse to a line joining the holes.

Inventors: ENDO KENJI, HAGAWA KIYOSHI, JINDOU KATSUYA, NAKANO FUMIHITO, WADA KOHEI, ENDO, KENJI, HAGAWA, KIYOSHI, JINDOU, KATSUYA, NAKANO, FUMIHITO, WADA, KOHEI
Applicants: TDK CORP, TDK CORPORATION
Application year: 1988
Main classification: H01P1
IPC codes: H01P7, H01P1