DIELECTRIC FILTER

Abstract: A dielectric filter having a stable characteristic but being without a gastight casing. The filter comprises an input means (345), an output means (346), a dielectric means (330) having a plurality of holes (331-336) extending from a top to a bottom surface thereof, each of the holes being covered with a first conductive material (337,338), a second conductive material (347) being electrically connected to the first con­ ductive material at the bottom surface and unconnected to the first conductive material at the top surface. An insulating weatherproof means such as an organic layer (360) is prov­ ided on the top surface.

Description: The present invention relates to a dielectric filter comprised of ceramic material, and in particular to a dielectric filter to which radio frequency signals (hereafter referred to as RF signals) having a frequency range from the ultra high frequency (UHF) bands to the relatively low frequency microwave bands can be coupled, and which is adapted for use as a band-pass filter coupling RF signals having a frequency range either from 825 mHz to 845 mHz or from 870 mHz to 890 mHz. A conventional dielectric filter structure is described in detail in U.S. Patent Nos. 4,386,328 and 4,283,697 assigned to the assignee of the present invention. The conventional dielectric filter as described above is generally sited in a conductive closed housing so as to sufficiently ground the filter and to prevent radiation generated by the filter from leaking and causing electrical interference to other electrical parts. The conductive closed housing comprises a main body and a lid and is further constructed as a gastight casing by virtue of a soldered joint made between the main body and the lid in a thermostatic and humidistatic atmosphere. As a result the characteristics of the filter are prevented from deteriorating as a result of changes in humidity. This type of filter needs many manufacturing processes, and is consequently expensive. Other known filters are described in WO 85/00929 and US-A-4 431 977. The filters are an interdigital bandpass type, and comprise a block of dielectric material in which are formed a plurality of through-holes whose internal walls are provided with conductive coatings to thereby form resonator rods. In accordance with the invention there is provided a filter comprising an input means; an output means; dielectric means comprised of a dielectric material having top, bottom and side surfaces, said dielectric means having a plurality of holes defining respective interior surfaces in the dielectric means, each hole extending from the top surface to the bottom surface thereof, the interior surfaces of each of the holes being covered with a first conductive material, such that each of the holes operates as a dielectric resonator; and a second conductive material provided on the exterior surface of the dielectric means, said second conductive material being electrically connected to the first conductive material at the bottom surface, said filter being characterised by, for each hole, a collar of conducting material on said top surface, each collar being electrically connected to the first conductive material of the respective hole, and being separated from the collars of adjacent hole or holes; and in that: 1) said second conductive material extends to cover only said bottom and side surfaces of said dielectric means, and is electrically unconnected to the first conductive material at the top surface; 2) said top surface is coated with a layer of epoxy resin material which layer is such as to prevent deterioration of the filter characteristics due to humidity, said layer being such as to cover at least those parts of said top surface which are not covered by conductive material. In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawings in which:- Figure 1 is a perspective view of a dielectric filter of a type which is well adapted for the teachings of the present invention; Figure 2 is a cross section of the filter of Figure 1 taken along lines 103-103; Figure 3 is a perspective view of a dielectric filter embodying the present invention; Figure 4 is a cross section of the filter of Figure 3 taken along lines 303-303; and Figure 5 is a drawing for illustrating the experimental weathering test results of the filters as shown in Figure 1 and Figure 3. In Figure 1, there is illustrated a dielectric filter which is particularly suitable for the application of the teachings of the present invention. The dielectric filter 100 has a substantially rectangular solid-shaped block 130 which is made of ceramic material. The block 130 has six parallel round holes 131-136, which respectively extend from the top surface to the bottom surface thereof and are spatially aligned. Each of the holes 131-136 is entirely covered with an electrically conductive material such as silver or copper as shown in Figure 2 which is a cross section of the dielectric filter in Figure 1 taken along lines 103-103, in which the holes 131 and 132 are each covered with an inner conductive layer indicated by the reference numerals 137 and 138, respectively. The inner conductive layers can be deposited on the surfaces of the holes by any conventional means such as printing or plating. The inner conductive layers are electrically connected with one another by means of a bottom conductive layer 147 such as baked silver or copper paste which is provided on the bottom surface of the block 130. The bottom conductive layer 147 is electrically connected with the outer conductive layer 148 which is provided on the side surface of the block 130. Each of the holes, being covered with the inner conductive layer and surrounded by dielectric material, which is itself covered with the outer conductive layer 148 connected with the inner conductive layer at the bottom thereof, will act as a dielectric resonator. The block 130 has conductive collared areas 139-144, each of which is provided on the top surface of the block 130 so as to surround the end of the corresponding hole, and connected with the corresponding inner conductive layers. The conductive collared areas 139-144 are shown as substantially rectangular shaped patterns in Figure 1, but are not limited to a rectangular shape and any shape of pattern such as a round shaped pattern can be selected. These conductive collared areas 139-144 act as an electromagnetic coupler for coupling with adjacent dielectric resonators. RF signals are capacitively and electromagnetically coupled to and from the filter 100 in Figure 1 by means of input and output electrodes 145, 146. The resonance frequency of each dielectric resonator depends mainly upon the height of the hole and the dimension of the conductive collared area associated with the hole, which are selected so as to construct substantially a quarter-wavelength coaxial resonator. The adjusting of the resonance frequency is accomplished by variation of the conductive collared area's dimension by means of a laser, sandblast trimmer or other suitable trimming manner. The amount of coupling (which can be expressed by a coupling coefficient) between adjacent dielectric resonators depends elementally upon the pitch (P) therebetween (Figure 2) and additionally upon the dimension of the conductive collared area. The fine adjustment of the coupling coefficient is easily performed by trimming the conductive collared area. The quality factor Q of the filter depends upon the number of dielectric resonators, or plated holes. The frequency characteristic becomes sharp as the number of dielectric resonators increases. Although there is illustrated a filter having six plated holes in Figure 1, any number of plated holes can be selected so as to obtain a desired frequency characteristic for the filter. The above mentioned dielectric filter 100 has a bare dielectric portion 150 which is provided on the block 130 and uncovered with a conductive material with the exception of the conductive collared areas 139-144, the input and output electrodes 145, 146, the inner conductive layers, the bottom conductive layer 147 and the outer conductive layer 148. In the filtering operation of the filter of Figure 1, when RF signals are applied to the input electrode 145, the first dielectric resonator having the hole 131 generates an electromagnetic field. This electromagnetic field is transferred through the area between adjacent conductive collared areas 139 and 140 to the second dielectric resonator having the hole 132, i.e. the energy of the electromagnetic field resulting from the first dielectric resonator concentrates in the area between the conductive collared areas 139 and 140. The electromagnetic field transferred to the second resonator is then transferred to the third resonator having the hole 133. In the same way, the electromagnetic field is transferred until the sixth dielectric resonator having the hole 136. Then, the energy of the electromagnetic field resulting from the sixth resonator is applied through the output electrode 146 to a load (not shown). The above mentioned filter's structure will be described in more detail in co-owned pending U.S. Patent Application Serial No. 780,649. In Figure 3, there is illustrated a dielectric filter embodying the present invention. The dielectric filter 300 has a substantially rectangular solid-shaped block 330 which is made of ceramic material. The block 330 has six parallel round holes 331-336, which respectively extend from the top surface to the bottom surface thereof. Each of the holes 331-336 is entirely covered with an electrically conductive material such as silver or copper as shown in Figure 4 which is a cross section of the dielectric filter of Figure 3 taken along lines 303-303, in which the holes 331 and 332 are covered with inner conductive layers 337, 338, respectively. The inner conductive layers are electrically connected with one another by means of a bottom conductive layer 347 such as baked silver or copper paste which is provided on the bottom surface of the block 330. The bottom conductive layer 347 is electrically connected with the outer conductive layer 348 such as baked silver or copper paste which is provided on the side surface of the block 330. The block 330 further has conductive collared areas 339-344, each of which is shown as a substantially rectangular shaped pattern and provided on the top surface of the block 330 so as to surround the end of the corresponding hole, and respectively connected with the corresponding inner conductive layers. These conductive collared areas 339-344 act as an electromagnetic coupler for coupling together adjacent dielectric resonators. RF signals are capacitively and electromagnetically coupled to and from the filter 300 by means of input and output electrodes 345, 346. The dielectric filter 300 further has a layer 360 of organic material comprised of an organic material such as an organic synthetic resin, preferably, a solder resist material which is a resist material containing epoxy resin. The organic material layer 360 covers that portion of the dielectric portion of the block 330 which is uncovered by the conductive material, with the exception of the conductive collared areas 339-344, the input and output electrodes 345, 346, the inner conductive layers, the bottom conductive layer 347 and the outer conductive layer 348. Here, the organic material layer 360 partially covers the input and output electrodes 345, 346 and the conductive collared areas 339-344 (see Figure 3). The organic material layer 360, the thickness of which is about from 10 to 20 micron, is obtained by the steps of depositing an organic material on the surface of the filter by means of screen printing and thence heating the deposited organic material at a temperature of around 150°C for thirty minutes so as to dry it. The adjusting of the resonance frequency and the coupling coefficient of this dielectric filter is accomplished by trimming the conductive collared area. The adjusting operation can be performed either before or, preferably so as to enable fine adjustment, after the organic material layer is deposited. In the case of the adjustment being performed after the organic material layer is deposited, the filter will have a re-bare dielectric portion again, but the re-bare dielectric portion is generally able to be disregarded because of being small and, as a result, exerting little influence on the deterioration of the filter characteristics due to humidity of the dielectric filter. If necessary, an organic material layer may be deposited on the re-bare dielectric portion. In respect of the filtering operation, the above mentioned dielectric filter 300 as shown in Figure 3 is substantially the same as the filter 100 as shown in Figure 1. In Figure 5, there is illustrated some experimental weathering test results of the filter as shown in Figure 3 in comparison with the filter as shown in Figure 1. The weathering test was carried out at a temperature each of 25°C and 50°C under a constant Relative Humidity (R.H.) of 90 percent and respectively measured Insertion Loss each of the filters. As a result of the weathering test, the Insertion Loss of both the filters was around minus 0.1 decibel (dB) at a temperature of 25°C, and at a temperature of 50°C the Insertion Loss of the filter shown in Figure 3 was around minus 0.7 dB (which is designated as ○ in Figure 5) and that of the filter as shown in Figure 1 was around minus 2.5 dB (which is designated as ▵ in Figure 5). The dielectric filter according to the present invention as shown in Figure 3 is thus superior in weatherproofness with respect to the filter of Figure 1 and has a stable characteristic without a gastight casing.

Claims: A filter (300) comprising:- an input means (345); an output means (346); dielectric means (330) comprised of a dielectric material having top, bottom and side surfaces, said dielectric means having a plurality of holes (331-336) defining respective interior surfaces in the dielectric means, each hole extending from the top surface to the bottom surface thereof, the interior surfaces of each of the holes being covered with a first conductive material (337,338), such that each of the holes operates as a dielectric resonator; and a second conductive material (347,348) provided on the exterior surface of the dielectric means, said second conductive material being electrically connected to the first conductive material at the bottom surface, said filter having for each hole, a collar (339-344) of conducting material on said top surface, each collar being electrically connected to the first conductive material of the respective hole, and being separated from the collars of adjacent hole or holes; and being characterised in that:- 1) said second conductive material extends to cover only said bottom and side surfaces of said dielectric means (330), and is electrically unconnected to the first conductive material at the top surface; 2) said top surface is coated with a layer (360) of epoxy resin material which layer is such as to prevent deterioration of the filter characteristics due to humidity, said layer being such as to cover at least those parts of said top surface which are not covered by conductive material. A filter according to claim 1, wherein the epoxy resin material is a deposited solder resist material on the top surface, which resist material has been subjected to a heat treatment. A filter according to claim 2, wherein the solder resist material is deposited by means of a screen printing process. A filter according to claim 1, wherein said epoxy material has a thickness of 10 to 20 micrometers. A filter according to claim 1, wherein said layer (360) of epoxy resin material covers those parts of said top surface which are not covered by conductive material and in addition overlaps over the edges of areas (339-346) of conductive material on said top surface so as to partially cover such areas. A filter according to claim 1, wherein said epoxy resin material covers all of the top surface of said dielectric means without extending beyond the top surface of said dielectric means (330).

Inventors: HORII, HIROYUKI OKI ELECTRIC INDUSTRY CO., LTD., IWASE, ICHIRO OKI ELECTRIC INDUSTRY CO., LTD., MATSUKURA, HISAO  OKI ELECTRIC INDUSTRY CO., LTD., SAKURAI, YOSHIMITU OKI ELECTRIC INDUSTRY CO., LTD., TAKINO, TETUJI OKI ELECTRIC INDUSTRY CO., LTD., YAMASAKI, TAKANORI OKI ELECTRIC INDUSTRY CO., LTD., YAMATO, OSAMU OKI ELECTRIC INDUSTRY CO., LTD., MATSUKURA, HISAO OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1987
Main classification: H01P1
IPC codes: H01P7, H01P1