Ceramic band-pass filter

Abstract

A dielectric filter is formed from a block of ceramic material with holes extending from a top surface toward a bottom surface. At least the bottom, both ends and one side surface are coated with conductive material. Also, the interior surfaces of the holes are coated with conductive material to form transmission line resonators. The uncoated side surface has an electrode pattern which allows coupling to the filter and between resonators of the filter. The elevation of the electrodes on the side surface between the top and bottom determines whether the coupling is capacitive, mixed inductive and capacitive, or inductive.

We claim: 
     
       1. A filter comprising: a body of dielectric material having (a) first and second surfaces on opposite sides of the body, (b) at least two side surfaces generally orthogonal to the first and second surfaces and connecting the edges of the first and second surfaces to each other, and (c) two end surfaces connecting the ends of the first, second and side surfaces to each other;   said body defining at least one hole with an interior surface which extends into said body from said first surface toward said second surface;   a conductive layer covering major portions of the second surface, one side surface of said body, both end surfaces, and the interior surface of said hole so as to form at least one transmission line resonator, the other side surface being generally free of said conductive layer; and   an electrically-conductive electrode pattern means located on the other side surface of said body for providing electrical signal coupling to and from the transmission line resonator by creating a field that penetrates the uncoated other side surface of the body to the interior surface of the hole, the coupling varying from (a) capacitive to (b) mixed capacitive and inductive to (c) inductive, depending on the relative location of the pattern means on the side surface between areas adjacent the first surface to areas adjacent the second surface, respectively.   
     
     
       2. A filter as claimed in claim 1 wherein there are at least two holes in the body forming at least two resonators, said pattern means extending along the other side surface from the vicinity of one of the resonators to the vicinity of another and providing electrical coupling between the resonators. 
     
     
       3. A filter as claimed in claim 2 further including an input lead connected to said pattern means in the vicinity of one resonator, and an output lead connected to said pattern means in the vicinity of another resonator so as to couple a signal into said filter on said input lead and to couple the signal out of said filter on said output lead. 
     
     
       4. A filter as claimed in claim 1 further including an electrically-conductive plate spaced from said other side surface by a certain gap and being electrically connected to the conductive layer on the other surfaces of said body, said conductive plate at least in part covering said other side surface. 
     
     
       5. The filter as claimed in claim 4, wherein the conductive plate is formed by a box-like shaped metal cover located over the other side surface so as to leave an air gap between the other side surface and the cover. 
     
     
       6. The filter as claimed in claim 4, wherein said air gap is filled with an insulating material and said conductive plate is formed by a metal film located on the insulating layer. 
     
     
       7. The filter as claimed in claim 5, wherein the distance of cover from the other side surface of body is adjustable to change the size of the air gap, whereby the bandwidth of the filter is adjusted. 
     
     
       8. The filter as claimed in claim 6, wherein the bandwidth of the filter depends, in part, on the dielectric constant of the insulating material. 
     
     
       9. The filter as claimed in claim 6, wherein the bandwidth of the filter depends, in part, on the thickness of insulating material. 
     
     
       10. The filter as claimed in claim 6, wherein the insulating material is Teflon®. 
     
     
       11. The filter as claimed in claim 5, wherein the cover has an inner surface that forms a cavity in which the body is retained, said cavity having shoulders projecting from the inner surface that engage the body to keep the inner surface of cover at the determined distance from the other side surface of the body. 
     
     
       12. The filter as claimed in claim 2, wherein there are four resonators, and further including a coupling electrode pattern located on said other side surface for coupling said resonators to create a phase cancellation with signals within the body so as to form at least one imaginary zero positioned so that the shape of the pass band of the filter is substantially equivalent to that of a band-pass filter with six resonators, but without an imaginary zero. 
     
     
       13. The filter as claimed in claim 6, wherein the insulating material is a printed circuit board, the filter body being mounted on the board with the other side surface toward the board, and the surface of the printed circuit board opposite the body being covered with the conductive plate. 
     
     
       14. A band-pass filter comprising: a body of dielectric material having first and second surfaces on opposite sides of the body from each other, end surfaces and side surfaces;   said body defining at least two holes extending from the first surface toward the second surface;   a conductive coating on said second, end and one side surfaces as well as the interior surfaces of the holes, to form at least two transmission line resonators the other side surface being generally free of said conductive coating;   an insulating plate having first and second surfaces with the first surface against the other side surface of the body; and   a conductive electrode pattern on the insulating plate for coupling between said two resonators, the electrode pattern providing coupling between the at least two resonators by creating a field that penetrates the uncoated other side of the body to the interior surface of the hole, said coupling being capacitive, mixed capacitive-inductive and inductive depending on whether the pattern is near and thereby closer to the first surface than to the second surface, or more equidistant between the first and second surfaces or near and thereby closer to the second surface than to the first surface, respectively.   
     
     
       15. The filter as claimed in claim 14, wherein the electrode patterns are on the first surface of said insulating plate against which the body is located. 
     
     
       16. The filter as claimed in claim 15, wherein the insulating plate is a multi-layer printed circuit board and the electrode patterns are in a conductive layer inside the multi-layer board. 
     
     
       17. The filter as claimed in claim 15, wherein, on the second surface of the insulating plate opposite from the body, at least in an area the size of the other side surface of the body, there is an electrically conductive plating that is electrically coupled to the conductive coating of body. 
     
     
       18. The filter as claimed in claim 14, wherein the body is fastened to the insulating plate by gluing. 
     
     
       19. The filter as claimed in claim 14, wherein the body is fastened to the insulating plate by soldering. 
     
     
       20. The filter as claimed in claim 14, wherein the body is mounted in a bracket which has been fastened to the insulating plate. 
     
     
       21. The filter as claimed in claims 1 or 14 wherein the first surface of the body is covered with the conductive layer, except for an area around the hole. 
     
     
       22. A filter as claimed in claims 1 or 14 wherein the electrode pattern includes a conductive strip connected to the conductive coating and located along at least one edge of the other side surface near one of the first and second surfaces, removal of a portion of said strip adjacent the resonator being effective to change the frequency of the resonator. 
     
     
       23. A filter as claimed in claim 1 or 14 wherein removal of a portion of the dielectric material on the first surface adjacent a resonator is effective to alter the frequency of the resonator. 
     
     
       24. A duplexer filter for a radio having an antenna, a transmitter and a receiver, comprising: first and second blocks of dielectric material, each block having first and second surfaces on opposite sides from each other, end surfaces and side surfaces; each block defining at least one hole extending from the first surface to the second surface;   a conductive coating over the second surface, end surfaces and one side surface, as well as over the interior of the hole of each block, so as to form at least one transmission line resonator in each block the other side surface being generally free of the conductive coating;   a conductive electrode pattern on the other side surface of each block for coupling directly to and from the resonator through the dielectric in each block by means of fields created at the electrodes that penetrate the other side surface and extend to the hole, the coupling varying from capacitive, to mixed capacitive-inductive, to inductive as the pattern is located respectively on the other side surface near and thereby closer to the first surface than to the second surface, to more equidistant between the first and second surfaces, to nearer and thereby closer to the second surface than to the first surface and   connecting means for connecting the first block between the transmitter and the antenna, and for connecting the second block between the receiver and the antenna.   
     
     
       25. A duplexer filter as claimed in claim 24 wherein the connecting means includes a portion of the electrode pattern on the other side surface. 
     
     
       26. A duplexer filter as claimed in claim 25 wherein the portion of the electrode pattern is an electrode strip one-quarter wavelength of the resonant frequency of the resonator in length. 
     
     
       27. A duplexer filter as claimed in claim 25 wherein the portion oft he electrode pattern forms a reactive component. 
     
     
       28. A duplexer filter as claimed in claim 24 wherein the electrode pattern for one of the blocks forms the block into a band-pass filter with at least one imaginary zero. 
     
     
       29. A duplexer filter as claimed in claim 24 wherein the electrode pattern for one of the blocks forms the block into a band-stop filter. 
     
     
       30. A duplexer filter as claimed in claim 24 wherein one of the first and second blocks has four holes and an electrode pattern that creates a four resonator band-pass filter with imaginary zeros at both sides of the pass-band, and the other block has three holes and an electrode pattern that creates a three resonator band-stop filter. 
     
     
       31. A filter comprising: first and second blocks of dielectric material, each block having first and second surfaces on opposite sides from each other, end surfaces and side surfaces; each block defining one hole extending from the first surface to the second surface;   a conductive coating over the second surface, end surfaces and one side surface, as well as over the interior of the hole of each block, so as to form one transmission line resonator in each block the other side surface being substantially free of the conductive coating:   conductive electrode patterns on the other side surface of each block for coupling to and from the resonator in each block by means of fields created by signals on the electrode patterns that penetrate the other side surface and extend to the hole, the coupling varying from capacitive, to mixed capacitive-inductive, to inductive as the pattern is located respectively on the other side surface near and thereby closer to the first surface than to the second surface, to more equidistant between the first and second surfaces, to nearer and thereby closer to the second surface than to the first surface; and   connecting means for connecting the first block to the second block between the receiver and the antenna.   
     
     
       32. A bandpass filter, comprising: a body of dielectric material having a plurality of surfaces including two opposite surface; and a side surface extending between said two opposite surfaces, said body defining at least one hole with an interior surface which extends into said body from each of said two opposite surfaces;   resonator means for producing at least one transmission line resonator, said resonator means including a conductive layer covering portions of at least some of said plurality of surfaces and an interior surface of said hole;   means for providing electrical signal coupling to and from said resonator means, said providing means including an electrically-conductive electrode pattern means on said side surface which varies said coupling from capacitive to mixed capacitive and inductive to inductive, depending on a relative location of the pattern means on the side surface between areas of said side surface adjacent to said two opposite surfaces;   an electrically conductive plate spaced from said pattern means so as to define a gap therebetween which determines a bandwidth; and   means for coupling said plate to said interior surface of said hole.   
     
     
       33. A bandpass filter as in claim 32, wherein said determining means including means for varying a size of said gap by enabling relative movement of said plate and said pattern means with respect to each other. 
     
     
       34. A bandpass filter as in claim 32, wherein said varying means includes a insulating element separating said pattern means from said plate, said bandwidth being determined partially based on a thickness and composition of said insulating element. 
     
     
       35. A bandpass filter as in claim 32, wherein said coupling is more capacitive than inductive. 
     
     
       36. A bandpass filter as in claim 32, wherein said coupling is more inductive than capacitive. 
     
     
       37. A bandpass filter as in claim 32, wherein said coupling provides mixed capacitance and inductance.