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.

Description: Technical Field The present invention relates to radio frequency signal band-pass filters made of ceramic materials and, more particularly, to ceramic block band-pass filters which have different characteristics depending on the pattern of conductive material that covers the ceramic block. Background of the Invention It is known, e.g., from U.S. Patent No. 3,505,618 of McKee, that a radio frequency band-pass filter may be formed from a generally right parallelepiped body of dielectric material having top, bottom, side, and end surfaces. Holes are formed in the body extending from the top surface toward the bottom surface. A conductive material is coated over the most of the outer surfaces, except perhaps the top surface, and extends into the holes in order to form transmission line resonators. The conductive material in the holes is electrically connected to the conductive material on the bottom surface of the dielectric block. However, at the top surface the conductive material of the holes is not connected to the conductive outer coating. As a result, the resonators have a short circuit end toward the bottom surface of the dielectric block and an open circuit end at the top surface. Means are provided for coupling a signal into and out of the endmost holes, e.g., by means of plug-type electrodes fitted into the open circuit ends of these holes. As an alternative to coupling into the dielectric block by means of plug-type electrodes, it is known to couple capacitively to the open circuit end of the resonator by means of conductive strips or electrodes formed on the top, end or side surfaces of the dielectric block. This type of coupling is described in U.S. Patents No. 4,431,977 of Sokola et al., No. 4,692,726 of Green et al. and No. 4,716,391 of Moutrie et al. Conductive electrode pads that are isolated from the other conductive material, are coated on one of these surfaces of the dielectric material adjacent one of the resonator holes. An input or output lead is also connected to the pad. By locating the pad toward the open circuit end of the resonator, the signal on an input lead affects the electric field surrounding the open circuit end of the resonator, and capacitively induces a signal into the dielectric block. Alternatively, the pad at the output intercepts the electric field and picks up a signal from the block which it induces in the output lead. In one embodiment disclosed in the Sokola et al. patent, an electrode is placed on an end surface near the short circuit end of the resonator. This electrode is coupled to the conductive material at the bottom of the block and an input lead is coupled to the electrode. As a result, the signal on the input lead forms a current that affects the magnetic field around the short circuit end of the resonator, and inductively induces a signal into the dielectric block. A similar output electrode and lead inductively pick up a signal from the block. The bandwidth of a dielectric filter can be adjusted by changing the physical width of the dielectric block. Fine adjustment of the bandwidth typically requires the dielectric body to be machined to some degree to set it at the optimal bandwidth. These filters are usually made of ceramic material formed in a mold. Since it is not practical to make blocks of different width in the same mold, changing the frequency the filter is designed for can be difficult and expensive. It is known that coupling between the resonators also controls the bandwidth. U.S. Patent No. 4,255,729 of Fukasawa et al. discloses a series of individual resonators coupled together to form a filter. The coupling into the endmost resonators and between resonators is achieved either by current carrying loops of wire near the short circuit end of each resonator, which produce inductive coupling, or by conductive plates positioned near the open circuit ends of each resonator, which produce capacitive coupling. The above-identified Sokola et al., Moutrie et al. and Green et al. patents illustrate that magnetic coupling between resonators in a single dielectric block can be controlled by unplated or plated holes through the block at locations between the resonators, and by grooves or slots on the surface of the body. Inductive coupling is also controlled by varying the dimensions of the dielectric body (e.g. by machining it) and varying the distance between resonators during manufacture. Capacitive coupling can be controlled by electrode patterns on the top or open circuit surface of the block. In addition to adjusting the inter-resonator coupling in order to control the filter characteristics, it may also be necessary to adjust the center frequency of the filter. The center frequency can be adjusted by changing the length of the resonators, i.e. the distance between the top and bottom surfaces when the resonator holes extend from one surface to the other. The relationship is as follows: f c = 300/l* e r where f c is the frequency in megahertz, l is the length and e r is the relative dielectric constant of the dielectric material. Since the body of dielectric material is typically a ceramic that is compressed in a mold, the height of a block can be varied without changing molds by controlling the amount of material placed in the mold and by making sure the open side of the mold corresponds to the top or bottom surface of the block. Another way of controlling the center frequency is by adding capacitance to the open circuit end of the resonators. See, Matthaei et al., Microwave Filters, Impedance-Matching Networks, and Coupling Structures , McGraw-Hill, pp. 497-506 (1964). In effect, this capacitance foreshortens the resonator in that it lowers the resonant or center frequency. This allows the length of the resonator for the desired frequency to shorter than that specified by the equation given above. This capacitance can be achieved by means of plates positioned above the open circuit ends of the resonators as shown in U.S. Patent No. 4,028,652 of Wakino et al. The capacitance can also be achieved by an electrode pattern on the open circuit surface of the dielectric block as shown in the Sokola patent. After the dielectric filter is formed the frequency can be adjusted by removing conductor material near the open circuit end to raise the frequency and at the short circuit end to lower the frequency. This is described in U.S. Patent No. 4,800,348 of Rosar. With the prior art techniques the coupling into and out of the filter structure, as well as between resonators in a single dielectric block, is generally either capacitive or inductive. Also, when this coupling is accomplished by electrode patterns on the dielectric block, the patterns are typically on the open circuit side. Because of the holes which open onto this side, the arrangement of patterns is limited. Further, electrode patterns on the open circuit side cannot create inductive coupling. The Japanese reference publication number JP 59119901 discloses a band stop filter where a non-conductive zone separates a transversal strip of conductive material from the conductive coating on one side surface of a dielectric body block. Summary of the Invention The filter according to the invention is defined in claim 1, whereas the duplexer filter according to the invention is defined in claim 21. The present invention is directed to the creation of a band-pass filter made of a dielectric material, which filter has electrical properties that are easily adjusted over a wide range of values without altering the dielectric body of the filter or the dimensions of the mold used to produce the body. This is achieved by locating, at least in part, an electrode pattern for controlling inter-resonator coupling on a side surface of the dielectric block, instead of on the top surface. An electrode pattern on the side of the dielectric block allows the inter-resonator coupling to be capacitive, inductive or mixed capacitive and inductive in the same filter block. In addition, coupling into or out of the block can also be achieved by means of electrodes on the side surface so that input/output coupling may also be capacitive, inductive or mixed. By utilizing the side surface of the dielectric block, the greatest surface area on the block and the area with the least number of obstructions, e.g. holes, is used for the electrode pattern. As a result, the maximum amount of design flexibility is provided to the filter designer. With this design flexibility the designer can change the filter characteristics, e.g. the bandwidth and center frequency, by changing the electrode pattern on the side of the filter block and without changing the mold in which the block is cast or the physical dimensions of the finished block. All that has to be done is to change the mask used to apply the coating of conductive material. Since mixed capacitive and inductive coupling can be used, the filter may be designed with imaginary zeros. Consequently, the number of resonators for equivalent performance can be reduced by about one-third. This allows for a corresponding reduction in the length of the filter. In an illustrative embodiment of the filter a block of ceramic material is molded in the form of a parallelepiped with top, bottom, side and end surfaces. A number of holes, e.g. four (4), are created in the block extending from the top or open circuit surface toward the bottom or short circuit surface. The bottom surface, both end surfaces and one side surface are completely covered with conductive material. The top surface may be uncoated or it may be mainly covered with conductive material, except for an area around each hole which is left uncoated. Conductive material is coated inside the holes and is connected with the conductive material at the bottom surface to form four (4) transmission line resonators. The uncoated side surface contains an electrode pattern that is used to achieve coupling into and out of the filter block, as well as to control coupling between the four (4) resonators. The pattern may take the form of loops located near the base of the input and output resonators, i.e. the endmost resonators. One end of each loop is connected to a lead, either an input or output lead, and the other end is connected to the conductive material near the bottom surface. This arrangement provides coupling into and out of the filter. An electrode projecting from the loop extends from the top of the loop at the endmost resonators to the next resonators to provide inductive coupling between them. An isolated electrode pad is located between the two middle resonator to capacitively couple them. Further, electrode strips extend from the conductive material near the top to the conductive material at the bottom, and extend between the projecting electrodes and the pad. These strips control the amount of capacitive coupling achieved with the pad. Conductive material is spaced at a distance from the side of the dielectric block with the electrode pattern. This material may be in the form of a conductor on the opposite side of a printed circuit board to which the filter is mounted or it may be a metal cover. When a printed circuit board is used, the conductive cover can be etched at the same time other patterns are formed. Further, instead of coating the electrode pattern on the side of the dielectric, it can be formed on the side of the printed circuit board in contact with the dielectric. This results in a savings in time in the formation of the filter. If a metal cover is used over the electrode pattern, it may be assembled to the filter block in such a manner that the spacing or air gap between the side and the cover is adjustable. Adjusting the size of the air gap is another means of adjusting the bandwidth of the filter to fine tune it. With the structure of the present invention, it is only necessary to alter the electrode pattern or coupling design on the side wall of the filter block in order to change the frequency response of the filter and the maximum points of attenuation formed at the upper and lower sides of the desired pass band of the filter. In practice this means that a few standard sizes of ceramic bodies or blocks can be used and, for a particular application, an electrode pattern is selected to create a filter with desired characteristics. Also, a much smaller filter can be formed. Brief Description of the Drawings The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention in which: Fig. 1 is a perspective diagram of one embodiment of a band-pass filter according to the present invention; Fig. 2 is a cross-sectional view of the band-pass filter presented in Fig. 1, taken alone line 2-2; Fig. 3 is the electrode pattern coupling design used in the band-pass filter of Fig. 1; Fig. 4 is a cross-sectional diagram of the another embodiment of a band-pass filter according to the invention; Fig. 5A and 5B show an equivalent circuit of a two resonator band-pass filter with imaginary zeros and a transfer characteristic for the filter; Fig. 6 is a perspective diagram of a still further embodiment of a band-pass filter mounted on a printed circuit board according to the invention; Fig. 7 is a cross-sectional diagram of the filter of Fig. 6, taken along line 7-7; Fig. 8 presents the electrode pattern coupling design used with the band-pass filter of Fig. 7; Figs. 9A-9C illustrate different electrode patterns; Figs. 10A-10C illustrate a block diagram of a duplexer filter structure and two transfer characteristics therefor; and Figs. 11A and 11B illustrate a technique for.mounting a filter on a printed circuit board so as to form an air gap. Description of Illustrative Embodiments Fig. 1 illustrates a ceramic band-pass filter according to one embodiment of the presented invention. Fig. 2 is a cross-sectional view of the filter taken along lines 2-2 in Fig. 1. The filter is made up of body 10, which is formed of a dielectric material that is selectively coated with a conductive material. The filter body 10 can be formed of any suitable type of dielectric material, e.g. a ceramic. The shape of body 10 is substantially a right parallelepiped, i.e. its surfaces are rectangular. These surfaces include a top surface 11, a bottom surface 12, two end surfaces 13 and two side surfaces 14, 15. In addition, body 10 has four (4) holes 16, 17, 18 and 19 which are along the longitudinal axis of the body. These holes extend from the top surface 11 of the body toward the bottom surface 12. The bottom surface, the top surface and side surface 14 are completely plated with an electrically conductive layer of material 21, except for the circular area 22, surrounding each of the holes 16, 17, 18 and 19 on the top surface of the body. If desired, area 22 can be increased until there is no conductive material on the top surface 11. In addition, each of the holes 16-19 is plated with conductive material 23, in such a way that the plating 23 at the bottom end of the hole is connected to the plating 21 on the bottom surface 12. However, at the top end of the holes the plating 23 is not connected to the plating 23 on the top surface 11 of the body because of the uncoated area 22 around each hole at the top. Thus the holes 16-19 form quarter wavelength transmission line resonators with the top surface of the body being at the open circuit end of the resonators and the bottom surface being at the short circuit end. When there is plating on the top surface 11, each plated hole 16-19 is capacitively coupled, at its open end, to the surrounding plating. This forms a foreshortened transmission line resonator. In particular, the length of each hole, and hence the height of the block, is less than a quarter wavelength of the resonant frequency of the resonator. Foreshortening can be avoided, however, by increasing the size of the uncoated area 22 until there is no conductive material on the top surface and the capacitance is effectively eliminated. The result will be that the height of the resonators, and hence the block, will have to be somewhat greater for a particular resonant frequency. The filter structure illustrated is a quarter wavelength comb-line filter. For it to operate, there must be an imbalance in the electrical and magnetic coupling between the resonators. Foreshortening achieves this. However, with the present invention, this imbalance can also be achieved with the electrode pattern, so foreshortening is not necessary. In a preferred embodiment of the invention, holes 16-19 are located off-center from the longitudinal axis of body 10 such that the holes are closer to the unplated side wall 15 of the body then to the plated side wall 14. On the unplated side wall 15 of the body, there are coupling designs 30 in the form of metal foil electrode patterns. These electrode patterns provide coupling into the filter, as well as coupling between the transmission line resonators. Fig. 3 is an example of a coupling designs on the side surface 15 of the band-pass filter of the present invention. Inductive coupling to or from a resonator is achieved by an electrode strip design that is positioned adjacent the resonator at about the mid-point of its height. A portion of the strip extends to the conductive layer on the bottom surface 12 of the body. This kind of inductive coupling design is illustrated by coupling designs 31 and 35 in Fig. 3 which are adjacent the endmost resonators 16, 19. Lateral ground strip electrode designs 33 and 37 are also located on the side surface 15. These strips 33, 37 extend from the conductive layer 21 on the top surface to the conductive layer 21 on the bottom surface 12. Ground strip electrodes 33, 37 are offset toward holes 17 and 18, respectively. These strips tend to control the capacitive coupling between resonators. The inductive input/output strips 31, 35 are connected to respective ground strips 33, 37 near the bottom surface 12. Purely capacitive coupling to a resonator or between two resonators can be achieved by using a detached conductive coupling pad, for example coupling design 34 in Fig. 3, which is located between resonators 17 and 18. Extensions 32 and 36 of inductive coupling designs 31 and 35, extend between the resonators 16 and 17 as well as resonators 18 and 19 to create a mixture of inductive and capacitive coupling between these resonators. This type of mixed coupling between two resonators can also be realized by simultaneously using separate inductive and capacitive coupling designs. Inductive coupling is the greatest close to the bottom end of the resonator, where the magnetic field of the resonator is the strongest. On the other hand, the capacitive coupling is the greatest close to the top end of the resonator, where the electric field is the strongest. In this way, both inductive and capacitive coupling can be adjusted by either changing the size of the coupling design or by changing the elevation of the coupling design along the side surface 15. For example, the widening and elevating of the inductive coupling pattern, decreases the inductance of the design, thus decreasing the coupling to the resonator. Equivalently, increasing the size of the capacitive coupling design or the elevating of it's position, increases the coupling to the resonator. The low end of the pass band can be affected by capacitive coupling and the high end of the pass band can be affected by inductive couplings. Since, by using inductive couplings, a low-pass type of filter can be achieved directly, the band-pass filter of the present invention can be realized with four transmission line resonators, while a minimum of six transmission line resonators was previously needed. In prior filters, it was necessary in order to produce steep attenuation at the edge of the pass band, and hence improve the selectivity of the filter, to create zeros at the upper and lower edges of the pass band. These zeros were created by additional resonators. However, the mixed inductive-capacitive coupling achieved by electrodes 31, 32 or 35, 36 of the present invention permits the creation of imaginary zeros. Thus, the two extra resonators required in the prior art to form the zeros at the upper and lower side of the band, can be eliminated with the present invention and the overall size of the filter can be reduced. The creation of imaginary zeros is actually a phase cancellation technique as described in Nagle, "High-Frequency Diversity Receiver From the 1930's", Ham Radio (April, 1980) pages 40-41. The basic idea is to have two coupling paths which, at a predetermine frequency, are opposite in phase, but equal in amplitude. In the present context there is magnetic coupling between the resonators through the dielectric body. To achieve phase cancellation, there is also coupling via electrodes 32, 36. These electrodes 32, 36 are arranged so that at particular frequencies, e.g. the upper and lower edges of the pass band, the signals travelling over the electrodes cancel the signals travelling through the body. This cancellation has the same effect as a band elimination filter or zero, but does not require a separate resonator. Hence it may be referred to as an "imaginary zero". There can be more than two imaginary zeros. Also, instead of being located on either side of the pass band, they may all be located above or below the pass band. Fig. 5A shows an equivalent circuit for a two resonator 61, 63 dielectric filter. Fig. 11B in solid lines shows the transfer characteristics for this filter. By utilizing the electrode pattern on the side surface, a capacitive connection 60 can be established between the input and output terminals 65, 67. The result of this capacitive coupling is to create imaginary zeroes at the edges of the pass-band. Thus, the transfer characteristic is changed to match that shown in dotted line in Fig. 5B. This sharpening of the pass-band due to the imaginary zero allows fewer resonators to be used. If the connection of electrodes 31, 35 to ground strips 33, 37 is broken the input/output pattern becomes capacitive This will change the position of the imaginary zeros, but they will still exist. Fig. 3 is meant only to illustrate the use of the coupling designs on the side surface of body 10, and an exemplary shape. The shapes and sizes used in a particular application are determined by the desired electrical specifications and the desired method of realization of a particular filter. In reference to Fig. 1 and 2, the side surface 15 of body 10 with the electrode pattern coupling designs on it, is covered with a moveable box-like metal cover 20, whose side surfaces, 20a and 20b are partially pushed onto the top and bottom surfaces 11, 12 of body 10 in contact with electrical conductive plating 21 which covers them. Thus cover 20 surrounds the side surface 15 which has the coupling design of its. The electrically conductive surface of the cover 20 is equivalent to plating 21. As a result, it provides a conductive cover on the side of the resonators and assures that the resonators function properly. On the inner surface of the sides of cover 20 are shoulders 20c, which come against the side surface of the body 10, thus determining the distance between the inner surface of cover 20 and the side surface 15. In the primary embodiment of the invention, there is an air gap 25 between the cover 20 and the side surface 15. By moving cover 20 and changing the size of the air gap 25, the bandwidth of the band-pass filter can be adjusted. If desired, the air gap 25 can be partially or wholly filled with a suitable dielectric material. In addition, in cover 20, there are one or more openings 29, through which coupling leads 28 extends inside the cover for connection to the coupling designs on the side surface 15 of body 10. Fig. 4 presents a cross-sectional diagram of another embodiment of a band-pass filter according to the present invention. The filter of Fig. 4 is equivalent to the band-pass filter of Figs. 1 and 2, and the same reference numbers used in Figs. 1 and 2 are used in Fig. 4 to indicate the same elements. The embodiment of Fig. 4 differs from that in Fig. 2 in that the side surface 15 of body 10, which is equipped with the electrode pattern coupling designs 30, is first covered with a suitable layer 26 of dielectric material, e.g. Teflon®. On top of this layer 26 of dielectric there is plated an electrically conductive metal film 24, which can be equivalent to plating 21 and which is formed simultaneously with plating 21. In addition, one or more openings 29' are left in the electrically conductive layer 24 and dielectric 26 to accommodate coupling leads 28. In this case, the electrically conductive layer 24 has exactly the same effect as cover 20, presented in Figs. 1 and 2. The bandwidth of the filter can, nevertheless, be adjusted only by changing the thickness of the dielectric material 26 during manufacture of the filter. Figs. 6 and 7 illustrate a still further embodiment of the invention in which the filter body or block 10 is mounted on its side on a printed circuit board 40. The filter block of Figs. 6 and 7 are substantially the same as the block of Figs. 1 and 2 and the same reference numbers will be used to indicate the same elements. In Figs. 6 and 7 the body 10 of a band-pass filter according to the invention is formed of dielectric material that has been selectively plated with a layer of conductive material 21. The shape of body 10, the holes 16-19, and an electrode pattern 30 are all as in Figs. 1 and 2. The difference, however, is that the block is mounted on its side 15 to printed circuit board 40. Thus, in terms of orientation in the drawings of Fig. 6 and 7, the top surface 11 (i.e. the resonator open circuit surface) is on the side and the uncoated side surface 15 is against the printed circuit board 40. Fig. 7 presents a cross-sectional diagram, taken across the line 7-7, of the ceramic dielectric body of Fig. 6, as fixed to printed circuit board 40, which board could be any type of insulation plate, but which is economically a printed circuit board. Instead of having the electrode pattern 30 on the side 15 of the body 10, it may advantageous be provided on the surface of board 40 that is in contact with side 15. The electrically conductive plating 21 on the ceramic body is economically coupled by a solder bead 44, to a conductive circuit pattern 42, which is located on the top surface of the board 40, substantially surrounding the perimeter of body 10. On the opposite side of the board from body 10, there is an area 46 of conductive material plated on the board. Area 46 is at least the size of the area of the side surface 15 of body 10 and forms an electrically conductive surface equivalent to plating 21 or cover 20 in Fig. 1 over the otherwise unplated side surface 15, so that the resonators 16-19 function properly. The conductive area 46 on the bottom side of the printed circuit board 40 in Fig. 7 is coupled to the conductive area 42 on the top of the board via a plated-through hole 48, and via a coupling of the plating 42 to plating 21 on body 10. Fig. 8 illustrates an exemplary coupling designs 30 on the board 40 for a band-pass filter according to the present invention. Inductive coupling to the endmost resonators is achieved by strip line design 31, 35. Unlike the embodiment on Figs. 1 and 2, there are no terminal pins 28. Instead leads 50, 52 form input and output lines, respectively, that are connected to one side of inductive patterns 31, 35, which are like those shown in Fig. 3. The other sides of these patterns 31, 35 are grounded to the plating 42 on the printed circuit board and/or to the plating 21 on the surface of body 10. Purely capacitive coupling to the resonator or between two resonators is achieved with separate conductive coupling pads or islands, for example, of the type shown in Fig. 8 as pad 34, which pads are located between resonators 17 and 18 in Fig. 6. The extensions 32 and 36 of the inductive coupling designs 31 and 35, which extend between the resonators 16 and 17 as well as resonators 18 and 19 in Fig. 5 create the same mixture of inductive and capacitive coupling that may be used to form imaginary zeros as discussed with respect to Fig. 3. As an alternative to the arrangement shown on the left side of Fig. 7, the printed circuit board 40 can be a multi-layer board 40, 41 as shown on the right side of Fig. 7. On the right side of Fig. 7 there are more than two conductive layers, i.e. layers 42, 46, 47 and the coupling designs 30 for coupling to the resonators are located on one of the center conductive layers 47 of the board. If, in this case, the metal plating 46' on the opposite side of the board, or on one of the center conductive layers of the multi-layer board that is farther away from the ceramic body than the above-mentioned coupling design, than the conductive layer 46' forms an electrical shield equivalent to conductive layer 46 on the left side of Fig. 7. Instead of fastening body 10 to the board 40 by soldering, it can also be fastened, for example, by gluing or by a separate fixing bracket in which body 10 is mounted and which in turn is fastened to the board. Figs. 9A-9C show filters with different electrode patterns 30 for coupling to and between resonators. These structures also show electrode patterns which assist in tuning the various resonators to desired frequencies. Fig. 9A illustrates a filter in which the top surface 11 is covered with conductive material, except for an area 22 around the open circuit end of each of the resonator holes 16-19. On the side surface which has the electrode pattern 30, there is a strip of conductive material 41 which extends along the bottom. The frequency of a particular resonator can be lowered by grinding or scratching away a portion of this conductive strip 41 adjacent the resonator. The frequency can be raised by adding additional conductive material to strip 41, for example, through the use of conductive paste or paint. The arrangement shown in Fig. 9B not only includes conductive strip 41 at the bottom, but also a conductive strip 43 which runs along the top of the side surface. Removing conductive material from strip 43 adjacent the resonator raises the frequency of that resonator. Thus, with the arrangement of Fig. 9A, the resonators are designed to have a frequency slightly above the desired frequency. Final tuning is then achieved by scratching away some of conductor 41 to lower the frequency to the exact value desired. With the arrangement of Fig. 9B, the resonators are designed to have the exact frequency which is desired. If the frequency is a little low or a little high, in practice, the material can be moved from conductors 41 and 43, respectively, to tune the frequency exactly. As an alternative, the frequency can also be reduced by removing a portion of the dielectric material from the top surface 11 adjacent the resonator. A gouging out of this material, as at 45, results in a increasing of the frequency. Further, by adding dielectric material adjacent a resonator on the upper surface 11, the frequency of the resonator can be lowered. The pattern shown in Fig. 9C is basically the same as in Fig. 9A, except it includes strip 43 with tuning tabs 78. These tabs can be scratched off to affect tuning without disrupting the grounding strip 43. While these techniques for tuning the frequency of the resonators are preferred, other tuning techniques can also be used. Two filters according to the present invention can be combined to form a duplex filter. A block diagram of such an arrangement is shown in Fig. 10A in which filter 50 is connected between a transmitter and an antenna 51 and a filter 52 is connected between a receiver and the antenna 51. The pass band of each of these filters is offset from each other such as shown, for example, in Fig. 10B, where the transmitter pass band is located below the receiver pass band. However, the opposite arrangement is also possible. The connection 53 between the filters and the antenna 51 may be made a quarter wavelength long in order to achieve phase and impedance matching. Alternatively, reactive components can be included in lines 53, so a full quarter wavelength line is not needed. A reactive component for combining two filters to form a duplexer may be formed by a portion of the electrode pattern 30 on the side surface of one or both of the resonators. In such a case, the block of ceramic material may be mounted in a metal bracket and installed in a printed circuit board without the need for discrete reactive components. Also, if a quarter wavelength structure is needed for combining filters 50 and 52, this structure can be provided in the form of an electrode pattern on the sides of the dielectric blocks. In addition to using two band-pass filters to achieve a duplexer structure, a band-pass and a band-stop filter may also be used. The transfer characteristic for this is shown in Fig. 10C. The advantage of using a band-stop filter is that it has the same insertion loss and isolation for the receiver band with three resonators, as does a four resonator band-pass filter. If the receiver pass-band filter is made using phase cancellation according to the present invention, only four resonators are needed, as opposed to the six resonators in a conventional band-pass filter. Thus, the duplexer structure using a band-stop arrangement has a total of seven resonators compared to twelve resonators using conventional band-pass arrangements. The circuit pattern shown in Fig. 9A is an arrangement for a receiver band-pass filter of a duplexer, i.e. for filter 52 of Fig. 10A. The input and output pads 72 capacitively couple to resonators 16 and 19, respectively. They also provide inductive coupling between resonators 16, 17 and resonator 18, 19 by means of grounded strips 74. These connections create the phase cancelling phenomenon that results in imaginary zeros. Pads 76 are connected by an external wire and allow capacitive coupling between resonators 17 and 18. The grounded strips 77 help to limit capacitive coupling between various portions of the electrode pattern 30. The pattern of Fig. 9B is for the transmitter filter 50 of Fig. 10A. It has capacitive input terminals or electrode pads 54 at the input and output ends. The pad at the output end is shown connected to a ground strip via a conductive lead 55. This lead, however, is made small so that at radio frequencies it does not diminish the capacitive effect of pad 54. Strip 55 is preferably a quarter wavelength long so that it appears like an open circuit at the resonant frequencies, as is the pad 54 at the input. By means of leads 57, capacitive coupling is provided between electrodes 16, 17 and 18, 19. Like the arrangement shown in Fig. 9A, there are small electrode strips 46 which can be connected by wire to form inter-resonator capacitive coupling as well as grounded electrode strips 47 which control coupling. Figs. 11A and 11B illustrate an alternative means for mounting the filter on a printed circuit board 40. In this arrangement the filter body 10 is in a metal casing 80 which is open at one side. The casing has side walls 82 which are longer than the width of the top wall 11 of the body. As a result, if the body 10 is at the upper end of the casing and the open end of the casing faces the printed circuit board, an air gap 25' is created between the side 15 of the body and the circuit board. The casing 80 may be soldered to a conductor pattern 42' on the top of the printed circuit board or it may be glued to the printed circuit board. Also, the electrode pattern is on the side 15 of the body. A conductive layer 46' is provided on the bottom of the board 40 to cover side 15 and assure that the resonators function properly. This layer 46' is connected to the casing 80 via plated-through hole 48', conductor 42' and solder weld 44'. The size of the air gap 25' and the thickness of the board 40 control the bandwidth of the filter. As an alternative, the effect of pattern 46' can be achieved by extending pattern 42' under the casing 80.. This alternative allows the pattern 46' and plated-through hole 48' to be eliminated.

Claims: A filter comprising a body (10) of dielectric material, having first (11) and second (12) surfaces on opposite sides of the body as well as two end surfaces (13) and two side surfaces (14, 15) generally orthogonal to the first and second surfaces, at least two holes (16, 17, 18, 19) extending through the body from the first surface toward the second surface and a conductive layer (21) covering major portions of the second, end and one side surfaces as well as on the interior surfaces of the holes (23), to form at least two transmission line resonators, characterized in that it comprises electrically conductive electrode pattern means (30, 31, 32, 33, 34, 35, 36, 37, 42, 54, 55, 72, 74, 77, 78) disposed adjacent to the other side surface (15) for providing electrical signal coupling between the transmission line resonators, depending on the relative location of said electrode pattern means between areas adjacent to the first surface and areas adjacent to the second surface, said coupling is one of the following: (a) capacitive, (b) mixed capacitive and inductive, (c) inductive, and the filter is of the band-pass type. A filter according to claim 1, characterized in that said electrically conductive pattern means additionally provide signal coupling to and from at least one of the transmission line resonators, wherein depending on the relative location of said electrode pattern means between areas adjacent to the first surface and areas adjacent to the second surface, said coupling is one of the following: (a) capacitive, (b) mixed capacitive and inductive, (c) inductive. A filter according to claim 1, characterized in that the holes (16, 17, 18, 19) are located closer to said other side surface (15) than the one side surface (14). A filter according to claim 1, characterized in that it comprises an input lead (50) connected to said electrode pattern means in the vicinity of one resonator and an output lead (52) connected to said electrode pattern means in the vicinity of another resonator; so as to couple a signal into the filter through said input lead and to couple a signal out of the filter through said output lead. A filter according to claim 1, characterized in that it comprises an electrically conductive plate (20, 46) spaced from said other side surface by a gap (25, 40) and being electrically connected to the conductive layer (21) that covers major portions of the second, end and one side surfaces, which conductive plate covers at least partly said other side surface. A filter according to claim 5, characterized in that said gap is filled with insulating material (26) and said conductive plate is in the form of a metal film located on the insulating material. A filter according to claim 6, characterized in that the insulating material is a printed circuit board (40), the filter body is mounted on one face of said printed circuit board with said other side surface (15) toward said printed circuit board and the face of the printed circuit board opposite to the filter body is covered with said conductive plate (46). A filter according to claim 5, characterized in that said conductive plate is formed by a box-like shaped metal cover (20) located over said other side surface so as to leave an air gap (25) between said other surface and said metal cover. A filter according to claim 8, characterized in that the distance of said cover (20) from said other side surface (15) of the filter body (10) is adjustable to change the size of said air gap. A filter according to claim 8, characterized in that said cover (20) has an inner surface that forms a cavity in which the filter body is retained, said cavity having shoulders (20c) projecting from said inner surface and engaging with the filter body (10) to keep the inner surface of said cover at a predetermined distance from said other side surface (15). A filter according to claim 1, characterized in that there are four transmission line resonators (16, 17, 18, 19) and said electrode pattern means are disposed so as to create a phase cancellation with signals within the filter body to form at least one imaginary zero in the frequency response of the filter. A filter according to claim 1, characterized in that said electrode pattern means (30, 31, 32, 33, 34, 35, 36, 37) are disposed directly on said other side surface. A filter according to claim 1, characterized in that said electrode pattern means are disposed on an insulating plate (40, 41) which is adjacent to said other side surface (15). A filter according to claim 13, characterized in that said electrode pattern means are disposed on that surface of said insulating plate which is against said other side surface. A filter according to claim 13, characterized in that said insulating plate is a multilayer printed circuit board (40, 41) and said electrode pattern means (30) are disposed on an inner layer thereof. A filter according to claim 13, characterized in that on that side of said insulating plate which is opposite to the side against the filter body, there is an electrically conductive layer (46) that is electrically coupled to the conductive layer (21) covering major portions of the second, end and one side surfaces of the filter body (10) and at least of the size in area of said other side surface (15). A filter according to claim 13, characterized in that the filter body is fastened to said insulating plate by glueing or soldering (44). A filter according to claim 13, characterized in that the filter body is mounted in a bracket which is fastened to said insulating plate. A filter according to claim 1, characterized in that the first surface (11) is covered by the conductive layer except for an area (22) around each hole. A filter according to claim 1, characterized in that said other side surface comprises a conductive strip (41, 43) connected to the conductive layer (21) covering major portions of the second, end and one side surfaces and located along one edge of said side surface (15) near one of the first (11) and second (12) surfaces. A duplexer filter for a radio having an antenna (51), a transmitter (Tx) and a receiver (Rx), said duplexer filter having an antenna port, a transmitter port and a receiver port and comprising a receiver branch filter (52) between the antenna port and the receiver port and a transmitter branch filter (50) between the transmitter port and the antenna port, characterized in that the receiver branch filter and the transmitter branch filter is a filter according to claim 1. A duplexer filter according to claim 21, characterized in that at least one of the antenna port, transmitter port and receiver port is coupled to said electrode pattern means through a certain conductive portion (53). A duplexer filter according to claim 22, characterized in that said certain conductive portion (53) is an electrode strip the length of which is one quarter of the wavelength on the resonant frequency of the transmission line resonators. A duplexer filter according to claim 22, characterized in that said certain conductive portion (53) forms a reactive component. A duplexer filter according to claim 21, characterized in that in at least one of the receiver branch filter (52) and the transmitter branch filter (50) said electrode pattern means is effective to create at least one imaginary zero in the frequency response of the filter. A duplexer filter according to claim 21, characterized in that exactly one of the receiver branch filter (52) and the transmitter branch filter (50) is a filter according to claim 1, and in that filter said electrode pattern means is effective to create imaginary zeros at both sides of the pass band in the frequency response of the filter, and the other filter is a band stop filter.

Inventors: NAEPPAE PAULI, TURUNEN AIMO, NAEPPAE, PAULI, TURUNEN, AIMO, Näppä, Pauli
Applicants: LK PRODUCTS OY, LK-PRODUCTS OY
Application year: 1990
Main classification: H01P1
IPC codes: H01P1