Dielectric Filter and Dielectric Duplexer Each Having a Plurality of Dielectric Resonators Connected in Series by a Dielectric Coupling Window

1. Field of the Invention

The present invention relates to dielectric filters and dielectric duplexers, and more particularly, to a dielectric filter for use in a communication unit in the microwave band and the millimeter-wave band.

2. Description of the Related Art

A conventional type of dielectric filter for use in a communication unit in the microwave band and the millimeter-wave band is a multiple-stage filter in which dielectrics having a low dielectric constant sandwich a plurality of TEM-mode coaxial resonators (see Japanese Unexamined Patent Publication No. 2-94903). In this type of dielectric filter, a TEM-mode coaxial resonator in each stage independently functions as a resonator.

Communication units using such a filter have been required to handle higher-frequency signals as more channels have been demanded, but at a high frequency (such as in the 3-GHz band), a dielectric filter formed of a plurality of TEM-mode coaxial resonators has a very low no-load Q value and high transmission losses.

To overcome the above problems, there has been proposed a waveguide-type dielectric filter 40 using the TE.sub.10 mode, as shown in FIG. 11 for example. This dielectric filter 40 has three TE.sub.10 -mode resonators 51 and two dielectric coupling windows 52. The TE.sub.10 -mode resonators 51 are connected in series with the dielectric coupling windows 52 disposed therebetween.

The resonators 51 and the dielectric coupling windows 52 are formed of a dielectric block 41 made from one material and having almost a rectangular-parallelepiped shape, on which an outer conductive member 43 is provided to cover almost the entire surface of the dielectric block 41. A pair of input and output electrodes 45 not electrically connected to the outer conductive member 43 with the a gap between the electrodes 45 and the member 43 are provided respectively at the two ends of the dielectric block 41. To set the central frequency of each resonator 51 to the desired value, the length of each resonator 51 needs to be set to approximately half the wavelength .lambda. of the central-frequency signal. To set the pass-band width of the dielectric filter 40 to the desired value, it is necessary to set the width W1 of the dielectric coupling windows 52 appropriately.

Although the proposed dielectric filter of FIG. 11 has a high no-load Q value and a small loss at a high frequency band, it is difficult to manufacture. Since the resonators 51 and the dielectric coupling windows 52 are made from the same dielectric material in the proposed dielectric filter, they have the same dielectric constant. Therefore, in order to manufacture a plurality of dielectric filters having the same central frequency and different pass-band widths, the width W1 of the dielectric coupling windows 52 needs to be changed and thereby the shape or the dimensions of the dielectric block 41 must be changed. A forming metal die is required for each of the plurality of dielectric filters.

There is a method for manufacturing a plurality of dielectric filters having different pass-band widths with the same forming metal die. A rectangular-parallelepiped dielectric block is formed by a metal forming die. Two pairs of grooves opposing each other with a gap therebetween are formed on both sides of the dielectric block by cutting with a dicing saw. A portion sandwiched between each pair of grooves in the dielectric block serves as a dielectric coupling window. To manufacture a plurality of dielectric filters having the same central frequency and different pass-band widths, it is also necessary in this method to change the blade-feeding distance (which equals the depth of the groove) of the dicing saw to modify the width of the dielectric coupling window for each of the plurality of dielectric filters.

Accordingly, it is an object of the present invention to provide a dielectric filter and a dielectric duplexer in which the pass-band width can be changed without modifying its shape and dimensions.

The foregoing object is achieved through the provision of a dielectric filter or a dielectric duplexer in which the dielectric constant of the dielectrics which form TE-mode resonators is different from that of the dielectric which forms a dielectric coupling window.

With the shape and dimensions of the dielectric coupling window not being changed, when the dielectric constant of the dielectric which forms the dielectric coupling window is set larger than that of the dielectrics which form the resonators, the amount of coupling between adjacent resonators increases and thereby the pass-band width of the dielectric filter increases. Conversely, with the shape and dimensions of the dielectric coupling window not being changed, when the dielectric constant of the dielectric which forms the dielectric coupling window is set smaller than that of the dielectrics which form the resonators, the amount of coupling between adjacent resonators decreases and thereby the pass-band width of the dielectric filter is reduced.

According to the present invention, by making the dielectric constant of the dielectric which forms the dielectric coupling window different from that of the dielectrics which form the TE-mode resonators, a dielectric filter or a dielectric duplexer having the same central frequency and a different pass-band width is obtained without changing the shape and dimensions of the dielectric coupling window.

As a result, a dielectric filters having the same central frequency and different pass-band widths and having the same dimensions and the same shape can be obtained by the use of the same forming metal forming die. Therefore, number of metal the types of forming dies is substantially reduced, and thereby the manufacturing cost also reduced.

FIG. 1 is a perspective view of a dielectric filter according to a first embodiment of the present invention.

FIG. 2 is an exploded perspective view of the dielectric filter shown in FIG. 1.

FIG. 3 is an exploded perspective view of a dielectric filter according to a second embodiment of the present invention.

FIG. 4 is an exploded perspective view of a dielectric duplexer according to a third embodiment of the present invention.

FIG. 5 is a perspective view of the dielectric duplexer of FIG. 4.

FIG. 6 is a perspective view of a dielectric filter according to a fourth embodiment of the present invention.

FIG. 7 is an exploded perspective view of a dielectric filter according to a fifth embodiment of the present invention.

FIG. 8 is an exploded perspective view of a dielectric filter according to a sixth embodiment of the present invention.

FIG. 9 is a perspective view of a dielectric filter according to a seventh embodiment of the present invention.

FIG. 10 is an enlarged sectional view of the coupling adjustment holes of the dielectric filter of FIG. 9.

FIG. 11 is a perspective view of a conventional dielectric filter.

Dielectric filters according to embodiments of the present invention will be described below by referring to the accompanying drawings. In each embodiment, the same symbols are assigned to the same components or the same portions.

FIG. 1 is a perspective view of a dielectric filter according to a first embodiment of the present invention. A dielectric filter 10 has a rectangular cross section and includes three TE.sub.10 -mode resonators 1a, 1b, and 1c, and two dielectric coupling windows 2a and 2b. The resonators 1a, 1b, and 1c are connected in series as a unit with the dielectric coupling windows 2a and 2b disposed therebetween. In order to set the central frequency of the resonators 1a, 1b, and 1c to the desired value, the length of each of the resonators 1a, 1b, and 1c is set to half the wavelength .lambda. of the central-frequency signal.

The resonators 1a, 1b, and 1c and the dielectric coupling windows 2a and 2b are formed as follows: As shown in FIG. 2, dielectrics 11 for the resonators 1a, 1b, and 1c and dielectrics 21 for the dielectric coupling windows 2a and 2b are prepared. The dielectrics 11 and 21 are made from dielectric powder kneaded with a binder to make slurry, shaped, and dried. The dielectric 11 and 21 have different dielectric constants. The dielectrics 11 and 21 are disposed in a metal forming die such that the dielectrics 11 sandwich the dielectrics 21. Heat and pressure are applied to the dielectrics 11 and 21 by the metal forming die. They are baked as a unit to form a rectangular-parallelepiped dielectric block 31. As shown in FIG. 1, an outer conductive member 5 is formed so as to cover almost the entire surface of the baked dielectric block 31. At respective ends of the dielectric block 31, input and output electrodes 7 and 8 are formed which are not electrically connected to the outer conductive member 5 and with a gap between the electrodes and the outer conductive member 5.

As described above, the resonators 1a, 1b, and 1c are formed of the dielectrics 11 and the outer conductive member 5 provided on the outer surface of the dielectrics 11, and the dielectric coupling windows 2a and 2b are formed of the dielectrics 21 and the outer conductive member 5 provided on the outer surface of the dielectrics 21. In the first embodiment, the width W1 of the dielectric coupling windows 2a and 2b is set equal to the width W2 of the resonators 1a, 1b, and 1c.

The obtained dielectric filter 10 has a different structure from that of the conventional multiple-stage filter in which dielectrics having a low dielectric constant sandwich a TEM-mode coaxial resonator independent in each stage. In other words, the TE.sub.10 -mode resonators 1a to 1c in the dielectric filter 10 function in the same way as a waveguide serving as a transfer area. The dielectric coupling windows 2a and 2b of the dielectric filter 10 function in the same way as a waveguide serving as a blocking area. In a waveguide, it is necessary in general to partition the waveguide serving as a transfer area at both end faces with electromagnetic boundaries having a large reflection coefficient in order to trigger resonance.

Therefore, if the dielectric coupling windows 2a and 2b of the dielectric filter 10 are made from a dielectric material having the same dielectric constant as that of a dielectric material forming the resonators 1a to 1c, boundaries having a large reflection coefficient are provided at both ends of the dielectric filter 10, namely, at the outer end face of each of the resonators 1a and 1c. The dielectric filter 10 serves as a filter having a one-stage resonator.

On the other hand, in the dielectric filter 10 according to the first embodiment, the dielectric coupling windows 2a and 2b are made from a dielectric material having a different dielectric constant from that of a dielectric material forming the resonators 1a to 1c. The resonators 1a to 1c sandwich members having a different dielectric constant, and thereby the resonators 1a to 1c function as resonators and the filter serves as a filter having a three-stage resonator. As described above, the dielectric coupling windows 2a and 2b function as electromagnetic boundaries having a large reflection coefficient for the resonators 1a to 1c as well as being electromagnetically coupled with the resonators 1a to 1c.

In the dielectric filter 10 configured as described above, when the dielectric constant of the dielectrics 21 which form the dielectric coupling windows 2a and 2b is set larger than that of the dielectrics 11 which form the resonators 1a, 1b, and 1c, the amount of coupling between the resonators 1a and 1b and that between the resonators 1b and 1c increase. Therefore, the pass-band width of the dielectric filter 10 increases. Conversely, when the dielectric constant of the dielectrics 21 which form the dielectric coupling windows 2a and 2b is set smaller than that of the dielectrics 11 which form the resonators 1a, 1b, and 1c, the amount of coupling between the resonators 1a and 1b and that between the resonators 1b and 1c are reduced, and thereby the pass-band width of the dielectric filter is reduced. As a result, even if the shape and the dimensions of the dielectric coupling windows 2a and 2b are not changed, when the dielectric constant of the dielectrics 21 which form the dielectric coupling windows 2a and 2b is made different from that of the dielectrics 11 which form the resonators 1a, 1b, and 1c, the dielectric filter 10 has the same central frequency and a different pass-band width. Since dielectric filters having the same central frequency and different pass-band widths can be manufactured by the use of the same metal forming metal die, the number of types of forming dies that are required is substantially reduced, and the manufacturing cost can thereby be reduced.

FIG. 3 is an exploded perspective view of a dielectric filter according to a second embodiment of the present invention. A dielectric filter 10 includes three TE.sub.10 -mode resonators 1a, 1b, and 1c, and two dielectric coupling windows 2a and 2b. The resonators 1a, 1b, and 1c are connected in series with the dielectric coupling windows 2a and 2b disposed therebetween. The dielectric filter 10 has substantially the same structure as that shown in FIG. 1 in the first embodiment.

The resonators 1a, 1b, and 1c, and the dielectric coupling windows 2a and 2b are formed as follows: Unbaked dielectrics 11 for the resonators 1a, 1b, and 1c and unbaked dielectrics 21 for the dielectric coupling windows 2a and 2b are prepared. The dielectrics 11 and 21 have different dielectric constants. The unbaked dielectrics 11 and 21 are put into a metal forming die separately. Heat and pressure are applied to the dielectrics 11 and 21 through the metal forming die to bake them. By changing the dielectric constant of the dielectrics 21 which form the dielectric coupling windows 2a and 2b to make it different from that of the dielectrics 11 which form the resonators 1a, 1b, and 1c, even if the shape and the dimensions of the dielectrics 21 are not changed, the dielectric filter 10 has the same central frequency and a different pass-bandwidth. Since, the dielectrics 21 can be manufactured with the use of the same metal forming die, the required number of types of metal forming dies is substantially reduced.

An outer conductive member 5 is formed so as to cover the baked dielectrics 11 completely except for surfaces to be in contact with the dielectrics 21 and portions where input and output electrodes 7 and 8 are to be formed. An outer conductive member 5 is also formed so as to cover the baked dielectrics 21 completely except for surfaces to be in contact with the dielectrics 11.

The dielectric coupling windows 2a and 2b are disposed between the resonators 1a, 1b, and 1c. The resonators 1a, 1b, and 1c are bonded to the dielectric coupling windows 2a and 2b with insulating adhesive such as glass glaze applied to each contact surface to form a dielectric filter. The dielectric filter may be formed such that the baked dielectrics 11 and 21 are bonded together with insulating adhesive in advance to form a rectangular-parallelepiped dielectric block, and then the outer conductive member 5 is formed on the surface of the dielectric block so as to cover almost the entire dielectric block.

A dielectric duplexer according to a third embodiment will be described below, which is for use in a mobile communication unit such as an automobile phone or a portable phone. As shown in FIG. 4, a dielectric duplexer 60 includes three TE.sub.10 -mode resonators 61a, 61b, and 61c, and two dielectric coupling windows 62a and 62b. The resonators 61a to 61c are connected in series as a unit with the dielectric coupling windows 62a and 62b disposed therebetween. The width W1 of the dielectric coupling windows 62a and 62b is set smaller than the width W2 of the resonators 61a to 61c. It is needless to say that the width W1 of the dielectric coupling windows 62a and 62b may also be set equal to the width W2 of the resonators 61a to 61c.

The resonators 61a to 61c and the dielectric coupling windows 62a and 62b are formed as follows: Unbaked dielectrics 71a to 71c for the resonators 61a to 61c and unbaked dielectrics 81a and 81b for the dielectric coupling windows 62a and 62b are prepared. The dielectric constant of the dielectrics 81a and 81b is different from that of the dielectrics 71a to 71c. In the dielectric 71b, two external coupling holes 72a and 72b passing through the upper and lower surfaces thereof are formed. Lead through holes 73a and 73b perpendicular to the two external coupling holes 72a and 72b, respectively, are also formed.

The unbaked dielectrics 71a to 71c, 81a, and 81b are put into a metal forming die separately. Heat and pressure are applied to the dielectrics 71a to 71c, 81a, and 81b by forming die to bake them. By making the dielectric constant of the dielectrics 81a and 81b which form the dielectric coupling windows 62a and 62b different from that of the dielectrics 71a to 71c which form the resonators 61a to 61c, even if the shape and the dimensions of the dielectrics 81a and 81b is not changed, the resulting dielectric duplexer 60 can be made with the same central frequency and a different pass-band width. Therefore, the dielectrics 81a and 81b can be manufactured with the use of the same metal forming die, and thereby the required number of types of forming metal dies is substantially reduced.

An outer conductive member 65 is formed so as to cover the surfaces of the sintered dielectrics 71a to 71c except for portions in contact with the dielectrics 81a and 81b, a transmitting electrode Tx and a receiving electrode Rx serving as input and output electrodes, and a portion where an antenna electrode ANT is formed. Inner conductive members 63 are formed on the entire inner surfaces of the external coupling holes 72a and 72b and the lead through holes 73a and 73b. The inner conductive members 63 are electrically connected to the outer conductive member 65 at both ends of the external coupling holes 72a and 72b, and are electrically connected to the antenna electrode ANT at one end of each of the lead through holes 73a and 73b. Therefore, the external coupling holes 72a and 72b are electrically connected to the antenna electrode ANT through the lead through holes 73a and 73b, respectively. In the same way, an outer conductive member 65 is formed so as to cover the surfaces of the sintered dielectrics 81a and 81b except for the portions in contact with the dielectrics 71a to 71c.

As shown in FIG. 5, the dielectric coupling windows 62a and 62b are disposed between the resonators 61a, 61b, and 61c. The resonators 61a to 61c are bonded to the dielectric coupling windows 62a and 62b with insulating adhesive such as glass glaze applied to each contact surface to form the dielectric duplexer 60. The dielectric duplexer may be formed by first bonding the sintered dielectrics 71a to 71c, 81a, and 81b together with insulating adhesive in advance to form a unit, and then forming the outer conductive member 65.

The dielectric duplexer 60 having the above structure includes a transmission filter (bandpass filter) 68A formed of the resonator 61a, the dielectric coupling window 62a, and approximately the left-hand half of the resonator 61b, and a receiving filter (bandpass filter) 68b formed of the resonator 61c, the dielectric coupling window 62b, and approximately the right-hand half of the resonator 61b. This dielectric duplexer 60 outputs a signal received from the antenna electrode ANT, through the receiving filter 68B from the receiving electrode Rx to a receiving circuit system not shown, and also outputs a transmission signal input from a transmission circuit system not shown to the transmitting electrode Tx, through the transmission filter 68A to the antenna electrode ANT.

A dielectric filter and a dielectric duplexer according to the present invention are not limited to the above embodiments. They can be changed in various ways within the scope of the invention.

In the first and second embodiments, the width W1 of the dielectric coupling windows 2a and 2b is set equal to the width W2 of the resonators 1a, 1b, and 1c. The setting of the widths is not so limited. As shown in FIGS. 6 and 7, the width W1 of the dielectric coupling windows 2a and 2b may be set smaller than the width W2 of the resonators 1a, 1b, and 1c. Also in this case, by making the dielectric constant of the dielectrics 21 which form the dielectric coupling windows 2a and 2b different from that of the dielectrics 11 which form the resonators 1a, 1b, and 1c, the dielectric filter 10 having the same central frequency and a different pass-band width is obtained without changing the shape and the dimensions of the dielectric coupling windows 2a and 2b.

Electrically conductive adhesive such as silver paste and solder paste may be used to bond the resonators 1a, 1b, and 1c to the dielectric coupling windows 2a and 2b in the second embodiment. In this case, as shown in FIG. 8, for example, electrically conductive adhesive 34 is applied to hatched areas, excluding circular windows 35, on both contact surfaces of each of the dielectric coupling windows 2a and 2b. Through the circular windows 35, the resonators 1a, 1b, and 1c are electromagnetically coupled.

A dielectric coupling window may be provided with a coupling adjustment hole. Specifically, as shown in FIG. 9, in the dielectric filter 10 according to the first embodiment, coupling adjustment holes 15a and 15b are formed in the dielectric coupling windows 2a and 2b. The coupling adjustment holes 15a and 15b have a circular cross section and pass through from the upper surfaces to the lower surfaces of the dielectric coupling windows 2a and 2b, respectively. Inner conductive members 17 (see FIG. 10) are formed on the inner walls of the coupling adjustment holes 15a and 15b.

Parts of the inner conductors 17 are removed to form portions 16a and 16b without inner conductors thereon, and thereby characteristics such as a pass-band width are adjusted. To adjust the pass-band width, characteristics of the dielectric filter 10 such as the pass-band width are measured first. Then, according to the measurement results, as shown in FIG. 10, a cutting tool 18 such as a router is inserted into the coupling adjustment holes 15a and 15b through the openings of the holes 15a and 15b, and the desired parts of the inner conductors 17, which are exposed on the inner surfaces of the coupling adjustment holes 15a and 15b, are removed to form the portions 16a and 16b where there are no inner conductors. Characteristics such as the pass-band width are adjusted by adjusting the sizes and positions of the portions 16a and 16b. Therefore, even after the dielectric filter 10 has been assembled, characteristics such as a pass-band width can be adjusted, further facilitating adjustment work.

The coupling adjustment holes 15a and 15b do not necessarily pass through from the upper surfaces to the lower surfaces of the dielectric coupling windows 2a and 2b. They may pass through from the front side face to the back side face, or may have their axes formed at angles with respect to the outer surfaces of the dielectric coupling windows 2a and 2b. The coupling adjustment holes 15a and 15b may have rectangular rather than circular cross-sections.

A three-stage dielectric filter in which three resonators are connected in series is described in the first and second embodiments. The number of stages is not limited to this case. It may be two, or four or more.

A dielectric filter of the present invention can have various shapes according to the specification. In addition to a rectangular cross section, the dielectric filter may have a circular cross section. It may be a coaxial line.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit of the invention.