Dielectric Filter Including at Least One Band Elimination Filter

This invention relates to a dielectric band elimination filter. More particularly, this invention relates to a dielectric band elimination filter adapted for use in a mobile communication apparatus such as a portable telephone.

As shown in FIG. 25, and also by an equivalent circuit diagram shown in FIG. 26 (wherein Z.sub.in indicates the input impedance), a prior art single-stage dielectric band elimination filter (BEF) is formed with a dielectric resonator 111 and a coupling capacitor 112 connected in series through a connector terminal 113 and having a coupling capacitance value of Ce (see FIG. 26). Its frequency-attenuation characteristic is shown in FIG. 27. FIG. 28 is a block diagram of a prior art mobile communication apparatus such as a portable telephone, with its transmitter side (T.sub.x) including an isolator I.sub.1, a power amplifier, a band pass filter (BPF) B, and a mixer M1 and its receiver side (R.sub.x) including a low noise amplifier, a band pass filter (BPF) and a mixer. A dielectric band elimination filter, as described above, is used in the transmitter circuit inside its duplexer D.sub.1. The transmitter frequency f.sub.TX and the receiver frequency f.sub.RX of this communication apparatus are indicated in the diagram of FIG. 27. It is adjusted such that the receiver frequency f.sub.RX and the trap frequency f.sub.T of the dielectric BEF match each other. As another example of prior art technology, a general prior art two-stage dielectric BEF is shown in FIG. 29. In FIG. 29, and FIG. 30 which is its equivalent circuit diagram, R indicates a resonator, C.sub.e indicates a trap capacitor, C.sub.t indicates a parallel capacitor, L indicates an inductor serving as a quarter-wavelength phase shifter, and numerals 121, 122, 123, 124 respectively indicate a case cover, a connector terminal, an inductor pattern substrate, and a common substrate as depicted in FIG. 29. FIG. 31 Shows the frequency-attenuation characteristic of this dielectric BEF, and also indicates the transmitter frequency f.sub.TX and the receiver frequency f.sub.RX when this filter is used in the transmitter side of the duplexer D.sub.1 of the communication apparatus shown in FIG. 28. In this application, too, the receiver frequency f.sub.RX and the trap frequency f.sub.T of the filter are adjusted to match each other.

With prior art mobile communication apparatus as described above, waves with frequency f.sub.s =f.sub.TX -(f.sub.RX -f.sub.TX) entering from the antenna A.sub.1 (see FIG. 28) into the transmitter side of the duplexer D.sub.1 cannot be stopped by a dielectric BEF with attenuation characteristic as given in FIG. 27 or FIG. 31 alone. This is why an isolator I.sub.1 is inserted into the transmitter circuit as shown in FIG. 28. In addition, a band pass filter (BPF) B.sub.1 is required in the transmitter circuit in order to attenuate waves with unwanted frequencies generated in the mixer M.sub.1 on the transmitter side.

Problems of this kind would not occur if a dielectric BPF were used in the place of the dielectric BEF in the transmitter circuit of the duplexer D.sub.1, but there would arise a different problem that insertion loss and attenuation characteristics obtainable by a dielectric BEF cannot be fully realized by a filter of a comparable size. In order to form a single-stage dielectric BEF as described above, furthermore, not only a dielectric resonator but also a coupling capacitor and a connector terminal for connecting the dielectric resonator and the coupling capacitor would be needed. Similarly, in order to form a two-stage dielectric BEF as described above, not only a dielectric resonator but also extra component parts such as a case cover, connector terminals, an inductor pattern substrate and a common substrate would be needed. In short, the number of required component parts and cost would increase, and the apparatus would become bulkier.

It is therefore an object of this invention to eliminate the problems described above and to provide dielectric BEFs which are capable of simplifying the circuit structure of mobile communication apparatus such as portable telephones, having only a small number of component parts and being compact in size.

A single-stage dielectric band elimination filter embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a dielectric block having its outer surfaces mostly covered by an outer conductor and two mutually coupled resonant lines formed therein, each having an open end which is insulated from the outer conductor and a shorted end which is connected to the outer conductor, and the open and shorted ends of the two resonant lines being oppositely oriented. The resonant lines are formed by providing inner conductors on the inner surfaces of throughholes formed through the block. The open ends of the resonant lines may be at end surfaces of the block where the throughholes open or at conductor-free portions of the inner surfaces of the throughholes.

A multi-stage dielectric filter embodying this invention may be characterized as having a plurality of single-stage band elimination filters formed inside a dielectric block, each of these single-stage filters being formed with an interdigitally coupled pair of resonant lines, each mutually adjacent pair of the single-stage band elimination filters being inter-digitally coupled or combline-coupled to each other with phase shift of II/2 therebetween.

Each single-stage band elimination filter may be structured as described above, each of its two resonant lines having an open end and a shorted end, and their open and shorted ends being oriented oppositely. Each open end may be formed at one of the end surfaces of the dielectric block, being connected to an electrode on the end surface and insulated from the outer conductor, or at an annular conductor-free area formed on the inner surface of the corresponding throughhole.

The resonant lines for forming the plurality of single-stage band elimination filters may be arranged in various ways. They may be arranged in two horizontal rows (the upper and lower rows) and many vertical columns, those on the upper and lower rows in each column forming a single-stage filter. With this arrangement of the resonant lines, all of the resonant lines on the upper row may be arranged to have their open ends pointing towards one of the end surfaces of the dielectric block, those on the lower row pointing to the other end surface. Alternatively, the resonant lines may be so arranged that the open ends of two mutually adjacent resonant lines on the same row are always oriented in opposite directions. Screening electrodes may be inserted between resonant lines which are next to each other on the same row.

The pairs of resonant lines forming single-stage band elimination filters need not all be arranged in the same direction. Two such filters with horizontally arranged resonant line may sandwich one with vertically arranged resonant line inside a horizontally elongated dielectric block with an upwardly protruding center part for forming therein one of the resonant lines for the vertically arranged filter.

The throughholes for containing the resonant lines may have a flattened shape such that the dielectric block can be made thinner.

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 is a schematic diagonal view of a single-stage dielectric BEF according to a first embodiment of this invention;

FIG. 2 is a circuit structure diagram of the filter of FIG. 1;

FIG. 3 is an equivalent circuit diagram of the filter of FIG. 1;

FIG. 4 is a sectional view of the filter of FIG. 1 taken along line 4--4 in FIG. 1 for explaining equivalent capacitances;

FIG. 5 is a diagram showing the input impedance characteristic of the filter of FIG. 1;

FIG. 6 is a frequency-attenuation characteristic of the filter of FIG. 1;

FIG. 7 is a schematic diagonal view of another single-stage dielectric BEF according to a second embodiment of this invention;

FIG. 8 is a schematic diagonal view of still another single-stage dielectric BEF according to a third embodiment of this invention;

FIG. 9 is a schematic diagonal view of a two-stage dielectric BEF according to a fourth embodiment of this invention;

FIG. 10 is a circuit structure diagram of the filter of FIG. 9;

FIG. 11 is an equivalent circuit diagram of the filter of FIG. 9;

FIG. 12 is a frequency-attenuation characteristic diagram for the filter of FIG. 9 and a prior art filter;

FIG. 13 is a schematic diagonal view of a five-stage dielectric BEF according to a fifth embodiment of this invention;

FIG. 14 is an equivalent circuit diagram of the filter of FIG. 13;

FIG. 15 is a schematic diagonal view of another five-stage dielectric BEF according to a sixth embodiment of this invention;

FIG. 16 is an equivalent circuit diagram of the filter of FIG. 15;

FIG. 17 is a schematic diagonal view of still another five-stage dielectric BEF according to a seventh embodiment of this invention;

FIG. 18 is a schematic diagonal view of a three-stage dielectric BEF according to an eighth embodiment of this invention;

FIG. 19 is a circuit structure diagram of the filter of FIG. 18;

FIG. 20 is an equivalent circuit diagram of the filter of FIG. 18;

FIG. 21 is a schematic diagonal view of another three-stage dielectric BEF according to a ninth embodiment of this invention;

FIG. 22 is a schematic diagonal view of still another three-stage dielectric BEF according to a tenth embodiment of this invention;

FIG. 23 is a schematic diagonal view of a six-stage dielectric BEF according to an eleventh embodiment of this invention;

FIG. 24 is a schematic diagonal view of a three-stage dielectric BEF according to a variation of the tenth embodiment of this invention;

FIG. 25 is an exploded diagonal view of a prior art single-stage dielectric BEF;

FIG. 26 is a circuit structure diagram of the prior art filter of FIG. 25;

FIG. 27 is a frequency-attenuation characteristic diagram of the prior art filter of FIG. 25;

FIG. 28 is a block circuit diagram of a mobile communication apparatus such as a portable telephone, using a prior art dielectric BEF;

FIG. 29 is an exploded diagonal view of another prior art dielectric BEF;

FIG. 30 is a circuit structure diagram of a general prior art dielectric filter; and

FIG. 31 is a frequency-attenuation characteristic diagram of the prior art filter of FIG. 29.

Throughout herein, the same or equivalent components and concepts are indicated by the same symbols and may not be repetitiously described what they are with respect to each of the figures in which they appear.

FIG. 1 shows a single-stage dielectric BEF according to a first embodiment of this invention, formed by a combination of two resonant lines. A rectangular dielectric block 101 has two circular cylindrical throughholes 102a, 103a formed near each other from one end surface to the opposite end surface. Inner conductors are formed on the inner surfaces of the throughholes 102a, 103a. The inner conductor inside throughhole 102a is connected to a rectangular electrode 102b on one of the end surfaces (first end surface) of the dielectric block 101. The inner conductor inside throughhole 103a is connected to another rectangular electrode 103b on the other end surface (second end surface) of the dielectric block 101. The outer surfaces of the dielectric block 101 are substantially entirely covered by an outer conductor, excluding conductor-free (or dielectric-exposing) areas 102c, 103c surrounding the electrodes 102b, 103b. The inner conductor inside throughhole 102a is connected to the outer conductor on the second end surface of the dielectric block 101 to form a quarter-wavelength resonant line 102. The inner conductor inside throughhole 103a is connected to the outer conductor on the first end surface of the dielectric block 101 to form another quarter-wavelength resonant line 103. The conductor-free areas 102c, 103c serve as open ends of these quarter-wavelength resonant lines 102, 103. It is to be noted that these two resonant lines 102, 103 are in a point-symmetric relationship with respect to the dielectric block 101.

FIG. 2 shows the circuit structure of the filter described above connected to a communication line between an input terminal and an output terminal, its equivalent circuit diagram being shown in FIG. 3, and FIG. 4 is a sectional view of the filter taken along line 4--4 in FIG. 1 to show how equivalent capacitors are formed. As indicated in FIG. 4, self-capacitance C.sub.11 per unit length is formed between each of the resonant lines 102, 103 and the outer conductor, and mutual capacitance C.sub.12 is formed between the two resonant lines 101, 103. In FIGS. 2 and 3, Z.sub.in indicates the input impedance. In FIG. 3, Z.sub.e and Z.sub.o respectively indicate the even-mode and odd-mode characteristic impedance give by:

where .epsilon..sub.r is the specific dielectric constant and v.sub.c is the speed of light. The coupling characteristic impedance Z.sub.k is defined as: ##EQU1## The phase angle .theta. is given by:

where .omega. indicates the angular frequency (or .omega.=2IIf where f is the frequency), and L indicates the length of each resonant line.

In FIG. 3, the equivalent circuit diagram shows a parallel connection of the even-mode characteristic impedance Z.sub.e and a series connection of the coupling characteristic impedance Z.sub.k and the even-mode characteristic impedance Z.sub.e between the input (output) and the ground.

FIG. 5 shows the input impedance characteristic of this filter, and FIG. 6 shows its frequency-attenuation characteristic. As shown in FIG. 5, the input impedance increased with frequency and reaches infinity, giving rise to the first peak shown in FIG. 6 on the lower-frequency side of the lower attenuation pole which corresponds to the first zero of the curve in FIG. 5. The next infinity point in FIG. 5 gives rise to the second peak shown in FIG. 6 on the higher-frequency side of the attenuation pole. The next zero on the curve of FIG. 5 corresponds to the higher-frequency attenuation pole shown in FIG. 6. In FIG. 6, the trap frequency f.sub.T is given by:

In FIG. 6, the solid line is for this invention; the broken line is for a prior art example shown in FIG. 27. FIG. 6 shows that increased attenuation is obtained by the present invention both in regions (indicated by double-headed arrows) on the higher frequency and lower frequency sides of the trap frequency.

If a single-stage dielectric BEF according to this invention having such a frequency-attenuation characteristic is used in the transmitter circuit of the duplexer D.sub.1 of the mobile communication apparatus shown in FIG. 28, it becomes possible to eliminate the isolator I.sub.1 for preventing unwanted waves from passing through the antenna A.sub.1 into the transmitter side of the duplexer D.sub.1 because sufficient attenuation is obtained on the lower frequency side of the trap frequency. Since attenuation is obtained both on the lower and higher frequency sides of the trap frequency, furthermore, the BPF B.sub.1 for attenuating waves with unwanted frequencies generated by the mixer M.sub.1 on the transmitter side can be either eliminated or replaced by a smaller, less costly BPF with fewer stages. Moreover, since the dielectric BEF according to this invention is formed with a single dielectric block providing its trap circuit by a mutually coupling pair of resonant lines, there is no need for a coupling capacitor to be connected or any connector terminal. In other words, the number of component parts can be reduced.

FIG. 7 shows another single-stage dielectric BEF according to a second embodiment of this invention, which is similar to the one described above except that the electrodes 103b for resonant line 103 and the outer conductor are removed from the second end surface. Components which are substantially identical or function substantially identically to those of the filter shown in FIG. 1 are indicated by the same numerals and are not repetitively described below.

The filter according to the second embodiment shown in FIG. 7 functions substantially like the first embodiment and is advantageous in that the number of electrode patterns is reduced and hence that it can be produced at a reduced cost.

FIG. 8 shows still another single-stage dielectric BEF according to a third embodiment of this invention, which is similar to the first embodiment described above with reference to FIG. 1 and of which components substantially identical or at least similar to those of the first embodiment are indicated in FIG. 8 by the same numerals. The third embodiment is different from the first embodiment in that two inner conductors (first and second inner conductors) are formed inside one of the throughholes (104a). One end of the first inner conductor is connected to the outer conductor on the first end surface of the dielectric block 101. One end of the second inner conductor is connected to the outer conductor on the second end surface of the dielectric block 101. Between the other ends of the two inner conductors, there is an annular conductor-free (or dielectric-exposing) area 104b formed on the inner surface of the throughhole 104a near the second end surface. The longer one of the inner conductors (or the first inner conductor in FIG. 8) serves as the resonant line 104, having its open end inside the throughhole 103a.

Although not separately illustrated in a figure, the conductor-free area 104b may be formed adjacent to the second end surface (there being no second inner conductor), as a variation of the third embodiment.

A single-stage filter according to the third embodiment of the invention also has functions similar to the first embodiment and is advantageous wherein it has better shielding effects because the outer surfaces of the dielectric block 1 are completely covered by the outer conductor except at the input and output portions.

FIG. 9 shows a two-stage dielectric BEF according to a fourth embodiment of this invention, comprising a rectangular dielectric block 10 having four circular cylindrical throughholes 1a, 2a, 3a, 4a formed therethrough near one another from one end surface to the opposite end surface of the block 10. Inner conductors are formed on the inner surfaces of the throughholes 1a, 2a, 3a, 4a. The inner conductor inside throughhole 2a is connected to an electrode 2b on one of the end surfaces (first end surface) of the block 10. The inner conductor inside throughhole 3a is connected to another electrode 3b on the other end surface (second end surface) of the block 10. The outer surfaces of the block 10 are substantially entirely covered by an outer conductor except conductor-free (or dielectric-exposing) areas 2c, 3c surrounding the electrodes 2b, 3b, respectively.

Inside throughhole 1a, an annular conductor-free (or dielectric-exposing) area 1c is formed on the inner surface near the second end surface. Inside throughhole 4a, another annular conductor-free (or dielectric-exposing) area 4c is formed on the inner surface near the first end surface. The inner conductors inside throughholes 1a, 3a, 4a are connected to the outer conductor on the first end surface, and the inner conductors inside throughholes 1a, 2a, 4a are connected to the outer conductor on the second end surface such that interdigital resonator lines 1, 2, 3 and 4 are formed by these throughholes 1a, 2a, 3a and 4a respectively. It is to be noted that the conductor-free areas 1c, 2c, 3c, 4c serve as open ends of the resonant lines 1, 2, 3 and 4 respectively.

Although not separately illustrated in figures, the annular conductor-free areas 1c, 4c may be formed adjacent respectively to the second end surface and to the first end surface of the block 10, as discussed with reference to the filter according to the third embodiment of the invention shown in FIG. 8.

As shown in FIG. 10, which is a circuit structure diagram of the filter shown in FIG. 9, resonant lines 1, 2 couple to each other interdigitally to together form a one-stage BEF 11, and resonant lines 3, 4 similarly couple to each other interdigitally to together form another single-stage BEF 12. These two BEFs 11, 12 are coupled to each other through a quarter-wavelength phase shifter formed between the resonant lines 2, 3 such that a two-stage dielectric BEF is formed as a whole. A dielectric BEF thus formed is capable of providing attenuation on both higher and lower frequency sides of the trap frequency, the electrodes 2b, 3b of the resonant lines 2, 3 serving as input and output lines as seen in FIG. 9. In FIG. 11, which is an equivalent circuit diagram of the filter of FIG. 9, Z.sub.e, Z.sub.k and .theta. again indicate the even-mode characteristic impedance, coupling characteristic impedance and phase shift angle of II/2, respectively. Each single-stage BEF 11, 12 is represented as a parallel connection of a series-connected parallel branch comprising (Z.sub.k, .theta.) and (Z.sub.e, .theta.) and another parallel branch comprising (Z.sub.e, .theta.). The filter shown in FIG. 9 is represented as a combination of two such single-stage BEFs connected through transmission lines Z.sub.k, .theta.).

In the frequency-attenuation characteristic diagram of FIG. 12, the solid line is for the filter described above, the broken line is for a prior art filter represented by FIG. 31. FIG. 12 shows that attenuation at the trap frequency f.sub.T is approximately the same but that increased attenuation is obtained by the present invention both on lower and higher frequency regions (shown by arrows) with respect to the trap frequency f.sub.T.

In the prior art example shown in FIG. 29, an LC-type II-circuit is adapted to serve both as a quarter-wavelength phase shifter and a low pass filter for obtaining attenuation outside the band. With an LC-type low pass filter, however, attenuation cannot be obtained on the lower frequency side, and attenuation on the higher frequency side is not sufficiently great, as compared to what is achievable by the present invention.

If a two-stage dielectric BEF having such frequency-attenuation characteristic is used in the transmitter circuit in the duplexer D.sub.1 of the mobile communication apparatus shown in FIG. 28, it is possible to eliminate the isolator I.sub.1 for preventing unwanted waves from passing through the antenna A.sub.1 into the transmitter side of the duplexer D.sub.1 because sufficient attenuation is obtained on the lower frequency side of the trap frequency. Since attenuation is obtained in fact both on the lower and higher frequency sides of the trap frequency, the BPF B.sub.1 for attenuating waves of unwanted frequencies generated by the mixer M.sub.1 on the transmitter side can be either eliminated or replaced by a smaller, less costly BPF with fewer stages.

FIG. 13 shows a five-stage combline-coupled dielectric BEF according to a fifth embodiment of this invention, comprising a rectangular dielectric block 20 having a total of ten circular cylindrical throughholes formed therethrough near one another from one end surface to the opposite end surface of the block 20, arranged geometrically in two horizontal rows such that resonant lines 21a-25a are formed in the five throughholes of the upper row and resonant lines 21b, 22b, 23b, 24b and 25b are formed in the five throughholes of the lower row.

On one of the end surfaces (first end surface) of the dielectric block 20, the resonant lines 21a-25a of the upper row each have a shorted end and the resonant lines 21b-25b of the lower row each have an open end. On the opposite end surface (second end surface) of the block 20, the resonant lines 21a-25a of the upper row each have an open end and the resonant lines 21b-25b of the lower row each have a shorted end. The outer surfaces of the dielectric block 20 are substantially entirely covered by an outer conductor excluding the open end surfaces. Inner conductors are formed on the inner surfaces of the throughholes forming the resonant lines 21a-25a, 21b-25b.

Each of pairs of upper-row and lower-row resonant lines 21a with 21b, 22a with 22b, 23a with 23b, 24a with 24b, 25a with 25b couples interdigitally to form one-stage BEFs 21, 22, 23, 24, 25. Each mutually adjacent pair of these one-stage BEFS is combline-coupled to each other according to a known mechanism. Input to and output from this dielectric filter are effected through the resonant lines 21b and 25b. An equivalent circuit diagram of this filter is shown in FIG. 14, showing single-stage BPFs, each represented as a parallel connection of a series-connected branch with (Z.sub.e, .theta.) and (Z.sub.k, .theta.) and another branch (Z.sub.e, .theta.), connected through shorted transmission lines (Z.sub.k, .theta.).

FIG. 15 shows a five-stage interdigitally coupled dielectric BEF according to a sixth embodiment of this invention, comprising a rectangular dielectric block 30 having a total of ten circular cylindrical throughholes formed therethrough near one another from one end surface to the opposite end surface of the block 30, arranged geometrically in two horizontal rows, resonant lines 31a, 32a, 33a, 34a and 35a being formed in the five throughholes of the upper row and resonant lines 31b-35b being formed in the five throughholes of the lower row. Inner conductors are formed on the inner surfaces of these ten throughholes for the resonant lines 31a-35a, 31b-35b.

Resonant lines 31a, 32b, 33a, 34b, 35a each have a shorted end on one of the end surfaces (first end surface) of the dielectric block 30 and an open end on the other end surface (second end surface). Resonant lines 31b, 32a, 33b, 34a, 35b each have an open end on the first end surface and a shorted end on the second end surface. The outer surfaces of the dielectric block 30 are substantially entirely covered by an outer conductor except at the aforementioned open ends. Each of the pairs of upper and lower resonant lines 31a with 31b, 32a with 32b, 33a with 33b, 34a with 34b, 35a with 35b couples to each other interdigitally to form a single-stage BEF 31, 32, 33, 34, 35. Each mutually adjacent pair of these one-stage BEFs is interdigitally coupled, as shown in the equivalent circuit diagram of FIG. 16. Since this equivalent circuit diagram is similar to the one explained above in FIG. 11, it is not repetitively explained here. Input to and output from this filter are effected through resonant lines 31b and 35b as depicted in FIG. 15. It is to be noted that the resonant lines 1, 2, 3 and 4 of FIG. 9 correspond respectively to resonant lines 31a, 3lb, 35b and 35a of FIG. 15. While the filter of FIG. 9 is of a two-stage type, that of FIG. 15 has five stages with three intermediate BEFs 32, 33 and 34. While the two resonant lines for each BEF are horizontally disposed with respect to each other in the filter of FIG. 9, those for each BEF of FIG. 16 are vertically separated with respect to each other. Since the only different between FIGS. 9 and 15 are the number of stages and the relative orientation of resonant lines for each BEF, the equivalent circuit shown in FIG. 16 can be explained similarly to that shown in FIG. 9.

FIG. 17 shows another five-stage combline-coupled dielectric BEF according to a seventh embodiment of this invention. This filter is similar to the one described above with reference to FIG. 13 except that screening electrodes 41 connected to the outer conductor are provided between each mutually adjacent pair of the resonant lines 21a, 22a, 23a, 24a and 25a of the upper row. In all other aspects, this filter is identical to the one shown in FIG. 13. Therefore, same numerals as used in FIG. 13 are used in FIG. 17 to indicate identical components.

FIG. 18 shows a three-stage interdigitally coupled dielectric BEF according to an eighth embodiment of this invention, comprising a rectangular dielectric block 50 having a total of six circular cylindrical throughholes formed therethrough near one another from one end surface to the opposite end surface of the block 50, arranged geometrically in two horizontal rows, resonant lines 51a, 52a and 53a being formed in the throughholes of the upper row and resonant lines 51b, 52b and 53b being formed in the throughholes of the lower row. Inner conductors are formed on the inner surfaces of these throughholes for the resonant lines 51a-53a, 51b-53b. The inner conductors of the resonant lines 51b, 52a, 53b are connected respectively to electrodes 51c, 52c, 53c on one of the end surfaces (first end surface) of the block 50 and to an outer conductor on the other end surface (second end surface). The inner conductors of the resonant lines 51a, 52b, 53a are connected respectively to electrodes 51d, 52d, 53d on the second end surface and to the outer conductor on the first end surface. The outer conductor covers the outer surfaces of the dielectric block 50 substantially entirely except conductor-free (or dielectric-exposing) areas 50a surrounding the electrodes 51c-53c, 51d-53d.

Screening electrodes 54 are provided between horizontally adjacent pairs of resonant lines of the upper row 51a with 52a, 52a with 53a for preventing coupling therebetween. Each pair of vertically adjacent resonant lines 51a with 51b, 52a with 53b, 53a with 53b of the upper and lower rows is interdigitally coupled to form single-stage BEFs 51, 52, 53. Mutually adjacent pairs of the resonant lines of the lower row 51b with 52b, 52b with 53b are interdigitally coupled with phase shift of II/2 such that the three single-stage BEFs 51, 52, 53 together form an interdigitally coupled dielectric BEF. FIG. 19 is its circuit structure diagram, and FIG. 20 is its equivalent circuit diagram. Explanations given above for FIGS. 15 and 16 should be referenced also for FIGS. 19 and 20.

FIG. 21 shows another three-stage interdigitally coupled dielectric BEF according to a ninth embodiment of this invention, comprising a rectangular dielectric block 60 having a protrusion and a total of six throughholes formed therethrough with inner conductors formed on the inner surfaces of these throughholes so as to provide six resonant lines 61a, 62a, 63a, 61b, 62b and 63b near one another. Resonant lines 62a and 62b are vertically adjacent to each other and interdigitally coupled to each other to together form a single-stage BEF 62. Pairs of resonant lines 61a with 61b, 63a with 63b are horizontally adjacent and interdigitally coupled to each other to form single-stage BEFs 61 and 63, respectively. The inner conductors of the resonant lines 61b, 62a, 63a are connected respectively to electrodes 61c, 62c, 63c on one end surface (first end surface) of the dielectric block 60 and to an outer conductor on the opposite end surface (second end surface). The inner conductors of resonant lines 61a, 62b, 63b are connected respectively to electrodes 61d, 62d, 63d on the second end surface and to the outer conductor on the first end surface. The outer conductor covers the outer surfaces of the dielectric block 60 substantially entirely except at conductor-free (or dielectric-exposing) areas 60a around the electrodes 61c-63c, 61d-63d. The three single-stage BEFs 61, 62, 63 are interdigitally coupled with phase shift of II/2 as in the preceding embodiment of the invention, forming an interdigitally coupled dielectric BEF. The circuit structure diagram and the equivalent circuit diagram of this filter are substantially the same as shown in FIGS. 19 and 20.

FIG. 22 shows still another three-stage interdigitally coupled dielectric BEF according to a tenth embodiment of this invention, comprising a rectangular dielectric block 70 having a total of six resonator-forming throughholes formed therethrough from one end surface to the opposite end surface of the block 70, arranged geometrically near one another so as to provide three circular cylindrical resonant lines 71a, 72a and 73a on an upper row and three others 71b, 72b and 73b on an lower row. Inner conductors are formed on the inner surfaces of these resonator-forming throughholes.

The inner conductors of the resonant lines 71b, 73b are respectively connected to electrodes 71c, 73c on one of the end surfaces (first end surface) of the dielectric block 70. Both ends of the inner conductors of the resonant lines 71a-73a, 71b-73b are connected to an outer conductor except at the ends of the resonant lines 71b, 73b on the first end surface. The outer conductor covers the outer surfaces of the dielectric block 70 substantially entirely except at conductor-free (or dielectric-exposing) areas 70a surrounding the electrodes 71c, 73c.

The resonant lines 71a, 72b, 73a are respectively provided with annular conductor-free (or dielectric-exposing) areas 71d, 72d, 73d near the opposite end surface (second end surface) of the dielectric block 70. The resonant line 72a is similarly provided with an annular conductor-free (or dielectric-exposing) area 72c near the first end surface of the dielectric block 70. These annular areas 71d-73d, 72c serve not only to divide the corresponding inner conductors into two parts but also as open ends of the corresponding resonant lines. Although not separately illustrated, these annular areas 71d-73d, 72c may each be formed adjacent to (rather than near) the first or second end surface.

Screening throughholes 70b are formed through the dielectric block 70 parallel to the aforementioned resonator-forming throughholes between the resonant lines 71a and 72a and also between the resonant lines 72a and 73a on the upper row. These screening throughholes 70b contain screening electrodes therein, in contact with the outer conductor at both ends so as to prevent coupling between the resonant lines 71a and 72a and between the resonant lines 72a and 73a. The vertically adjacent pairs of resonant lines 71a with 71b, 72a with 72b, 73a with 73b are interdigitally coupled to each other to form three single-stage BEFs 71, 72, 73. The mutually adjacent pairs of resonant lines on the lower row 71b with 72b, 72b with 73b are each interdigitally coupled with phase shift of II/2 such that the three single-stage BEFs 71, 72, 73 together form an interdigitally coupled dielectric BEF. FIG. 19 shows its circuit structure diagram, and FIG. 20 shows its equivalent circuit diagram.

FIG. 23 shows a six-stage interdigitally coupled BEF according to an eleventh embodiment of this invention, comprising a rectangular dielectric block 80 having a total of twelve circular cylindrical throughholes formed from one end surface to the opposite end surface of the dielectric block 80, geometrically arranged in three horizontal rows and four vertical columns, having inner conductors formed on the inner surfaces of the throughholes so as to serve as resonant lines 81a, 82a, 83a, 84a, 85a, 86a, 81b, 82b, 83b, 84b, 85b and 86b. Resonant lines 81a, 82b, 83a, 84a, 85b, 86a each have an open end on one of the end surfaces (first end surface) of the dielectric block 80 and a shorted end on the opposite end surface (second end surface) of the dielectric block 80. Resonant lines 81b, 82a, 83b, 84b, 85a, 86b each have a shorted end on the first end surface and an open end on the second end surface. The outer surfaces of the dielectric block 80 are substantially entirely covered by an outer conductor except at the aforementioned open ends. Screening electrodes 80a connected to the outer conductor are provided between mutually adjacent pair of resonant lines 81b and 86a of the lower row and between mutually adjacent pair of resonant lines 82b and 85a of the middle row. Horizontally adjacent pairs of resonant lines 81a and 81b, 82a and 82b, 83a and 83b, 84a and 84b, 85a and 85b, 86a and 86b couple to each other interdigitally within themselves to form single-stage BEFs 81, 82, 83, 84, 85, 86, respectively. Mutually adjacent pairs of these single-stage BEFs 81-86 couple interdigitally each other with phase shifts of II/2 and thereby form altogether an interdigitally coupled dielectric BEF. Input to and output from this filter are effected through the resonant lines 81b and 86a.

Although the present invention has been described above with reference to only a limited number of examples, these examples are not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of this invention. For example, throughholes, whether for forming resonant lines therein or for containing a screening electrode, need not be circular in cross-section. If the throughholes are made in the shape of a horizontally elongated rectangle of flattened ellipse, the filter as a whole can be made thinner. FIG. 24, for example, shows a variation of the filter according to the tenth embodiment of this invention shown above in FIG. 22, having all its throughholes formed in an elliptical shape. Since the filters shown in FIGS. 22 and 24 are different only in the cross-sectional shapes of their throughholes and are identical in all other aspects, same numerals are used to indicate corresponding components. In all examples, furthermore, it is to be understood that input and output connections can be formed in any known manners.

In summary, sufficient attenuation can be obtained both on the lower and higher frequency sides of the trap frequency by a dielectric BEF according to this invention. If such a filter is used in a mobile communication apparatus such as a portable telephone, it is possible to simplify the circuit structure by eliminating the isolator and the BPF which used to be necessary. Since the number of component parts becomes reduced, the production cost is also reduced. If the number of components to be soldered is reduced, reliability is improved, individual variations in characteristics are reduced among the products, and the yield is increased.