Interdigital filter

An interdigital filter comprising a plurality of resonators each comprising a resonant conductor rod coupled to each other in the even and odd modes of a transverse electromagnetic wave, wherein each resonant conductor rod is enclosed with a dielectric material so as to increase the ratio of the odd mode characteristic impedance to the even mode characteristic impedance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an interdigital filter, and more 
specifically to an interdigital band-pass filter comprising a plurality of 
resonators coupled in the even and odd modes of a transverse 
electromagnetic wave. 
2. Description of the Prior Art 
Interdigital band-pass filters have been utilized in transmitters, for 
example, because of the high quality factor Q thereof. Such an 
interdigital filter utilizes a plurality of resonators coupled to each 
other not in the harmonic modes but in the even odd modes of the 
transverse electromagnetic wave. 
FIGS. 1A and 1B show an example of a prior art interdigital band-pass 
filter, wherein FIG. 1A shows a plan view of the filter with a cover 
removed and FIG. 1B shows a sectional view of the filter taken along the 
line IB--IB. Such prior art filters may be seen, for example, in pages of 
the book entitled "Microwave Filters, Impedance-Matching Networks, and 
Coupling Structures" published by Mcgraw-Hill Book Company. Referring to 
FIGS. 1A and 1B, upper and lower conductor plates 1 and 2 are kept in 
parallel with each other spaced apart from each other by the distance H. 
The upper and lower conductor plates 1 and 2 may be made of a metal plate 
such as an aluminum plate and serve as a ground conductor. Metal spacers 
1a are provided on the lower surface of the upper conductor plate 1 at 
both sides i.e. the upper and lower sides of the upper conductor plate, as 
viewed in FIG. 1A and metal spacers 2a are provided on the upper surface 
of the lower conductor plate 2 at both sides, i.e. the upper and lower 
sides of the lower conductor plate 2 as viewed in FIG. 1A. A plurality of 
resonant conductor rods 3, 3, 3 . . . are provided between the metal 
spacers 1a and 2a so as to extend alternately from either side of the 
conductor plates 1a and 2a in the transversal direction of the conductor 
plates 1 and 2 with a predetermined distance d from each other. The length 
of the resonant conductor rods 3, 3, 3 . . . is selected to be l which is 
shorter than the width L between the spacers at both sides. As a result, a 
cut-off space 9 is formed between the terminal end of the resonant 
conductor rod 3 and the metal spacers in the opposite side of the 
conductor plates 1a and 2a. Thus, the resonant conductor rod 3, 3, 3 . . . 
are arranged in the so-called interdigital manner, as seen in FIG. 1A. 
Referring further to FIG. 1A, an input coupler 41 is provided in parallel 
with and in the vicinity of the left end resonant conductor rod 3 as 
viewed in FIG. 1A, while an output coupler 42 is provided in parallel with 
and in the vicinity of the right end resonant conductor rod 3 as viewed in 
FIG. 1A. The input coupler 41 is coupled to an input coaxial connector 51 
through an impedance matching terminal, while an output coupler 42 is 
coupled to an output coaxial connector 52 through an impedance matching 
terminal. Such an arrangement is packed to provide a complete interdigital 
filter 10. As is well known, the resonant conductor rods 3, 3, 3 . . . are 
coupled to each other in the even and odd modes of the transverse 
electromagnetic wave in such an interdigital filter 10. As a result, the 
interdigital filter 10 exhibits a resonance characteristic as shown in 
FIG. 2, wherein the ordinate shows an attenuation and the abscissa shows 
the frequency. 
Such is interdigital filter 10 as described in the foregoing was not able 
to be made compact, because the distance H, the width L and the length W 
were not able to be made small due to a restriction to a requirement in 
terms of the characteristics of the filter. Generally, it is required that 
such a filter be of a high quality factor Q which makes it difficult to 
make the effective distance H smaller than a predetermined value, inasmuch 
as a decreased distance H decreases the quality factor of the filter. In 
addition, if the distance d between the adjacent resonators and thus the 
resonant conductor rods 3 becomes too small, the degree of mutual coupling 
of the resonators become too large, which makes too broad the band width 
of the frequency characteristic of the filter. Furthermore, the width L is 
restricted because of the inherent length l of the resonant conductor rod 
3 and the cut-off space at the open end of the resonant conductor rod. A 
filter of a narrow band width could be provided by decreasing the degree 
of the mutual coupling between the resonant conductor rods. In such a 
situation, however, it is necessary to increase the distance d and thus 
the length W, which degrades a temperature characteristic although the 
quality factor remains high. More specifically, it could happen that if 
the band width is made narrow the central frequency fo could vary greatly 
by virtue of the temperature variation. Thus, in spite of a demand for a 
compact interdigital filter, there has been difficulty in miniaturizing 
such a prior art interdigital filter. Accordingly, this difficulty to in 
miniaturizing of such interdigital filters has been a hindrance to 
compactness of the whole system where such interdigital filter is 
utilized. 
SUMMARY OF THE INVENTION 
Brifely described, the present invention comprises an interdigital filter 
including a plurality of resonators each being enclosed with a dielectric 
material, with a spacing provided therebetween or a low dielectric 
material inserted therebetween, thereby to modifying the ratio of the 
characteristic impedance. 
Therefore, a principal object of the present invention is to provide a 
compact interdigital filter, wherein the above described difficulty in 
achieving compactness has been eliminated. 
Another object of the present invention is to provide an improved 
interdigital filter, wherein a plurality of resonant conductor rods are 
arranged, with each being enclosed with a dielectric material and with a 
low dielectric material portion being formed between the adjacent 
resonators. 
A further object of the present invention is to provide an improved 
interdigital filter, wherein the degree of mutual coupling between two 
adjacent resonators therein can be selected as desired with ease. 
Still a further object of the present invention is to provide an improved 
interdigital filter, wherein a temperature characteristic has been 
stabilized. 
These objects and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3A shows a perspective view of an interdigital filter in accordance 
with the present invention, FIG. 3B shows a plan view of the FIG. 3A 
filter, with a cover removed, and FIG. 3 shows a sectional view of the 
inventive filter taken along the line III--III in FIG. 3B. The embodiment 
shown comprises the upper and lower conductor plates 1 and 2, and a 
plurality of resonant conductor rods 3, 3, 3 . . . arranged between the 
upper and the lower conductor plates 1 and 2 so as to extend alternately 
from either side of the conductors 1 and 2 in the transversal direction 
with a predetermined distance from each other. Each of the resonant 
conductor rods is covered with a dielectric material block 6 the 
dielectric material comprising a titanium oxide group ceramic or 
forsterite. The dielectric material block 6 is shaped in a square 
parallelepiped. An electrode 6a is formed on the top surface of the 
dielectric material block 6 and another electrode 6b is formed on the 
bottom surface of the dielectric material block 6. These electrodes 6a and 
6b are in electrical contact with the upper and the lower conductor plates 
1 and 2 in the assembled state. In order to improve such electrical 
contact, the dielectric material block 6 formed with the electrodes 6a and 
6b may be fired in an electric furnace. Such a combination of one resonant 
conductor rod 3 and the dielectric material 6 constitutes a single 
resonator. In the embodiment shown, such resonators are arranged so as to 
be coupled to each other in the even and odd modes. In the embodiment 
shown, these resonators are arranged with a spacing 7 between two adjacent 
resonators. 
In accordance with the present invention, each of the the resonant 
conductor rods 3, 3, 3 . . . is surrounded by or enclosed with the 
corresponding dielectric material blocks 6, 6, 6 . . . of a square 
parallelepiped. As a result, the characteristic impedance Zo, i.e. the 
characteristic impedance Zoo for the odd mode and the characteristic 
impedance Zoe for the even mode, between the respective adjacent 
resonators becomes small as a whole. On the other hand, enclosing the 
respective resonant conductor rods with a dielectric material increases 
the degree of mutual coupling between the adjacent resonators. Therefore, 
if and when the respective resonant conductor rod is simply covered with a 
dielectric material, only the length l of the resonant conductor rod 3 can 
be made small, assuming that a quarter wave resonator of the same 
characteristic is to be constituted. On the contrary, however, the degree 
of mutual coupling is increased, which necessitates broadening of the 
distance d between the adjacent resonant conductor rods, with the result 
that the length W may be increased. In accordance with the present 
invention, therefore, the above described spacing 7 is formed, in order to 
decrease the degree of mutual coupling between the adjacent resonators and 
thus in order to make the same or to make narrow the band width which is 
dependent on the coupling. Thus, the fact that each resonator is formed in 
a dielectric resonator and a spacing is formed between adjacent resonators 
decreases the degree of mutual coupling of the resonators. 
In general, the degree of coupling is determined by the ratio of the 
characteristic impedance Zoo in the odd mode of the transverse 
electromagnetic wave to the characteristic impedance Zoe in the even mode 
of the transverse electromagnetic wave. Therefore, if and when the above 
described spacing 7 is formed between the adjacent resonators, only the 
even mode characteristic impedance Zoe is decreased while the odd mode 
characteristic impedance Zoo is not substantially changed, with the result 
that the ratio can be increased as a whole. Accordingly, the degree of 
coupling which is dependent on the ratio of the characteristic impedances 
is decreased and thus the band width becomes narrow. This means that if 
and when the same band width characteristic is maintained the distance d 
between the adjacent resonator conductor rods can be decreased and if and 
when the same distance d is maintained the band width can be narrowed. 
Furthermore, since the resonator is formed with a dielectric material, an 
adverse affect of the coefficient of linear expansion of the respective 
metalic conductors can be eliminated by properly selecting the temperature 
coefficient of the dielectric material 6, with the result that the 
temperature characteristic of the filter is extremely improved 
Accordingly, even if the filter is implemented in a narrower band width, 
there is no fluctuation of the resonance frequency and hence a stabilized 
operation can be achieved. 
FIG. 4 shows a sectional view of another embodiment of the present 
invention. In comparison with the FIG. 3 embodiment, the embodiment shown 
in FIG. 4 has protuberances 1b and 2b protruded from the upper and lower 
conductor plates 1 and 2 at the position of the spacing 7 for the purpose 
of adjusting the degree of mutual coupling between the adjacent 
resonators. Since the remaining portions in the FIG. 4 embodiment are the 
same as those depicted in FIG. 3, it is not believed necessary to describe 
them again in more detail. Since the protuberances 1b and 2b formed at the 
position of the spacing 7 achieves adjustment of the degree of mutual 
coupling, the distance between the adjacent resonators can be further 
decreased. 
FIG. 5A shows a sectional view of a further embodiment of the present 
invention taken along the line VA--VA in FIG. 3B and FIG. 5B shows a 
sectional view of the FIG. 5A embodiment taken along the line VB--VB in 
FIG. 5A. The embodiment shown comprises a coupling adjusting screw 11 
provided through a rear cover 10a of the package of the filter 10 such 
that the screw 11 is protruded into the cut-off space 9 of the resonator 
at the position intermediate the adjacent resonators. According to the 
embodiment shown, the degree of mutual coupling between the adjacent 
resonators can be adjusted as desired as a function of the amount of 
protrusion of the screw 11 into the cut-off space 9 at the position 
intermediate the adjacent resonators. Accordingly, the embodiment shown in 
FIGS. 5A and 5B may be employed also in the FIG. 4 embodiment. 
FIG. 6 shows a sectional view of still a further embodiment of the present 
invention. In comparison with the embodiments described in the foregoing, 
the FIG. 6 embodiment comprises a dielectric material 12 which is 
integrally formed to the respective resonators such that each of the 
resonant conductor rods 3 is covered with the dielectric material 12 at 
the corresponding portions. Such continuous dielectric block 12 is 
provided with conductor plates 12a and 12b on the upper and lower surfaces 
of the dielectric material 12, by means of a firing process of a silber 
paste for example, whereby the conductor plates 12a and 12b are in 
electrical contact with the corresponding conductors 1 and 2 in the 
completed filter. One feature to be noted in the FIG. 6 embodiment is that 
the gaps 12c are formed at the positions intermediate the adjacent 
resonators so as to correspond to the spacings 7 in the embodiments 
described previously. It has been observed that an integral dielectric 
material block common to all the resonators with the gaps 12c formed at 
the positions intermediate the adjacent resonators also provides 
substantially the same characteristic as that attained in the embodiments 
described previously. According to the FIG. 6 embodiment, the dielectric 
material block 12 can be fabricated as a single block, which simplifies 
the manufacturing process. 
In the embodiments described in the foregoing, a gap or a spacing was 
formed in the dielectric material covering the resonant conductor rods at 
the position intermediate the adjacent resonators, in order to form a low 
dielectric portion therein. Alternatively, however, separate dielectric 
pieces having a smaller dielectric coefficient may be inserted in such gap 
or spacing. It is further pointed out that the embodiments described in 
conjunction with FIGS. 3 through 6 may be properly combined in practicing 
the present invention. 
FIG. 7 shows a sectional view of still a further embodiment of the present 
invention and FIG. 8 shows a perspective view of only a single resonator 
for use in the FIG. 7 embodiment. In comparison with the FIG. 3 
embodiment, the embodiment shown in FIGS. 7 and 8 includes the following 
features. One feature to be noted is that side wall electrodes 6d and 6e 
are formed on the side surfaces such that the side surface electrode 6d is 
in electrical contact with the upper electrode 6a and the side surface 
electrode 6e is in electrical contact with the lower electrode 6b, while 
an opening 17 of the width w is formed therebetween in the horizontal 
direction as viewed in FIG. 8. The opening 17 formed between the side 
surface electrodes 6d and 6e functions as an opening for mutual coupling 
of the adjacent resonators in the even and odd modes. Another feature to 
be noted in the FIGS. 7 and 8 embodiment is that a plurality of the 
electric resonators 6 are arranged so as to be contiguous to each other at 
the side surface electrodes 6d and 6e. In the embodiment shown, the mutual 
coupling between the adjacent dielectric resonators is achieved through 
the above described coupling opening 17 and the degree of coupling is 
determined as a function of the width w of the above described coupling 
opening 17. 
A further feature to be noted in the FIGS. 7 and 8 embodiment is that the 
dielectric material block 6 is formed of a central bore or aperture 
extending in the longitudinal direction of the resonator means and the 
resonant conductor is implemented as a hollow conductor layer 6c formed on 
the inner wall of the central aperture. 
The length l of the respective electric resonator is selected to be a 
quarter or a half of the wave length of the electromagnetic wave. If the 
length l of the dielectric resonator 6 is selected to be a quarter of the 
wave length, a short circuit electrode 6f is formed at the short circuit 
end of the resonator 6. 
According to the embodiment shown in FIGS. 7 and 8, mutual coupling between 
the adjacent dielectric resonators is achieved through a coupling opening 
17 formed on the side surfaces of the respective resonators, which enables 
arrangement of the resonators without such a gap between the adjacent 
dielectric resonators as seen in the embodiments shown in FIGS. 3 through 
6. As a result, the length in the longitudinal direction of the inventive 
interdigital filter can be made small. 
FIG. 9 is similar to FIG. 8 but shows a perspective view of a single 
dielectric resonator of another embodiment for alternative use in the FIG. 
7 embodiment. In comparison with the FIG. 8 embodiment, the FIG. 9 
embodiment has the side surface electrodes 6g and 6h formed on the side 
surface at the left and right end portions, with a similar coupling 
opening, 17 formed therebetween extending in the vertical direction as 
viewed in FIG. 9, although the FIG. 8 embodiment has the side surface 
electrodes 6d and 6e formed on the side surface at the upper and lower end 
portions, with a coupling opening 17 formed therebetween extending in the 
horizontal direction as viewed in FIG. 8. Because of a similar structure 
of the coupling opening, substantially the same effect is achieved by the 
FIG. 9 embodiment as that described in conjunction with the FIG. 8 
embodiment. 
Although the embodiments shown in FIGS. 7 through 9 were described as 
adapted such that the degree of coupling is adjusted as desired by 
adjusting the width w of the coupling opening 17 for mutual coupling 
between the adjacent dielectric resonators, alternatively the width d' of 
the respective resonators may be selected for the purpose of adjustment of 
the degree of coupling, while the width w is kept constant. If desired, 
the FIGS. 8 and 9 embodiments may be employed simultaneously. 
In forming the above described side surface electrodes as well as the upper 
and lower electrodes, a silber paste deposited on a ceramic in a 
predetermined pattern may be fired, or alternatively an electrode layer 
formed on the whole surface may be removed to form the coupling opening in 
a chemical manner, i.e. by means of a photoetching process, for example, 
or in a mechanical manner. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only and is not to be taken by way of limitation, 
the spirit and scope of the present invention being limited only by the 
terms of appended claims.