Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2000-03-17
2001-11-06
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S202000
Reexamination Certificate
active
06313721
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to high performance ceramic block filters with a novel arrangement of resonators allowing for a smaller package than possible with conventional filters. In particular, by moving one or more resonators off of the straight line bisecting the two or more other resonators, it is possible to delete one or more resonator traps while maintaining performance.
BACKGROUND OF THE INVENTION
A ceramic body with a coaxial hole bored through its length forms a resonator that resonates at a specific frequency determined by the length of the hole and the effective dielectric constant of the ceramic material. The holes are typically circular, or elliptical. Combining multiple resonators may form a dielectric ceramic filter. The holes in a filter must pass through the entire block, from the top surface to the bottom surface. This means that the depth of a hole is the exact same length as the axial length of a filter. The axial length of a filter is selected based on the desired frequency and specified dielectric constant of ceramic.
The ceramic block functions as a filter because the resonators are inductively coupled and/or capacitively coupled between every two adjacent resonators. These couplings are formed by the electrode pattern designed on the top surface of the ceramic block, plated with a conductive material such as silver or copper. More specifically and with reference to
FIGS. 10A-D
, a ceramic block
101
is shown with two holes
103
and
105
. All surfaces, except for the front open face
107
through which the two holes
103
and
105
extend, are plated with silver. Due to the size of the holes, their proximity and the conductive coating, the two holes
103
and
105
are inductively and capacitively coupled to each other. However, block
101
will not perform as a filter because these couplings cancel each other out.
To form a filter, a pattern of conductive material is printed on face
107
, as shown in FIG.
10
B. In this embodiment the patterns A and A enhance the capacitive coupling between holes
103
and
105
. While the capacitive coupling is enhanced, the inductive coupling remains substantially unaffected. This is because inductive coupling is mostly a function of the hole diameter, shape and spacing between holes. These parameters are the same in
FIGS. 10A and 10B
.
The capacitive coupling can be regulated in
FIG. 10B
by adjusting parameters L and G. By decreasing G or increasing L, the capacitive coupling is strengthened. The capacitive coupling can also be weakened such that the inductive coupling is stronger, by printing line M on open face
107
. The simple line M in
FIG. 10C
has a greater diminishing effect on the capacitive coupling of the block filter
101
, than the broken line M of FIG.
10
D.
Ceramic filters are well known in the art and are generally described for example in U.S. Pat. Nos. 4,431,977; 4,716,391; 4,954,796 and 5,783,980, all of which are hereby incorporated by reference as if fully set forth herein.
With respect to its performance, it is known in the art that the band pass characteristics of a dielectric ceramic filter are sharpened as the number of holes bored in the ceramic block are increased. The number of holes required depends on the desirable attenuation properties of the filter. Typically a simplex filter requires at least two holes and a duplexer needs more than three holes. This is illustrated in
FIG. 1
where graph
10
represents the filter response with fewer holes than graphs
12
and
14
. It is apparent that graph
14
which is the response of the filter with the most holes, is the sharpest of the three responses shown. Referring to
FIG. 2
, it can be seen that the band pass characteristic of a particular dielectric ceramic filter is also sharpened with the use of trap holes bored through the ceramic block. Solid line graph
21
represents the response of a filter without a high end trap. Dashed line graph
23
represents the response of the same filter with a high-end trap.
Trap holes, or traps as they are commonly referred to are resonators which resonate at a frequency different from the primary filter holes, commonly referred to simply as holes. They are designed to resonate at the undesirable frequencies. Thus, the holes transmit an input signal at the desirable frequencies while the traps remove the input signal at the undesirable frequencies, whether low end or high end. In this manner the characteristic of the filter is defined, i.e. high pass, low pass, or band pass. The traps are spaced from holes a distance greater than the spacing between holes so as to avoid mutual interference between the holes and traps. As shown in
FIG. 3
, whereas holes
31
are separated from each other a distance D, a distance of 2 D is placed between trap
33
and the hole nearest to trap
33
. The precise distance D is one of design choice for achieving a specified performance. D typically falls within a preferred range of 1 to 10 mm. Traditionally, the traps will be spaced from 1.5 D to 2 D from the holes.
Conventionally the holes
41
and traps
43
in a ceramic filter are positioned along a straight line, as shown in FIG.
4
. This design together with the spacing requirements addressed above limits the extent to which a filter may be reduced in size. Specifically, the performance characteristics of a given filter are a function of its width, length, number of holes and diameter of holes. The usual length is 2 to 20 mm. The width of a filter is a function of the number of holes in the filter. Typically, the width of the block filter ranges from 2 to 70 mm. Reducing the number of holes, the diameter of the holes, or the spacing between holes, will effect the performance of the filter. Accordingly, it is desirable to design a dielectric ceramic filter which can effectively reduce the size of a given filter while maintaining its given performance characteristics.
SUMMARY OF THE INVENTION
Accordingly, it is desirable to design a block filter in the smallest package possible without significant loss in performance as compared with a filter with a given number of holes, as conventionally designed. In accordance with the present invention a dielectric ceramic filter with a given performance characteristic can be reduced in size by eliminating one or more trap holes and positioning one or more holes off of the straight line bisecting at least two holes, as conventionally designed. The additional interactions between holes which are realized when one or more of the holes are positioned off the center line, provides the attenuating properties heretofore only possible with additional trap holes.
In one specific embodiment of the present invention the performance characteristics of a conventional five-hole ceramic filter can be realized in a three-hole filter wherein the holes are positioned so as to form the vertices of a triangle and within specific dimensions. The width of the resulting filter is approximately 50% of the width of the conventional five-hole filter it can replace. Other geometric arrangements are possible as well.
REFERENCES:
patent: 4431977 (1984-02-01), Sokola et al.
patent: 4450421 (1984-05-01), Meguro et al.
patent: 4716391 (1987-12-01), Moutrie et al.
patent: 4954796 (1990-09-01), Green et al.
patent: 5428325 (1995-06-01), Jachowski et al.
patent: 5675301 (1997-10-01), Nappaet et al.
patent: 5739733 (1998-04-01), Cameron
patent: 5783980 (1998-07-01), Blair et al.
patent: 593635 (1993-01-01), None
Harada Nobuhiro
Kitajima Masahiko
Nishimura Kosuke
Nguyen Patricia T.
Pascal Robert
Ube Electronics Ltd.
Waldaum Maxim H.
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