U.S. patent number RE34,898 [Application Number 08/139,982] was granted by the patent office on 1995-04-11 for ceramic band-pass filter.
This patent grant is currently assigned to LK-Products Oy. Invention is credited to Pauli Nappa, Aimo Turunen.
United States Patent |
RE34,898 |
Turunen , et al. |
April 11, 1995 |
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.
Inventors: |
Turunen; Aimo (Oulu,
FI), Nappa; Pauli (Kempele, FI) |
Assignee: |
LK-Products Oy (Kempele,
FI)
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Family
ID: |
26158567 |
Appl.
No.: |
08/139,982 |
Filed: |
October 19, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
532018 |
Jun 1, 1990 |
05103197 |
Apr 7, 1992 |
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Foreign Application Priority Data
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Jun 9, 1989 [FI] |
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892855 |
Jun 9, 1989 [FI] |
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892856 |
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Current U.S.
Class: |
333/206; 333/134;
333/202 |
Current CPC
Class: |
H01P
1/2056 (20130101) |
Current International
Class: |
H01P
1/20 (20060101); H01P 1/205 (20060101); H01P
001/202 (); H01P 001/213 () |
Field of
Search: |
;333/202,204-207,246,222,219.1,134 ;455/78,82 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0208424 |
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Jan 1987 |
|
EP |
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0364931 |
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Apr 1990 |
|
EP |
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0401839 |
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Dec 1990 |
|
EP |
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58-114503 |
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Jul 1983 |
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JP |
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59-101902 |
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May 1984 |
|
JP |
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59-119901 |
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Jul 1984 |
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JP |
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60-216601 |
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Oct 1985 |
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JP |
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61-161806 |
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Jul 1986 |
|
JP |
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62-120703 |
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Jun 1987 |
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JP |
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63-124601 |
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May 1988 |
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JP |
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63-311801 |
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Dec 1988 |
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JP |
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64-53601 |
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Mar 1989 |
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JP |
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64-60006 |
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Mar 1989 |
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JP |
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2139427 |
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Nov 1984 |
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GB |
|
2184608 |
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Jun 1987 |
|
GB |
|
2212671 |
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Jul 1989 |
|
GB |
|
Other References
Matthaei et al, "Microwave filters, Impedence-Matching networks,
and Coupling structures", McCraw-Hill, pp. 497-506, 1964. .
Nagle, "High Frequency Diversity Receiver from the 1930's", Ham
Radio, pp. 34-43, Apr. 1980. .
Patent Abstract of Japan-vol. 12, No. 465(E-690)[3312] Dec. 7,
1988, 1 page..
|
Primary Examiner: Ham; Seungsook
Attorney, Agent or Firm: Darby & Darby
Claims
We claim:
1. A filter comprising:
a body of dielectric material having (a) first and second surfaces
on opposite sides of the body, (b) at least two side surfaces
generally orthogonal to the first and second surfaces and
connecting the edges of the first and second surfaces to each
other, and (c) two end surfaces connecting the ends of the first,
second and side surfaces to each other;
said body defining at least one hole with an interior surface which
extends into said body from said first surface toward said second
surface;
a conductive layer covering major portions of the second surface,
one side surface of said body, both end surfaces, and the interior
surface of said hole so as to form at least one transmission line
resonator, the other side surface being generally free of said
conductive layer; and
an electrically-conductive electrode pattern means located on the
other side surface of said body for providing electrical signal
coupling to and from the transmission line resonator by creating a
field that penetrates the uncoated other side surface of the body
to the interior surface of the hole, the coupling varying from (a)
capacitive to (b) mixed capacitive and inductive to (c) inductive,
depending on the relative location of the pattern means on the side
surface between areas adjacent the first surface to areas adjacent
the second surface, respectively.
2. A filter as claimed in claim 1 wherein there are at least two
holes in the body forming at least two resonators, said pattern
means extending along the other side surface from the vicinity of
one of the resonators to the vicinity of another and providing
electrical coupling between the resonators.
3. A filter as claimed in claim 2 further including an input lead
connected to said pattern means in the vicinity of one resonator,
and an output lead connected to said pattern means in the vicinity
of another resonator so as to couple a signal into said filter on
said input lead and to couple the signal out of said filter on said
output lead.
4. A filter as claimed in claim 1 further including an
electrically-conductive plate spaced from said other side surface
by a certain gap and being electrically connected to the conductive
layer on the other surfaces of said body, said conductive plate at
least in part covering said other side surface.
5. The filter as claimed in claim 4, wherein the conductive plate
is formed by a box-like shaped metal cover located over the other
side surface so as to leave an air gap between the other side
surface and the cover.
6. The filter as claimed in claim 4, wherein said air gap is filled
with an insulating material and said conductive plate is formed by
a metal film located on the insulating layer.
7. The filter as claimed in claim 5, wherein the distance of cover
from the other side surface of body is adjustable to change the
size of the air gap, whereby the bandwidth of the filter is
adjusted.
8. The filter as claimed in claim 6, wherein the bandwidth of the
filter depends, in part, on the dielectric constant of the
insulating material.
9. The filter as claimed in claim 6, wherein the bandwidth of the
filter depends, in part, on the thickness of insulating
material.
10. The filter as claimed in claim 6, wherein the insulating
material is Teflon.RTM..
11. The filter as claimed in claim 5, wherein the cover has an
inner surface that forms a cavity in which the body is retained,
said cavity having shoulders projecting from the inner surface that
engage the body to keep the inner surface of cover at the
determined distance from the other side surface of the body.
12. The filter as claimed in claim 2, wherein there are four
resonators, and further including a coupling electrode pattern
located on said other side surface for coupling said resonators to
create a phase cancellation with signals within the body so as to
form at least one imaginary zero positioned so that the shape of
the pass band of the filter is substantially equivalent to that of
a band-pass filter with six resonators, but without an imaginary
zero.
13. The filter as claimed in claim 6, wherein the insulating
material is a printed circuit board, the filter body being mounted
on the board with the other side surface toward the board, and the
surface of the printed circuit board opposite the body being
covered with the conductive plate.
14. A band-pass filter comprising:
a body of dielectric material having first and second surfaces on
opposite sides of the body from each other, end surfaces and side
surfaces;
said body defining at least two holes extending from the first
surface toward the second surface;
a conductive coating on said second, end and one side surfaces as
well as the interior surfaces of the holes, to form at least two
transmission line resonators the other side surface being generally
free of said conductive coating;
an insulating plate having first and second surfaces with the first
surface against the other side surface of the body; and
a conductive electrode pattern on the insulating plate for coupling
between said two resonators, the electrode pattern providing
coupling between the at least two resonators by creating a field
that penetrates the uncoated other side of the body to the interior
surface of the hole, said coupling being capacitive, mixed
capacitive-inductive and inductive depending on whether the pattern
is near and thereby closer to the first surface than to the second
surface, or more equidistant between the first and second surfaces
or near and thereby closer to the second surface than to the first
surface, respectively.
15. The filter as claimed in claim 14, wherein the electrode
patterns are on the first surface of said insulating plate against
which the body is located.
16. The filter as claimed in claim 15, wherein the insulating plate
is a multi-layer printed circuit board and the electrode patterns
are in a conductive layer inside the multi-layer board.
17. The filter as claimed in claim 15, wherein, on the second
surface of the insulating plate opposite from the body, at least in
an area the size of the other side surface of the body, there is an
electrically conductive plating that is electrically coupled to the
conductive coating of body.
18. The filter as claimed in claim 14, wherein the body is fastened
to the insulating plate by gluing.
19. The filter as claimed in claim 14, wherein the body is fastened
to the insulating plate by soldering.
20. The filter as claimed in claim 14, wherein the body is mounted
in a bracket which has been fastened to the insulating plate.
21. The filter as claimed in claims 1 or 14 wherein the first
surface of the body is covered with the conductive layer, except
for an area around the hole.
22. A filter as claimed in claims 1 or 14 wherein the electrode
pattern includes a conductive strip connected to the conductive
coating and located along at least one edge of the other side
surface near one of the first and second surfaces, removal of a
portion of said strip adjacent the resonator being effective to
change the frequency of the resonator.
23. A filter as claimed in claim 1 or 14 wherein removal of a
portion of the dielectric material on the first surface adjacent a
resonator is effective to alter the frequency of the resonator.
24. A duplexer filter for a radio having an antenna, a transmitter
and a receiver, comprising:
first and second blocks of dielectric material, each block having
first and second surfaces on opposite sides from each other, end
surfaces and side surfaces; each block defining at least one hole
extending from the first surface to the second surface;
a conductive coating over the second surface, end surfaces and one
side surface, as well as over the interior of the hole of each
block, so as to form at least one transmission line resonator in
each block the other side surface being generally free of the
conductive coating;
a conductive electrode pattern on the other side surface of each
block for coupling directly to and from the resonator through the
dielectric in each block by means of fields created at the
electrodes that penetrate the other side surface and extend to the
hole, the coupling varying from capacitive, to mixed
capacitive-inductive, to inductive as the pattern is located
respectively on the other side surface near and thereby closer to
the first surface than to the second surface, to more equidistant
between the first and second surfaces, to nearer and thereby closer
to the second surface than to the first surface and
connecting means for connecting the first block between the
transmitter and the antenna, and for connecting the second block
between the receiver and the antenna.
25. A duplexer filter as claimed in claim 24 wherein the connecting
means includes a portion of the electrode pattern on the other side
surface.
26. A duplexer filter as claimed in claim 25 wherein the portion of
the electrode pattern is an electrode strip one-quarter wavelength
of the resonant frequency of the resonator in length.
27. A duplexer filter as claimed in claim 25 wherein the portion
oft he electrode pattern forms a reactive component.
28. A duplexer filter as claimed in claim 24 wherein the electrode
pattern for one of the blocks forms the block into a band-pass
filter with at least one imaginary zero.
29. A duplexer filter as claimed in claim 24 wherein the electrode
pattern for one of the blocks forms the block into a band-stop
filter.
30. A duplexer filter as claimed in claim 24 wherein one of the
first and second blocks has four holes and an electrode pattern
that creates a four resonator band-pass filter with imaginary zeros
at both sides of the pass-band, and the other block has three holes
and an electrode pattern that creates a three resonator band-stop
filter.
31. A filter comprising:
first and second blocks of dielectric material, each block having
first and second surfaces on opposite sides from each other, end
surfaces and side surfaces; each block defining one hole extending
from the first surface to the second surface;
a conductive coating over the second surface, end surfaces and one
side surface, as well as over the interior of the hole of each
block, so as to form one transmission line resonator in each block
the other side surface being substantially free of the conductive
coating:
conductive electrode patterns on the other side surface of each
block for coupling to and from the resonator in each block by means
of fields created by signals on the electrode patterns that
penetrate the other side surface and extend to the hole, the
coupling varying from capacitive, to mixed capacitive-inductive, to
inductive as the pattern is located respectively on the other side
surface near and thereby closer to the first surface than to the
second surface, to more equidistant between the first and second
surfaces, to nearer and thereby closer to the second surface than
to the first surface; and
connecting means for connecting the first block to the second block
between the receiver and the antenna.
32. A bandpass filter, comprising:
a body of dielectric material having a plurality of surfaces
including two opposite surface; and a side surface extending
between said two opposite surfaces, said body defining at least one
hole with an interior surface which extends into said body from
each of said two opposite surfaces;
resonator means for producing at least one transmission line
resonator, said resonator means including a conductive layer
covering portions of at least some of said plurality of surfaces
and an interior surface of said hole;
means for providing electrical signal coupling to and from said
resonator means, said providing means including an
electrically-conductive electrode pattern means on said side
surface which varies said coupling from capacitive to mixed
capacitive and inductive to inductive, depending on a relative
location of the pattern means on the side surface between areas of
said side surface adjacent to said two opposite surfaces;
an electrically conductive plate spaced from said pattern means so
as to define a gap therebetween which determines a bandwidth;
and
means for coupling said plate to said interior surface of said
hole.
33. A bandpass filter as in claim 32, wherein said determining
means including means for varying a size of said gap by enabling
relative movement of said plate and said pattern means with respect
to each other.
34. A bandpass filter as in claim 32, wherein said varying means
includes a insulating element separating said pattern means from
said plate, said bandwidth being determined partially based on a
thickness and composition of said insulating element.
35. A bandpass filter as in claim 32, wherein said coupling is more
capacitive than inductive.
36. A bandpass filter as in claim 32, wherein said coupling is more
inductive than capacitive.
37. A bandpass filter as in claim 32, wherein said coupling
provides mixed capacitance and inductance.
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. Pat. 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. Pat. Nos. 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 at. 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. Pat. 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 at., 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: ##EQU1## where f.sub.c is the frequency in megahertz, l is
the length and e.sub.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. Pat. 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. Pat. 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.
SUMMARY OF THE INVENTION
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
interresonator 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 its 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. 5B 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 on 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.RTM.. 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 443.
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. 1 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 arid
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 interresonator 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.
While the present invention has been particularly shown and
described with reference to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
* * * * *