Balanced dielectric filter

Details Balanced dielectric filter Balanced dielectric filter

1999-02-23

2001-04-24

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S202000, C333S204000, C333S026000

Reexamination Certificate

active

06222431

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a balanced dielectric filter mainly used for high-frequency circuits, such as those used for radio communication apparatuses.
PRIOR ART
In accordance with the recent progress of mobile communications including the cellular phone system, filters have been required to be more compact in size and higher in performance, and dielectric filters suited for these requirements have been used widely. Such dielectric filters are used in a microwave band ranging from a few hundred megahertzs to about five gigaherzs, which are mounted on circuit boards in communications equipment, in particular, in the cellular phones. For this purpose, ceramic-multilayered filters have been used in larger quantity to be suited to be made especially smaller and thinner.
FIG. 9B
shows the structure of a conventional unbalanced dielectric filter in which five ceramic dielectric layers
61
to
65
are laminated into a multilayer structure. Between the dielectric layers
63
and
64
, a resonator comprising a pair of strip lines
66
,
66
as a resonator is formed on a plane in the structure, and the strip lines have lengths of a quarter of a resonant wavelength with short-circuited ends.
In this example, an input capacitance electrode
68
is coupled to one end of one of the strip lines
66
, and an output capacitance electrode
69
is coupled to one end of the other strip lines
66
. The two strip lines as resonating elements are disposed in parallel and coupled electrostatically through an interstage-coupling capacitance electrode
70
.
The resonator of the strip lines
66
and
66
is interposed between two shield electrodes
71
,
72
through dielectric layers, thereby forming a tri-plate structure. The strip lines
66
and
66
in a pair are grounded through a loading capacitance electrode
67
. Furthermore, an input terminal
73
(
11
) and an output terminal
74
(
51
) are connected to the one and the other of the strip lines
66
, respectively, through the input capacitance electrode
68
and the output capacitance electrode
69
, respectively. Moreover, grounding terminals
75
,
76
(
4
) are connected to the shield electrodes
71
,
72
and the above loading-capacitance electrode
67
so that they are grounded.
FIG. 9A
is a view showing the connections of the terminals to the conventional dielectric filter. A high-frequency input signal is applied between the input terminal
11
and the grounding terminal
4
, and then an output signal is delivered between the output terminal
51
and the grounding terminal
4
.
In the dielectric filter as described above, the two strip lines
66
and
66
of the resonator are first coupled electromagnetically to each other to form a comb-line type filter. The loading-capacitance electrode
67
is used to connect a capacitance in parallel with the strip lines, thereby lowering the resonant frequency for the strip line having the same length.
In this filter, the input and output stages of the filter are capacitance coupling, and parallel-plate capacitors are formed at the portions of the input/output capacitance electrode
68
,
69
on the dielectric layer opposed to the strip-line resonator
66
. The interstage-coupling capacitance electrode
70
can attain the interstage coupling between the strip line resonators by combining electromagnetic field coupling with electric field coupling, then, generating an attenuation pole in transfer characteristics (see Japanese Patent Publication JP-A 5-95202, for example).
An unbalanced filter using stepped impedance resonators integrally formed in a dielectric ceramic-multilayered structure is disclosed in Japanese Patent Publication JP-A 7-312503. In this filter, a pair of strip lines are stepped impedance resonators, each comprising a first line portion, one end of which is grounded, and a second line portion, one end of which is open, and which has a characteristic impedance lower than that of the first line portion. The coupling factor between the first line portions and the coupling factor between the second line portions are changed to control the transfer characteristics of the filter circuit.
Furthermore, as still another conventional example, an attempt to balance the output and/or input terminal arrangement of the filter has already been proposed in PCT international publication WO92/02969, as shown in FIG.
10
. In this example, the filter comprises two split ring resonators
80
and
81
of a microstrip-line type, wherein one of the resonators is connected to an unbalanced input terminal
82
through an input coupling capacitance
85
, and the other resonator is connected to balanced output terminals
83
,
84
. The split ends of the rings are connected to each other by loading capacitances
86
and
87
, respectively. In this arrangement, the split-ring resonators
80
,
81
are coupled electromagnetically to form a filter.
In the conventional strip-line type filters described above, since the input terminals are unbalanced, a balance-unbalance transformer (BALUN) is required to connect the unbalanced terminals to a high-frequency balanced amplifier or semiconductor integrated circuit. Furthermore, in the unbalanced circuit, current flows in the grounding circuit thereof, thereby causing a problem of having low resistance against electromagnetic interference and being easily susceptible to noise or the like.
Furthermore, the PCT international publication WO92/02969 discloses another embodiment of a filter using a pair of the ring strip resonators with balanced input and output terminals. This balanced filter must occupy a large area on the dielectric substrate to locate the twin resonators on the same surface. Therefore, a new type of balanced filters with more compact sizes is eagerly desired and it is required to arrange the resonators compactly for fabricating a balanced filter.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dielectric filter having balanced input and output terminals, being highly resistant against electromagnetic interference and capable of being designed easily.
Another object of the present invention is to provide a balanced dielectric filter having excellent characteristics for connection to balanced circuits or balanced integrated circuits.
Yet another object of the present invention is to provide a balanced dielectric filter being mountable efficiently on circuit boards.
The balanced dielectric filter in accordance with the present invention comprises two resonators, each comprising plural TEM mode resonating elements which are disposed in parallel and mutually coupled electromagnetically, two input terminals each of which is connected to the corresponding resonator, and two output terminals each of which is connected to the corresponding resonator, functioning as balanced input and output terminals, wherein the two resonators are disposed in a dielectric to face each other and to have mirror symmetry with each other.
In the balanced dielectric filter in accordance with the present invention, both the input and output terminals are of a balanced type, and the two resonators are disposed so as to be mirror images of each other. When high-frequency signals opposite in phase are applied to the two input terminals of the two resonators, an electric wall having the zero potential is formed in the mirror-symmetry plane between the two resonators. Therefore, both the input and output sides of the filter are balanced excellently. External electromagnetic interference is cancelled, and not produced on the output side. As a result, it is possible to configure a filter highly resistant against external electromagnetic interference.
In the present invention, the TEM mode resonating element comprises a strip line formed of a thin conductor embedded in a dielectric.
A hollow resonator included in a dielectric may be used as another type of TEM mode resonating element. In this case, holes, acting as resonant cavities, arranged in a dielectric block may be used to constitute a dielectric block type balanced filter.

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