Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-07-27
2003-05-20
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S133000, C310S31300R
Reexamination Certificate
active
06566981
ABSTRACT:
This application is based on application Nos. 2000-229400, 2000-259672, 2000-259673, 2000-259674, 2000-264524, and 2000-264525 filed in Japan, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a duplexer, a weak signal extracting filter or the like, and more particularly to a surface acoustic wave device in which a plurality of SAW filter elements are housed in a single container.
2. Description of Related Art
Recently, a surface acoustic wave filter formed by surface acoustic wave elements (hereinafter referred to as SAW filter), is used in a variety of communication fields. Because of its characteristics of high-performance, compact design, mass-productivity and the like, this SAW filter plays an important role of the diffusion of small-size communication devices such as cellular phones or the like.
The requirements for miniaturization and higher performance of small-size communication devices are increasing constantly in severity. Recently commercialized is a product in which a single container houses a plurality of SAW filters (e.g., a front-end SAW filter and an inter-stage SAW filter).
Examples of the product in which a single container houses a plurality of SAW filters, include a duplexer in which a signal in the reception frequency band is separated from a signal in the transmission frequency band, and a dual-or triple-band weak signal extracting filter in which signals in a plurality of frequency bands are separated from one another.
For example, a duplexer has, in a single container, a SAW filter element serving as a reception filter, and a SAW filter element serving as a transmission filter. A weak signal extracting filter comprises a front-end SAW filter element, an amplifying circuit and an inter-stage SAW filter element, and the front-end and inter-stage SAW filter elements are disposed in a cavity of the same container.
In any case, it is important to restrain floating capacitance among a plurality of SAW filter elements to prevent crosstalk from occurring. Particularly, in the weak signal extracting filter, the front-end SAW filter element and the inter-stage SAW filter element have bands of which center frequencies are approximately identical with each other, and these two SAW filter elements are required to be driven at the same time. Accordingly, it becomes important to achieve isolation (reduce crosstalk) between these two SAW filter elements as much as possible.
More specifically, an increase in crosstalk introduces the following problem. When the front-end SAW filter element and the inter-stage SAW filter element are connected in cascade to each other by external wirings, there cannot often be obtained the total attenuation amount which can theoretically be expected from the individual attenuation amounts of the two SAW filter elements. This is because the total attenuation amount of the two SAW filter elements is restricted by the crosstalk between the front-end SAW filter element and the inter-stage SAW filter element. Actually, such a device is often used with an amplifying circuit interposed between the two SAW filter elements. In such a case, too, a similar problem arises.
A method of achieving isolation between two SAW filter elements as above-mentioned is for example disclosed by Japanese Patent Laid-Open Publication No. 11-205077.
According to the technique in the Publication above-mentioned, a surface acoustic wave device is arranged such that two SAW filter elements are housed in a cavity of a container, and that the input pads, output pads, and ground pads of the container are respectively electrically connected to the input electrodes, output electrodes and ground electrodes of the SAW filter elements. It is particularly disclosed that a plurality of ground pads are disposed in the cavity of the container and that each ground electrode of one SAW filter element and each ground electrode of the other SAW filter element are respectively connected to different ground pads.
The prior art above-mentioned prescribes only the connection relationship between the SAW filter elements and the ground pads formed in the cavity.
The inventors of this application had conducted a various tests and found that there occurred crosstalk resulting from the connection means between the ground electrodes of the SAW filter elements and the ground pads of the container. More specifically, the inventors found that crosstalk occurred as resulting from not only the bonding wires, but also the shapes of ground conductor films formed on the SAW filter mounting surface of the cavity in which the SAW filter elements are mounted, and the arrangement of the ground external terminals.
(A) Ground Conductor Film
A surface acoustic wave device of prior art is shown in
FIGS. 16 and 17
.
FIG. 16
is a perspective view of the surface acoustic wave device with its metallic lid omitted, and
FIG. 17
is a perspective view thereof with SAW filter elements omitted.
Shown in
FIGS. 16 and 17
are a front-end SAW filter element
100
, an inter-stage SAW filter element
200
, a concaved container
300
, a ground pad
136
formed on a step portion of the container
300
and connected to a ground electrode
101
of the front-end SAW filter element
100
through a bonding wire, a ground pad
139
formed on a step portion of the container
300
and connected to a ground electrode
102
of the inter-stage SAW filter element
200
through a bonding wire, a ground conductor film
500
formed substantially on the entire surface of the mounting surface of a cavity
317
of the container
300
, and ground external terminals
145
,
146
,
147
,
148
formed on the external lateral sides of the container
300
and connected to a metallic lid (not shown).
As shown in
FIG. 17
, the ground conductor film
500
is connected to the ground pad
136
and electrically connected to the ground external terminal
145
formed on the external lateral side of the container
300
. Likewise, the ground conductor film
500
is connected to the ground pad
139
and electrically connected to the ground external terminal
146
. The ground conductor film
500
is connected to a ground pad
138
and electrically connected to the ground external terminal
147
, and the ground conductor film
500
is connected to a ground pad
137
and electrically connected to the ground external terminal
148
. The ground external terminals
145
,
146
,
147
,
148
are respectively disposed at a plurality positions. This is for assuring the ground potential of the ground conductor film
500
.
According to the arrangement above-mentioned, for example when the ground pad
136
connected to the front-end SAW filter element
100
and the ground pad
139
connected to the inter-stage SAW filter element
200
, are commonly connected to the ground conductor film
500
, the ground electrodes of the two SAW filter elements
100
,
200
are consequently commonly connected to the ground conductor film
500
in the container
300
.
When a surface acoustic wave device having this ground conductor film
500
is mounted on a circuit board (not shown), there are disadvantageously generated, between the ground potential of the ground conductor film
500
and the ground potential of the circuit board, parasitic impedances resulting from the ground external terminals
145
,
146
,
147
,
148
and parasitic impedances resulting from the ground conductor film
500
. These parasitic impedances are generated, between the ground potential of the ground conductor film
500
and the ground potential of the circuit board, through the ground conductor film
500
commonly electrically connected to the SAW filter elements
100
,
200
. Accordingly, the parasitic impedances generated at the side of one SAW filter element
100
also exert influence on the other SAW filter element
200
.
Accordingly, the connection of the ground external terminals formed on the external sides of the container to the ground conductor film, causes
Maruta Kouichi
Noguchi Kenji
Urabe Masashi
Hogan & Hartson LLP
Kyocera Corporation
Summons Barbara
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