Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-06-08
2002-04-30
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S133000, C333S195000, C310S31300R, C310S31300R
Reexamination Certificate
active
06380827
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a surface acoustic wave filter (hereinbelow, referred to as an SAW filter) for use in a radio frequency unit of mobile communication equipment, particularly, a portable telephone and relates to a branching filter comprising the filter. This application is a counterpart application of Japanese Application Serial Number 160636/1999, filed Jun. 8, 1999, the subject matter of which is incorporated herein by reference.
(ii) Description of the Related Art
Hitherto, as an art regarding an SAW filter, for example, there is one disclosed in “Structure of transmitting SAW filter for branching filter” of Japanese Patent Application Laid-Open No. 10-256869 (1998).
FIG. 8
shows the structure of a branching filter comprising conventional SAW filters. As shown in
FIG. 8
, the branching filter comprising the SAW filters is constructed as described hereinbelow. An antenna-side matching circuit
7
is coupled to an antenna terminal
1
and an earth terminal
2
. Between a transmitting terminal
3
, an earth terminal
4
, and the antenna-side matching circuit
7
, a transmission-side branching circuit
8
, a transmission-side SAW resonator type filter
9
, and a transmission-side matching circuit
10
are coupled. On the other hand, between a receiving terminal
5
, an earth terminal
6
, and the antenna-side matching circuit
7
, a reception-side branching circuit
11
, a reception-side SAW resonator type filter
12
, and a reception-side matching circuit
13
are coupled.
FIG. 9
shows the construction of a radio frequency unit (RF unit) including a branching filter constructed by using general SAW resonators. As shown in
FIG. 9
, a branching filter
15
is coupled to an antenna
14
. An amplifier
16
and a reception-side SAW filter
17
are coupled to the reception side of the branching filter
15
. A power amplifier
18
and a transmission-side SAW filter
19
are coupled to the transmission side of the branching filter
15
.
In this instance,
FIG. 10
shows the structure of the reception-side SAW resonator type filter
12
as an example. The reception-side SAW filter
12
shown in
FIG. 10
has a ladder-shaped six-stage structure using SAW resonators. Between an input terminal
21
and an output terminal
22
, a first serial-arm SAW resonator
23
, a second serial-arm SAW resonator
24
, and a third serial-arm SAW resonator
25
are coupled. Between the input terminal
21
and the ground potential, a first parallel-arm SAW resonator
26
is coupled. Between the ground potential and node between the first and second serial-arm SAW resonators
23
and
24
, a second parallel-arm SAW resonator
27
is coupled. Between the ground potential and node between the second and third serial-arm SAW resonators
24
and
25
, a third parallel-arm SAW resonator
28
is coupled. Between the output terminal
22
and the ground potential, a fourth parallel-arm SAW resonator
29
is coupled.
As kinds of the branching filters, two kinds of branching filters, namely, a branching filter comprising dielectric filters for use in a communication terminal such as a portable telephone and an SAW branching filter for use in a card terminal can be mainly mentioned so far. Since a demand for miniaturization of a communication terminal device itself is increased in association with the spread of communication terminals such as portable telephones in recent years, a demand for miniaturization of high frequency parts built in the communication terminal device is also increased. Consequently, the SAW branching filter is used in place of the dielectric filer also in the communication terminal such as a portable telephone. Requested standards for the SAW branching filter as a device come to be equivalent to those for the branching filter comprising the dielectric filters.
When the SAW branching filter is used in the communication terminal such as a portable telephone, as compared with the case where it is used in the card terminal, the electric power resistance of comb teeth electrodes of the SAW filter in the SAW branching filter is regarded as important. It is strongly desired that the SAW branching filter satisfies standards regarding an amount of attenuation in the communication terminal such as a portable telephone. Further, the above-mentioned electric power resistance of the SAW branching filter is obtained, and on the other hand, it is desired that an amount of attenuation in the vicinity of a frequency pass band regarding the reception-side SAW filter, namely, in a frequency pass band regarding the transmission-side SAW filter is obtained as much as possible while realizing the miniaturization of the chip size of the SAW filter. Also in the SAW branching filter using a cascade-connecting dual-mode SAW filter, while the electric power resistance of comb teeth electrodes is obtained, and on the other hand, it is desired that an amount of attenuation in the vicinity of the frequency pass band regarding the reception-side SAW filter, namely, in the frequency pass band regarding the transmission-side SAW filter is obtained as much as possible while realizing the miniaturization of the chip size of the SAW filter.
SUMMARY OF THE INVENTION
According to the present invention, it is an object to provide an SAW filer in which while realizing the miniaturization of the chip size of a SAW filter, the electric power resistance of comb teeth electrodes in the SAW filter can be obtained, and an amount of attenuation in the vicinity of a frequency pass band regarding one SAW filter, namely, in a frequency pass band regarding the invention other SAW filter can be obtained as much as possible and provide a branching filter utilizing it.
To accomplish the above object, according to the present invention, there is provided an SAW filter comprising: a first serial-arm SAW resonator coupled to an input terminal; and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and an output terminal, wherein the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal.
To accomplish the above object, according to the present invention, there is provided an SAW branching filter comprising: a reception-side surface acoustic wave filter which is coupled between an antenna terminal and an amplifier; and a transmission-side surface acoustic wave filter which is coupled between the antenna terminal and a power amplifier, wherein an input terminal of the reception-side surface acoustic wave filter is coupled to the antenna terminal and an output terminal (of the reception-side surface acoustic wave filter) is coupled to the amplifier, the reception-side surface acoustic wave filter has a first serial-arm SAW resonator coupled to the input terminal and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and the output terminal, the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third comb-shaped interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal.
REFERENCES:
patent: 5592135 (1997-01-01), Taguchi et al.
patent: 5717367 (1998-02-01), Murai
patent: 5864262 (1999-01-01), Ikada
patent: 5874869 (1999-02-01), Ueda et al.
patent: 9-186553 (1997-07-01), None
patent: 10-256869 (1998-09-01), None
Frank Robert J.
Oki Electric Industry Co. Ltd.
Pascal Robert
Sartori Michael A.
Takaoka Dean
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