Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-10-19
2003-01-07
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S196000, C310S31300R, C310S31300R
Reexamination Certificate
active
06504453
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transversal surface acoustic wave filter and more particularly, to a transversal surface acoustic wave filter having a reflector or a reflection end surface provided on the outer surface in the surface acoustic wave propagation direction of at least one of an interdigital transducer (hereinafter, referred to as “IDT”) on the input side and an IDT on the output side.
2. Description of the Related Art
Surface acoustic wave filters have been widely used as band filters used in cellular telephones or other telecommunications devices. More specifically, as band filters used in the IF stages of such devices, transversal surface acoustic wave filters are known.
The transversal surface acoustic wave filters are characterized in that the ground delay time deviation curve is flat, and the attenuation near the pass band is sufficient. Accordingly, the transversal surface acoustic wave filters are suitable for use as IF filters. In recent years, the IF filters in cellular telephones operating in the CDMA system have been required to have a wide band characteristic. The transversal surface acoustic wave filters can easily meet such requirement for the wide band characteristic.
However, the transversal surface acoustic wave filters have a large insertion loss, and have a very large size.
Thus, conventionally, various attempts have been made to reduce the insertion loss of the transversal surface acoustic wave filters and reduce the size thereof (for example, see Japanese Unexamined Patent Application Publication No. 11-186865 and EP 0140618B1).
FIG. 12
is a schematic plan view of an example of a conventional transversal surface acoustic wave filter. In this filter, an IDT
101
on the input side and an IDT
102
on the output side are arranged in the surface acoustic wave propagation direction on a piezoelectric substrate. The IDT on the input side is thinning-out weighted so that a desired characteristic can be obtained, and moreover, a unidirectional electrode
103
is disposed inside the IDT to reduce the insertion loss.
FIG. 13
is a schematic plan view of another example of the conventional transversal surface acoustic wave filter, and illustrates the configuration disclosed in Japanese Unexamined Patent Application Publication No. 11-186865. In a transversal surface acoustic wave filter
201
, an IDT
203
on the input side, an IDT
204
on the output side, and a reflector
205
are arranged in the surface acoustic wave propagation direction, and moreover, similarly, an IDT
206
on the input side, an IDT
207
on the output side, and a reflector
208
are formed and arranged in the surface acoustic wave propagation direction on the side of the configuration in which the IDT
203
on the input side, the IDT
204
on the output side, and the reflector
205
are arranged. The polarities of the IDTs
206
and
207
are opposite to those of the IDTs
203
and
204
.
The IDTs
203
and
206
on the input side are connected in common, that is, are connected to an input terminal IN. The IDTs
204
and
207
are connected in common, that is, are connected to an output terminal OUT.
The IDT
203
on the input side and the IDT
204
on the output side are arranged at an interval W
1
, and the IDT
206
on the input side and the IDT
207
on the output side are arranged at an interval W
3
. The intervals W
1
and W
2
are equal to each other.
The interval W
2
between the IDT
204
on the output side to the reflector
205
in the surface acoustic wave propagation direction is not equal to the interval W
4
between the IDT
207
on the output side to the reflector
208
. That is, the intervals W
2
and W
4
satisfy W
2
−W
4
=&lgr;/4, in which &lgr; is the wavelength of a surface acoustic wave.
Referring to the transversal surface acoustic wave filter
201
, electrical signals are input to the input IDTs
203
and
206
and are converted to surface acoustic waves. These surface acoustic waves are propagated toward the output IDTs
204
and
207
. The output IDTs
204
and
207
are arranged so that the surface acoustic waves can pass through the output IDTs
204
and
207
while the waves are not converted to electrical signals. The surface acoustic waves passed through the output IDTs
204
and
207
are reflected by the reflectors
205
and
208
. The reflected surface acoustic waves are converted to electrical signals by the output IDTs
204
and
207
, and are output therefrom. According to this configuration, the polarities of the IDTs
206
and
207
are opposite to those of the IDTs
203
and
204
, respectively. Since the intervals W
2
and W
4
satisfy W
2
−W
4
=&lgr;/4, the phase of the surface acoustic waves is the same as those of the surface acoustic waves reflected by the reflectors
205
and
208
. Thus, the electrical signals having the same phase are output from the output IDTs
204
and
207
.
The transversal surface acoustic wave filter
201
shown in
FIG. 13
utilizes the weighting of the input IDTs
203
and
204
and the output IDTs
206
and
207
, the characteristics determined by the position in which the reflection electrode of the unidirectional electrode or the like, and moreover, the reflection characteristics of the reflectors
205
and
208
. Thus, the attenuation near the pass band can be increased.
In recent years, the reduction in size and weight of mobile communication devices such as cellular telephones or other devices has been advanced. Accordingly, the IF filters for use in the mobile communication devices have been required to have much smaller sizes.
However, the bandwidth of the pass band of the transversal surface acoustic wave filter shown in
FIG. 12
is determined mainly by the number of the electrode finger pairs of each IDT. Thus, when the size of the transversal surface acoustic wave filter is reduced, a required bandwidth must be considered. Accordingly, it has been difficult to reduce the number of the paired electrode fingers of each IDT.
Moreover, to obtain the characteristic of the attenuation steeply changing near the pass band, it is necessary to sufficiently weight the IDTs on the input and output sides. As a result, the total number of the electrode finger pairs of the IDTs must be increased.
Referring to the transversal surface acoustic wave filter
201
shown in
FIG. 13
, surface acoustic waves excited by the input IDTs
203
and
206
pass through the output IDTs
204
and
207
in which the surface acoustic waves are electrically cancelled out, and are reflected by the reflectors
205
and
208
. Thereafter, the surface acoustic waves are converted to electrical signals by the output IDTs
204
and
207
. On the other hand, no mechanical reflections from the output IDTs
204
and
207
should be generated so that the surface acoustic waves can pass through the output IDTs
204
and
207
as they are. Thus, it has been necessary that the output IDTs
204
and
207
comprise split electrodes only, which generate no mechanical reflections.
However, only the surface acoustic waves reflected by the reflectors
205
and
208
, that is, only the surface acoustic waves propagated from the input side and entering the reflectors
205
and
208
are output as electrical signals from the output IDTs
204
and
207
. Therefore, the bidirectional loss 3dB in the transversal surface acoustic wave filter is caused. It is impossible to reduce the insertion loss.
To sufficiently increase the attenuation near the pass band, it is not satisfactory to simply consider the reflection characteristics of the reflectors
205
and
208
. The surface acoustic wave filter
201
is also required to increase the number of the paired electrode fingers of the IDTs
203
and
204
and
205
and
206
on the input and output sides similarly to the transversal surface acoustic wave filter shown in FIG.
12
. Thus, the insertion loss is large, and it is difficult to reduce the size of the elements.
SUMMARY OF THE INVENTION
In order to overcome the problems
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Pascal Robert
Summons Barbara
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