Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2003-01-22
2004-11-09
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C333S193000
Reexamination Certificate
active
06815870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, for example, delay lines, filters, and other electronic components, and to a communication device including a surface acoustic wave device.
2. Description of the Related Art
Since electronic devices have become smaller in size and lighter in weight in recent years, there has been an increasing demand for electronic devices with multiple functions. With such a background, there has also been an increasing demand for surface acoustic wave filters (hereinafter referred to as “SAW filters”) incorporated in surface acoustic wave devices used in communication devices, such as cellular phones, to have an unbalanced-to-balanced conversion function so that the SAW filters can be directly connected to an IC which operates by using a balanced input, and active research has been intensively carried out in this area.
In particular, in a SAW filter formed of a resonator filter, having an unbalanced-to-balanced conversion function, the amplitude difference and the phase difference on the balanced side are important characteristics (hereinafter referred to as “balanced characteristics”), and it is necessary that the amplitude difference be 0 dB and the phase difference be 180° on the balanced side. However, in practice, since the tendencies of the balanced characteristics also differ according to the structure of the SAW filter, it is not possible to accurately achieve an amplitude difference of 0 dB and a phase difference of 180°, and improvement of the balanced characteristics with respect to the structure of the SAW filter has become an important objective in such devices.
For the surface acoustic wave device having an unbalanced-to-balanced conversion function, there are various structures depending on the type and purpose of an IC. For example, for the surface acoustic wave device in which the matching impedance on the balanced side is approximately four times as large as the matching impedance on the unbalanced side, the structure such as that shown in
FIG. 12
is widely used.
A surface acoustic wave device shown in
FIG. 12
is configured in such a manner that a resonator filter
100
and a resonator filter
101
, which is 180° out of phase with the resonator filter
100
, are provided on a piezoelectric substrate (not shown).
The resonator filter
100
is provided with a comb-shaped electrode (an interdigital transducer, hereinafter abbreviated as an “IDT”)
101
, and IDTs
102
and
103
are arranged on the right and left sides (the right and left direction along the propagation direction of the surface acoustic wave) of the IDT
101
. Furthermore, in the resonator filter
100
, reflectors
104
and
105
are arranged so as to sandwich the IDTs
101
,
102
, and
103
from the right and left (the right and left direction along the propagation direction of the surface acoustic wave), respectively.
The IDT is formed from a metal thin-film of aluminum, etc., and functions as a surface acoustic wave conversion section which converts an input electrical signal (AC) into surface acoustic waves (SAW energy) so that the surface acoustic waves propagate on the piezoelectric substrate, and which converts the propagated surface acoustic waves into electrical signals and outputs them. The reflector is used to reflect the propagated surface acoustic waves back in the direction from which they came in order to improve the conversion efficiency.
In such an IDT, it is possible to set signal conversion characteristics and a passband by setting the length and the width of each comb-shaped electrode finger, the spacing between adjacent comb-shaped electrode fingers, and the finger overlap indicating the opposing length in an interdigitated state between mutual comb-shaped electrode fingers. In the reflector, reflection characteristics can be set by adjusting the width of each reflector electrode finger and the spacing between fingers.
A resonator filter
110
is provided with an IDT
111
in which the hot side (signal side) and the ground side of the electrode finger are reversed with respect to the IDT
101
in the resonator filter
100
so that the phase differs by 180° with respect to the resonator filter
100
. IDTs
112
and
113
are provided on the right and left (the right and left direction along the propagation direction of the surface acoustic wave) of the IDT
111
. Furthermore, in the resonator filter
110
, reflectors
114
and
115
for reflecting surface acoustic waves in order to improve the conversion efficiency are arranged so as to sandwich the IDTs
111
,
112
, and
113
from the right and left, respectively.
More specifically, there are provided an unbalanced signal terminal
170
to which the IDTs
102
and
103
in the resonator filter
100
and the IDTs
112
and
113
in the resonator filter
110
are connected in parallel, and balanced signal terminals
180
and
190
which are connected in series to the IDTs
101
and
1111
, respectively. That is, one side of each of the resonator filters
100
and
110
which are 180° out of phase with each other is connected, and a surface acoustic wave device having an unbalanced-to-balanced conversion function is configured in such a manner that the connected unbalanced signal terminal
170
is an unbalanced terminal and the balanced signal terminals
180
and
190
which are not connected are balanced terminals. For this surface acoustic wave device, high attenuation and sharpness of attenuation outside the passband are required.
Therefore, as shown in
FIG. 13
, in each of the terminals
170
,
180
, and
190
of the surface acoustic wave device shown in
FIG. 12
, by arranging in series trapping resonators
130
,
140
, and
150
having resonators
131
,
141
, and
151
, respectively, high attenuation and sharpness of attenuation can be obtained outside the passband.
However, the above-described structure has factors that deteriorate balanced characteristics. For example, since the hot side and the ground side of the electrode finger in the central IDT
111
of the resonator filter
110
are reversed with respect to those of the IDT
101
in the resonator filter
100
so that the phase differs by 180° with respect to the resonator filter
100
, the number of the hot side and the ground side of the resonator filter
100
and that of the resonator filter
110
differ, and the ground side and the hot side are aligned in the IDT-IDT interface, causing an unwanted electric-field to be generated. As one of the countermeasures to improve these balanced characteristics, there is a method of grounding the hot electrode finger, but still a problem remains in the balanced characteristics.
For the surface acoustic wave device shown in
FIG. 12
, an ideal state is that only the phase difference is 180° between the resonator filter
100
and the resonator filter
110
in the vicinity of the passband. However, after all, for the reasons described above, in practice, the impedance, etc., also differs. As a result, the balanced characteristics deviate from the ideal amplitude difference of 0 dB and the ideal phase difference of 180°. Such an inconvenience is a problem which inherently occurs in a surface acoustic wave device having an unbalanced-to-balanced conversion function, including 2-system filter sections which are 180° out of phase with each other. Furthermore, there is no effective practical method for solving such problems when the balanced characteristics experience these problems.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device having an unbalanced-to-balanced conversion function, in which balanced characteristics are greatly improved.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes a first surface acoustic wave element having a plurality of interdigital transducers arranged along the propagation direction of a surface acoustic wave, and a se
Shibata Osamu
Takamine Yuichi
Addison Karen Beth
Dougherty Thomas M.
Keating & Bennett PLLC
Murata Manufacturing Co. Ltd.
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