Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-04-09
2004-11-09
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06815868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus for use in, for example, a filter having an unbalanced-to-balanced conversion function, and also relates to a communication unit using the SAW apparatus described above.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and weight of communication units, such as cellular telephones. As the frequency used in cellular telephones is increased, smaller SAW apparatuses are used for filters in the communication units. Additionally, multi-functional components are being developed to reduce the size and the number of the individual components in communication units.
In view of the above background, research is being actively conducted on SAW filters provided with a balanced-to-unbalanced conversion function, i.e., a so-called “balun function”, used in the RF stage of cellular telephones. Such SAW filters are used mostly in Global System for Mobile communications (GSM) cellular telephones.
A known SAW filter having a balanced-to-unbalanced conversion function and an input impedance and output impedance that are substantially the same is shown in FIG.
67
.
In the SAW filter apparatus shown in
FIG. 67
, an interdigital transducer (hereinafter referred to as an “IDT”)
101
is provided on a piezoelectric substrate
100
. IDTs
102
and
103
are provided on the left and right sides of the IDT
101
(in a SAW propagating direction). With this configuration, a three-IDT-type longitudinally-coupled-resonator-type SAW apparatus having a balanced-to-unbalanced conversion function is provided.
In the above-described SAW apparatus, reflectors
104
and
105
are arranged to sandwich the IDTs
102
,
101
, and
103
therebetween. Terminals
106
and
107
are provided as balanced signal terminals, and a terminal
108
is provided as an unbalanced signal terminal.
Another type of SAW apparatus, which is provided with a balanced-to-unbalanced conversion function and has an input impedance and output impedance that differ by, for example, four times, is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-117123.
The SAW apparatus disclosed in the above-identified publication includes, as shown in
FIG. 68
, a first SAW filter device
111
and a second SAW filter device
112
provided on a piezoelectric substrate. An output signal of the first SAW filter device
111
is 180° out of phase with an output signal of the second SAW filter device
112
. The piezoelectric substrate is not shown in FIG.
68
. With this configuration, the above-described SAW filter apparatus provides not only a filtering function, but also a balanced-to-unbalanced conversion function.
The first SAW filter device
111
includes cascade-connected three-IDT-type longitudinally-coupled-resonator-type SAW filters
118
and
124
which are symmetrical to each other with respect to the symmetrical line extending along the SAW propagating direction. That is, the first SAW filter device
111
is defined by two stages of filters.
In the longitudinally-coupled-resonator-type SAW filter
118
, IDTs
114
and
115
are arranged to sandwich a central IDT
113
on the left and right sides thereof (along the SAW propagating direction), and reflectors
116
and
117
are arranged to sandwich the IDTs
114
,
113
, and
115
. Similarly, in the longitudinally-coupled-resonator-type SAW filter
124
, IDTs
120
and
122
are arranged to sandwich a central IDT
119
from the left and right sides, and reflectors
122
and
123
are arranged such that they sandwich the IDTs
120
,
119
, and
121
.
The second SAW filter device
112
includes a cascade-connected longitudinally-coupled-resonator-type SAW filter
128
, which is the same type as the longitudinally-coupled-resonator-type SAW filter
124
, and a longitudinally-coupled-resonator-type SAW filter
127
. The longitudinally-coupled-resonator-type SAW filter
127
is provided with a central IDT
133
whose phase is inverted (i.e., about 180°) by inverting the direction of the central IDT
113
of the longitudinally-coupled-resonator-type SAW filter
118
.
One terminal
129
of the first SAW filter device
111
and one terminal
130
of the second SAW filter device
112
are electrically connected in parallel to each other, and the other terminals
131
and
132
are electrically connected in series to each other. The parallel-connected terminals
129
and
130
define an unbalanced terminal
108
, while the series-connected terminals
131
and
132
define balanced terminals
107
and
108
.
In the SAW apparatus having a balanced-to-unbalanced conversion function, the transmission characteristics within the pass band between the unbalanced terminal
108
and each of the balanced terminals
106
and
107
must have equal amplitude characteristics and 180°-out-of-phase characteristics. Such amplitude characteristics and phase characteristics are referred to as “the amplitude balance level” and “the phase balance level”, respectively.
The amplitude balance level and the phase balance level are defined as follows. When the above-described SAW apparatus having a balanced-to-unbalanced conversion function is a three-port device, and when the unbalanced input terminal is port
1
and the balanced output terminals are port
2
and port
3
, the amplitude balance level |A| and the phase balance level |B| are defined as follows:
A=|
20log(
S
21
)|−|(20log(
S
31
)| (1)
B=|∠S
21
−
∠S
31
| (2)
where S
21
indicates the transfer factor from port
1
to port
2
, and S
31
indicates the transfer factor from port
1
to port
3
. Ideally, in the pass band of a SAW apparatus, the amplitude balance level is 0 dB, and the phase balance level is 180 degrees.
In the above-described SAW apparatus having balanced signal terminals, the balance levels between the balanced signal terminals are reduced. One of the reasons for this is as follows. The distance (indicated by
109
in
FIG. 67
) between the electrode finger connected to the balanced signal terminal
106
and the signal electrode finger of the IDT
102
is different from the distance (indicated by
110
in
FIG. 67
) between the electrode finger connected to the balanced signal terminal
107
and the signal electrode finger of the IDT
103
by 0.5 times the wavelength, which is determined by the pitch of the electrode fingers.
Then, the total capacitance of the electrode fingers connected to the balanced signal terminal
106
is different from that of the electrode fingers connected to the balanced signal terminal
107
, and the conversion efficiency between an electrical signal and a SAW also is different between the balanced signal terminals
106
and
107
. As a result, the balance levels are reduced.
Accordingly, as shown in
FIG. 70
, the amplitude characteristics of the frequency output from the balanced signal terminal
106
shown in
FIG. 67
were measured by grounding the balanced signal terminal
107
. As shown in
FIG. 71
, the amplitude characteristics of the frequency output from the balanced signal terminal
107
were measured by grounding the balanced signal terminal
106
shown in FIG.
67
. The difference between the amplitude characteristics output from the balanced signal terminal
106
and the amplitude characteristics output from the balanced signal terminal
107
is shown in FIG.
69
.
FIG. 69
shows that there is a large difference between the amplitude characteristics, and this difference causes a reduction in the balance levels.
In the SAW apparatus having cascade-connected filter devices shown in
FIG. 68
, the polarities of adjacent electrode fingers of two adjacent IDTs are not symmetrical between the first SAW filter device
111
and the second SAW filter device
112
. This further reduces the balance levels.
More specifically, in the IDT
113
, the portions located adjacent to the IDTs
1
Sawada Yoichi
Shibata Osamu
Takamine Yuichi
Watanabe Hiroki
Yata Masaru
Addison Karen Beth
Dougherty Thomas M.
Keating & Bennett
Murata Manufacturing Co. Ltd.
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