Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-01-23
2004-08-03
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06771003
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus having balanced signal terminals provided at least for an input side and an output side and, more particularly, to a SAW apparatus having a balanced-to-unbalanced conversion function, and also relates to a communication unit including the above-described SAW apparatus.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and the weight of communication apparatuses, such as cellular telephones. This progress has been achieved by reducing the number and the size of the individual components of cellular telephones. As the frequency used in cellular telephones increases to, for example, a GHz band, SAW apparatuses are important because the sizes of the individual components of the SAW apparatuses can be reduced.
Some SAW apparatuses require impedance matching devices, such as phase shifters, at the input/output terminals. Cited reference 1 (Japanese Unexamined Patent Application Publication No. 8-195645: Device-Mounting Package) discloses the following type of SAW apparatus. An impedance matching device is provided on a matching circuit substrate, and the matching circuit substrate is bonded to a SAW-device-mounted substrate. With this configuration, the resulting package is simplified.
Cited reference 2 (Japanese Unexamined Patent Application Publication No. 6-97761: Branching Filter and Method Therefor) discloses the following type of SAW apparatus. In a branching circuit, an impedance matching device and a phase matching device are embedded in a portion of a package other than a portion in which a SAW filter device is accommodated. With this arrangement, the size of the SAW apparatus is reduced while maintaining the isolation of the filter device.
To reduce the number of components, research has been actively conducted on SAW apparatuses provided with a balanced-to-unbalanced conversion function, i.e., a so-called “balun function”. Such SAW apparatuses are most widely used in Global System for Mobile communications (GSM) cellular telephones, which are used on a world-wide basis, particularly in Europe.
A balun is a circuit for matching a balanced line and an unbalanced line. More particularly, when a balanced line, such as a twin-lead type feeder, and an unbalanced line, such as a coaxial cable, are directly connected, a balanced current disadvantageously flows to cause the feeder itself to operate as an antenna. Accordingly, the balun circuit is provided to prevent the generation of a balanced current, thereby matching the balanced line and the unbalanced line.
Several patent applications concerning SAW apparatuses provided with the above-described balun function have been filed. As a SAW device used in a SAW apparatus provided with the balun function in which the input impedance and the output impedance are substantially the same, the configuration shown in
FIG. 18
is commonly used.
In the SAW device shown in
FIG. 18
, on a piezoelectric substrate
100
, a comb-like electrode (which is also referred to as an “interdigital electrode” and is hereinafter referred to as an “IDT”)
101
is provided as the balanced portion. IDTs
102
and
103
are respectively provided on the left and right sides of the IDT
101
(in a SAW propagating direction) as the unbalanced portion. Reflectors
104
and
105
for improving transmission efficiency by reflecting the propagated SAW thereon are arranged such that they sandwich the IDTs
101
,
102
, and
103
therebetween. A SAW device having three IDTs arranged along a SAW propagating direction is referred to as a “3 IDT-type extensionally-coupled-resonator-mode SAW device”.
In the above-described SAW device, reflectors
107
and
108
are arranged such that they sandwich an IDT
106
therebetween to define a SAW resonator. The SAW resonator is connected in series to the IDTs
102
and
103
. Additionally, balanced signal terminals
109
and
110
connected to the IDT
101
and an unbalanced signal terminal
111
connected to the IDT
106
are also provided.
In a SAW apparatus provided with the above-described balun function, equal amplitude characteristics and 180°-inverted phase characteristics are required as transmission characteristics in a pass band between the unbalanced signal terminal
111
and each of the balanced signal terminals
109
and
110
. The amplitude characteristics and the phase characteristics described above are referred to as the “amplitude balance degree” and the “phase balance degree”, respectively.
The amplitude balance degree and the phase balance degree are defined as follows. When the above-described SAW device having a balun function is used as a three-port device, and when the unbalanced input terminal is a first port, and the balanced output terminals are a second port and a third port, the amplitude balance degree [A] is defined as A=[20log(S21)]−[(20log(S31)], and the phase balance degree is defined as [B-180] in which B is [∠S21−∠S31], where S21 is the transfer factor from the first port to the second port, and S31 is the transfer factor from the first port to the third port, and where the symbol [] represents an absolute value.
Ideally, in the pass band of a SAW apparatus, the amplitude balance degree is 0 dB, and the phase balance degree is 0 degrees. As one example of a SAW apparatus having a balun function, the SAW device shown in
FIG. 18
is mounted on a package
200
shown in
FIGS. 20 and 21
.
On the reverse surface (external surface) of the package
200
, as shown in
FIG. 19
, an external terminal
201
, which is an unbalanced signal terminal, and external terminals
202
and
203
, which are balanced signal terminals, are arranged along the peripheral portions of the reverse surface.
Within the package
200
, as shown in
FIG. 20
, a die attach portion
204
is provided for holding the SAW device shown in FIG.
18
and for electrically connecting the SAW device to external devices. In wiring patterns of the die attach portion
204
, the external terminal
201
and a wiring pattern
302
, which are disposed adjacent to one another, are connected, the external terminal
202
and a wiring pattern
303
, which are disposed adjacent to one another, are connected, and the external terminal
203
and a wiring pattern
304
, which are disposed adjacent to one another, are connected.
In the known SAW apparatus, however, the following problems are presented.
In the package
200
shown in
FIG. 19
, the balanced-signal external terminals
202
and
203
cannot be symmetrically disposed with respect to the unbalanced-signal external terminal
201
. Accordingly, the balance degrees between the balanced-signal external terminals
202
and
203
are decreased.
The reason for this is, for example, as follows. Because of the arrangement of the external terminals
201
,
202
, and
203
on the reverse surface of the package
200
, the distance from the unbalanced-signal external terminal
201
to the balanced-signal external terminal
202
is different from the distance from the unbalanced-signal external terminal
201
to the balanced-signal external terminal
203
. Accordingly, the bridge capacitance generated between the unbalanced-signal external terminal
201
and the balanced-signal external terminal
202
is different from the bridge capacitance between the unbalanced-signal external terminal
201
and the balanced-signal external terminal
203
.
In the wiring patterns on the die attach portion
204
of the package
200
, electrodes that are symmetrical with respect to the individual components cannot be provided, thereby decreasing balancing characteristics.
Additionally, in the SAW device shown in
FIG. 18
, the electrical polarity between the adjacent electrode fingers of the IDT
102
and the IDT
101
is different from that of the adjacent electrode fingers between the IDT
103
and the IDT
101
. Accordingly, the balance degree of the
Addison Karen
Dougherty Thomas M.
Keating & Bennett LLP
Murata Manufacturing Co. LTD
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