Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-01-22
2004-06-22
Budd, Mark (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06753641
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device having a filter characteristic, especially, a balance-unbalance conversion function, and a communication device including such a novel surface acoustic wave device.
2. Description of the Related Art
In recent years, techniques for reducing the sizes and weights of portable telephones have remarkably progressed. In order to achieve such reduction in size and weight, composite elements each having a plurality of functions combined with each other have been developed in addition to reduction of the number and sizes of components. Under these circumstances, recently, surface acoustic wave filters having balance-unbalance conversion functions, namely, balum functions for use in the RF stages of portable telephones have been intensively studied. The GSM system (Global System for Mobile Communications) and other such telecommunications systems have been used.
When balanced lines such as a two parallel line feeder are connected directly to unbalanced lines such as a coaxial cable, undesirably, unbalanced currents flow, and the feeder itself acts as an antenna. Thus, the balun includes a circuit for preventing the unbalanced currents and matching the balanced and unbalanced lines with each other.
Some surface acoustic wave devices having the above-described balance-unbalance conversion functions have been the subject of recent patent applications. As a surface acoustic wave device which has a balance-unbalance conversion function and of which the input and output impedances are substantially equal to each other, a device having the configuration shown in
FIG. 20
is widely known.
In the surface acoustic wave device shown in
FIG. 20
, a comb-like electrode (also called an interdigital electrode, defining an Inter-Digital Transducer which is hereinafter referred to as “IDT”)
101
is provided on a piezoelectric substrate
100
, and IDTs
102
and
103
arranged on the right and left sides (in the surface acoustic wave propagation direction) of the IDT
101
.
Moreover, in the aforementioned surface acoustic wave device, reflectors
104
and
105
for reflecting a surface acoustic wave to enhance the conversion efficiency are arranged so as to sandwich the IDTs
101
,
102
, and
103
from the right and left sides thereof. Moreover, balanced-signal terminals
106
and
107
, and an unbalanced signal terminal
108
are provided.
The above-described surface acoustic wave device is called a longitudinally-coupled resonator type surface acoustic wave device of a 3-IDT type. The device has a balance-unbalance conversion function by which a surface acoustic wave is converted between the IDTs
101
,
102
, and
103
.
The surface acoustic wave device having a balance-unbalance conversion function is required to have the following transmission characteristics in the pass-band between the unbalanced-signal terminal and the respective balanced-signal terminals: the amplitude characteristics are the same and the phases are inverted with respect to each other by 180°. These characteristics are represented by the amplitude-balance-degree and the phase-balance-degree between the balanced-signal terminals, respectively.
The above-described surface acoustic wave device having a balance-unbalance conversion function is assumed to be a three-port device. The unbalanced-input terminal is a first port, and the balanced-output terminals are the second and third ports. The amplitude-balance-degree and the phase-balance-degree are defined as follows: Amplitude-balance-degree=[A]. A=[20 log(S21)]−[20 log(S31)], phase-balance-degree=[B−180], B=[ZS21−ZS31]. S
21
represents the coefficient of transfer from a first port to the second port, and S
31
represents the coefficient of transfer from the first port to the third port. The symbol of [ ] in the aforementioned formulae represents an absolute value.
Referring to the balance degrees between signals at the aforementioned balanced-signal terminals, ideally, the amplitude-balance-degree in the transmission band of the surface acoustic wave device is 0 dB, and the phase-balance-degree is 0 degrees.
However, in the configuration of the related art shown in
FIG. 20
, a problem arises in that the balance degrees between the balanced-signal terminals are deteriorated. Several reasons for this deterioration are speculated. The inventor of the present invention discovered that a capacitance generated between the balanced-signal terminal
106
and the signal line
109
, which is a bridging capacitance and is present in the vicinity of the balanced-signal terminal
106
, is one of the greatest causes for the deterioration.
Hereinafter, the deterioration of the balance degrees between the balanced-signal terminals, caused by the generation of the bridging capacitance, will be described with reference to
FIGS. 21 and 22
. The respective characteristics of transmission from the unbalanced-signal terminal
108
to the respective balanced-signal terminals
106
and
107
were excluded from the matching impedance and identified by simulation, and the positions of the resonant modes were examined.
FIGS. 21 and 22
show the results.
No bridging capacitance is assumed to be added between the balanced-signal terminal
106
and the signal line
109
in FIG.
20
. Then,
FIG. 21
shows the difference between the resonant mode of a signal transmitted from the unbalanced-signal terminal
108
to the balanced-signal terminal
106
and that of the signal transmitted from the unbalanced-signal terminal
108
to the balanced-signal terminal
107
.
FIG. 22
is a graph showing the difference between the resonant modes occurring when a bridging capacitance of 0.30 pF is assumed to be added between the balanced-signal terminal
106
and the signal line
109
. In
FIGS. 21 and 22
, the solid lines represent the characteristics of transmission from the unbalanced-signal terminal
108
to the balanced-signal terminal
106
, and the dotted lines represent the characteristics of transmission from the unbalanced-signal terminal
108
to the balanced-signal terminal
107
.
Comparison of
FIGS. 21 and 22
shows that when the bridging capacitance is added between the balanced-signal terminal
106
and the signal line
109
, the difference between the resonant modes of signals at the balanced-signal terminals
106
and
107
which are two balanced-output terminals is larger.
It is generally known that in a longitudinally-coupled resonator type surface acoustic wave device, resonant modes having a phase difference of 180° are acoustically coupled to produce the characteristic of the device. In this case, the phases of the resonant modes seen from the balanced-signal terminal
106
side and those seen from the balanced-signal terminal
107
side are different by 180° in any case. This is simply expressed by symbols as follows. For example, when three resonant modes seen from the balanced-signal terminal
106
side are expressed by the symbols −, +, −, the three modes seen from the balanced-signal terminal
107
side are expressed by the symbols +, −, +.
When a bridging capacitance having a certain phase angle is added, the resonant mode of a transmission characteristic of a signal transmitted via the balanced-signal terminal
106
and that of the signal transmitted via the balanced-signal terminal
107
are influenced in completely opposite directions by the bridging capacitance, and the balance degrees between the balanced-signal terminals
106
and
107
are deteriorated.
The inventor of the present invention made various examinations including simulation. As a result, the inventor identified the cause for the deterioration of the balance degrees between signals the balanced-signal terminals
106
and
107
.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface a
Budd Mark
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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