Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-03-05
2004-05-04
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S133000
Reexamination Certificate
active
06731188
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter device, for use as, for example, a band-pass filter provided in, for example, portable telephones. More particularly, the present invention relates to a surface acoustic wave filter device in which the characteristic impedance on the input side differs from the characteristic impedance on the output side and which has an unbalanced-to-balanced conversion function.
2. Description of the Related Art
In recent years, to decrease the size of portable telephones, composite components in which the number of component parts and the sizes thereof are reduced and which have multiple functions have been developed.
In such a surface acoustic wave filter used in the RF stage of a portable telephone, one having an unbalanced-to-balanced conversion function has been used.
In a portable telephone, the portion from the antenna to the surface acoustic wave filter as a band-pass filter is defined by an unbalanced circuit, and generally has a characteristic impedance of 50 &OHgr;. In contrast, an amplifier used in a later stage of the surface acoustic wave filter has a balanced terminal and often has an impedance of 150 &OHgr; to 200 &OHgr;. Therefore, a surface acoustic wave filter device having a balanced-to-unbalanced conversion function that converts an unbalanced input of 50 &OHgr; into a balanced output of 150 &OHgr; to 200 &OHgr; has been used.
An example of this type of surface acoustic wave filter is disclosed in Japanese Unexamined Patent Application Publication No. 10-117123. Referring to
FIG. 19
, this conventional surface acoustic wave filter device is described below. In a surface acoustic wave filter device
500
, surface acoustic wave filters
511
and
512
are connected to an unbalanced input terminal
500
a
. The surface acoustic wave filter
511
is configured such that the surface acoustic wave filters
511
a
and
511
b
are cascaded in two steps. Similarly, the surface acoustic wave filter
512
is configured such that the surface acoustic wave filters
512
a
and
512
b
are cascaded in two steps. The output ends of the surface acoustic wave filters
511
and
512
are provided as balanced output terminals
500
b
and
500
c
, respectively.
In the filter having a balanced-to-unbalanced conversion function, there is a demand for the transmission characteristics in the pass band between an unbalanced terminal and one of the balanced terminals to have an equal amplitude and a phase which is inverted by 180° with respect to the transmission characteristics in the pass band between the unbalanced terminal and the other balanced terminal. Such relationships of amplitude and phase between two transmission characteristics are called “degree of amplitude balance” and “degree of phase balance”, respectively.
The above-described degree of amplitude balance and degree of phase balance are expressed by the following equations when a filter device having balanced-to-unbalanced input/outputs is a device of three ports and when the unbalanced input terminal is denoted as “port
1
” and the balanced output terminals are denoted as “port
2
” and “port
3
”:
Degree of amplitude balance=|A|
where A=|20×log S
21
|−|20×log S
31
|, and where S
21
represents the amplitude of the transmission characteristics between port
1
and port
2
, and S
31
represents the amplitude of the transmission characteristics between port
1
and port
3
.
Degree of phase balance=|
B−
180|
where B=|∠S
21
−∠S
31
|, and where ∠S
21
represents the phase between port
1
and port
2
, and ∠S
31
represents the phase between port
1
and port
3
.
Ideally, in the pass band of the filter, the degree of amplitude balance is 0 dB and the degree of phase balance is zero degrees. However, in practice, the degree of amplitude balance only needs to be 1.5 dB or less, and the degree of phase balance only needs to be 20 degrees or less. Preferably, the degree of amplitude balance is 1.0 dB or less, and the degree of phase balance is 10 degrees or less.
In the conventional surface acoustic wave filter having a balanced-to-unbalanced conversion function, such as the above-described Japanese Unexamined Patent Application Publication No. 10-117123, to invert the phase of the transmission characteristics between an unbalanced terminal and one of the balanced terminals with respect to the transmission characteristics between the unbalanced terminal and the other balanced terminal, a method in which the polarity of an IDT (Interdigital Transducer) is inverted or a method in which, when the wavelength of the surface wave is denoted as &lgr;, the IDT-to-IDT spacing is widened by 0.5&lgr;, is used. As a consequence, a difference occurs in the electrical characteristics due to the difference between the configuration between an unbalanced terminal and one of the balanced terminals and the configuration between the unbalanced terminal and the other balanced terminal. Therefore, the degree of amplitude balance and the degree of phase balance in the pass band are often unsatisfactory.
Furthermore, in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, to prevent the deterioration of the above-described degree of balance, four surface acoustic wave filter devices
511
a
,
511
b
,
512
a
, and
512
b
are used, and each of the surface acoustic wave filter devices
511
and
512
has a two-step configuration. As a result, the size of the surface acoustic wave filter device
500
is substantially increased, and thus, size reduction is difficult. Furthermore, since the number of surface acoustic wave filter devices which can be manufactured from one wafer is decreased, the manufacturing costs substantially increase.
SUMMARY OF THE INVENTION
In order to overcome the above-described problems, preferred embodiments of the present invention provide a surface acoustic wave filter device which has a balanced-to-unbalanced conversion function, in which input and output impedances are different, which has a satisfactory degree of amplitude balance and degree of phase balance, and having a greatly reduced size and cost.
According to a first preferred embodiment of the present invention, a surface acoustic wave filter device includes a first surface acoustic wave filter element having a plurality of IDTs provided along the propagation direction of a surface-acoustic wave on a piezoelectric substrate and having first and second terminals, and a second surface acoustic wave filter element having a plurality of IDTs arranged along the propagation direction of a surface-acoustic wave on a piezoelectric substrate, the spacing between input/output IDTs is greater than that of the first surface acoustic wave filter element by approximately half the wavelength of the surface-acoustic wave, and having first and second terminals, the first terminals of the first and second surface acoustic wave filter elements being commonly connected to an unbalanced terminal, and the second terminals of the first and second surface acoustic wave filter elements being connected to a balanced terminal, wherein the total of the electrostatic capacitance values of the plurality of IDTs of the second surface acoustic wave filter element is greater than the total of the electrostatic capacitance values of the plurality of IDTs of the first surface acoustic wave filter element.
In the surface acoustic wave filter device in accordance with the first preferred embodiment of the present invention, when the total of the electrostatic capacitance values of the plurality of IDTs of the second surface acoustic wave filter element is denoted as C
2
and when the total of the electrostatic capacitance values of the plurality of IDTs of the first surface acoustic wave filter element is denoted as C
1
, the condition C
1
<C
2
<1.20(C
1
) is preferably satisfied.
W
Sawada Yoichi
Takamine Yuichi
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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