Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-07-26
2004-11-09
Tokar, Michael (Department: 2819)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S193000, C333S196000, C310S31300R, C310S31300R
Reexamination Certificate
active
06816036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter having filtering characteristics and, in particular, a balance-to-unbalance conversion function, and to a communication apparatus including the surface acoustic wave filter.
2. Description of the Related Art
Recently, there has been significant technological progress in reducing the size and weight of cellular telephones. In order to achieve these advantages, in addition to reducing the number and size of components, multi-function components are being increasingly developed. In the state of the art, recent research has been actively conducted regarding surface acoustic wave (SAW) filters having a balance-to-unbalance conversion function, i.e., a so-called “balun” function, which are used in the RF (radio frequency) stage of cellular telephones. Such SAW filters are used mostly for GSM (global system for mobile communications) compliant devices.
If a balanced line such as a twin lead feeder is directly coupled to an unbalanced line such as a coaxial cable, an unbalanced current flows, thereby, undesirably, causing the feeder itself to operate as an antenna. A circuit called a balun is thus required for preventing the unbalanced current in order to match the balanced line to the unbalanced line.
Many patent applications pertaining to SAW filters having a balance-to-unbalance conversion function have been filed. A well-known SAW filter having a balance-to-unbalance conversion function and having an input impedance and output impedance which are substantially equal to each other is illustrated in FIG.
19
.
In the SAW filter shown in
FIG. 19
, a comb-like electrode (also called an interdigital transducer, hereinafter referred to as an “IDT”)
101
is provided on a piezoelectric substrate
100
. IDTs
102
and
103
are positioned on the left and right sides of the IDT
101
, respectively, in the surface acoustic wave (SAW) propagation direction.
Reflectors
104
and
105
for reflecting a surface acoustic wave in order to improve the conversion efficiency are further arranged in the SAW filter in such a manner that the reflectors sandwich the IDTs
101
,
102
, and
103
therebetween from the left and right. The SAW filter further includes balanced signal terminals
106
and
107
, and an unbalanced signal terminal
108
.
The SAW filter of this type is called a three-IDT longitudinally coupled resonator SAW filter having a balance-to-unbalance conversion function by performing conversion between the IDTs
101
,
102
, and
103
using a surface acoustic wave.
In a SAW filter having a balance-to-unbalance conversion function, it is required that the transmission characteristics in the pass band between an unbalanced signal terminal and each balanced signal terminal have equal amplitude characteristics and 180°-out-of-phase characteristics. The amplitude characteristic and the phase characteristic are referred to as “amplitude balance” and “phase balance” between the balanced signal terminals, respectively.
The amplitude balance and the phase balance are defined as follows. If the SAW filter having a balance-to-unbalance conversion function is a three-port device consisting of, for example, a first port corresponding to an unbalanced input terminal and second and third ports corresponding to balanced output terminals, the amplitude balance=[A], where A=|20log(S
21
)|−|20log(S
31
)|, and the phase balance=B−180, where B=|∠S
21
−∠S
31
|. In the above equations, S
21
denotes the transfer coefficient from the first port to the second port, S
31
denotes the transfer coefficient from the first port to the third port, and | | indicates an absolute value.
In balance between the balanced signal terminals, ideally, the amplitude balance and the phase balance are 0 dB and 0 degree, respectively, in the pass band of a SAW filter.
However, the SAW filter in the conventional art shown in
FIG. 19
has a problem in that balance between the balanced signal terminals is low. As one possible reason, the distance (indicated by a portion
109
in
FIG. 19
) between an electrode finger connected to the balanced signal terminal
106
and a signal electrode finger of the IDT
102
is different from the distance (indicated by a portion
110
in
FIG. 19
) between an electrode finger connected to the balanced signal terminal
107
and a signal electrode finger of the IDT
103
by 0.5 times the wavelength defined by the pitch between the electrode fingers.
This leads to additional problems in that the total capacitance of the electrode fingers connected to the balanced signal terminal
106
is different from the that of the electrode fingers connected to the balanced signal terminal
107
, and that the conversion efficiency between an electrical signal and a surface acoustic wave is different between the balanced signal terminals
106
and
107
, resulting in low balance.
FIG. 20
shows that the balanced signal terminal
107
shown in
FIG. 19
is connected to the ground, and
FIG. 21
shows that the balanced signal terminal
106
shown in
FIG. 19
is connected to the ground.
FIG. 22
shows the difference between the frequency-amplitude characteristic output from the balanced signal terminal
106
in FIG.
20
and the frequency-amplitude characteristic output from the balanced signal terminal
107
in FIG.
21
. There is a large difference between the two amplitude characteristics, and this difference causes low balance.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a balance-to-unbalance conversion function that compensates for the above-described difference between balanced signal terminals, which causes low balance between the balanced signal terminals, and that achieves satisfactory balance between the balanced signal terminals, and also the preferred embodiments of the present invention provide a communication apparatus including such a SAW filter.
According to a preferred embodiment of the present invention, a SAW filter includes a piezoelectric substrate, an IDT set disposed on the piezoelectric substrate, and a balanced signal terminal connected to the interdigital transducer set. The IDT set includes, along a SAW propagation direction, a plurality of IDTs for converting an input electrical signal into a surface acoustic wave and outputting the surface acoustic wave, and for converting a propagating surface acoustic wave into an output electrical signal and outputting the output electrical signal. The IDT set is asymmetric with respect to an imaginary axis which is located at the center of the IDT set in the SAW propagation direction and which is substantially perpendicular to the SAW propagation direction.
The IDT set may include an odd number of IDTs. The imaginary axis may be located at the approximately central portion of the IDT which is centrally located among the plurality of the IDTs, and may be substantially perpendicular to the SAW propagation direction.
The distances between adjacent IDTs in the IDT set are different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
The ratios (duties) of the electrode finger widths relative to the pitches of the IDT set may be different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
The pitches of the interdigital transducer set are different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
The IDT set may include narrower-pitch electrode finger portions at portions where two IDTs are adjacent to each other, each narrower-pitch electrode finger portion including several electrode fingers having a smaller pitch than that of the other electrode fingers. The pitches of the narrower-pitch electrode finger portions may be different such that the interdigital transducer se
Keating & Bennett LLP
Khai Nguyen
Murata Manufacturing Co. Ltd
Tokar Michael
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