Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-11-20
2004-07-27
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S133000, C310S31300R, C310S31300R
Reexamination Certificate
active
06768397
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter used as, for example, a band pass filter in a portable telephone, and more particularly, to a surface acoustic wave filter having a balanced-to-unbalanced conversion function, namely, a balun function.
2. Description of the Related Art
With the reduction in size and weight of portable telephones in recent years, the number and size of components included in portable telephones have been greatly reduced. In addition, components combining a plurality of functions are being developed.
Accordingly, surface acoustic filters used for the RF(radio frequency) stage of portable telephones, having a balanced-to-unbalanced conversion function, namely, a balun function, have been developed, and are primarily used in GSM portable telephones.
FIG. 11
is a schematic plan view showing the electrode structure of a conventional surface acoustic wave filter
100
having a balanced-to-unbalanced conversion function.
In the surface acoustic wave filter
100
, first to third IDTs (interdigital transducers)
101
to
103
are arranged on a piezoelectric substrate (not shown) along the propagation direction of a surface acoustic wave. Reflectors
104
and
105
are provided on the opposite sides of the IDTs
101
to
103
in the direction of a surface acoustic wave.
One end of each of the IDTs
101
and
103
is electrically connected to an unbalanced signal terminal
108
, and the other end of each IDT
101
and
103
is connected to the ground potential. One end of the IDT
102
is electrically connected to a first balanced signal terminal
106
, and the other end thereof is electrically connected to a second balanced signal terminal
107
.
The magnitude of the attenuation outside the pass band of a surface acoustic wave filter having a balanced-to-unbalanced conversion function depends upon the degree of balance of the surface acoustic wave filter. The degree of balance is represented as the difference between the transmission characteristic between the unbalanced signal terminal and the first balanced signal terminal, and the transmission characteristic between the unbalanced signal terminal and the second balanced signal terminal. Of these differences in the transmission characteristic, the difference in the amplitude characteristic is called “amplitude balance degree” and the difference in the phase characteristic is called “phase balance degree”.
When a surface acoustic filter having a balanced-to-unbalanced conversion function is defined by a device having first to third ports, and, for example, the unbalanced input terminal thereof is the first port, and the first and second balanced output terminals thereof are the second and third ports, respectively, the amplitude balance degree and the phase balance degree are represented by the following expressions:
Amplitude balance degree=|
A|, A=|
20·log(
S
21)|−|20·log(
S
31)|, and
Phase balance degree=|
B−
180
|, B=|∠S
21−∠
S
31|
Here, S
21
denotes the transmission coefficient from the first port to the second port, and S
31
denotes the transmission coefficient from the first port to the third port.
Optimal values of the amplitude-balance degree and the phase balance degree in the outside of the pass band of the filter are 0 dB for the amplitude balance degree, and 0° for the phase balance degree. The magnitude of the attenuation outside the pass band in the filter having these optimal degrees of balance is infinity. Therefore, as the amplitude balance degree and the phase balance degree approach 0 dB and 0 degrees, respectively, the attenuation outside the pass band is greatly increased.
In the surface acoustic wave filter
100
shown in
FIG. 11
, although a balanced-to-unbalanced conversion function can be achieved, the degree of balance is unsatisfactory, and thus, the attenuation outside the pass band is insufficient.
SUMMARY OF THE INVENTION
To overcome the above-described problems and drawbacks, preferred embodiments of the present invention provide a longitudinally-coupled resonator-type surface acoustic wave filter that improves the degree of balance outside the pass band in addition to having a balanced-to-unbalanced conversion function, and that greatly increases the attenuation outside the pass band.
A surface acoustic wave filter according to a first preferred embodiment of the present invention includes a piezoelectric substrate, a plurality of IDTs provided on the piezoelectric substrate and arranged along the propagation direction of a surface acoustic wave, and a balanced-to-unbalanced conversion function. Among the plurality of IDTs, the IDTs on opposite sides are disposed in an approximate point-symmetry arrangement relative to the IDT positioned at the approximate center in the propagation direction of a surface acoustic wave.
A surface acoustic wave filter according to a second preferred embodiment of the present invention includes a piezoelectric substrate, first to third IDTs provided on the piezoelectric substrate, and sequentially arranged along the propagation direction of a surface acoustic wave, an unbalanced signal terminal connected to the first and third IDTs, and first and second balanced signal terminals each connected to the opposite ends of the second IDT. In this surface acoustic wave filter, the first to third IDTs have first and second end portions located on opposite sides thereof in a direction that is substantially perpendicular to the propagation direction of a surface acoustic wave, the first end portion of the first IDT and the second end portion of the third IDT are each electrically connected to the unbalanced signal terminal, and the second end portion of the first IDT and the first end portion of the third IDT are each connected to the ground potential.
A surface acoustic wave filter according to a third preferred embodiment of the present invention includes a piezoelectric substrate, first to third IDTs provided on the piezoelectric substrate and sequentially arranged along the propagation direction of a surface acoustic wave, an unbalanced signal terminal connected to the second IDT, and first and second balanced signal terminals connected to the first and third IDT. In this surface acoustic wave filter, the first to third IDTs have first and second end portions located on opposite sides thereof in a direction that is substantially perpendicular to the propagation direction of a surface acoustic wave, the first end portion of the first IDT and the second end portion of the third IDT are each electrically connected to a first balanced signal terminal, and the second end portion of the first IDT and the first end portion of the third IDT are each electrically connected to a second balanced signal terminal.
In the above-described conventional surface acoustic wave filter
100
, since the degree of balance outside the pass band is unsatisfactory, the attenuation outside the pass band is insufficient. The reason for this is as follows. Since the balanced signal terminal
106
is surrounded by the signal line connected to the unbalanced signal terminal
108
, a parasitic capacitance inserted between input and output terminals in parallel has a large influence, and, since the balanced signal terminal
107
is adjacent to the wiring connected to the ground terminal, a parasitic capacitance added between the signal line and the ground line in parallel has a large influence. That is, the parasitic capacitances added to the balanced signal terminals
106
and
107
differ from each other, and thereby the degree of balance outside the pass band is unsatisfactory.
Due to the above-described problems, the present inventor found that, if an electrode is provided such that substantially equal parasitic impedances are each added to a pair of balanced signal terminals, the degree of balance outside the pass band is greatly improved, and the attenuation outside the pass band
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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