Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-12-24
2004-10-26
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R
Reexamination Certificate
active
06809614
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device for use, for example, as a band-pass filter, and more particularly, to a surface acoustic wave device of which at least one of the input terminal and the output terminal is an unbalanced signal terminal.
2. Description of the Related Art
In recent years, the demand for surface acoustic wave devices for use in the RF stages of portable telephones and having a balanced-to-unbalanced function, a so-called balun function, have increased, and the use of such devices in the RF stages of GSM system portable telephones has started.
Regarding the balanced-to-unbalanced function, the difference between the amplitudes at a pair of terminals provided on the balance side (hereinafter, referred to as an amplitude balance characteristic) is required to be zero, and the difference between the phases (hereinafter, referred to as a phase balance characteristic) is required to be about 180°.
FIG. 6
is a schematic view of an example of a related art surface acoustic wave device having a balanced-to-unbalanced function.
In a surface acoustic wave device
200
shown in
FIG. 6
, longitudinally-coupled resonator type SAW filters
201
and
202
, and 1 port type SAW resonators
203
and
204
are composed of Al electrodes on a piezoelectric substrate. The input sides of the SAW filters
201
and
202
are connected in parallel to each other and also are connected to an unbalanced signal terminal
205
. The output sides of the SAW filters
201
and
202
are connected in series with the SAW resonators
203
and
204
, respectively. The SAW resonators
203
and
204
are connected to balanced signal terminals
206
and
207
, respectively.
In the SAW filter
201
shown in
FIG. 6
, IDTs
201
a
and
201
b
which are positioned on the unbalanced sides and are adjacent to reflectors have inverted-shapes with respect to IDTs
202
a
and
202
b
which are adjacent to the reflectors in the SAW filter
202
. As a result, a signal output from the balanced signal terminal
206
and the signal output from the balanced signal terminal
207
have a phase difference of 180°.
According to the configuration shown in
FIG. 6
, of a plurality of the electrodes of the IDT
201
a
and the IDT
201
b
contained in the SAW filter
201
, the electrode fingers
201
d
and
201
e
which are adjacent to the center IDT
201
c
are connected to the unbalanced signal terminal
205
, respectively. On the other hand, of a plurality of the electrodes of the IDT
202
a
and the IDT
202
b
of the SAW filter
202
, the electrode fingers
202
d
and
202
e
which are adjacent to the center IDT
202
c
are connected to the ground potential, respectively. Accordingly, the phase characteristic of the SAW filter
202
is not exactly equal to the inverted characteristic obtained when the phase of the SAW filter
201
is inverted by 180°. Thus, the phase balance characteristic of the surface acoustic wave
200
device can not be equal to 180° which is an ideal value.
Moreover, in some of the related art surface acoustic wave devices having a balanced-to-unbalanced conversion function, the layout thereof such as a wiring structure ranging from an unbalanced signal terminal to a pair of balanced signal terminals is not symmetrical with respect to the unbalanced signal terminal. In this case, the parasitic capacitances of the lines extending to a pair of the balanced signal terminals become different. This readily causes deterioration of the phase balance characteristic.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device having a balanced-to-unbalanced function and a superior phase balance characteristic.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate, at least one interdigital transducer IDT arranged on the piezoelectric substrate, an input terminal and an output terminal, one of the input terminal and the output terminal including an unbalanced signal terminal, the other of the input terminal and the output terminal including first and second balanced signal terminals, and a delay line provided on the unbalanced signal terminal side in the path between the unbalanced signal terminal and the first balanced signal terminal. Accordingly, the phase of a signal in the line in which the delay line is introduced is delayed, due to the introduction of the delay line. Thus, the phase balance characteristic is greatly improved. As a result, the surface acoustic wave device having a balanced-to-unbalanced function with a superior phase balance characteristic is provided.
Preferably, the delay line is disposed on the piezoelectric substrate. In this case, the surface acoustic wave device can be easily designed compared to the case where the delay line is disposed on the package. Moreover, the delay line can be formed easily with high precision on the piezoelectric substrate using a photolithography technique, for example. Moreover, the design of the delay line can be easily changed.
According to a second preferred embodiment of the present invention, a surface acoustic wave device includes a first surface acoustic wave filter element including a first piezoelectric substrate, and at least two interdigital transducers (IDTs) arranged along the surface acoustic wave propagation direction on the first piezoelectric substrate, a second surface acoustic wave filter element including a second piezoelectric substrate, and at least two interdigital transducers (IDTs) arranged along the surface acoustic wave propagation direction on the second piezoelectric substrate, the second surface acoustic wave filter element having a transmission phase characteristic that is different by about 180° from that of the first surface acoustic wave filter element, the first and second surface acoustic wave filter elements each having an input terminal and an output terminal, certain ones of the input terminals and the output terminals of the first and second surface acoustic wave filter elements being commonly connected to each other at a common connection point, an unbalanced signal terminal being connected to the common connection point, the other terminals of the input terminals and the output terminals of the first and second surface acoustic wave filter elements defining first and second balanced signal terminals, respectively, and a delay line provided between the common connection point and the first surface acoustic wave filter element. Thus, the phase of a signal on the first surface acoustic wave filter element side is delayed compared to the phase of the signal on the other surface acoustic wave filter element side, and hence, the phase balance characteristic is greatly improved.
Also, preferably, the delay line is disposed on the piezoelectric substrate. In this case, the surface acoustic wave device can be easily designed compared to the case where the delay line is disposed on the package. Also, the cost can be greatly reduced. Moreover, the delay line can be formed easily with high precision on the piezoelectric substrate using a photolithography technique, for example. Further, the design of the delay line can be easily changed.
Preferably, the length of the signal line between the common connection point and the IDTs of the first surface acoustic wave filter element electrically connected to the common connection point is different from the length of the signal line between the common connection point and the IDTs of the second surface acoustic wave filter element electrically connected to the common connection point. This line length difference defines the delay line.
Also, preferably, the device is configured so that the phases of input-output signals in the line between the unbalanced signal terminal and the first balanced signal terminal and, those of the input-output signals in the line between the unbalanced signal terminal and the second bala
Fujii Yasuhisa
Ouchi Minefumi
Yata Masaru
Keating&Bennett, LLP
Murata Manufacturing Co. Ltd.
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