Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-06-26
2004-04-06
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S133000, C333S195000, C310S31300R
Reexamination Certificate
active
06717489
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 longitudinally connected resonator type surface acoustic wave filter.
2. Description of the Related Art
In recent years, advances have been made in the reduction in size and weight of portable telephones. Therefore, a reduction in the number of components defining a portable telephone and miniaturization of the components are required. To meet these requirements, development of components combining a plurality of functions is also progressing.
Surface acoustic filters used at the RF(radio frequency) stage of portable telephones provided with a balanced-unbalanced conversion function, or a balun function, have been developed, and are in use as GSM-type portable telephones.
Such surface acoustic wave filters having a balanced-unbalanced conversion function are disclosed in, for example, Japanese Unexamined Patent Application publication Nos. 6-204781 and 11-97966.
FIG. 22
is a schematic plan view showing the electrode structure of a conventional surface acoustic wave filter having a balanced-unbalanced conversion function.
In this surface acoustic wave filter
100
, longitudinally connected resonator type surface acoustic wave filters
101
and
102
are used. These longitudinally connected resonator type surface acoustic wave filters
101
and
102
include three IDTs
101
a
to
101
c
, and
102
a
to
102
c
, respectively, and reflectors
101
d
and
101
e
, and
102
d
and
102
e
, respectively.
One-side of the IDTs
101
a
and
101
c
of the longitudinally connected resonator type surface acoustic wave filter
101
are commonly connected to an unbalanced signal terminal
104
. Likewise, one-side of the IDTs
102
a
and
102
c
of the longitudinally connected resonator type surface acoustic wave filter
102
are commonly connected to an unbalanced signal terminal
104
.
The central IDTs
101
b
and
102
b
of the surface acoustic filters
101
and
102
are connected to balanced signal terminals
105
and
106
, respectively.
The phase of the IDT
101
b
is opposite to that of IDT
102
b
. Hence, the phases of signals outputted from the terminal
105
and
106
are different from each other by approximately 180°. Thereby, unbalanced signals input from the terminal
104
are transformed into balanced signals, and are output from the terminals
105
and
106
.
FIG. 23
is a schematic plan view showing the electrode structure of the surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 06-204781. In this surface acoustic wave filter
200
, three IDTs
200
a
to
200
c
are arranged in the propagation direction of a surface acoustic wave, and reflectors
200
d
and
200
e
are disposed on opposite sides of the area where these IDTs
200
a
to
200
c
are arranged. The phase of the IDT
200
a
is opposite to that of the IDT
200
c
, and thereby the phases of signals output from the terminals
202
and
203
connected to the respective IDTs
200
a
and
200
c
are different from each other by approximately 180°. Therefore, unbalanced signals input from an unbalanced terminal
201
connected to the IDT
200
b
are transformed into balanced signals, and are output from the terminals
202
and
203
.
FIG. 24
is a schematic plan view showing the electrode structure of the surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 11-97966. In this surface acoustic wave filter
300
, IDTs
300
a
to
300
c
are arranged in order along the propagation direction of a surface acoustic wave. Also, reflectors
300
d
and
300
e
are disposed on opposite sides of the area where these IDTs
300
a
to
300
c
are arranged.
Herein, one-side end of the IDTs
300
a
and
300
c
are commonly connected to an unbalanced signal terminal
301
.
On the other hand, one-side comb electrode of the central IDT
300
b
is divided into comb electrodes
300
b
1
and
300
b
2
, and these comb electrodes
300
b
1
and
300
b
2
are connected to terminals
302
and
303
, respectively.
In the surface acoustic wave filter
300
, the phase of the IDT
300
c
is opposite to that of the IDT
300
a
. Hence, the phases of signals output from the terminal
302
and
303
are different from each other by approximately 180°. Thereby, unbalanced signals input from the terminal
301
are output from the terminals
302
and
303
as balanced signals.
In any of the above-described surface acoustic wave filters
100
,
200
, and
300
, the output impedance is about four times greater than the input impedance. In these surface acoustic wave filters
100
,
200
, and
300
, when switching the positions of the input terminal and the output terminal, the input impedance becomes four times greater than the output impedance. As a result, a filter providing a balanced-unbalanced output is produced.
A filter having a balanced-unbalanced conversion function requires the transmission characteristics in the pass band between an unbalanced signal terminal and one of the balanced terminals to be equal in the amplitude characteristics to, and different in the phase by 180°. from, the transmission characteristics between an unbalanced signal terminal and the other of the balanced terminals. These requirements are called “amplitude balance degree” and “phase balance degree”, respectively.
Where a filter device having the above-described balanced-unbalanced conversion function is a three-port device, letting an unbalanced input terminal be port
1
, and letting two balanced output terminals be each ports
2
and
3
, the amplitude balance degree and the phase balance degree is defined as follows:
Amplitude balance degree=|
A|, A
=|20·log(
S
21
)|−|20·log(
S
31
)|
Phase balance degree=|
B−
180|,
B=|∠S
21
−∠
S
31
|
Ideal values of the amplitude balance degree and the phase balance degree in the pass band of the filter, are considered to be 0 dB for the amplitude balance degree, and 0° for the phase balance degree. The current market demand for the amplitude balance degree is about 2.0 dB, and that for the phase amplitude balancing is about 20°.
In reality, however, in any of the surface acoustic wave filters
100
,
200
, and
300
, deviations exist in the above-described balance degrees, and therefore the balance degrees are not sufficient for practical use.
This is because, in the surface acoustic wave filter
100
, the electrode fingers of the IDT
101
b
adjacent to the IDTs
101
a
and
101
c
define a ground electrode, whereas the electrode fingers of the IDT
102
b
adjacent to the IDTs
102
a
and
102
c
define a signal electrode, thereby causing significant differences in the frequency characteristics between the surface acoustic wave filter
101
and
102
.
FIG. 25
shows the differences in the frequency characteristics between the surface acoustic wave filters
101
and
102
in the surface acoustic wave filter
100
shown in FIG.
22
. In
FIG. 25
, the solid lines show the frequency characteristics of the surface acoustic wave filters
101
, and the broken lines show those of the surface acoustic wave filters
102
. In any of the surface acoustic wave filters
101
and
102
, impedance matching has been achieved with 100 &OHgr;. In the figure, the right side scale of the vertical axis shows enlarged frequency characteristics.
As can be seen from
FIG. 25
, the frequency characteristics of the surface acoustic wave filters
101
and
102
differ greatly. Particularly on the higher frequency side of the pass band range, a significant difference is observed. This difference is a large factor contributing to the deterioration of the above-described balance degrees, when a surface acoustic wave device having a balanced-unbalanced conversion function is made using the surface acoustic wave
101
and
102
.
Also, in each of
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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