Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-10-05
2003-07-22
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S133000, C310S31300R
Reexamination Certificate
active
06597262
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave filters suitable for use in mobile communication apparatuses such as mobile phones. More particularly, the present invention relates to wiring structures of longitudinally-coupled resonator-type surface acoustic wave filters.
2. Description of the Related Art
As a conventional band pass filter used in the RF stage of a mobile communication apparatus such as a mobile phone, a surface acoustic wave filter is well known. In general, capabilities required in such a band pass filter include characteristics of low loss, high attenuation, a broad band, and other known characteristics. Thus, in order to improve these characteristics of the surface acoustic wave filter, there have been many inventions developed in the past.
For example, in order to obtain high attenuation in a longitudinally-coupled resonator-type surface acoustic wave filter, a method is known in which a surface acoustic wave filter is constituted by longitudinally connecting a plurality of longitudinally-coupled resonator-type surface acoustic wave elements, as described in Japanese Unexamined Patent Application Publication No. 5-335881. In this method, since the plurality of surface acoustic wave elements are longitudinally connected to each other, there is a disadvantage that insertion loss within the pass band increases. On the other hand, high attenuation outside the pass band can be obtained.
However, when constituting a filter having a broad pass bandwidth by longitudinally connecting the longitudinally-coupled resonator-type surface acoustic wave elements as mentioned above, waves within the pass band and the voltage standing wave ratio (VSWR) both become large. Next, the cause of the problem will be explained with reference to FIG.
6
.
FIG. 6
is a plan view of a conventional surface acoustic wave filter
100
constructed by longitudinally connecting two longitudinally-coupled resonator-type surface acoustic wave elements. In this figure, the reference numeral
138
denotes a piezoelectric substrate made of LiTaO
3
. The reference numerals
101
and
102
denote longitudinally-coupled resonator-type surface acoustic wave elements arranged on the piezoelectric substrate
138
. The surface acoustic wave element
101
is composed of an IDT (interdigital transducer)
103
, two IDTs
104
and
105
located on each side of the IDT
103
, and two reflectors
106
and
107
disposed on each side of the arrangement of the IDTs
104
,
103
, and
105
. In the same manner, the surface acoustic wave element
102
is composed of IDTs
108
,
109
, and
110
, with reflectors
111
and
112
. The two surface acoustic wave elements
101
and
102
are longitudinally connected to each other to constitute the surface acoustic wave filter
100
. The filter
100
is electrically connected to bonding pads
123
to
128
on a package
137
via bonding wires
129
to
136
. The bonding pad
124
is used as an input terminal, the bonding pad
127
is used as an output terminal, and the bonding pads
123
,
125
,
126
, and
128
are used as ground terminals. In addition, electrode pads
113
to
120
are provided on the piezoelectric substrate
138
so as to correspond to the bonding pads
123
to
128
disposed on the package
137
. The electrode pad
114
is used as an input terminal, the electrode pad
119
is used as an output terminal, and the electrode pads
113
,
115
,
116
,
117
,
118
, and
120
are used as ground terminals. Furthermore, in order to longitudinally connect the surface acoustic wave elements
101
and
102
, an electrode pattern
121
for connecting the IDTs
104
and
109
and an electrode pattern
122
for connecting the IDTs
105
and
110
are disposed on the piezoelectric substrate
138
.
In the surface acoustic wave filter
100
having the above-described arrangement, parallel capacitances are generated between the electrode pad
116
as a ground terminal and the electrode patterns
121
and
122
and between the electrode pad
117
as a ground terminal and the electrode patterns
121
and
122
, respectively. When the parallel capacitances are generated between the ground-terminal pads and the electrode patterns through which a passing signal is transmitted, the impedance of each element viewed from the interstage junction tends to be capacitive.
Primarily, in a surface acoustic wave filter having a broad pass bandwidth, the impedance tends to be capacitive. As shown in the conventional filter in
FIG. 6
, in the structure in which the parallel capacitances are generated at the interstage junction of the longitudinally connected surface acoustic wave elements, the impedance tends to be more capacitive. As a result, waves within the pass band and the VSWR both become larger, resulting in the filter characteristics being significantly deteriorated.
SUMMARY OF THE INVENTION
In order to solve the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a broad pass bandwidth that minimizes generation of parallel capacitances at the interstage junctions of longitudinally connected surface acoustic wave elements so that the impedance of each element is hardly capacitive.
According to a preferred embodiment of the present invention, a surface acoustic wave filter includes a piezoelectric substrate, a plurality of longitudinally-coupled resonator-type surface acoustic wave elements having a plurality of interdigital transducers (IDTs) disposed on the piezoelectric substrate in a direction in which a surface acoustic wave propagates, electrode pads defining input/output terminals of the longitudinally-coupled resonator-type surface acoustic wave elements, and wiring patterns for electrically connecting the plurality of longitudinally-coupled resonator-type surface acoustic wave elements. In this filter, at least one pair of the plurality of longitudinally-coupled resonator-type surface acoustic wave elements is longitudinally connected to each other via the wiring patterns, and at least one of the electrode pads is arranged between the longitudinally connected surface acoustic wave elements.
In this arrangement, since at least one of the electrode pads defining input/output terminals is arranged between the longitudinally connected surface acoustic wave elements, parallel capacitances generated at the interstage junction of the surface acoustic wave elements are minimized and eliminated.
In addition, according to another preferred embodiment of the present invention, a surface acoustic wave filter includes the structure of the filter according to the preferred embodiment described in the preceding paragraph. In this filter, at least one of the longitudinally-coupled resonator-type surface acoustic wave elements is one of a balanced-input type and a balanced-output type so as to achieve a balance-unbalance conversion function.
When applying the structure of the filter of the first preferred embodiment to the filter having the balance-unbalance conversion function according to the other preferred embodiment, parallel capacitances generated at the interstage junction are minimized, thereby improving balancing in the balance-unbalance conversion, which is an additional advantage that is achieved in this structure.
Furthermore, according to another preferred embodiment of the present invention, a communication apparatus includes the surface acoustic wave filter according to one of the above-described preferred embodiments.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention.
REFERENCES:
patent: 5790000 (1998-08-01), Dai et al.
patent: 5874869 (1999-02-01), Ueda et al.
patent: 5994980 (1999-11-01), Tada
patent: 6160339 (2000-12-01), Takagi et al.
patent: 6255915 (2001-07-01), Edmonson
patent: 0 884843 (1998-12-01), None
patent: 4-113712 (1992-04-01), None
patent: 05-335881 (1993-12-01), None
patent:
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
LandOfFree
Surface acoustic wave filter and communication apparatus... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Surface acoustic wave filter and communication apparatus..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface acoustic wave filter and communication apparatus... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3080074