Surface acoustic wave filter and communication device using...

Details Surface acoustic wave filter and communication device using... Surface acoustic wave filter and communication device using...

2002-07-15

2004-01-06

Summons, Barbara (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S195000, C333S133000, C310S31300R

Reexamination Certificate

active

06674345

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a longitudinal-mode surface acoustic wave filter, a method of manufacturing a surface acoustic wave filter, and a communication device.
2. Related Art of the Invention
In recent years, surface acoustic wave filters have been widely used in mobile communication devices. Surface acoustic wave filters of a longitudinal mode type or a ladder type are used as a filter in a radio frequency (RF) stage. With the improvement in performance of communication devices such as portable telephones, there has been an increasing demand for reducing the loss and increasing the attenuation in surface acoustic wave filters.
A conventional longitudinal-mode surface acoustic wave filter will be described.
FIG. 12
shows a configuration of a conventional longitudinal-mode surface acoustic wave filter. As shown in
FIG. 12
, the surface acoustic wave filter has a piezoelectric substrate
801
, first, second, and third interdigital transducer (IDT) electrodes
802
,
803
, and
804
, and first and second reflector electrodes
805
and
806
, the IDT electrodes and the reflector electrodes being formed on the substrate. The upper electrode fingers of each of the second and third IDT electrodes
803
and
804
is connected to an input terminal IN, while the lower electrode fingers of each of the second and third IDT electrodes
803
and
804
is grounded. The lower electrode fingers of the first IDT electrode
802
is connected to an output terminal OUT, while the upper electrode fingers of the first IDT electrode
802
is grounded. The distances between centers of adjacent pairs of the electrode fingers of the first, second, and third IDT electrodes
802
,
803
, and
804
, represented by the distance indicated by P in
FIG. 12
(hereinafter referred to as “pitch”), are equal to each other. The longitudinal-mode surface acoustic wave filter is thus constructed.
In the above-described surface acoustic wave filter, the electrode fingers are arranged with a constant pitch in order that the acoustic velocity of a surface acoustic wave be constant through the arrangement of the first, second, and third IDT electrodes
802
,
803
, and
804
. In many instances, however, the number of electrode fingers of the first IDT electrode
802
and that of each of the second and third IDT electrodes
803
and
804
are set different from each other according to a design considering the bandwidth and impedance. Ordinarily, the surface acoustic wave filter is designed so that the number of electrode fingers of the first IDT electrode
802
is larger than that of each of the second and third IDT electrodes
803
and
804
.
A longitudinal-mode surface acoustic wave filter has also been used which is designed so that the electrode fingers of each of electrodes have different pitches as shown in
FIG. 13
to achieve a reduction in loss for example. The conventional longitudinal-mode surface acoustic wave filter shown in
FIG. 13
is based on a design in which each IDT electrodes have an electrode finger pitch different from that in a main region.
Referring to
FIG. 13
, the surface acoustic wave filter has a piezoelectric substrate
1201
, first, second, and third IDT electrodes
1202
,
1203
, and
1204
, and first and second reflector electrodes
1205
and
1206
, the IDT electrodes and the reflector electrodes being formed on the substrate. The upper electrode fingers of each of the second and third IDT electrodes
1203
and
1204
is connected to an input terminal IN, while the lower one of each of the second and third IDT electrodes
1203
and
1204
is grounded. The lower electrode fingers of the first IDT electrode
1202
is connected to an output terminal OUT, while the upper electrode fingers of the first IDT electrode
1202
is grounded.
Also, referring to
FIG. 13
, if the pitch in a region indicated by
1
a
in the first IDT electrode
1202
is P, P is ½ wavelength. If the pitch in a region indicated by
1
b
is P′, P′ is smaller than ½ wavelength. The pitch in a region indicated by
2
a
in the second IDT electrode
1203
is P, and P is ½ wavelength. The pitch in a region indicated by
2
b
is P′, and P′ is smaller than ½ wavelength. Similarly, the pitch in a region indicated by
3
a
in the third IDT electrode
1204
is P, and P is ½ wavelength. The pitch in a region indicated by
3
b
is P′, and P′ is smaller than ½ wavelength.
Thus, in each of the first IDT electrode
1202
, the second IDT electrode
1203
, and the third IDT electrode
1204
, different electrode finger pitches are set between the electrode fingers in the same IDT electrodes.
Also in many instances relating to the arrangement shown in
FIG. 13
, the number of electrode fingers of the first IDT electrodes
1202
and that of each of the second and third IDT electrodes
1203
and
1204
are set different from each other according to a design considering the bandwidth and impedance. Ordinarily, the surface acoustic wave filter is designed so that the number of electrode fingers of the first IDT electrode
1202
is larger than that of each of the second and third IDT electrodes
1203
and
1204
.
There is a problem in such a surface acoustic wave filter that there is a limit to improvement in filter characteristics in achieving a wide-band characteristic.
SUMMARY OF THE INVENTION
In view of the above-described problem, an object of the present invention is to provide a wide-band surface acoustic wave filter having a steep out of-band attenuation characteristic, a method of manufacturing the surface acoustic wave filter, and a communication device.
One aspect of the present invention is a surface acoustic wave filter comprising:
a piezoelectric substrate;
at least an input IDT electrode arranged on said piezoelectric substrate; and
at least an output IDT electrode arranged on said piezoelectric substrate,
wherein a pitch of electrode fingers of said input IDT electrode and a pitch of electrode fingers of said output IDT electrode are different from each other.
Another aspect of the present invention is the surface acoustic wave filter, wherein the pitch of electrode fingers of the IDT electrode larger in number of electrode fingers in said input and output IDT electrode is larger than the pitch of electrode fingers smaller in number of electrode fingers.
Still another aspect of the present invention is a surface acoustic wave filter comprising:
a piezoelectric substrate;
at least an input IDT electrode arranged on said piezoelectric substrate; and
at least an output IDT electrode arranged on said piezoelectric substrate,
wherein the metalization ratio of said input IDT electrodes and the metalization ratio of said output IDT electrodes are different from each other.
Yet still another aspect of the present invention is the surface acoustic wave filter, wherein the metalization ratio of an IDT electrode larger in number of electrode fingers in said input and output IDT electrodes is lower than the metalization ratio of an IDT electrode smaller in number of electrode fingers.
Still yet another aspect of the present invention is the surface acoustic wave filter, wherein if an IDT electrode has a plurality of electrode finger pitches, the pitch of main excitation electrode fingers is set as a basic pitch.
A further aspect of the present invention is the surface acoustic wave filter, wherein a peak frequency of a radiation characteristic of said input IDT electrode is substantially equal to a peak frequency of a radiation characteristic of said output IDT electrode.
A still further aspect of the present invention is the surface acoustic wave filter, wherein one of said input IDT electrode and said output IDT electrode comprises a first IDT electrode including a pair of electrode fingers opposed to each other;
the other of said input IDT electrode and said output IDT electrode comprises a second IDT electrode including a pair of electrode fingers opposed to each other, and a third IDT electrod

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