Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-09-20
2001-12-11
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R
Reexamination Certificate
active
06329888
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a surface acoustic wave transducer (hereinafter to be referred to as an IDT electrode) and a surface acoustic wave device structured by using the same, and relates, more particularly, to a reflection reversed type surface acoustic wave transducer having three electrodes disposed in one wave length of an excited surface acoustic wave thereby to have improved a drop in attenuation generated at a high-pass side near a pass band and have improved spurious signal response in this area.
BACKGROUND ART
In recent years, a surface acoustic wave device (hereinafter to be referred to as a SAW device) has been used in many communications fields, and has been playing the role in reducing the sizes of portable telephones and the like because of its excellent characteristics in high frequency, compactness and easiness in mass production.
FIG.
5
(
a
) is a top plan view for showing one example of an electrode pattern of a conventional primary-third order longitudinally coupled double mode SAW filter (hereinafter to be referred to as a double mode SAW filter). On a main surface of a piezoelectric substrate
11
, there are provided three normal IDT
12
, IDT
13
and IDT
14
closely disposed to each other along a propagation direction of a surface wave, with reflectors
15
a
and
15
b
disposed on both sides of these IDT's.
Each of the IDT
12
, IDT
13
and IDT
14
is structured by a pair of comb electrodes having a plurality of electrode fingers, with each electrode finger inserted into a space of the other comb electrode. One of the comb electrodes of the IDT
12
is connected with an input terminal and the other comb electrode is grounded. One of the comb electrodes of the IDT
13
and one of the comb electrodes of the IDT
14
are mutually connected with each other and are connected to an output terminal. The other comb electrode of the IDT
13
and the other comb electrode of the IDT
14
are mutually connected with each other and are grounded.
The double mode SAW filter shown in FIG.
5
(
a
) operates as follows as known. A plurality of surface waves excited by the IDT's
12
,
13
and
14
are trapped between the reflectors
15
a
and
15
b
, and an acoustic coupling is generated among the IDT's
12
,
13
and
14
. As a result, two longitudinally coupled resonance modes of primary and third-order are excited strongly, and the filter works as a double mode SAW filter utilizing these two modes. It is known that a passing band of the double mode SAW filter is proportional to a frequency difference between the primary-order resonance mode and the third-order resonance mode.
Having a plurality of the double mode SAW filters disposed on the piezoelectric substrate and having these filters connected in cascade to improve a shape factor and guaranteed attenuation of the filter, is also a means well-known in the art.
FIG.
5
(
b
) shows an example of frequency characteristics obtained as a result of simulating a double mode SAW filter, taking the IDT electrode pattern shown in FIG.
5
(
a
) as an example, by using 360° Y-cut X-propagation LiTaO
3
as a piezoelectric substrate, using 18 pairs of the IDT
12
, using 18 pairs of the IDT
13
, using 18 pairs of the IDT
14
, using
500
reflectors, assuming a pitch ratio L
t
/L
R
of the reflectors to the IDT's as 0.990, assuming a center frequency of 1.5 GHz, and assuming a necessary pass band width as 24 MHz.
However, when an attempt is made to realize a wide-band double mode SAW filter by using the conventional normal IDT electrode pattern, the following trend is observed as is clear from the filter characteristic shown in FIG.
5
(
b
). The shape factor at a high-pass side standardized by a center frequency is not as good as the shape factor at a low-pass side standardized by a center frequency, and the attenuation level is decreased by 13 dB at around 1.54 GHz and then increases. (A small ripple near the pass band shown in FIG.
5
(
b
) is attributable to the reflectors
15
a
and
15
b
, and this becomes smaller after being converted into a bulk wave in the actual product. Therefore, this has no practical problem.
In order to overcome the above problem, a plurality of double mode SAW filters having similar characteristics are connected in cascade, whereby to improve the characteristics. However, it is not possible to eliminate the above-described attenuation level decreasing characteristic itself by this method, and there has been a problem that an insertion loss increases two times or three times by the known cascade connection.
Further, there has recently been a demand for narrowing an interval between channels from the needs of effective utilization of frequencies. There has also been a demand for low loss and high attenuation in the filters used. However, these demands cannot be met by the conventional double mode SAW filters that have what is called the attenuation level decreasing characteristic at the high-pass side in the vicinity of the passing band.
In order to solve the above problems, there is also proposed a ladder type SAW filter. However, this also has a problem that the use of this filter is limited as it is not possible to obtain sufficient attenuation in the frequency at a distance from the center frequency of the pass band.
With a view to solving the above-described problems, it is an object of the present invention to provide a structure of IDT electrodes and an SAW using this structure that has improved both the attenuation at a high-pass side near the passing band and spurious signal response in this area.
DISCLOSURE OF THE INVENTION
In order to achieve the above object, according to a first aspect of the present invention, there is provided a reflection reversed type surface acoustic wave transducer, structured by a repetition of a plurality of unit segments formed on a piezoelectric substrate, each segment comprising: a first electrode finger having a width W
1
; a second electrode finger having a width W
2
disposed adjacent to the first electrode finger with a gap g
1
; a third electrode finger having a width W
3
disposed adjacent to the second electrode finger with a gap g
2
; and a space of (g
3
)/2 disposed on both end sides of the first electrode finger and the third electrode finger respectively, wherein the first and third electrode fingers are set in-phase, with the second electrode finger set in an opposite phase, the width W
1
of the first electrode finger and the width W
3
of the third electrode finger are set equal to each other, and the gap g
1
and the gap g
2
are set equal to each other. (Corresponding to claim
1
)
According to a second aspect of the invention, there is provided a longitudinally coupled multi mode SAW filter, wherein a plurality of surface acoustic wave transducers in the above first aspect of the invention are disposed close to each other along a propagation direction of a surface wave, with reflectors disposed on both sides thereof. (Corresponding to claim
2
)
According to a third aspect of the invention, there is provided a longitudinally coupled multi mode SAW filter, wherein a plurality of surface acoustic wave transducers in the above first aspect of the invention are disposed close to each other along a propagation direction of a surface wave, with reflectors disposed on both sides thereof, and a ratio of a pitch of the reflectors to a pitch of the IDT's is set larger than 1. (Corresponding to claim
3
).
According to a fourth aspect of the invention, there is provided a reflection reversed type surface acoustic wave converter, wherein, in a unit segment of IDT electrodes disposed on a piezoelectric substrate, a combined vector which is a combination of reflection vectors from each end surface of each electrode finger within the segment has a positive phase. (Corresponding to claim
4
).
REFERENCES:
patent: 3870975 (1975-03-01), Vasile
patent: 4249146 (1981-02-01), Yen et al.
patent: 4642506 (1987-02-01), Lewis
patent: 4902925 (1990-02-01), Wright
patent: 5793146 (1998-08-01), Wright
pat
Koda & Androlia
Pascal Robert
Summons Barbara
Toyo Communication Equipment Co., Ltd.
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