Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-05-15
2003-09-16
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S196000, C333S133000, C310S31300R
Reexamination Certificate
active
06621380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a longitudinally coupled resonator type surface acoustic wave filter, and more particularly, to a longitudinally coupled resonator type surface acoustic wave filter, in which at least three IDTs are arranged in a surface wave propagating direction.
2. Description of the Related Art
Recently, with the increasing numbers of customers and the diversifying services, many more mobile phone systems use transmission-side frequency bands and reception-side frequency bands close to each other. In addition, to prevent interference with other communication systems, there is a demand for increased attenuation very close to a pass band.
As a result, there is a strong demand for obtaining an attenuation band very closely to the pass band in surface acoustic wave filters widely used as band pass filters for an RF stage in a mobile phone.
On the other hand, recently, to reduce the number of components, there has been a growing demand for a balance-unbalance conversion function, the so-called balun function. Thus, as the band pass filter for an RF stage in a mobile phone, longitudinally coupled resonator type surface acoustic wave filters easily adaptable to the balance-unbalance conversion function have been used. Various types of longitudinally coupled resonator type surface acoustic wave filters (for example, Japanese Unexamined Patent Application Publication No. 5-267990, etc.) have been provided.
However, in the longitudinally coupled resonator type surface acoustic wave filter, an undesired response referred to as a transversal response appears on a frequency side higher than a pass band. Thus, for example, it is impossible to obtain large attenuation on the pass band high frequency side, which is required in a mobile phone system of the PCS system.
FIG. 10
shows a schematic plan view illustrating the electrode structure of a longitudinally coupled resonator type surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 10-190394. In the longitudinally coupled resonator type surface acoustic wave filter
101
, IDTs
102
to
104
are arranged in a surface wave propagating direction. On both sides of the IDTs
102
to
104
reflectors
105
and
106
are provided. In this case, an IDT gap P between the IDTs
102
and
103
is differentiated from an IDT gap Q between the IDTs
103
and
104
. With this arrangement, the above transversal response is suppressed. In the conventional longitudinally coupled resonator type surface acoustic wave filter of the 3-IDT type, in addition to a zero-order mode and a second mode, a resonance mode (hereinafter referred to as a resonance mode generated by the IDT to IDT gap) is generated at a peak in the strength distribution of the surface acoustic wave in the gaps between the IDTs. These three resonance modes are used to produce the pass band of the filter.
In the technology described in the conventional art, by differentiating the IDT gap P from the IDT gap Q the resonance mode caused by the IDT to IDT gap is generated in each of the gaps between the IDTs. The two resonance modes are used to produce the pass band. In this technology, attenuation generated near the pass band, particularly, on the high frequency side thereof, is reduced to a greater extent than in the conventional longitudinally coupled resonator type surface acoustic wave filter.
However, the IDT gaps P and Q must be substantially differentiated from 0.50 times the wavelength determined by electrode-finger pitches. As a result, since the propagation continuity of a surface acoustic wave is lost, insertion loss in the pass band is greatly deteriorated.
SUMMARY OF THE INVENTION
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a longitudinally coupled resonator type surface acoustic wave filter which eliminates the transversal response, and thereby obtains sufficient attenuation on a pass band high frequency side, and which greatly reduces insertion loss in the pass band.
A longitudinally coupled resonator type surface acoustic wave filter according to preferred embodiments of the present invention includes a piezoelectric substrate and first, second and third IDTs each having a plurality of electrode fingers, provided on the piezoelectric substrate and arranged in a surface wave propagating direction such that the second IDT is interposed between the first and the third IDTs.
The first and second IDTs have narrow electrode-finger pitch sections which have an electrode-finger pitch that is narrower than the remaining electrode-finger pitches, at respective end portions of the first and second IDTs that are adjacent to each other. The second and third IDTs have narrow electrode-finger pitch sections that have an electrode-finger pitch that is narrower than the remaining electrode-finger pitches, at respective end portions of the second and third IDTs, which are adjacent to each other.
According to the first preferred embodiment of the present invention, the electrode-finger pitch of the narrow electrode-finger pitch sections in the first and second IDTs is different from the electrode-finger pitch of the narrow electrode-finger pitch sections in the second and third IDTs.
According to the second preferred embodiment of the present invention, the number of the electrode fingers of the narrow electrode-finger pitch sections in the first and second IDTs is different from the number of the electrode fingers of the narrow electrode-finger pitch sections in the second and third IDTs.
According to the third preferred embodiment of the present invention, only the first and second IDTs or only the second and third IDTs have narrow electrode-finger pitch sections which have an electrode-finger pitch narrower than the remaining electrode-finger pitches, at respective end portions of the first and second IDTs adjacent to each other, or at the respective end portions of the second and third IDTs adjacent to each other.
According to the unique arrangements of these preferred embodiments, transversal response is greatly minimized. In addition, by adjusting the pitches of the narrow electrode-finger pitch sections, without increasing the insertion loss of the pass band, the pass-band width is easily adjusted. It is possible to combine first and second preferred embodiments to further reduce the transversal response.
The first to third preferred embodiments of the present invention are successfully applied to a longitudinally coupled resonator type surface acoustic wave filter including first and second longitudinally coupled resonator type surface acoustic wave filter units. More specifically, the pitches and/or the numbers of electrode fingers of the narrow pitch electrode finger sections are different between the first and second longitudinally coupled resonator type surface acoustic wave filter units. Alternatively, the narrow pitch electrode finger sections may be provided only in either first or second longitudinally coupled resonator type surface acoustic wave filter units. With the above-described configurations, the bandwidth is easily adjusted without degrading the insertion loss.
In the longitudinally coupled resonator type surface acoustic wave filter according to each preferred embodiment of the present invention, when the second IDT has an even number of electrode fingers or when each of the second and fifth IDTs has even numbers of electrode fingers, the difference of the electrode-finger pitches between the narrow electrode-finger pitch sections on both sides of the central IDT can be reduced. Thus, as compared with the case in which the central IDT has odd electrode fingers, the narrowest pitch between the electrode fingers is widened. As a result, the longitudinally coupled resonator type surface acoustic wave filter of each of the first and second preferred embodiments of the present invention are easily produced in a stable manner. Furthermore, as shown in the aforementioned third preferred embodi
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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