Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-02-22
2002-12-31
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C310S31300R, C310S31300R, C333S193000, C333S195000
Reexamination Certificate
active
06501208
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a surface acoustic wave filter utilizing surface acoustic waves, in particular to a surface acoustic wave filter that has a two-port acoustic wave resonator utilizing longitudinal mode coupling and a surface acoustic wave resonator connected thereto and formed on the same piezoelectric substrate therewith.
BACKGROUND ART
RF SAW (Surface acoustic wave) filters employed in vehicle telephones and portable telephones have pass bands mainly in the frequency range of from several hundred MHz to several GHz. The frequency and the necessary frequency bandwidth depend on the mobile telephone system being employed. In general, the bandwidth is required to be several percent in terms of specific bandwidth.
In addition, a low insertion loss is required for the SAW filters employed in these mobile telephones or the like. Therefore, the following SAW filters have been dominantly used for these applications. These are ladder type filters in which SAW resonators are connected in ladder as disclosed in Japanese Patent Laid-open Application No. HEI 5-183380, longitudinal mode coupled resonant SAW filters in which a plurality of inter digital transducers (IDTs) having comb-shaped electrodes are put between reflectors as disclosed in Japanese Laid-open Application No. HEI 4-207615, and a combination of these two type filters.
Any one of the SAW filters mentioned above can realize filters of relatively small insertion loss. When these SAW filters are employed in RF filters of portable telephones, piezoelectric substrates possessing relatively large electromechanical coupling factor k
2
such as 36° Y-X LiTaO
3
, 64° Y-X LiTaO
3
, 41° Y-X LiTaO
3
, or like, have been widely used. The reason is that the possible pass bandwidth of these SAW filters largely depends on the electromechanical coupling factor k
2
of piezoelectric substrates formed thereon.
These piezoelectric substrates, however, have the following disadvantages. The conversion loss from a surface acoustic wave to a bulk wave on a substrate of these piezoelectric substrates increases as the thickness of the conductive films forming IDTs or reflectors increases. For this reason, the SAW filters show a large insertion loss when the filters are formed with an increased film thickness. On the other hand, when the film thickness decreases, the electrical resistance of the electrodes forming IDTs on the piezoelectric substrate increases. Therefore, the filters show a large insertion loss also when the filters are formed with a decreased film thickness. Thus there is a range of its optimum values for the conductive film thickness h formed on one of these piezoelectric substrates. The range of the optimum values in terms of normalized film thickness (h/&lgr;) normalized by the wavelength &lgr; of the SAW that propagates on the piezoelectric substrate is approximately several percent (approximately 3 to 8 percent).
Normally, a SAW filter in which a two-port SAW resonator utilizing longitudinal mode coupling and another SAW resonator are connected, is formed by patterning a conductive thin film for instance Al thin film or slightly Si and Cu containing Al film or like on a piezoelectric substrate. Then the two SAW resonators are formed with the same film thickness.
The bandwidth of the frequency pass-band required for the mobile communication system is broad such that the width is approximately several percent in terms of specific bandwidth. Therefore, the two-port SAW resonator utilizing longitudinal mode coupling is formed on a piezoelectric substrate possessing high electromechanical coupling factor k
2
chosen from the substrates described above with a normalized conductive film thickness in the prescribed range. In addition, the electrode finger width W divided by the disposed electrode finger pitch P (W/P, abbreviating as a duty hereinafter) is set at approximately 50 to 65 percent for the purpose of reducing insertion loss in many cases.
The SAW resonator connected to the two-port acoustic wave resonator has an effect of attenuating signals in the vicinity of the two-port SAW resonator pass band. Since the transmission band and the reception band are located in a very close vicinity in mobile communication systems, the SAW resonator connected to the two-port SAW resonator is largely used in order to have the filter characteristics of attenuating either one of the transmission band or the reception band.
The SAW resonator connected to the two-port SAW resonator utilizing longitudinal mode coupling, however, is formed with the same film thickness as that of the two-port SAW resonator in which broad bandwidth is required. Accordingly, the SAW resonator connected to the two-port SAW resonator is apt to have a relatively large frequency difference between the resonance frequency and the anti-resonance frequency thereof.
In addition, since the wiring of the two-port SAW resonator utilizing longitudinal mode coupling is relatively complicated in its structure, the wiring carries a certain amount of inductance component. Other inductance components are added by the package, the bonding wire and so on.
As these inductance components are loaded to the SAW resonator connected to the two-port SAW resonator, the substantial frequency difference between the resonance frequency and the anti-resonance frequency of the resonator further increases.
The SAW resonator connected to a series arm of the two-port SAW resonator can obtain steep attenuation characteristics at the anti-resonance frequency and pass characteristics at the neighborhood of the resonance frequency of the connected oscillator. When the SAW filter is required to have an attenuation band at a frequency range in the vicinity of the high frequency side of the pass band, the anti-resonance frequency of the series arm SAW resonator is needed to locate in a vicinity of a low frequency edge of the attenuation band.
In this case, there is no problem when the resonance frequency of the SAW resonator connected to a series arm of the two-port resonator is located approximately at the center of the pass band of the two-port SAW resonator utilizing longitudinal mode coupling. However, when the resonance frequency of the SAW resonator connected to the two-port SAW resonator shifts towards the lower frequency side of the pass band, the filter loss at the high frequency side of the pass band increases. As the result, the pass band characteristics of the SAW filter as a whole have pass band characteristics having unfavorably dull shoulder. That is to say, the SAW filter loss at the high frequency side of the pass band becomes large in the case when the filter is required to have an attenuation band at a frequency range in the vicinity of the high frequency side of the pass band.
The present invention is carried out to solve these problems.
The object of the present invention is to provide a SAW filter having low loss and excellent cut-off characteristics, in particular to provide a SAW filter for mobile communication applications excellent in attenuation characteristics in the vicinity of high frequency side of a pass band.
DISCLOSURE OF INVENTION
A SAW filter of the present invention comprises a SAW filter, comprising a piezoelectric substrate, a first SAW resonator disposed on the piezoelectric substrate so as to have a first duty, the first SAW resonator being a two-port SAW resonator utilizing longitudinal mode coupling and having IDTs each comprising a pair of comb-shaped electrodes, and a second SAW resonator on the piezoelectric substrate so as to have a second duty smaller than the first duty, the second SAW resonator being connected to a series arm of the first SAW resonator on the piezoelectric substrate and having an IDT comprising a pair of comb-shaped electrodes.
The first duty and the second duty can be adjusted so that reflectance per electrode finger constituting the IDT of the second SAW resonator becomes smaller than that of the first SAW resonator.
In addition, the first duty and the second duty can be adjusted so that the electromechanica
Dougherty Thomas M.
Gonzalez Julio
Kabushiki Kaisha Toshiba
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