Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-10-15
2002-04-02
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000
Reexamination Certificate
active
06366179
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a composite filter for vehicular communication employing surface acoustic waves, in particular relates to a two-port surface acoustic wave resonant filter utilizing longitudinal resonance mode coupling disposed on one substrate and having different passbands.
In RF SAW filters being used in cellular telephones and portable telephones, ones that have passbands thereof mainly in the frequency band of from several hundreds MHz to several GHz are used.
Frequencies of SAW filters and necessary passbands thereof are decided according to systems to be incorporated and in general the specific bandwidths of several % are required.
The SAW filters of aforementioned usage are also necessary to be low in insertion loss. For example, ladder filters connected surface acoustic wave resonators in a ladder as disclosed in Japanese Patent Laid-open Application (KOKAI) No. HEI 5-183380, or longitudinal mode-coupled resonant filters in which a plurality of interdigital transducers (hereinafter refer to as IDT) are sandwiched by reflectors, as disclosed in Japanese Patent Laid-open Application (KOKAI) No. HEI 4-207615, have been mainly used. Further, these have been frequently used in combination (for instance, Japanese Patent Laid-open Application (KOKAI) No. HEI 8-65097). In any constitutions thereof, filters of relatively low loss can be constituted.
In applying these SAW filters in RF filters for portable telephones, passband widths that can be obtained depend largely on electromechanical coupling factors (k
2
) of piezoelectric substrates thereon the filters are formed. Accordingly, the piezoelectric substrates such as 36° Y-X LiTaO
3
, 64° Y-X LiNbO
3
, or 41° Y-X LiNbO
3
and so on of relatively large electromechanical coupling factor have been largely used.
In many of these piezoelectric substrates, when the thickness of a metal film formed thereon is increased, the conversion loss from surface acoustic waves to bulk waves increases, resulting in an increase of the insertion loss of the filter formed with that thickness.
When the thickness of a metal film is made thin, the electric resistance of an electrode formed on a piezoelectric substrate increases, resulting in an increase of the insertion loss.
As the result of these, in these piezoelectric substrates, a normalized film thickness of approximately several % (from approximately 3 to approximately 8%) becomes the best value, the normalized film thickness being one that is normalized by a wavelength of a propagating surface acoustic wave.
RF filters for portable telephones are being adopted in the following places.
One is a portion called a duplexer (transmission/reception switching element) that sends signals from an antenna to a receiving circuit or signals from a transmitting circuit to an antenna.
In a receiving stage, a RF filter is used as a receiving filter that after the signals from an antenna are amplified through a low noise amplifier, eliminates unnecessary signals. Similarly in a transmitting circuit, the RF filter is used as a filter of a transmitting stage.
In the receiving stage, the signals passed through the receiving filter are further mixed with signals from another high-frequency oscillator and are further sent to an intermediate-frequency filter. At this time, also to the signals from the aforementioned high-frequency oscillator, the RF filters are used.
So far, in such portable telephones, except for the duplexer, one filter is used for one passband. There are, however, cases where filters containing a plurality of passbands in one filter are desired.
For instance, there is a case when one filter is demanded to correspond to a plurality of portable telephone systems.
A second case is that the frequency band that has been used is made broader due to an increase of subscribers and a new band exists in distanced frequency from the original one. Even in this case, for all one system, the filter is required to have characteristics as if corresponding to two systems.
In still another case, upon forwarding small size of portable telephones, a compact filter in which a plurality of filters are accommodated together in one package is demanded.
Thus, there are strong demands for composite surface acoustic wave elements in which a plurality of filter characteristics are contained in one package.
Such composite filters are required further to be smaller. On the other hand, in order to constitute a smaller composite filter with surface acoustic wave filters there are the following problems.
First, from a viewpoint of smaller size, it is desirable to form a plurality of surface acoustic filters on a single piezoelectric substrate. Even when considered manufacturing processes, it is desirable to form, through filming and patterning of one conductive film, patterns of IDTs and reflectors from which a plurality of filters are formed.
However, in a composite filter, it is necessary to form a plurality of filters having passbands in different frequencies. A two-port surface acoustic wave resonant filter is liable to be affected on performance in particular by the normalized thickness. Accordingly, in the composite filter in which the two-port surface acoustic wave resonators are used, unless two frequency bands are adjacent (for instance, approximately 80 MHz apart in the neighborhood of 800 MHz), the filter with a single film thickness has been formed with difficulty.
In the case of the frequency bands being distanced more than the above, it is necessary to implement partial additional etching to vary a film thickness of an electrode for each constituting part of a filter. Accordingly productivity of the surface acoustic wave elements is deteriorated.
SUMMARY OF THE INVENTION
The present invention has been made in view of the problems mentioned above.
An object of the present invention is to provide a surface acoustic wave element having a structure that is low in insertion loss, is excellent in attenuation outside the band, can correspond to a plurality of systems or a plurality of frequency bands, is compact, and is high in productivity.
In order to accomplish the aforementioned object, a surface acoustic wave element of the present invention is provided with the following constituent elements and conditions.
A surface acoustic wave element of the present invention comprises a piezoelectric substrate, a first surface acoustic wave resonant filter, and a second surface acoustic wave resonant filter. The first surface acoustic wave resonant filter has a first IDT group consisting of a conductive film disposed on the piezoelectric substrate, and a first passband. The second surface acoustic wave resonant filter has a second IDT group consisting of the conductive film disposed on the piezoelectric substrate, and a second passband higher in frequency than that of the first passband. Here, the number of IDTs constituting the first IDT group is more than that of the IDTs constituting the second IDT group.
By constituting thus, a lower frequency filter necessary for a larger specific bandwidth is increased in kinds of higher modes that can be taken out. Thereby, the lower frequency filter is formed in a structure more appropriate for a broader band.
A surface acoustic wave resonant filter of higher frequency side is easy to make high attenuation in the passband of a surface acoustic wave resonant filter of lower frequency side. Thereby, when two surface acoustic wave resonant filters are connected, the insertion loss of the lower frequency filter can be prevented from increasing.
As the first surface acoustic wave resonant filter and the second surface acoustic wave resonant filter, a two-port surface acoustic wave resonant filter utilizing longitudinal mode coupling, for instance, can be cited.
Thereby, two-port surface acoustic wave resonant filters utilizing longitudinal mode coupling having passbands of different frequencies can be combined to correspond to a plurality of systems or a plurality of frequency bands.
Grounding electrodes of IDTs constituting the first IDT gro
Kabushiki Kaisha Toshiba
Pascal Robert
Summons Barbara
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