Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-04-25
2002-10-08
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S196000, C310S31300R
Reexamination Certificate
active
06462632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a one port type SAW resonator and a surface acoustic wave (SAW) filter that are included in, for example, a band-pass filter or other filter for use in portable telephones and other communication devices.
2. Description of the Related Art
SAW filters are widely used as band-pass filters in portable telephones and other communication devices. In recent communication systems, such as portable telephones, the transmitting frequency band and the receiving frequency band are close to each other. Therefore, attenuation characteristics in the vicinity of the ends of the passband, that is, sharpness in the attenuation characteristics, are increasingly required.
To meet the above-described requirements, a composite SAW filter is disclosed in Japanese Unexamined Patent Application Publication No. 7-131290. In this composite SAW filter, a first SAW resonator is connected in parallel to one of an input terminal filter and an output terminal of a SAW filter, and a second SAW resonator is series-connected thereto.
For the above-mentioned prior art, a high impedance in the vicinity of an antiresonant frequency in the first SAW resonator series-connected is used to provide the sharpness in cutoff characteristics of the high-band side of the passband of the SAW filter. It is also described that a low impedance in the vicinity of a resonant frequency in the second SAW resonator connected in parallel is used to provide the sharpness in cutoff characteristics of the low-band side of the passband of the SAW filter.
In the above-described method according to the prior art, the antiresonant frequency in the first SAW resonator must be arranged to be closer to the passband on the high-band side of the passband. Also, the resonant frequency in the second SAW resonator must be arranged to be closer to the passband on the low-band side of the passband.
However, when the antiresonant frequency in the first SAW resonator is arranged to be closer to the passband, a high impedance in the vicinity of the antiresonant frequency affects the high-band side of the passband. This increases insertion losses on the high-band side of the passband. Similarly, when the resonant frequency in the second SAW resonator is arranged to be closer to the passband, the low impedance in the vicinity of the resonant frequency influences the low-band side of the passband. This increases the insertion loss on the low-band side of the passband.
That is, the above-described method according to the prior art causes a problem in that the insertion loss in the passband is increased when the amount of attenuation in the vicinity is very close to the passband.
FIG. 25
is a graph showing frequency-amplitude characteristics, which is used to explain the aforementioned inverse effects that occur when the SAW resonator is connected in parallel to the SAW filter.
In
FIG. 25
, broken lines indicate a frequency-amplitude characteristic of a simple substance of the SAW filter, and solid lines indicate a characteristic provided when a SAW resonator of which an impedance-frequency characteristic indicated by a broken line in
FIG. 15
is connected in parallel to the above-described SAW filter.
A graph on a magnified scale shows the characteristics magnified on a scale on the right of the vertical axis. The figure showing a frequency-amplitude characteristic, which is referred to below, is similarly presented.
As is apparent in
FIG. 25
, when the SAW resonator is connected in parallel, amounts of attenuations increase in the vicinity of the high-band side of the passband, particularly, in frequency zones where amounts of attenuations increase from 10 dB.
However, when the resonant frequency of the SAW resonator is arranged to be close to the passband, the low-band side of the passband is influenced according to the influence of the low impedance in the vicinity of the resonant frequency. This indicates that, as shown by the solid lines, the insertion loss increases. As a result, when the sharpness on the low-band side of the passband is judged using the frequency pitch in positions where amounts of attenuations are 3 dB and 20 dB as a criteria, the frequency interval in the simple substance of the SAW filter is 3.3 MHz while it is only 3.6 MHz when the SAW resonator is connected in parallel. Thus, no improvement in the sharpness has been achieved.
FIG. 26
is a graph showing frequency-amplitude characteristics, which is used to explain the aforementioned inverse effects that occur when the SAW resonator is series-connected to the SAW filter. In
FIG. 26
, broken lines indicate a frequency-amplitude characteristic of the simple substance of the SAW filter, and solid lines indicate a characteristic provided when a SAW resonator of which an impedance-frequency characteristic indicated by a broken line in
FIG. 18
is series-connected to the SAW filter.
As is apparent in
FIG. 26
, when the SAW resonator is series-connected to the SAW filter, amounts of attenuations increase in the vicinity of the low-band side of the passband, particularly, in the vicinity of 913 MHz, which corresponds to the antiresonant frequency of the SAW resonator. However, similar to the above, the high-band side of the passband is affected by the influence of the high impedance in the vicinity of the antiresonant frequency. When the sharpness in the frequency-amplitude characteristic on the high-band side of the passband is judged using the frequency pitch at positions where amounts of attenuations are 3 dB and 8 dB as a criteria, the frequency interval in the simple substance of the SAW filter is 2.2 MHz while it is 3.4 MHz when the SAW resonator is series-connected. Thus, no improvement in the sharpness has been achieved.
To prevent the decrease in the passband, that is, the adverse effects in the insertion loss, in the case of the parallel connection, the vicinity of the antiresonant frequency in the SAW resonator is simply arranged so as to agree with the passband. In the case of the series connection, the vicinity of the resonant frequency in the SAW resonator is simply arranged so as to agree with the passband. As a result of the actual connection, as described above, however, the resonant frequency in the case of the series connection is farther from the vicinity of the passband, thereby disabling large amounts of attenuations in the vicinity very close to the passband. That is, according to the conventional method in which the SAW resonator is connected to the SAW filter, large amounts of attenuations in an area that is very close to the passband and preferable insertion losses in the passband are incompatible.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a SAW resonator and a SAW filter including the SAW resonator, the SAW resonator being arranged to control the frequency interval between the resonant frequency and the antiresonant frequency and adapted to define a ladder circuit and various other types of SAW filters, and furthermore, being adapted to be connected to the SAW filter in the above-mentioned composite SAW filter.
Preferred embodiments of the present invention also provide a composite SAW filter in which the SAW resonator of the present invention is series-connected to and/or connected in parallel to the SAW filter, thereby achieving sharpness in filter characteristics in the vicinity of the passband, and concurrently, achieving insertion losses in the passband.
A SAW resonator according to a preferred embodiment of the present invention includes a piezoelectric substrate and an interdigital transducer (which is abbreviated as an “IDT”, hereinbelow) on the piezoelectric substrate, the IDT including first and second comb-shaped electrodes having one or more electrode fingers which are interdigitated with each other, wherein, when the first comb-shaped electrode is connected to a positive potential, the second comb-shaped electrode is connected to a negative potential, and the electrode finger
Fujii Yasuhisa
Takamine Yuichi
Keating & Bennett LLP
Lee Benny
Murata Manufacturing Co. Ltd.
Summons Barbara
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