Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1996-02-07
2001-07-03
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06255916
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a band-pass resonator-type surface-acoustic-wave filter having a plurality of surface-acoustic-wave resonators connected in series and parallel with one another. More particularly, this invention is concerned with a resonator-type surface-acoustic-wave filter for reducing the signal height of a spurious peak that causes the extra-passband suppressibility to deteriorate and occurs especially at a frequency outside a passband.
(2) Description of the Related Art
A known high-frequency band pass filter includes a surface-acoustic-wave (SAW) resonator in which different electrode patterns are formed in reflectors and an InterDigital Transducer (IDT) on a piezoelectric element. With the recent popularity of mobile telephones and portable telephones, there is an increasing demand for SAW resonators as small, thin radio-frequency (RF) filters.
Generally, the SAW resonator element has a pair of reflectors each of which has a plurality of reflector electrodes arranged in the form of a grid on a substrate made of a piezoelectric material or a magnetostrictive material. An IDT having numerous pairs of screen-like electrodes is interposed between the two reflectors. Resonance resulting from reciprocation of a surface acoustic wave between the two reflectors is fed to an electric circuit via input/output terminals. The pitch between adjoining reflector electrodes, which is referred to as an inter-electrode pitch, in the reflectors shall be defined as &lgr;
REF
, and the pitch between adjoining interdigital electrodes in the IDT, which is referred to as an inter-electrode pitch, shall be defined as &lgr;
IDT
. The values of the inter-electrode pitches are determined according to a resonant frequency. In the prior art SAW resonator element, &lgr;
REF
and &lgr;
IDT
have an equal value.
A SAW filter that has been adopted widely to date has a transversal structure or what is referred to as a cascade structure in which transversal structures are cascaded in two stages. For example, Japanese Unexamined Patent Publication (Kokai) No.57-99813 has disclosed a transversal SAW filter, and Japanese Unexamined Patent Publication (Kokai) No.4-94208 has disclosed a cascaded SAW filter. In each of SAW filters shown in these documents, reflectors are arranged in both sides of an IDT.
A SAW resonator with a pair of terminals includes an equivalent circuit electrically equivalent to a crystal oscillator. As already known, a resonator-type filter can be realized by connecting the SAW resonators in series and parallel with one another in the form of a multistage structure. Therefore, the resonator-type filter is available in various modes dependent on connection arrangements. In a resonator-type filter based on SAW resonators, each SAW resonator having a pair of terminals is treated as a pure reactance element in order to utilize a resonant effect. Impedance occurring at any of frequencies in a passband can therefore be limited. This obviates the necessity of an external matching circuit and realizes a reduced insertion loss. The resonator-type filter acts as a narrow-band band pass filter because of the generally large capacity ratio among resonators. Owing to characteristics similar to those of a narrow-band band-pass filter, the resonator-type filter is used as an RF filter for a mobile telephone or a portable telephone.
Thus, the SAW resonator having a pair of terminals and the resonator-type filter based on the SAW resonators differ from each other even in terms of the design technology. The present invention relates to a resonator-type filter composed of SAW resonators each having a pair of terminals. Hereafter, the resonator-type filter composed of SAW resonators each having a pair of terminals is referred to as a resonator-type SAW filter.
The resonator-type SAW filter offers great freedom in specifying constants for an individual resonator in the process of design or in selecting a combination of resonators to be connected in series or parallel with one another. This leads to a low loss and a relatively large stop-band attenuation ratio. Thanks to these advantages, resonator-type SAW filters are taking the place of conventional filters.
One of the important characteristics of a band pass filter is stop-band suppressibility. The stop-band suppressibility indicates to what extent signals with frequencies outside a passband can be blocked relative to signals with frequencies within the passband. When a band pass filter is employed as an RF filter for radio communications, if the stop-band suppressibility is small, interference and other problems may occur. Therefore, a large stop-band suppressibility is usually desired.
In a conventional resonator-type SAW filter, a spike or what is referred to as a spurious peak occurs at a certain frequency outside a passband because of the structure of the filter. The spurious peak causes the stop-band suppressibility to drop or the attenuation ratio to rise like a spike. In equipment using the resonator-type SAW filter, when the frequency causing the spurious peak corresponds to a frequency that must be suppressed, the suppressibility deteriorates by a degree corresponding to the height of the spurious peak.
When a resonator-type SAW filter is needed, a resonator-type SAW filter permitting high suppressibility at all stop-band frequencies should be adopted on the assumption that stop-band suppressibility may deteriorate due to the spurious peak. In this case, occurrence of a spurious peak would pose no problem. Alternatively, a spurious peak is reduced in signal height by adjusting the combination of or the number of stages of SAW resonators each having a pair of terminals to be connected in series or parallel with one another. However, it is very hard to manufacture a resonator-type SAW filter permitting high suppressibility at all stop-band frequencies. Adjusting the combination of or the number of stages of SAW resonators each having a pair of terminals requires considerable labor and is quite time-consuming.
SUMMARY OF THE INVENTION
The present invention attempts to solve the aforesaid problems. An object of the present invention is to realized a resonator-type SAW filter permitting higher extra-passband suppressibility by minimizing the occurrence of an unwanted spurious peak.
In a resonator-type SAW filter according to the present invention, a plurality of SAW resonators each having a pair of terminals as well as reflectors and an IDT are connected in series and parallel with one another. In at least one of the SAW resonators, particularly in at least one of the SAW resonators having a pair of terminals and being connected in series with another, an inter-electrode pitch &lgr;
REF
in the reflectors has a different value than inter-electrode pitch &lgr;
IDT
in the IDTs.
In a resonator-type SAW filter in the second mode of the present invention, only in SAW resonators each having a pair of terminals and being connected in series with the other, is the inter-electrode pitch &lgr;
SREF
in the reflectors a different value to the inter-electrode pitch &lgr;
SIDT
in the IDT.
Further, in a resonator-type SAW filter in the third mode of the present invention, in all SAW resonators, the inter-electrode pitch &lgr;
REF
in the reflectors is different value to the inter-electrode pitch &lgr;
IDT
in the IDTs.
For a conventional resonator-type SAW filter, the inter-electrode pitch in the reflectors in each SAW resonator having a pair of terminals is set to the same value as the inter-electrode pitch in the IDT therein. In contrast, for a resonator-type SAW filter of the present invention, in at least one of SAW resonators, particularly, in at least one of SAW resonators each having a pair of terminals and being connected in series with the other, the inter-electrode pitch &lgr;
REF
in the reflectors is set to a slightly different value from the inter-electrode pitch &lgr;
IDT
in the IDTs. It has been found that this structure can reduce the height of a spurious peak. Moreover, as
Minemura Sen
Nakamura Yoshitaka
Yamada Sumio
Fujitsu Limited
Lee Benny
Staas & Halsey , LLP
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