Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2002-01-22
2004-10-05
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C333S195000, C310S31300R
Reexamination Certificate
active
06801100
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inter-digital transducer provided with a balanced type terminal, a longitudinally coupled mode type surface acoustic wave filter and a communication apparatus using the same.
2. Description of the Related Art
As the electromechanical function parts using surface acoustic wave (SAW) have been noted with attention in the current of making the hardware high density because the acoustic velocity of wave is several kilometers/second, and the wave energy has properties to be concentrated on the surface of the propagation medium. Due to the development of inter-digital transducer (IDT) electrode and progress of the thin film preparing technique and surface processing technique, the SAW has been practically utilized for delay line for radar, band-pass filter for television receiver, etc. At present, the SAW is extensively used as RF and IF stage filters for receiving and transmitting circuits in radio-communication apparatus.
Conventionally, there are known transversal SAW filter and two kinds of longitudinally coupled mode coupled and transverse mode coupled type SAW filters. The transversal SAW filter has small group delay variation characteristic, but it has large insertion loss and large element size. On the other hand, the mode coupled type SAW filter presents steep attenuation characteristic, shows small insertion loss, and is small in element size, but its group delay variation characteristic is inferior to that of transversal type SAW filter. The longitudinally coupled mode type SAW filter is characterized by having relatively wide pass band, and the transverse mode type SAW filter is characterized by having very narrow pass band characteristic. In view of the above characteristics, as the RF filter for the mobile communication apparatus, the longitudinally coupled mode coupled type filter, and as the IF filter, the transverse mode coupled type SAW filter which is miniature in size and excellent in attenuation characteristic have been widely used.
Furthermore, balancing of semiconductor parts such as IC has advanced in recent years in order to reduce noise characteristics and balancing is also required for surface acoustic wave filters used for the RF stage. Furthermore, from the standpoint of impedance matching with ICs placed before and after, control over impedance of surface acoustic wave is required in recent years.
A surface acoustic wave filter used for the RF stage will be explained first as a first conventional technology.
A longitudinally coupled mode type surface acoustic wave filter having a conventional balanced type input/output terminals will be explained below.
FIG. A
12
shows a configuration of a longitudinally coupled mode type surface acoustic wave filter having conventional balanced type input/output terminals. In FIG. A
12
, the surface acoustic wave filter is constructed by forming input IDT electrodes
3002
a
and
3002
b
, reflector electrodes
3003
a
and
3003
b
and output IDT electrode
3004
on a piezoelectric substrate
3001
. The electrode fingers of the input IDT electrodes
3002
a
and
3002
b
on the same side are connected to an input terminal S and the other fingers are grounded. Furthermore, the upper electrode digit of the output IDT electrode
3004
is connected to one output terminal T
1
and the lower electrode digit of the output IDT electrode
3004
is connected to the other output terminal T
2
. By adopting the above-described configuration, a surface acoustic wave filter having balanced type output terminals is obtained.
Then, a surface acoustic wave filter used for the IF stage will be explained as a second conventional technology.
A conventional transversal mode coupled type surface acoustic wave filter will be explained below.
FIG. B
24
is a block diagram showing a transversal mode coupled resonator type surface acoustic wave filter according to a conventional technology. In FIG. B
24
, reference numeral
241
denotes a single crystal piezoelectric substrate and by forming an electrode pattern on this piezoelectric substrate
241
, it is possible to excite surface acoustic wave. Reference numeral
242
a
denotes an IDT electrode formed on the piezoelectric substrate
241
and reflector electrodes
242
b
and
242
c
are placed on both sides of the IDT electrode forming thereby an energy-containment type surface acoustic wave resonator. Furthermore, an IDT electrode
243
a
, reflector electrodes
243
b
and
243
c
form a similar surface acoustic wave resonator on the piezoelectric substrate
241
. These two resonators are placed close to each other and a surface acoustic wave filter is formed by generating acoustic coupling between these resonators.
In the surface acoustic wave filter configured as shown above, two kinds of surface acoustic wave mode frequencies to be excited on the piezoelectric substrate are determined according to the aperture length of the electrode fingers of the IDT electrodes and the distance between the two surface acoustic wave resonators placed close to each other and the passing bandwidth of the filter is determined.
However, the surface acoustic wave filters explained in the first and second conventional technologies are required to have impedance matching with IC elements in the preceding and following stages, and since the input/output impedance of the above-described conventional filters is dependent on the number of pairs of electrode fingers included in the IDT electrodes and aperture length closely associated with the filter characteristic, there is a problem that it is difficult to attain a desired filter characteristic and a desired impedance value simultaneously.
In view of the above-described problems, the present invention has as its object to provide an inter-digital transducer, a surface acoustic wave filter having an optimal balancing and controlling input/output impedance and communication apparatus using it.
In view of the above-described problems, it is another object of the present invention to provide an inter-digital transducer, a surface acoustic wave filter having desired input/output impedance and communication apparatus using it.
SUMMARY OF THE INVENTION
The 1st invention of the present invention is an inter-digital transducer comprising:
a piezoelectric substrate; and
an IDT (inter-digital transducer) electrode having a pair of upper bus bar electrode and lower bus bar electrode placed facing each other on said piezoelectric substrate and a plurality of electrode fingers placed on said piezoelectric substrate each being led out from either said upper bus bar electrode or said lower bus bar electrode toward the other bus bar electrode,
wherein said IDT electrode is constructed of a plurality of divisional IDT electrodes and connected to a balanced type terminal.
The 2nd invention of the present invention is the inter-digital transducer according to 1st invention, wherein at least two of said plurality of divisional IDT electrodes are equivalently connected in series and at least one of the remaining divisional IDT electrodes is connected in parallel with said at least two divisional IDT electrodes.
The 3rd invention of the present invention is the inter-digital transducer according to 2nd invention, wherein the ratio in the number of electrode fingers among said plurality of divisional IDT electrodes is adjusted beforehand so as to have a predetermined impedance value.
The 4th invention of the present invention is the inter-digital transducer according to 3rd invention, wherein said plurality of divisional IDT electrodes is placed in an same/reverse phase relationship so that charges of the divisional IDT electrodes do not cancel out each other.
The 5th invention of the present invention is the inter-digital transducer according to 4th invention, wherein said plurality of divisional IDT electrodes is constructed of first, second and third divisional IDT electrodes.
The 6th invention of the present invention is the inter-digital transducer according to 5th invention,
wherein said second
Ishizaki Toshio
Mita Narihiro
Nakamura Hiroyuki
Nishimura Kazunori
Yamada Toru
Matsushita Electric - Industrial Co., Ltd.
Smith , Gambrell & Russell, LLP
Summons Barbara
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