Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1997-06-17
1999-03-02
Pascal, Robert
Wave transmission lines and networks
Coupling networks
Electromechanical filter
333194, 333195, 310313B, 310313D, H03H 964
Patent
active
058776613
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a surface acoustic wave (SAW) filter to be used in mobile communication, etc.
BACKGROUND ART
Presently surface acoustic wave (SAW) filters are widely used in instruments of mobile communication, etc. because they have the advantages of small size, light weight, and no adjustment requirement, as well as other advantages. For mobile communication which requires characteristics, such as low propagation losses and high attenuation, SAW resonator filters are suitable.
A SAW resonator filter includes an input interdigital transducer (input IDT), an output interdigital transducer (output IDT), and a reflector. The SAW resonator filter uses resonance phenomena of surface acoustic waves confined in the transducer structure including the input IDT, the output IDT, and the reflector to have a passband of a frequency range which satisfies resonance conditions. A bandwidth of the SAW resonator filter is proportional to an electromechanical coupling coefficient (k.sup.2) of the piezoelectric substrate.
The SAW resonator filters using one resonance can only have narrow bandwidths. In order for a SAW resonator filter to have a wide bandwidth, it is effective to couple a plurality of resonances so that the SAW resonator filter has a wider bandwidth than that using one resonance.
For example, standing waves of acoustic surface waves confined by the IDTs and the reflector can have a wider bandwidth by coupling two resonances (so-called the zeroth mode resonance and the first mode resonance) than by the use of one resonance. A SAW resonator filter including three IDTs to couple the zero-order mode and the secondary mode resonances is also effective.
On the other hand, to realize a SAW resonator filter having high attenuation, it is effective to concatenate two electrode structure arrays.
FIG. 1 shows one example of the SAW resonator filter including concatenated two electrode structure arrays.
A first electrode structure array is disposed on a piezoelectric substrate. The first electrode structure comprises an input/output IDT 1 having N1 pairs of electrodes, and receipt IDTs 2, 2' disposed outside the input/output IDT 1 and having N2 pairs of electrodes which are substantially the same pitch as the input/output IDT and, and reflectors 3, 3' disposed outside the receipt IDTs 2, 2'. The input/output IDT 1 and the receipt IDTs 2, 2' are arranged with the adjacent electrodes spaced from each other by a gap L between the centers thereof.
A second electrode structure array is disposed on the piezoelectric substrate. The second electrode structure comprises an input/output IDT 11 having N1 pairs of electrodes and disposed on the piezoelectric substrate, receipt IDTs 12, 12' disposed outside the input/output IDT 11 and having N2 pairs of electrodes which are substantially the same pitch as the input/output IDT 11, and reflectors 13, 13' disposed outside the receipt IDTs 12, 12'. The input/output IDT 11, and the receipt IDTs 12, 12' are arranged with the adjacent electrodes spaced from each other by a gap L between the centers thereof.
The receipt IDTs 2, 2' of the first electrode structure array, and the receipt IDTs 12, 12' of the second electrode structure array are connected respectively by lines 4, 4' to concatenate the first and the second electrode structure arrays. The lines 4, 4' are connected with each other by a line 5 and have the same potential. The line 5 is provided as required.
Input signals are inputted to the input/output IDT 1 of the first electrode structure array in the first stage. Acoustic surface waves excited by the input/output IDT 1 are multi-reflected and received by the receipt IDTs 2, 2'. Energy of the received surface acoustic waves is converted to electric signals by the receipt IDTs 2, 2' and is supplied to the second electrode structure array on the second stage by the line 4, 4'.
In the second electrode structure array in the second stage, the surface acoustic waves are excited by the receipt IDTs 12, 12' and received by the input/output IDT 11. E
REFERENCES:
A translated copy of the International Search Report for corresponding International Application No. PCT/JP96/00691, Jun. 10, 1996.
Abe Hidenori
Honmo Hiroshi
Ohmura Masashi
Kinseki Limited
Pascal Robert
Summons Barbara
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