Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1997-03-20
1998-12-15
Pascal, Robert
Wave transmission lines and networks
Coupling networks
Electromechanical filter
333195, 310313B, 310313D, H03M 964
Patent
active
058501673
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a surface acoustic wave device which is used in intermediate frequency filters, etc. of mobile communication equipments.
Filters in the form of surface acoustic wave devices are used in communication equipments, broadcasting equipments, measuring devices, etc. because of their small sizes, lightness and high achievement. Conventionally, intermediate frequency filters used in the analog communication system have been required narrow-band characteristics, in addition, small deviation of center frequency due to temperatures, and large attenuation characteristics. In this application transversal surface acoustic wave filters using ST cut quartz substrates and having 3 dB specific band widths (hereinafter abbreviated as specific band widths) of 0.33-1.0% have been used.
The analog communication system is being changed to the digital communication system in mobile communication for effective use of frequencies and confidentiality. Intermediate-frequency filters used in the digital communication system have a relatively wide specific band width of 0.2-0.5%, flat group delay times, and attenuation characteristics which exceed those of the inter-mediate filters of the analog communication system. Particularly portable instruments for the mobile communication must be compact and require low power consumption, and the surface acoustic wave filters must be accordingly compact, have low minimum insertion loses, attenuation characteristics, and good shape factors. For these requirements various studies have been conventionally made.
As a first conventional art, transversal surface acoustic wave filters having two interdigital transducers are known. These filters have a merit that required amplitude characteristics and phase characteristics can be designed independently of each other.
The transversal surface acoustic wave filters, however, have large insertion loses. To obtain specific band widths of 0.2-0.5%, they require a large number of electrodes, which increases device size and makes attenuation characteristics insufficient.
As surface acoustic wave filters having low losses and compact sizes, surface acoustic wave resonator filters are known. The surface acoustic wave resonator filters have had their structures decided simply on consideration of insertion loss in the passband and attenuation characteristics, and have found it difficult to obtain the required group delay time characteristics.
As a second conventional art, a two terminal pair surface acoustic wave filter using a plurality of pairs of interdigital transducers (IDTs) and reflectors is described in Yahei Oyamada, Masakichiro Yoshikawa, "TWO TERMINAL PAIR SURFACE ACOUSTIC WAVE RESONATOR USING A PLURALITY OF PAIRS OF IDTS", Journal of Japan Acoustics Association, vol. 33, No. 10, pp. 557-564, October, 1977.
This filter comprises two interdigital transducers of a perfect periodical structure and reflectors arranged outside of both ends of two interdigital transducers formed on a quartz substrate. It has a 1.25% normalized aluminium film thickness, a 15 L aperture length (L is a reflector pitch), 300 pairs of the interdigital electrodes of the IDTS, and 200 electrodes of the reflectors. When a terminal impedance is 50 .OMEGA., filter characteristics of a 3 dB minimum insertion loss, a 0.033% specific band width, and a 30 dB attenuation level are obtained. Furthermore, to obtain good attenuation levels, two above-described electrode structure rows are connected in cascade. Filter characteristics of a 6 dB minimum insertion loss, a 0.028% specific band width and a 70 dB attenuation level are obtained.
Group delay time characteristics of this conventional art example, however, are unknown, and the specific band width is narrow. The insertion loss is large, and decrease of the insertion loss has been required.
As a third conventional art is known a longitudinal double mode SAW resonator filter which is described in Masaki Tanaka, Takao Morita, Kazuo Ono, and Yuzo Nakazawa, "HIGH FREQUENCY LOW LOSS FILTER PROVIDED BY CONCATENATE
REFERENCES:
patent: 5699028 (1997-12-01), Hiraishi et al.
A translated copy of the International Search Report for corresponding application No. PCT/JP96/01002; Jul., 1996.
Kinseki Limited
Pascal Robert
Summons Barbara
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