Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1996-05-15
1999-09-21
Pascal, Robert
Wave transmission lines and networks
Coupling networks
Electromechanical filter
333195, 310313B, 310313D, H03H 925, H03H 952, H03H 964
Patent
active
059559330
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of International Application No. PCT/JP95/02293, filed Nov. 10, 1995.
TECHNOLOGICAL FIELD
The present invention relates to a surface acoustic wave filter of a ladder type, especially to an improvement of its power-withstanding characteristics.
BACKGROUND TECHNOLOGY
Recently, the market of mobile communication apparatus represented by, for example, portable telephones are rapidly expanding, and the demand for small and efficient parts to be used in these apparatus has become strong. Because the high power-withstanding characteristic is required in an antenna switch located in a front end of radio frequency circuit of the mobile communication terminal, there has been conventionally employed a dielectric filter. However, in order to make the apparatus further smaller it is necessary to replace the dielectric filter with a surface acoustic wave (referred to hereinafter as a SAW) filter. On the other hand, width of electrodes of the SAW filter becomes minute as the operation frequency rises; accordingly, further power-withstanding characteristic has come to be required.
The SAW filter of a ladder type is a filter in which SAW resonators having mutually different resonance characteristics (single-pair terminal resonator) are arranged in the parallel arm and in the series arm, where insertion loss can be decreased very much owing to the use of this ladder type SAW filter compared with the case using a SAW filter of a transversal type in which the comb teeth electrode pair is connected in multiple stages. Details of the SAW resonator will be explained later.
Impedance of the resonator is zero at the resonance frequency fr, and is maximum at the antiresonance frequency fa. On the contrary, the admittance is maximum at the resonance frequency fr, and is zero at the antiresonance frequency fa. That is, the resonator is tuned in two ways.
Such resonators are connected in an L shape arrangement so as to constitute a two-pair terminal resonator, where the resonance frequency of resonator Rp in the parallel arm and resonator Rs in the series arm are set such that the antiresonance frequency faP of resonator Rp of the parallel arm conforms substantially to the resonance frequency frs of resonator Rs in the series arm, whereby, there is formed a band-pass filter having a center frequency which is the resonance frequency frs of resonator Rs in the series arm. Then, a band-pass filter characteristic which satisfies the specification, such as of portable telephones, is accomplished by connecting the two-pair terminal resonators in multiple stages to form a ladder structure as shown in an equivalent circuit of FIG. 3A.
When the SAW filter is built in a radio frequency circuit, such as of portable telephones, an electric power withstanding characteristic is required corresponding to the maximum transmitting power of the apparatus. A consideration to Joule heat is severely required, particularly in the use in an antenna duplexer (a transmitter/receiver switch) to which the transmitting power output from the output amplifier stage is applied. The antenna duplexer is a device for sharing the antenna by transmitter and receiver by the utilization of the difference between the transmitting frequency and the receiving frequency, and consists of a transmitter band-pass filter in which the transmitting frequency band is in the pass-band and a receiver band-pass filter in which the receiving frequency band is in the pass-band.
In the prior art SAW filter, there was a problem in that the temperature rose in the filter chip which formed the resonator and the characteristic was apt to deteriorate when a signal was input at an attenuation band (a stop band) which is at the lower frequency side in the filter characteristic (bandpass characteristic). That is, power-withstanding characteristics at the attenuation band at the lower frequency side was low compared with power withstanding characteristics at other frequency bands.
Therefore, when an antenna duplexer employed in, for instance, AMPS (Automatic Message
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Ikata Osamu
Matsuda Takashi
Nishihara Tokihiro
Satoh Yoshio
Uchishiba Hidema
Fujitsu Limited
Pascal Robert
Summons Barbara
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