Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1997-01-29
1998-12-15
Pascal, Robert
Wave transmission lines and networks
Coupling networks
Electromechanical filter
333191, 310320, 310312, 310367, H03H 9205, H03H 956
Patent
active
058501665
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a piezoelectric ceramic filter circuit and a piezoelectric ceramic filter.
BACKGROUND ART
Piezoelectric ceramic filters of this type are used, for example, as IF filters in various mobile communication devices and FM sound multiplex receivers. To date, the following types of piezoelectric ceramic filter circuits are well known: multistage type piezoelectric ceramic filter circuits and ceramic filters with trapped energy type multiple mode piezoelectric resonators having approximately the same center frequencies connected in cascade.
Insertion loss and group delay characteristics are important factors in piezoelectric ceramic filter circuits and piezoelectric ceramic filters. The insertion loss must be maintained at a low level in order to hold down signal attenuation through filtering. It is desirable that the group delay characteristics are as flat as possible within the pass band so that transmission time difference depending on frequency will not be created in the signal transmission within the pass band. Since piezoelectric ceramic filters are minimum phase shift devices, it is not possible to control the amplitude characteristics and the group delay characteristics independently. Previous attempts, therefore, to improve the group delay characteristics have been limited to keeping the mechanical quality factor Q of the piezoelectric resonator as low as possible.
However, with the prior art, wherein the group delay characteristics are improved by keeping the mechanical quality factor Q of the piezoelectric resonator low, the amplitude characteristics degrade and the insertion loss becomes extremely large, resulting in lowered filter transmission efficiency. In order to lower the insertion loss, Q must be high, thereby degrading the group delay characteristics. In other words, with the prior art, it has been difficult to improve the insertion loss and the group delay characteristics at the same time.
DISCLOSURE OF THE INVENTION
It is the object of the present invention to provide a piezoelectric ceramic filter circuit and a piezoelectric ceramic filter, wherein lowered insertion loss and improved group delay characteristics can be realized at the same time.
In order to achieve the afore-mentioned object, the piezoelectric ceramic filter circuit according to the present invention comprises a plurality of piezoelectric ceramic filter elements connected in cascade with each other, of which at least two piezoelectric ceramic filter elements have different center frequencies and satisfy the condition 0<.vertline.df.sub.0 .vertline./BW.sub.3 <0.8.
Here .vertline.dF.sub.0 .vertline. is the absolute value of the frequency difference dF.sub.0 between the center frequencies of said piezoelectric ceramic filter elements, and BW.sub.3 is the individual band width of each of the ceramic filter elements in which the amplitude loss is 3 dB.
Also, the piezoelectric ceramic filter according to the present invention has a plurality of trapped energy type filter elements on the same piezoelectric ceramic substrate. At least two of these trapped energy type filter elements have different center frequencies, are connected in cascade and satisfy the condition 0<.vertline.dF.sub.0 .vertline./BW.sub.3 <0.8.
Here .vertline.df.sub.0 .vertline. is the absolute value of the frequency difference dF.sub.0 between the center frequencies of said trapped energy type ceramic filter elements, and BW.sub.3 is the individual band width of each of the ceramic filter elements in which the amplitude loss is 3 dB.
Since at least two of the piezoelectric ceramic filter elements have different center frequencies and since they are connected in cascade with each other, it is possible, depending upon the selected center frequencies, to synthesize the group delay characteristics of the multi-type piezoelectric ceramic filter circuit and to obtain total stage group delay characteristics which are flat within the pass band.
Especially with a piezoelectric ceramic filter circuit that s
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patent: 3866155 (1975-02-01), Kobayashi et al.
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patent: 4511202 (1985-04-01), Kasai
Katoh Ikuo
Momiyama Tsuyoshi
Pascal Robert
Summons Barbara
TDK Corporation
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