Low loss surface wave filter

Details Low loss surface wave filter Low loss surface wave filter

1992-09-23

1994-04-26

Pascal, Robert J.

Wave transmission lines and networks

Coupling networks

Electromechanical filter

333196, 310313R, H03H 900

Patent

active

053070356

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a low loss microwave surface acoustic wave filter.
2. Discussion of the Background
The type of high performance, low loss filter to which the invention relates is, for example, as described in the article by Messrs. DUFILIE, HODE and DESBOIS: "A low loss high performance filter structure", IEEE 1988 Ultrasonic Symposium (Chicago), pages 1 to 4. The filter described in this article comprises:
two unidirectional input transducers placed symmetrically on either side of a first bidirectional coupling transducer and acoustically connected to the latter;
a second bidirectional coupling transducer electrically connected to this first bidirectional coupling transducer;
two unidirectional output transducers, placed symmetrically on either side of this second bidirectional coupling transducer and acoustically connected to the latter; and
two input and output terminals the first electrically connected to the two input transducers and the other to the two output transducers respectively.


SUMMARY OF THE INVENTION

The invention aims to improve the performance characteristic of this type of surface wave filter. To this end it relates to a surface wave filter comprising, in a very general way:
one or more input transducers, electrically connected to the input terminal of the filter, and acoustically connected to at least a first coupling transducer, whether weighted or not;
at least one second coupling transducer, weighted or not, electrically connected to this first coupling transducer; and acoustically connected to one or more output transducers, which are themselves electrically connected to the output terminal of the filter;
this filter being such that the positions of the input and/or output transducers with respect to the coupling transducers and/or with respect to the reflectors with which these various transducers may be associated, &re chosen, having regard to the weightings of the coupling transducers, in order to obtain the desired transfer function for this filter.
According to one feature in accordance with the invention, the filter involved comprises one or more unidirectional transducers with "O coupler", this filter being such that the transducer which is surrounded by this O coupler is offset with respect to the axis of this coupler, axis which is normal to the direction of propagation of the surface waves, by a distance which is equal to an eighth of the propagation wavelength of the surface waves, increased by a whole number of half-wavelengths.
According to another feature, the filter involved comprises two pairs of unidirectional transducers with O coupler, with a first pair associated with the input of the filter and a second pair associated with its output, and is characterised in that the internal diameters, taken in the direction of propagation of the surface waves, of the two O couplers of the first and/or of the second of these transducer couplers differ for the two O couplers of the same pair by a propagation half-wavelength of the surface waves, to within a whole number M of wavelengths, this number M possibly being different for the first and the second pair.
According to another feature, the filter involved comprises, on the input side, either two lateral input transducers acoustically connected to a central coupling transducer, or a central input transducer acoustically connected to two lateral coupling transducers, and comprises, on the output side, a configuration of transducers similar to that of the input side, and is characterised in that, on the input side and/or on the output side of the filter, the respective distances between the axis of each input, or output, transducer, and that of the coupling transducer which is associated with it differ, for the same pair of distances relative to the input or to the output of the filter, by a whole number of wavelengths, this whole number possibly being different between the input side and the output side.
According to yet another feature, the filter inv

REFERENCES:
IEEE Ultrasonics Symposium, vol. 1 2-5 Oct. 1988, (Chicago, Ill., US), P. Dufilie et al.: "A low loss high performance filter structure", pp. 127-130.
1982 Ultrasonics Symposium Proceedings, vol. 1, 27-29 Oct. 1982, (San Diego), C. L. West: "Saw convolver employing unidirectional transducers for improved efficiency", pp. 119-123.

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