Surface wave transducer, especially of split finger design, with

Details Surface wave transducer, especially of split finger design, with Surface wave transducer, especially of split finger design, with

1992-09-29

1994-04-19

Pascal, Robert J.

Wave transmission lines and networks

Coupling networks

Electromechanical filter

333193, 310313R, 310313B, H03H 900

Patent

active

053049659

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a surface wave for suppressing reflections of terminal transducer fingers transducer.
In order to suppress undesired reflections of the acoustic wave in the transducer and on its transducer fingers, it has been known for a long time for such (transmitting/receiving) transducers to be constructed with interdigitally arranged transducer fingers, especially as split finger transducers. In comparison with a simple interdigital transducer with adjacent transducer fingers having alternating polarity, such transducers have in each case two (if required, also in each case three) adjacent fingers which have the same polarity, that is to say they are electrically connected to one and the same busbar. In a normal, regular design of such a split finger transducer, there follow alternating with one another two (or three) transducer fingers connected to the one busbar and two (or three) transducer fingers connected to the other busbar. The center distances between successive transducer fingers (irrespective of which polarity, that is to say including the fingers within a "split finger" and apart from points at which transducer fingers are omitted) are a quarter (or a sixth) of the acoustic wavelength of the surface wave to which the relevant transducer is tuned, to be precise the wavelength of the acoustic wave in the substrate material of the transducer. The center distances of adjacent groups of two (or three) adjacent fingers of the same polarity, that is to say the center distances between alternating split fingers, is once again equal to half the wavelength, as in the case of simple transducers.
A further transducer type with suppression of internal reflections of the ultrasound waves is the third-lambda finger arrangement. This is likewise an interdigital transducer in which, as in the case of the split finger transducer, the one finger comb (with the busbar) has doubled transducer fingers and there is in each case a single finger of the finger comb of the other busbar between adjacent groups of such double transducer fingers.
For the internal regions of such a split finger transducer or third-lambda transducer, the neutralization or suppression of internal reflections in the transducer is virtually complete with correspondingly precise dimensioning. However, this unfortunately does not apply to the terminal split finger groups and finger groups. In order to achieve suppression of reflections for these as well, it is known (European reference EP-A-0,088,400) for the relevant transducers to be provided with terminal transducer fingers which are dimensioned such that their finger length is reduced in steps, in such a manner that terminal transducer fingers interact in an interfering manner with one another with respect to the reflections. A disadvantage of such an (additional) measure is, however, that such an end of stepped construction of a transducer causes distortions in the phase front of the acoustic wave of the transducer. In addition, not all the fingers of a transducer which is weighted in an overlapping manner, as is often the case, are affected completely by the acoustic wave which is received by the transducer which is operated as the receiving transducer. The efficiency of such a measure for suppressing undesired reflections occurring at the end of a transducer is also only limited.
Another method of suppressing reflections occurring on terminal fingers of an interdigital transducer is that which is described in German reference DE-B-2,839,851. Weighted fingers in the region of the transducer end are directed with essentially longitudinal components of these fingers at an oblique angle with respect to the major axis of the wave propagation, so that these fingers reflect ultrasound waves which impinge on them away in an oblique direction, by means of these obliquely positioned components of their finger lengths, that is to say said fingers do not intrinsically reflect back.
A further method (Trans. IECE Jap. Vol 64-C (1981) pages 437-438) is t

REFERENCES:
patent: 4162465 (1979-07-01), Hunsinger et al.
patent: 4263569 (1981-04-01), Moellmer
patent: 4701657 (1987-10-01), Grassl
"Efficient Modeling of Transduction & Reflection Properties of Finite Aperiodic SAW Transducers" by O. Manner et al, IEEE Ultrasonic Symposium (1985), pp. 78-81.

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