Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-08-16
2001-11-06
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S196000, C029S025350, C310S31300R
Reexamination Certificate
active
06313717
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave filters capable in particular of using one-directional DART type transducers.
2. Discussion of the Background
This type of transducer has the advantage of sending or receiving an acoustic wave essentially in one direction. This greatly improves the efficiency of the transducer. It is formed by a set of transducer centers and reflection centers whose distribution makes it possible to obtain very valuable filtering characteristics. However, owing to their design, DART filters are devices whose length is relatively great, typically several hundreds of &lgr;, if &lgr; is the wavelength at the central frequency of operation of the device.
Consequently, a filter comprising two DART type transducers, one at input and the other at output, requires the use of a substrate whose length is appreciably greater than its width. It then becomes necessary to use a non-standard package, itself having a length appreciably greater than its width, which is difficult to incorporate into the usual systems of implantation, and which furthermore has weaknesses in torsion and deflection.
SUMMARY OF THE INVENTION
In this context, the invention proposes a surface acoustic wave filter using transducers into which there are integrated DART type transduction and reflection functions, the filter comprising two electrically connected channels, each channel comprising an input transducer and an output transducer, said channels having structures optimized so as to be able to attain very satisfactory performance levels in terms of filtering band and rejection band by reducing the length of the filter, a parameter which, at present, is decisive in terms of the cost of filters.
More specifically, an object of the invention is a surface acoustic wave filter comprising transducers, each transducer being defined by a weighting function that represents the density of the reflection and transduction within the transducer as well as the phase of the reflections and transductions. This weighting function may be achieved physically on the transducer either by a discretization method, well known to those skilled in the art, consisting of the choice, in each zone of the transducer, of a density of reflectors and a density of sources proportional to those given by the weighting function or by a so-called method of weighting by aperture consisting of the placing, in the transducer, of the sources and reflectors on an aperture given by the weighting function, said filter having a given passband and a given rejection band and comprising:
a first channel comprising at least one first input transducer and one first output transducer defining a first transfer function, said first channel having a first resultant weighting function;
a second channel comprising at least one second input transducer and one second output transducer defining a second transfer function, said second channel having a second resultant weighting function;
the first and second input transducers being electrically connected to each other;
the first and second output transducers being electrically connected to each other;
wherein:
the resultant weighting functions of the two channels are different and such that the transfer functions of the first channel and of the second channel are in phase in the passband of the filter and in antiphase in the rejection band of the filter.
Advantageously, the input or output transducers may be identical from one channel to another to limit the problems of coupling, due to the diffraction between transducers, which generally cause the appearance of minor lobes in the rejection band of the filter.
According to one alternative embodiment of the invention, the input transducers and/or output transducers are series-connected.
They comprise a first series of electrodes connected to an external bus of a first differential input (+V), a second series of electrodes connected to an external bus of a second differential input (−V), a third series of electrodes connected to a central bus with a floating potential, said third series separating said first and second series of electrodes. The filter can also work in non-differential mode, the potential (+V) being replaced by the ground.
According to another variant of the invention, the input and/or output transducers are parallel-connected.
The transducers of the first channel comprise a first series of electrodes connected to an external bus of a first differential input (+V) and a second series of electrodes, the transducers of the second channel comprise a third series of electrodes connected to an external bus of a second differential input (−V) and a fourth series of electrodes, a sub-set of electrodes of the second series of electrodes being connected to a sub-set of electrodes of the third series of electrodes by means of connection pads, a sub-set of electrodes of the fourth series of electrodes being connected to a sub-set of electrodes of the first series of electrodes by means of connection pads.
An object of the invention is also a method for the making of a filter according to the invention with a given template, namely a given passband and a given rejection band.
This method especially comprises the following steps:
the determining of the weighting functions of the transducers of the first channel and of the transducers of the second channel by an iterative process so as to define transfer functions of said first channel and of said second channel in phase in the passband of the filter and in antiphase in the rejection band;
a step of discretization of the weighting functions determined beforehand to achieve electrode masks constituting the set of transducers of the first and second channels.
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Chamaly Stéphane
Dufilie Pierre
"Thomson-CSF"
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pascal Robert
Summons Barbara
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