Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1992-08-28
1995-06-13
Lee, Benny
Wave transmission lines and networks
Coupling networks
Electromechanical filter
310313, H03H 964
Patent
active
054246970
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the correction of the amplitude response in the time domain or in the frequency domain for a class of linear, reciprocal filters which are based on surface acoustic waves (SAW). The signal is carried in the form of a SAW over the surface of the component. The SAW will follow a path from an input transducer where the SAWs are generated to an output transducer where the SAWs are detected and the signal is made available to the user in electric form. Only a part of each signal carrying SAW is converted to electric form by the output transducer; such part will be referred to part of the useful acoustic signal or saw.
Existing correction methods are based on the fact that the useful acoustic signal follows different paths over the component's surface determined by the signal frequency if the correction is to be made in the frequency domain or dependent on the signal time delay if the correction is made in the time domain.
Correction of the phase response for SAW filter has been done and is being done today by the deposition of a metal film which influences the velocity of the SAW. After electrical measurement of the component's response, part of this metal film may be removed by a deliberate selective etching until a correct phase response has been reached as described by V. S. Dolat, J. H. C. Sedlacek and D. J. Ehrlich in the paper "Laser direct write compensation of reflective array compressors", IEEE 1987 Ultrasonic Symposium Proceedings, p. 203-208.
Correction of the amplitude response for reflector array compressor (RAC) chirp filters has been done earlier by the deposition of a resistance film (CERMET) which will attenuate the SAW as described in the above Dolat et al. paper. After electrical measurement of the response of the RAC filter, the attenuating film has been made partially inactive by laser induced oxidation until a correct amplitude response has been reached. This method requires a strong piezo-electric coupling for the waves in the area where the film is located. This is not possible for RAC chirp lines on quartz, in particular not if the crystal has a so-called ST-cut. As described by M. B. Schulz, B. J. Matsinger and M. G. Holland in the paper "Temperature dependence of surface acoustic wave velocity on alpha-quartz"; J. Appl. Phys, Vol 41, 2755-2765 orientation of the SAW plane related to the crystal axis results in a low temperature dependence of the component, but the crystal cut lacks piezo-electric coupling for SAWs in the transversal direction where the attenuating CERMET film has to be located. Other SAW materials like LiNbO.sub.3 has a sufficient piezo-electric coupling, however, it represents a general problem to produce a CERMET film with a suitable resistance and a sufficient stability.
By using the invention, correction of the amplitude response on a time selective or a frequency selective basis is obtained by changing the substrate surface properties in a deliberate pattern transversely to the propagation of the wave to obtain desired SAW velocity changes. These velocity changes have to be made in that area of the surface where the signal path depends on frequency or on time delay. The resulting change in the transfer of the useful signal can be used for amplitude correction of the filter response by a procedure of the present invention that will be described below.
A RAC filter, or in principle any other filter, may also be corrected by making a cascade connection to a correction filter which has been designed to cancel the response errors of the RAC filter to be corrected. This method is described in the U.S. Pat. No. 4,857,870: "Method of manufacturing a surface wave dispersive filter and a filter manufactured in accordance with this method". The additional filter, or the correction filter can be integrated with the primary filter on the same piezo-electric substrate or on an additional substrate dependent on what is the most practical and economic solution. In any case the correction of a filter by the design of a particular correction filter
REFERENCES:
patent: 4336514 (1982-06-01), Paige
patent: 4745321 (1988-05-01), Raschke
patent: 4916416 (1990-04-01), Desbois
Anderson et al, Attenuating Thin Films for SAW Devices Nov. 1980, 442-445.
Forbe Radio AS
Lee Benny
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