Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1980-11-21
1982-10-19
Nussbaum, Marvin L.
Wave transmission lines and networks
Coupling networks
Electromechanical filter
333196, H03H 964, H03H 925
Patent
active
043552900
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to filters and, more particularly, to acoustic surface wave filters.
DESCRIPTION OF THE PRIOR ART
There is known an acoustic surface wave filter comprising a piezoelectric substrate which carries an input interdigitated converter with electrodes having overlapping portions to produce acoustic surface waves for propagation in a first acoustic channel, a coupling element to transmit acoustic surface waves from the first acoustic channel to a second acoustic channel, and an output interdigitated converter with electrodes having overlapping portions to receive acoustic surface waves propagating in the second acoustic channel (cf. U.S. Pat. No. 3,836,876, of Sept. 17, 1974).
In this filter, the coupling element is a multistrip system of electrodes whereof each overlaps both acoustic channels. The electrode portions in one channel are parallel with those in the other channel and orthogonally oriented to the direction of acoustic surface wave propagation in this channel.
Both converters of the filter under review may have electrodes with overlapping portions of a variable length. As a result, the amplitude-frequency characteristic of the filter can be formed both by the input and output converter. The resultant amplitude-frequency characteristic of such a filter is the product of the amplitude-frequency characteristics formed by the input and output converters.
In cases requiring a high level of signal suppression beyond the filter passband, the amplitude-frequency characteristic of the filter under review turns out to be unsatisfactory. Besides, this filter can be manufactured only from a limited range of materials, because the multistrip electrode system of its coupling element can only be arranged on substrates of piezoelectric materials possessing a high electromechanical coupling constant, such as lithium niobate. If the substrate is of a material possessing a relatively low electromechanical coupling constant, such as quartz, an exorbitant number of electrodes is required in the multistrip system to ensure effective transmission of acoustic surface waves from one channel to the other.
There is further known an acoustic surface wave device which can employ a substrate of any piezoelectric material. The coupling element of this device comprises a system of electrodes whereof each is galvanically coupled to every other electrode (cf. UP Pat. No. 1,389,023, Cl. H3U, of Apr. 3, 1975).
Like the above filter, the amplitude-frequency characteristic of this device is often unsatisfactory, because it is formed only by the acoustic surface wave converters incorporated in the device.
There is known an acoustic surface wave filter whose amplitude-frequency characteristic is much better than that of the filter and device described above.
This filter comprises a piezoelectric substrate which carries an input interdigitated converter with electrodes having overlapping portions to produce acoustic surface waves for propagation in a first acoustic channel, a coupling element to transmit acoustic surface waves from the first channel to a second acoustic channel and simultaneously form the amplitude-frequency characteristic of the filter, and an output interdigitated converter with electrodes having overlapping portions to receive acoustic surface waves propagating in the second acoustic channel (cf. Application No. 2,003,353, Cl. H3U, advertized by the British patent office on Mar. 7, 1979).
The coupling element of this filter comprises a first row of electrodes, which is arranged in the first acoustic channel and composed of a first group of electrodes and a second group of electrodes; these are parallel with the electrodes of the input converter and arranged so that the electrodes of the second group are interposed between those of the first group so as to form overlapping portions. The coupling element further contains a second row of electrodes which is arranged in the second acoustic channel and composed of a third group of electrodes and a fourth group of electrodes that are parallel wit
REFERENCES:
patent: 3987379 (1976-10-01), DeVries
patent: 4004254 (1977-01-01), DeVries
patent: 4162415 (1979-07-01), Andreev et al.
Bagdasarian Alexandr S.
Gulyaev Jury V.
Kmita Anatoly M.
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