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Subminiature Broadband Filters.

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The third period of this program witnessed two major developments in acoustic surface-wave devices. A surface wave filter with only 3-dB total insertion loss, including matching losses, and 0.05-dB passband ripple was demonstrated with unidirectional transducers developed in the first part of this contract. The 3-dB value is the lowest insertion loss ever reported for a multiphase unidirectional filter, and tests indicate that the loss can be reduced even further. The second development is the invention of the capacitively weighted transducers CWT, a new technique for weighting surface-wave transducers. In the CWT, the charges on the electrodes and therefore the excitation strength of the electrodes, are controlled by thin-film weighting capacitors connected in series with the electrodes. Primary advantages of the CWT are the electrodes are of equal length, thus reducing diffraction effects that degrade apodized transducer performance and both input and output transducers can be weighted, in contrast to overlap weighting Apodization techniques. A CWT design procedure was developed during the third period and is presently being used to design the first CWT. The procedure is unique in that it includes the effects of partial capacitances in the array in addition to the capacitances between adjacent electrodes. This should lead to better filter performance. Author

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Interim technical rept. no. 3, 1 May-30 Nov 73,



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