GB1260525A - An acoustic electric filter system - Google Patents

Abstract

1,260,525. Electromechanical filters; frequency discriminators. ZENITH RADIO CORP. 31 March, 1969, No. 16856/69. Headings H3A and H3U. A filter suitable for embodying in integrated circuitry comprises, Fig. 1, a substrate 14 of piezoelectric material having on its surface transducers 13, 15 constructed as interleaved comb-type electrode arrays. The arrays 13, 15 are formed by vacuum depositing gold, the distance between the centres of two consecutive strips in each array being ¢ the wavelength or an integral multiple of ¢ the wavelength, of the signal that is to be propagated. The electric field set up by the transducer 13 coupled to the signal source 10 produces an acoustic surface wave which is detected by the transducer 15 and applied to a load 18. The source 10 may be the tunable front end of a TV receiver and the load 18 may be three stages of the receiver subsequent to the IF selector. Inductor coils 12, 16 form tuned circuits with the capacitance of the arrays 13, 15. In another embodiment, Fig. 3, the two arrays having terminals A, B and C, D, respectively, are circular. To reduce the 3 db loss associated with each transducer, the transmitted surface wave may be reflected from an end surface of the substrate 14 of Fig. 1 by bevelling one end of the substrate (Fig. 4, not shown). In another method of reducing the 3 db loss, Fig. 5, additional passive arrays 27, 28 are provided on the substrate body 30. In another embodiment (not shown) the surface of the substrate is coated with photoconductive material and the interleaved comb type arrays are formed by projecting light rays through transparencies having the desired pattern. The pattern may be readily changed by using different transparencies to produce a tuning feature. Another readily tuned embodiment, Fig. 6, comprises transducers 47, 50 printed on a carrier 48 of low acoustic impedance placed under a polarized piezoelectric bar 49. Since the stripes of the electrode arrays 47, 50 are not perpendicular to the bar 49, lateral movement of the bar permits tuning. An FM discriminator embodying the filter comprises, Fig. 7, a transducer 31 coupled to source 10 and output transducers 32, 33 coupled through diodes 36, 37 to a load 40. The resonant frequencies of transducers 32, 33 are respectively below and above the resonant frequency of transducer 31. In another embodiment of FM discriminator (Fig. 8, not shown) there is a single input transducer and single output transducer. Fig. 10 shows in block diagram form the IF channel of a colour TV receiver employing elements SWIF No.1... SWIF No. 4 each of which is as described with reference to Fig. 1 or Fig. 7.