GB475275A - Electrical wave filters - Google Patents

Abstract

475,275. Impedance networks ; piezoelectric crystals. STANDARD TELEPHONES & CABLES, Ltd. May 22, 1936, Nos. 14547 and 14548. Convention dates, July 2, 1935 and July 27, 1935. [Classes 40 (iii) and 40 (v)] A lattice section comprises a pair of fourelectrode crystals or a pair of two-winding magnetrostrictive oscillators, each element providing two arms of the lattice. In the arrangement shown in Fig. 1, a lattice section comprises two crystals 10, 11, each of which carries two pairs of plates, those of crystal 10 being connected so as to form the line arms while those of crystal 11 form the lattice arms of the section. The connections may be such that the crystals vibrate in the flexural mode. The line and lattice arms may have a resonance frequency in common, Fig. 9 (not shown) ; otherwise a line anti-resonance coincides with a lattice resonance and vice versa, Figs. 8, 9 (not shown). Inductances Lo are added in order to broaden the pass band. Instead of the crystals 10, 11 and inductances Lo, two-coil magnetostrictive resonators in series with condensers may be used, Fig. 7 (not shown). The crystals may be adapted to vibrate in a plane mode, Fig. 2, or a flexural mode, Figs. 10, 12. According to the Specification as open to inspection under Sect. 91 a lattice section may comprise four four-electrode crystals vibrating in the flexural mode, the electrodes of each crystal being cross-connected, but this subject-matter does not appear in the Specification as accepted. Piezo-electric crystals. A crystal, Fig. 2, for use in a wave filter has two pairs 12, 121 and 13, 13<1> of opposite electrodes. The electrodes are formed by silver-plating the faces of the crystal and removing the plating along the centre lines and edges of the faces. The Specification gives instructions for dimensioning flexurally - vibrating X-cut crystals with their widths variously inclined to the Z axis and mechanical axis of the mother crystal. Crystals may be adapted to vibrate in flexural modes at relatively low frequencies. Thus the crystal 61, Fig. 10, having two opposite pairs 62, 64 and 63, 65 of electrodes, and four pairs of knife-edged clamps 74, vibrates flexurally about two nodal lines 66, 67. A crystal shaped like a tuning-fork, Fig. 12, has two pairs 35, 37 and 36, 38 of electrodes, and two pairs of clamps 41, 42 and vibrates flexurally about a nodal line 39. Magnetostrictive vibrators. A magnetostrictive vibrator with two operative windings 18, 19, Fig. 4, which may be side by side or wound one over the other, has a tubular core 16 slotted longitudinally to prevent eddy currents, and a polarizing-magnet 21 with polarizing-winding 22.