Patent GB-467724-A (GB)
Title: Piezo electric crystal apparatus
IPC: H03H9/13
Filed: 1936-11-20
Abstract: 467,724. Piezo-electric crystal mountings. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.) Nov. 20,1936, No. 31805. [Class 40 (v)] A number of crystals which may have different frequencies are mounted in one holder in such a way as to reduce mechanical and capacitive coupling between them, and various improvements are made in connection with the electrodes and the contacts for these. In the arrangement shown in Fig. 1, two crystals 10, 20, adapted to vibrate in the fundamental longitudinal mode, have each two pairs of electrodes such as 34, 35, and are clamped each between two pairs of cantilever springs such as 44, 45, the springs serving also to connect the electrodes respectively to eight lugs such as 74, 75, which are internally connected to form a bridge. The pressure on the crystals is determined by the width of the spacers 60. The electrodes 34, 35 may be formed by the application in vacuo of vaporizedaluminium, platinum or chromium on platinum. The springs 44, 45 may be of steel, phosphor bronze or phenol fibre. If made of insulating material the springs are coated with tin laid on by spraying. The springs grip the crystals by means of projections 50, which may be of gold or silver, or may have a core 51, Fig. 4, of soft brass or of phenol product sprayed with a heavy layer of tin or other soft metal or plated with gold or silver. They may have rounded edges and a flat contact surface of square, circular or other shape, with a nipple 57 fitting. a recess at a nodal point in the crystal face. The fixed ends of the springs may be spaced apart so that the pairs of springs form a V. In a modification, an inner pair of springs such as 41, 44 may be replaced by single springs. The assembly may be housed in an earthed copper can 82 which may be dried or evacuated and sealed to eliminate moisture. In a modification the crystals may be mounted to vibrate to a harmonic of the longitudinal mode, Figs. 5 and 6 (not shown). The springs can be turned about the clamping screws at their fixed ends to suit different modes of vibration. The crystals may be adapted for flexural vibration, the space between the electrodes being an oblique line, Figs. 7 and 8 (not shown), or a horizontal line with the clamping points just above and just below it. The crystals may be of the U-shaped type, Figs. 9, 10 11 (not shown). In a further modification, four crystals having only two electrodes each may be clamped between five springs and connected to form a bridge, Figs. 12, 13 (not shown). In another embodiment, Fig. 14, four crystals 142 are held by springs 201 against four projections 221, which may all be separate or may be the ends of two metal rods passing through the phenol fibre block 209, through which also passes the block 208 carrying the springs 201. In a further modification, Fig. 16, two crystals 360, 361, are gripped along nodal areas between pairs of clamping-contacts 350 mounted on phenol-fibre blocks 330, 331, 332, which are pressed together by springs 340, 341. acting upon roller rods or pins 344. The clamping-contacts 350 are connected by pins 380 and by conductors (which may be metal coatings deposited on the fibre) to form a lattice filter section. The assembly is attached to the cover 312 of a sealed container 310. The crystals may be cut as described in Specification 437,294. In a modification, Figs. 19, 20, 21 (not shown) four separate crystals are accommodated and the assembly is attached to the cover 312 by leaf springs instead of rigid supports 326. In a further modification, Figs. 22, 23 and 24 (not shown), four crystals may be mounted separately between pairs of blocks similar to 330, 331. The electrodes may be undivided, or each may be divided into two parts by means of a flat ribbon or wire which protects a strip of the surface during the imposition of the coating and is subsequently removed. The electrodes may cover only part of the crystal faces in order that the driving efficiency of the crystals may be increased and internal capacity reduced. The electrodes may be upon the central area of the crystal faces to prevent voltage breakdown across the edges. Electric condensers. The anti - resonance frequencies of piezo-electric crystals may be adjusted by means of shunt condensers of small capacity consisting of short lengths of brass tubing with coaxial brass pins or screws, longitudinally adjustable, the condensers being inserted in cavities in an insulating block upon which the crystals are mounted, Fig. 14.