GB542452A - Wave transmission networks - Google Patents

Abstract

542,452. Impedance networks ; piezo-electric crystals. STANDARD TELEPHONES & CABLES, Ltd. Aug. 23, 1940, No. 13399. Convention date, Nov. 10, 1939. [Classes 40 (iii) and 40 (v)] A filter or other transmission network comprises a single crystal having two or more independent simultaneously effective resonances of different pre-determined frequencies; The crystal may have a rectangular major face, one frequency being determined mainly by the length of the face and the other by its width. Two such crystals are described in detail. In one the major face is parallel to an electric axis and at an angle of 50 degrees to the optic axis, the length dimension of the face being inclined at 45 degrees with respect to the electric axis, and the length being about twice the width of the face ; one mode of vibration is in the direction of the width of the face and the other is a harmonic of the vibration in the direction of its length. In the other crystal, the major face is perpendicular to the electric axis, the length dimension being inclined minus 19.5 degrees with respect to the mechanical axis, and the ratio of the width to the length being between 0.21 and 0.26; one mode of vibration being longitudinal in the direction of its length and another being a harmonic of a flexure vibration in the plane of the major face. The crystals may have two pairs of electrodes 1, 2 and 3, 4, Figs. 1 and 2, or the two electrodes 2, 4 may be replaced by a single electrode 5, Fig. 6. The crystal connected as in Fig. 2 will have the equivalent lattice network shown in Fig. 3, in which each line branch consists of a crystal 2QL shunted by a capacitance C13, and each diagonal branch consists of a crystal 2QW shunted by a capacitance C14. The crystal 2QL has twice the impedance of the whole crystal fully plated and a resonance frequency equal to twice that of the fundamental vibration in the direction of its length. The crystal 2QW has twice the impedance of the whole crystal and a frequency equal to that of the fundamental vibration in the direction of its width. The two modes of vibration are (Figs. 4 and 5, not shown), substantially uncoupled. The connections between the output electrodes may be reversed (Figs. 4 and 5, not shown) and the balanced circuits of Fig. 2 and (Fig. 4 not shown) may be connected to unbalanced circuits, Fig. 6, and Figs. 7, 8 and 9 (not shown). In order to determine the impedance of the crystals, the electrodes may be arranged as in Figs. 10, 11, 14, 15. To reduce the capacity C13 between electrodes 1, 3 on the same side of the crystal, a grounded strip may be run from one side of the crystal to the other between the electrodes 1, 3 and connected to the electrode 5, Fig. 6. The circuit shown in Fig. 2 may be completed by adding series inductances L, Fig. 16, shunt capacitances C1, and bridging capacitances C2. Similar circuits corresponding to Fig. 4 (not shown), Fig. 6 and Fig. 8 (not shown) are described in connection with Figs. 18, 20, 22 (not shown) respectively, their equivalent circuits being shown in Figs. 19, 21, 23 (not shown). Filters using two doubly resonant crystals connected in parallel are described in connection with Figs. 24 ... 27 (not shown). Specification 437,294 is referred to.