GB434819A - Wave filters - Google Patents

Abstract

434,819. Impedance networks; piezo-electric crystal mountings. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.-(Western Electric Co., Inc. ; 195, Broadway, New York, U.S.A.) March 10, 1934, No. 7658. [Classes 40 (iii) and 40 (v)] In band-pass lattice or bridged-T wave-filters utilizing piezo-electric crystals, (1) the pass-band may be broadened by insertion in both line and lattice arms of inductance in series or in shunt to the crystals ; (ii) the crystals may be coupled to the filter arms by step-down transformers so that crystals and impedances of convenient size may be used ; and (iii) in order that mass-produced crystals of equal size may have differently pitched frequency characteristics, they may be tuned by means of series and shunt condensers. Band filters utilizing crystals. A crystal being equivalent to an acceptor circuit shunted by a condenser, Fig. 2 (not shown), the Specification shows how a band-pass filter can be made with a lattice section, Fig. 4 (not shown) which has equal line arms and equal lattice arms. the line arms being suitably dimensioned in relation to the lattice arms and unequal to them. The band can be widened by the addition of inductance in series and capacity in shunt with each crystal, Fig. 6, or by the addition of inductance in shunt with each crystal, Fig. 7 (not shown) : or there may be inductance and capacity both in series and in shunt with each crystal, Fig. 9 (not shown). Where the added impedances are numerous the attenuation characteristic may become distorted by dissipation, but this result may be prevented by, for instance, substituting for the section shown in Fig. 6 the equivalent section shown in Fig. 12, provided that LO1=LO2=LO and C1>C2. The resistances of the coils marked ¢LO raise the characteristic impedance of the reaction, but this can be reduced by means of resistance shunted across the terminals of the section. When the filter is to be used in an unbalanced amplifier, a bridged-T section, equivalent to the balanced lattice section, may be employed. Coupling of crystals. The dimensioning of the above circuits may necessitate inconveniently large inductances or thin crystals. This necessity is obviated by coupling the crystal to the filter arms by means of step-down transformers instead of conductive connections. An economy of crystals can be effected by means of the double inductive couplings shown in Fig. 14. Tuning crystal circuits. The impedance characteristic of a crystal may be varied by the use of series and shunt condensers, Figs. 15, 16. Thus mass-produced crystals all having the same dimensions can be tuned to have different frequency characteristics. Mounting of crystal. A crystal having electrodes P, P, Fig. 1, is supported by median knife-edges 15 parallel to the optic axis O O<1> and in the plane of the electric axis E E<1>.