GB538946A - Improvements in electric filter circuits - Google Patents

Abstract

538,946. Impedance networks. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and STARR, A. T. Dec. 13, 1939, No. 32162. [Class 40 (iii)] An unbalanced wave filter comprises a threeelectrode crystal which provides two shunt arms, the two electrodes on one side being bridged by a series impedance Z. An additional pair of series or shunt impedances Z 1 as shown may be provided in certain cases. The electrodes may be formed by partial plating, as described in Specification 529,131, and an earthed screen may, if desired, be placed between the two electrodes on one side to reduce any direct capacitance between them. The filters are shown to be equivalent to a balanced lattice filter. The impedances Z, Z, may take various forms, such as crystals, inductances, capacitances or combinations of such elements, according to the requirements of the filter. In particular, if Z is a crystal and Z 1 an inductance (either series or shunt) a much lower image impedance can be obtained than with the equivalent lattice form. In another arrangement, Z is a series-tuned circuit with parallel capacitance, and Z 1 = 0, a broad pass-band being obtained of which the upper cut-off may be adjusted by varying the parallel capacitance. Reference has been directed by the Comptroller to Specification 531,662.