GB412942A - Frequency selective filtering circuits - Google Patents

Abstract

412,942. Impedance networks. HAZELTINE CORPORATION, 15, Exchange Place, Jersey City, New Jersey, U.S.A.-(Assignees of Wheeler, H. A. ; 18, Melbourne Road, Great Neck, New York, U.S.A.) Jan. 3, 1933, Nos. 208 and 209. Convention dates, Feb. 1, 1932 and April 25, 1932. [Class 40 (iii).] In a tunable coupling system, signal energy is transferred over two paths so arranged that the two paths deliver equal and opposite voltages at a frequency differing from the signal currents by a constant amount which is the image frequency in the case where the coupling is used as the input to a superheterodyne receiver. Fig. 1 shows a tuned transfer path Z1, LC transferring a voltage E1 from the source 10 in series with an aperiodic path Z2 which transfers a voltage E2. The phase of the voltage E1 may be changed by interchanging the impedances LC. The impedances Z1, Z2 may be inductive, capacitive or mixed couplings and the ratio of the impedances Z1, Z2 varies with the tuning of the circuit Z1, LC so that the energy transferred by the aperiodic path equals the energy transferred by the tuned path at the particular undesired frequency. Fig. 6 shows an arrangement in which the valve cathode is ungrounded to reduce capacity effects across its input and exact suppression is obtained at two points by adjustment of C1 and M1. A variable resistance 21 in the aerial circuit serves to broaden the tuning of the aerial circuit. Numerical data are given of the values of impedances used for suppressing the image frequency in superheterodyne receivers operating on intermediate frequencies of 175 and 260 kilocycles.