GB556774A - Image suppression systems for radio receivers and the like - Google Patents

Abstract

556,774. Transformers. JOHNSON LABORATORIES, Inc. March 19, 1942, No. 3665. Convention date, May 13, 1941. [Class 38 (ii)] [Also in Group XL] In an arrangement for suppressing image frequencies in a superheterodyne radio receiver, the aerial circuit is coupled to the first tuned circuit by a transformer 3, 4 the windings of which are in series opposition and are grounded through a common condenser 7 such that at the image frequency the voltage due to mutual inductance is equal and opposite to that across the condenser, and this condition is maintained as the circuit is tuned over a range of frequencies. Tuning is effected by the movement of a powdered ferromagnetic core, as described in Specifications 366,475 and 455,573, [Group XXXVI], and in order to obtain the desired variation of mutual inductance the greater part of the primary winding 3 is concentrated at the end. of the secondary 4 remote from the core 10 and resonance to the lowest frequency is obtained before the core enters the concentrated part. The Specification gives an example of suitable dimensions for the transformer and for the condenser 7.