GB675382A - Improvements in and relating to interference reduction circuits - Google Patents

Abstract

675,382. Radio receiving systems; frequency modulation. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 18, 1950 [Jan. 25, 1949], No. 1314/50. Class 40 (v). In an A.M. or F.M. receiver the frequency modulation and amplitude modulation of the carrier wave due to interference are both detected and one used to balance out the other. The signal detector or discriminator has in parallel a discriminator or detector circuit for detecting carrier modulation, due to interference, of opposite type to the signal modulation and comprising a differentiating circuit 5, 6 (in an F.M. receiver, as shown) or integrating circuit (in an A.M. receiver, Fig. 3, not shown) and amplifier 7 so that the interference varies equally in amplitude with that in the main circuit as the frequency α varies, where α is the difference in frequency between the carrier and the interfering wave or between the carrier and the noise induced side-bands, respectively. Further the main and auxiliary circuits include phase shifters 8, 9 producing phase shifts of # and # + 90 degrees of constant phase difference compensating the 90 degree phase shift due to the differentiating or integrating circuit, so that the interference voltages are of phase to balance out. To this end the phase shifters 8, 9 may be of the type producing phase shift proportional to the logarithm of the frequency α of the input.