GB660192A - Improvements in or relating to radio receivers - Google Patents

Abstract

660,192. Radio receiving circuits. STANDARD TELEPHONES & CABLES, Ltd. Feb. 11, 1949 [Feb. 13, 1948], No. 4262/48. Class 40 (v). In a receiver having a band-width greater than necessary for the modulation received, filter and pulse-shaping means are used to derive the high or low-frequency components of noise pulses and counteract static. A demodulated wave with superimposed noise pulse, as shown (Fig. 1a), may be applied to a valve 8 (Fig. 3), with a high-pass filter 16 as cathode load, terminating with a resistance 19 and diode 17. The diode input takes the form shown in Fig. 1 (b), its output across cathode load resistance 18 eliminating the negative overswing to produce the pulse (c). The negative overswing contains low-frequency components of the noise pulse frequency spectrum which cancel corresponding low-frequency components in the positive swing. Removal of the negative overswing therefore leaves the pulse (c) containing both low and high frequencies of the original pulse. On combining the latter developed across resistance 18, therefore with a phaseinverted output of the original demodulated wave developed across the anode load 12 of the valve 8 and suitably attenuated by a tapped potentiometer 21 in series with the resistance 18, the tapped output at 22 of the potentiometer is free from noise pulse. In a modification (Fig. 4, not shown), the high-pass filter comprises simply a condenser 23, and a diode 24 clipping the base of the pulse replaces the resistance 19. In a further modification (Fig. 7, not shown), the valve 8 is dispensed with, a condenser 30 acts as a high-pass filter and the diode 17 is reversed so that across its anode load resistance 18 only the negative overswing, containing only low-frequency components of the noise pulse reversed in phase, is produced. On combination with a shunt demodulation output across a tapped potentiometer 21 the low-frequency noise components are eliminated. A final shunt condenser 34 eliminates the high-frequency noise components. Contact rectifiers may replace the diodes. Specification 641,211 is referred to.