GB702152A - Improvements in or relating to circuits for reducing pulse interference in radio receivers - Google Patents

Abstract

702,152. Limiters. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Feb. 26, 1951 [March 1, 1950], No. 4647/51. Class 40 (6) In a noise limiter circuit of the type in which an input impedance is connected in series with a rectifying component, e.g. a diode, and a capacitor which shunts the output terminals of the circuit, the rectifying component being normally so biased that desired signals can be transmitted through the circuit but pulses having steep wave-fronts cause the rectifymg component to become momentarily non-conductive, the input impedance is the cathode resistance of a discharge tube arranged as a cathode follower whereby both desired signals and interfering pulses are fed into the circuit. As shown in Fig. 1, to suppress pulses of negative polarity a diode 3 is connected to the cathode resistor 2 of the cathode-follower 11, the diode 3, triode 11, and a high resistance 6 shunted by a capacitor 4 being in series with the H.T. supply so that a small direct current component flows through the diode 3. This permits signal currents to be transmitted but negative pulses are limited by the diode 3. Both positive and negative pulses may be eliminated by two oppositely connected diodes in series, Fig. 2 (not shown). In an alternative arrangement, Fig. 3, the triode portion of the diode-triode 8 has the same function as the resistor 6 of Fig. 1 and under the normal conditions the diode 3 passes a current primarily determined by the impedance of the triode which is chosen to be high. Through the resistor 9 a positive bias is applied to the diode of the valve 8 so that the resistance of this diode for all signal voltages is negligible. The value of the resistor 9 considerably exceeds the impedance of the capacitor 10 and this auxiliary circuit passes a current proportional to the variation of the input voltage with time. An interfering negative pulse makes both diodes non-conducting and the pulse voltage on the control grid of the triode renders it also non-conducting. The capacitor 4 is thus isolated and retains a voltage approximating to that of the signal at the instant when the pulse was suppressed. This arrangement may be made more sensitive by including an additional diode in series with the triode 8, Fig. 4 (not shown). In a modification for suppressing positive pulses, Fig. 5, the auxiliary diode 21 is connected to a negative voltage source through a resistor 9 and the diode 14 is connected to a positive voltage source through a resistor 15, the D.C. path for the diodes 3, 14, being completed by a resistor 23. Under normal conditions all the diodes are conducting, but when a positive pulse occurs the diodes 3 and 21 cease to conduct, and the valve 8 becomes highly conducting causing its anode potential to fall and the diode 14 to become non-conducting. The output capacitor 4 is thus isolated and retains its charge during the occurrence of the pulse.