GB552943A - Wave amplitude limiting device for signal transmission systems - Google Patents

Abstract

552,943. Wireless receiving circuits. ZENITH RADIO CORPORATION. Feb. 3, 1942, No. 1424, Convention date, Dec. 26, 1940. [Class 40 (v)] In a limiter comprising two valves each having means for preventing one set of peaks of the applied signal from reaching the corresponding anode, the second valve is a screen grid valve having means for decreasing its plate voltage when the applied signal is inadequate whereby both the plate resistance and the noise output of the valve are substantially decreased. Fig. 1 shows the invention applied to a frequency modulation receiver in which the output from the intermediate frequency transformer 26 is fed to limiter valves 22, 23 and from these through a discriminator circuit 38 and double diode 46 to audio frequency stages 18, 19, and loudspeaker 21. The limiter valves 22,23 operate without self bias and serve to limit the negative, and positive peaks of the initial wave successively by anode current cut-off. Coupling between valves 22, 23 is by a large condenser 34 and parallel resistor 33 giving a predominantly capacitative load at the intermediate frequency. Resistor 33 reduces noise at low signal levels in co-operation with a large resistor 41 in the anode circuit of valve 23. With no signal resistor 33 makes grid 31 go positive and the increased anode current causes the voltage of anode 36 to fall due to resistor 41. This not only limits the anode current but reduces the anode resistance of valve 23 thus effectively shunting the discriminator input and thus reducing the audio output.