GB577191A - Improvements in or relating to radio telegraph receiving arrangements - Google Patents

Abstract

577,191. Automatic gain control. CREED & CO., Ltd., and MASON, F. P. Jan. 5, 1944, No. 196. Addition to 557,225. [Class 40 (v)] In a telegraph receiver as claimed in the parent Specification, the automatic volume control voltage is dependent upon the average of the voltage across two condensers charged during successive half-waves of received signals and each discharged to an extend dependent upon the amplitude of the half-wave succeeding the half-wave during which it was charged: The Figure shows the last H.F. stage of a telegraph receiver which receives carrier signals at 2 from an aerial or transmission line, and the main output of which passes to recording apparatus. A portion of the output from 1 is fed through transformer 3 having a secondary 5, 6 with earthed centre tap to rectifiers 7, 8, valves 9, 10, condensers 11, 12 and resistance 18 arranged as shown. During alternate half cycles condenser 11 is charged through rectifier 7, valve 9 being cut off by negative bias. During the other half-cycle valve 9. is rendered conductive and condenser 11 is discharged through resistance 18 whose valve is adjusted so that the condenser is substantially discharged in half a cycle. Condenser 12 is charged during the half cycles when condenser 11 is being discharged and vice versa, valves 9, 10 being conductive during alternate half cycles. Thus the average potential across condensers 11, 12 is constant for a fixed amplitude signal and is a measure of the signal strength. This average potential is taken at the junction of two equal resistances 13, 14 and used to control the anode current of a valve 16 and thus provide at the lower end of a cathode resistance 17 a negative potential which varies with signal strength and which is fed by line 4 as an automatic gain control potential for amplifier 1. With this arrangement slow changes of signal strength during marking periods cause condensers 11 and 12 to by charged to a greater or lesser potential during successive half cycles, and the gain of amplifier 1 is altered accordingly. During the short space between successive marks, both valves 9, 10 are cut off and the gain control potential across condensers 11, 12 is held at its adjusted value except for slow leakage through a high resistance 19. At the end of a signal, the charge on condensers 11, 12 slowly leaks through resistance 19 to allow the gain to rise to a predetermined valve, which is below that at which noise potentials would operate the receiving apparatus. If this were not so, a strong signal would set the gain at a low value and prevent reception of a subsequent signal.