Patent GB-516495-A (GB)
Title: Improvements in or relating to radio receivers
IPC: H03G3/22
Filed: 1938-06-30
Abstract: 516,495. Automatic volume control. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 30, 1938, No. 19441. Convention date, June 30, 1937. [Class 40 (v)] In a radio-receiver with a plurality of amplifying stages in cascade to each of which automatic - volume - control potential is applied, in order to avoid the overloading of later tubes on strong signals whilst improving the signalto-noise ratio on weak signals the reduction of gain is greater on the later tubes for weak signals and greater on the earlier tubes for strong signals. The rapidity of control when one tube is varying is different from that when another tube is varying. In one arrangement, Fig. 1, in which A.V.C. potential derived from the output circuit 14 of an intermediatefrequency amplifier through a rectifier 18 and amplifier 21 is applied through resistances 29, 29<1>, 29<11> to the grids of a radio-frequency amplifier 1 and two intermediatefrequency amplifiers 3, 4 respectively, the grids of the tubes are normally biased to the same potential by resistances 15, 16, 17 in descending order of magnitude connected respectively to points 31, 32, 24 of a bleeder resistance P.. The amplifier 21 is normally biased beyond cut-off by resistances 22, 19. For relatively-weak signals, when this bias is overcome, almost the whole of the A.V.C. potential on lead 28 is effective on tube 4 owing to the small value of resistance 17. When a predetermined strength of signal has been reached, a diode 35 becomes conductive and effectively prevents any further increase of bias on tube 4. On a further rise in strength, a second diode 34, more strongly biased than 35, also becomes conductive and thereafter the whole of any further increase in the A.V.C. potential is effective only on valve 1. In a second arrangement, Fig. 2 (not shown), the resistances 15 .. 17 are equal but the screen grids of tubes 1, 3, 4 are connected through resistances decreasing in magnitude to points on resistance P of progressively-decreasing positive potential. In a third arrangement, Fig, 4, (not shown), a radio-frequency amplifier and a single intermediate-frequency amplifier are associated with diodes which are normally conducting to short-circuit the A.V. C. potential from the control grids. For relatively-weak signals the diode associated with the intermediate-frequency amplifier becomes non- conducting and the A.V.C. potential is effective only on this tube. For stronger signals, the other tube becomes non-conducting so that both amplifiers are subject to the A.V.C. potential but that on the intermediatefrequency amplifier is smaller due to a connection from the grid of this tube to a tapping on resistance P. Specification 426,803 is referred to. The Specification as open to inspection under Sect. 91 comprises also an arrangement Fig. 4 (Cancelled) in which the A.V.C. potential is at first only applied to tube 4. At successive stages of increase of signal strength, diodes 63, 62 become conducting to apply the control to tubes 3 and 1, and diodes 35, 34 operate as in Fig. 1 to remove in succession any further control from tubes 4 and 3. This subjectmatter does not appear in the Specification as accepted.