GB631380A - Improvements in or relating to electric discharge for receiving frequency-modulated signals - Google Patents

Abstract

631,380. Electron discharge apparatus. PHILIPS LAMPS, Ltd. July 24, 1946, No. 22011. Convention date, July 27, 1945. [Class 39 (i)] [Also in Group XL (c)] The invention relates to electric discharge tubes having two control electrodes and three screen grids, and which are adapted for demodulating a frequency-modulated signal and suppressing amplitude modulation, wherein two voltages differing in phase by an amount depending on the instantaneous frequency duration of the signal are fed to the control grids to affect the distribution of the electron current among the electrodes but not its total strength. Each control voltage causes the anode current to obtain full strength or zero strength according as it deviates above or below a predetermined level, so that the duration of anode current pulses depends on the phase relation between the two control voltages ; this condition usually obtains if the voltage of the control electrodes affects the distribution of the electron stream, but not the cathode current. According to the invention, the distance between each control grid and the subsequent, screen grid is smaller than that between the control grid and the preceding screen grid. In Fig. 5, suction grid 21 has a constant positive voltage to produce saturation of electron current from cathode 19. The electron beam is concentrated by rods 22. Screen grid 25 is situated between control grid 23 and grid 21 and a second. screen grid 25a is placed between the control grid 23 and a second control grid 24. A screen grid 26 is provided between grid 24 and the anode. In accordance with the invention grid 23 is nearer grid 25a than to grid 25, and grid 24 is nearer to grid 26 than to grid 25a. Fig. 6 (not shown) describes a similar arrangement but in which the other side of the cathode is also coated, so that an additional electron stream may be provided for another purpose. In another modification, the electron stream is formed into a beam by means of screens attached to the cathode. The shape of each control grid may be made irregular so that the slope of the anode-current, grid-voltage characteristic decreases as the grid is made more negative. U.S.A. Specification 2,343,263 is referred to. The Specification as open to inspection under Sect. 91 describes further examples in which the desired effect is produced by means of electrode construction to produce mirror-deflection effect and in which the effect is obtained by velocity control of the electron stream. Reference is also made to the use of an electron lens between two deflection electrodes. This subject-matter does not appear in the Specification as accepted.