Patent GB-721497-A (GB)
Title: Improvements in electron beam tubes
IPC: H01J21/14
Filed: 1952-08-08
Abstract: 721,497. Electron beam apparatus. ZENITH RADIO CORPORATION. Aug. 8, 1952 [Aug. 18, 1951; Sept. 15, 1951; Dec. 28, 1951]. No. 20071/52. Class 39 (1). A beam deflection tube comprises an electron gun including a cathode having an elongated emissive surface for projecting a sheet-like electron beam of substantially rectangular cross-section, a deflection-control means responsive to. an input signal for subjecting the electron beam to a transverse deflection field, first and second output electrode means associated with different portions of the beam and disposed in overlapping alignment in a direction parallel to the elongated surface of the cathode, and anode means to collect spare electrons not collected by the output electrode means. The tube shown in Fig. 1 is designed to separate the synchronizing pulses and to provide automatic gain control signals in a television receiver. Composite video signals are applied to deflector plates 4, 5 which are biased so that the normal video signals do not deflect the beam sufficiently for part of it to flow through either of the apertures 7, 8 in the anode 6. Normal synchronizing signals deflect the beam so that it passes through aperture 7 to the collecting electrode 9; noise pulses of larger amplitude than synchronizing signals deflect the beam past the aperture 7 so that the beam is again intercepted by the anode 6. Aperture 8 is slightly displaced with respect to the aperture 7 so that the signal pulses above a certain level cause the beam to pass through to the electrode 10 and generate gain control signals; again noise pulses deflect the beam past the aperture and thus do not generate gain control signals. To prevent the receivers being paralysed by a strong signal when it is first switched on, the aperture 8 is provided with an extension of smaller height than the main part, as shown in the figure, which ensures that a strong signal produces some gain control output but thereby reduces the immunity of the gain control to 'noise pulses. In a modification, Fig. 4, the opposite face of the cathode is also electronemissive and provides a beam which passes between deflection plates 17, 18 on to anodes 15, 16; thus a sine wave applied to plates 17, 18 will provide a square wave output from either of the anodes 15, 16. The electrode 9 of Fig. 1 is replaced by three electrodes 11, 12, 13, the electrode 11 performing the same function as electrode 9 in Fig. 1 and electrodes 12 and 13 being connected to the deflection plates 17 and 18 and to a source of sinusoidal potential. If the latter is out of phase with the signal pulses a potential is developed between electrodes 12 and 13 and hence between the plates 17 and 18 which speeds up or slows down the square wave output from the anodes 15, 16; the arrangement thus provides automatic frequency control. In a modification, Fig. 5 (not shown), electrodes 11, 12 and 13 are replaced by two L shaped electrodes 19, 20, as in Fig. 9, which together with the anode 6 form a lens which focusses the beam at a point independent of which part of the aperture 7 the beam traverses. In the arrangement shown in Fig. 9 the electron gun is at an angle # with respect to the axis of the output electrodes, # being about 20 degrees. Deflection electrode 4 is biased so that with no signal on electrode 5 substantially the whole beam is collected by electrode 4; signals applied to electrode 5 deflect the beam upwards towards the aperture 7. This arrangement reduces current flow to electrode 5 and thus reduces the load on the input.