Patent GB-611222-A (GB)
Title: Improvements in television analysers with slow moving electrons
IPC: H01J31/34
Filed: 1945-10-09
Abstract: 611,222. Television. COMPAGNIE POUR LA FABRICATION DES COMPTEURS ET MATERIEL D'USINES A GAZ. Oct. 9, 1945, Nos. 26336 and 26337. Convention dates, June 8, 1944, and June 21, 1944. [Class 40 (iii)] [Also in Group XL (a)] In a television analyzer operating with slowspeed electrons, the scanning beam is modulated by a carrier oscillation applied to suitable electrodes. Fig. 1 shows an image analyzing tube V in which the electron beam F, concentrated by field H and deflected so as to scan the mosaic M by means of the mutually perpendicular fields h1, h2, is modulated at a frequency f applied between the Welnelt cylinder W and the cathode K from an oscillator Os in addition to a negative polarizing voltage P. A positive potential U is applied to the final accelerating anode A to which an alternating voltage of frequency b may also be applied to complete the modulation or to provide a corrective focusing field. An image of the object to be televised is focused on to the transparent signal plate S, the output from which is fed to a transformer T tuned to frequency f and designed to pass the full frequency band comprising.the video signals. A disadvantage inherent in this arrangement, namely the effect of direct induction between the modulation circuit and the output circuit, is substantially eliminated, without the use of complicated screens, by the circuit of Fig. 2 in which two separate oscillations of frequency f1, f2 are applied between the cathode K and the Wehnelt cylinder W. In one method of operation, the biassing voltage applied to W is such that the mean operating point occurs in the lower bend of the beam current/cylinder voltage characteristic. The amplitudes of f1, f2 are made sufficiently large to result in detection mixing, thus providing a component modulation of the beam at frequency f =f1 - f2. The output circuit is then free from the effects of induction or radiation, from the input circuit, a slight residue of frequency f only reaching the output circuit in the absence of light on the mosaic. Another method of operating utilises the straight portion of the characteristic, the beam modulation then consisting of two superimposed waves of frequencies f1, f2, but containing no frequency between f1 and f2. The beam has maximum intensity, due to the modulation, when f1 and f2 are in phase, this maximum occurring at intervals T=1/#1-#2, and consequently the mean voltage across condenser C will have a frequency f=f1-f2 successive maxima of which decrease if an elemental area of the mosaic is considered. When the characteristic of the. cylinder W is not strictly a straight line, a slight residual component of frequency between f1, f2 exists in the beam. In a further method, one source f1 is applied to W, the other source f2 being applied to the output circuit so as to transmit by capacity to the elements of the mosaic oscillations of frequency f2. The portion of the beam which falls on these elements will then give rise to a discharge which includes a frequency between f1 and f2. The frequency f can also be introduced by modulating a higher frequency by the frequency f and detecting this latter frequency either by the curve of the characteristic or by providing a suitable time constant in the output circuit.