GB666221A - Improvements in or relating to electron discharge apparatus - Google Patents

Abstract

666,221. Cathode-ray tubes. WESTERN ELECTRIC CO., Inc. July 9, 1948 [July 10, 1947], No. 18495/48. Class 39 (i). [Also in Groups XL (b) and XL (c)] A cathode-ray tube for converting pulse position modulated signals into amplitude modulated signals comprises a plurality of target electrodes 33, Fig. 3, arranged in two concentric circular rows in staggered relation, the width of each target taken along its respective row when projected on to a line passing between the two rows, falling between the projections on to this line of the two adjacent targets in the other row, means for projecting to the targets a beam of transverse dimension no greater and preferably about the same as the transverse dimensions of the targets, and means for deflecting the beam to produce a stepped trace so that it sweeps in succession from a target in one row to the next adjacent target in the other row, the trace passing also from substantially the centre of each target to a point substantially midway between the two adjacent targets in the other row. The concentrated and focused beam is rotated in a circle by two pairs of deflecting plates in quadrature and is then subjected to a square waveform applied between electrode 41 supported from shield 28, 29 by three wires 40, and a conductive wall coating 19, Fig. 1, which causes the beam trace to assume the desired path T, Fig. 4. The targets 33 are surrounded by a shield comprising annular plate 28 perforated with rectangular or square apertures 31 corresponding to the targets 33 and whose dimensions are approximately equal to the beam diameter and cylindrical portions 29 formed with pairs of radially aligned slots 30; the shield is treated to minimize secondary emission. The targets 33 are carried by ceramic tubes 35 seated in the slots 30, and the tubes 35 are cemented at 36 to the target flanges 34 and shield portions 29. Thus the targets and shield structure constitute a unitary assembly. The targets 33 have a secondary emission ratio of greater than unity, and secondaries are collected by the shield 28, 29. For example the targets may be of an alloy of 99 per cent silver-and 1 per cent magnesium, or the parts 33 may be coated with secondary emissive material. Shields extending between adjacent targets are affixed by tabs to the member 28, Fig. 2 (not shown). The tube comprises two vitreous portions 10A, 10B, Fig. 1, joined at their abutting edges, one portion 10A carrying the electron gun 22 ... 25 and deflecting plates 26, 27, and having a wall coating 18 of colloidal graphite, and the other portion 10B carrying the target and shield structure and the square wave deflecting structure 19, 41. The wall coating 19 has a cap terminal 21 and the individual leads 37 to the targets are brought out through the tubes 35, Fig. 3, to terminals 17 mounted in insulating disc 16 carried by dished sheet 14 cemented to the envelope wall at 15. The shield structure 28, 29 is carried by wires 32 connected together and to terminal 13. The output couplings to the individual target terminals may be of resistance-condenser form or of transformer type. In operation the input signal comprising position modulated pulses corresponding in position to signal sample amplitudes are applied to control electrode 22; control electrode 22 may be biased negatively so as to suppress the beam, the beam being switched on by each pulse, or alternatively the beam may be normally on and be extinguished by each pulse. It is shown that the output from the targets is amplitude modulated in accordance with the pulse position modulation and consequently is a reproduction of the original complex input at the transmitter. The output is passed through a low-pass filter having a cut-off frequency of half the sampling frequency. In a modified construction the apertures 31 in the outer row of shield 28 are made larger than the inner ones and the beam diameter is increased when it is at the outer parts of its trace by controlling the potentials of the focusing electrode 23 in accordance with the square waveform applied to electrodes 19, 41, for example by means of a feed-back arrangement, Fig. 5 (not shown). Specification 607,610, [Group XL (c)], is referred to.