US2837689A - Post acceleration grid devices - Google Patents

Description

June 3, 1958 c, DU 2,837,689

POST ACCELERATION GRID DEVICES I Filed Dec. 1, 1955 POST ACCELERATION GRIDDEVICES (Iharles Dufour, Paris, France, assignor to Compagnie Generale de Telegraphic Sans Fil, a corporation of France Application December 1, 1955, Serial No. 550,413

Claims priority, application France December 2, 1954 2 Claims. (Cl. 315-914) The present invention relates to post acceleration grid devices for oscilloscopes and other tubes, such as the Graphecon. A device of this type has been describedin the co-pending application Serial No. 497,311 filed on March 28, 1955, now abandoned, by Charles Dufour for: Improvements in Oscilloscopes Systems. It comprises a thin grid of high transparency, positioned at a small distance from the tube target (for instance to 30 mm.) and subjected to a low voltage with respect to the anode voltage of 10,000 volts. It can be proved that such a device will not introduce any geometrical distorsion into the system, while operating with a much higher sensitivity than that of conventional post-accelerating cathoderay tubes. The device has, however, the following disadvantages: the impact of the electrons on the postaccelerating grid extracts secondary electrons, a certain number of which, in oscilloscopes for instance, are picked up by the screen which is the seat of a much higher potential than that of the grid. An electron in the beam will then produce a small luminous patch on the screen, instead of a luminous point, causing the curves or images represented to be slightly blurred. It has been thought possible to avoid the blurring effect by increasing the transparency of the grid or by reducing the number of meshes. The results of these experiments, however, were unsatisfactory.

The object of the invention is a post-accelerating grid device usable in oscilloscopes or other tubes which does not present this disadvantage.

The device according to the invention comprises a post-accelerating grid and a second grid placed between the post-accelerating grid and the target, said second grid being spaced from said post-accelerating grid by a distance which is small with respect to its distance from the target, said second grid being impressed with a voltage slightly lower than that of the first grid.

According to one feature of the invention, the wires of one of the grids are inclined at an angle of 45 with respect to the wires of the other grid.

According to another feature of the invention, the

two grids form a single unit comprising a grid-shaped insulating support having a metal film coated on either side thus forming first and second grids.

The invention will be better understood with the aid of the appended description and with reference to the annexed drawings, wherein:

Fig. 1 shows a diagram of a cathode ray tube embodyin g the post-accelerating device according to the invention.

Fig. 2 is an explanatory diagram of operation of the tube of Fig. 1.

Fig. 3 shows the preferred relative position of themeshes of the two grids.

Fig. 4 shows a cross-sectional view of a detail of a preferred embodiment of the post-accelerating device according to the invention.

In Fig. l, the cathode ray tube represented comprises a glass envelope 1, coated with a metal film 2 raised to a voltage of l500volts with respect to the'electron emisgrid 3. This screen grid'5 is raised to a voltage of 1,000

volts, i. e. at 500 volts with respect to grid 3; Grids 3 and 5 are very thin and comprise 20 square meshes per mm. Besides these-elements, the tube according to the invention comprises all other parts common to conventional'cathode-ray tubes, i; e.: theabove-mentioned electrongunl 16 with focusing and' accelerating electrodes (not shown), and-horizontal and'vertical deflecting plates 6, 7 and 8, 9. A direct current source 10 provides the necessary voltages required by the various elements.

The cathode-ray tube of Fig. 1 operates as follows i (for a better understanding of the operation, Figure 2 has been drawn on a larger scale) Screen 4, post-accelerating grid 3 and screen grid 5 are, as mentioned, raised to a voltage of 11,500, 1,500 and 1,000 volts, respectively. The trajectory of a primary electron emitted by gun "16 appears at points 11, 13 and 15 of grids 3 and 5 and screen 4 respectively; the trajectories of the secondary electrons generated by said electron through the impact on the two grids are shown at 11-12 and lit-14. The screen grid 5 will thus repel the secondary electrons leaving the, post-accelerating grid 3 with zero initial velocity. These electrons will fall back on the latter grid 3. Also, said post-accelerating grid 3, close to screen grid 5, will intercept the secondary electrons produced through the impact of the primary beam on the latter. reach the screen and the post-acceleration process will not be substantially modified thereby.

A moir or watered silk effect on the oscilloscope could eventually occur, were one of the grids to become slightly displaced with respect to the other, if the wires constiv tuting the meshes of said two grids were parallel to one another. This is avoided by arranging the two grids in the manner shown in Figure 3, wherein one of the grids is represented in solid lines and the other in dotted lines, the wires of one of the grids being inclined at an angle of 45 with respect to the wires of the other grid.

Finally, grids 3 and 5 may also form a single unit as shown in cross-section in Figure 4: a grid-shaped support 17, made of insulating material, is mounted in the tube, perpendicular to the axis of gun 16, between the gun 16 and the screen 4. On the side of the support facing the gun, a film of conductive material is deposited, constituting grid 3. On the opposite side, a second film is deposited constituting grid 5.

Perfect parallelism between the grids 3 and 5 is thus insured. Any short-circuit resulting fromth'e application of different potentials to said two grids and any shift of one of the grids with respect to the other are Patented June 3, 1958 No secondary electron will thus 3 tive with respect to said cathode and highly negative with respect to said target, and means for causing an electron beam issued from said cathode to scan said tar- .get, said tube further comprising an additional grid disposed between said post-accelerating grid and said target, substantially nearer to the post-accelerating grid than to the target, and means for raising said additional grid to a slightly negative potential with respect to said postaccelerating grid, whereby secondary emission electrons from both grids are prevented from reaching said target, said grids being constituted of fine wires, the wires of said first grid being disposed at an angle of 45 with respect to the wires of said second grid.

2. A monochromatic cathode ray tube of the type including a cathode, a target, a post-accelerating grid disposed in the vicinity of said target, means for raising said post-accelerating grid to a potential slightly positive with respect to said cathode and highly negative with respect to said target, and means for causing an electron beam issued from said cathode to scan said target, said tube further comprising an additional grid disposed between said post-accelerating grid and said target, substantially nearer to the post-accelerating grid than to the target, and means for raising said additional grid to a slightly negative potential with respect to said post-accelerating grid, whereby secondary emission electrons from both grids are prevented from reaching said target, further comprising a grid-shaped insulating support having a first and a second sides, said first and second sides being respectively coated with a first and second films of metal constituting respectively said first and said second grids.

References Cited in the file of this patent UNITED STATES PATENTS 2,315,367 Epstein Mar. 30, 1943 2,580,250 Smith Dec. 25, 1951 2,602,145 Law July 1, 1952 2,692,532 Lawrence Oct. 26, 1954 2,728,024 Ramberg Dec. 20, 1955 2,734,146 Noskowicz Feb. 7, 1956

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