Numerical Indicator Tube

a display cathode-ray tube on whose screen an arbitrary designation of an incoming electrical signal glows in the form of a character (symbol, letter, or digit) at the point of incidence of the ray. Numerical indicator tubes are used when the high-speed reproduction and revision of data in the form of characters is necessary— for example, to supplement, in coded form, radar data on atmospheric conditions, and in electronic landing systems at large airports. A distinction is made between instantaneous-operation tubes (the Charactron) and visual storage tubes (the Typotron).

In instantaneous-operation numerical indicator tubes, a character is reproduced only while the electron beam is producing the character and the screen’s afterglow persists. In visual storage tubes, once the characters are written they shine on the screen for a virtually unlimited time, until they are artificially erased. The electron-optical system is the same in both types.

In instantaneous-operation numerical indicator tubes (see Figure 1) the beam from the electron gun G passes through an

Figure 1. Electron-optical system of a numerical indicator tube: (G) electron gun, (S₁) deflection system for selecting the character, (M) matrix of characters (letters or digits), (C) focusing coil, (D) compensating deflection system, (S₂) address deflection system

electrostatic deflection system 5, that selects the character (the primary address system) and strikes a particular place on the matrix M, which is a metal plate with openings in the shape of various characters. The beam covers only one character on the matrix at a time. After passing through the matrix, the cross section of the beam takes on the shape of the character through which it has passed. After passing through a focusing coil C, it strikes a compensating deflection system/), which rotates it to the tube’s axis, and then passes through a magnetic or electrostatic secondary address deflection system S2, which directs it to a specific place on the tube’s screen, where the character corresponding to the inci-dent signal will glow.

The screen of a numerical indicator tube is usually a two-layer structure. A layer of phosphor, which upon irradiation glows yellow with a persistence ranging from tenths of a second to several dozen seconds, is applied directly to the glass of the tube. A second layer is applied to the first; when the second layer is struck by an electron beam, a blue light is produced in it that excites a glow in the first layer. The screen is usually circular, with a diameter up to 500 mm.

Visual storage tubes differ from instantaneous numerical indicator tubes in that they have an additional storage grid target in front of the screen of the same type as in a memory tube with a visible image on the screen and a broad electron-beam source (produced by a gun) that simultaneously dis-plays on the screen all the characters written on the target.

an electronic vacuum device for the visual presentation of information, displayed as luminous images of numerals and other symbols. They are used mainly in computers, digital-readout measuring instruments, and control panels.

The most widely used types of numerical indicator tubes are gas-discharge devices with several cathodes (each having the shape of a symbol to be displayed) and a common anode in the form of a grid. The tube is filled with neon at a pressure of several tens of millimeters of mercury; mercury vapor is sometimes added to the neon to increase the stability of operating parameters. The glow discharge surrounding the cathodes is used for the visual display of the symbols. The glow appears between the anode and one of the cathodes whenever the voltage reaches the value required for firing the discharge. The current in the anode circuit is selected in such a way that the cathode surface is completely covered by the glow. Operational control of a numerical indicator tube, that is, the switching of the cathodes, is accomplished with the aid of various switching devices, such as relays and mechanical, electromechanical, and electronic switches. Electronic switches frequently operate in conjunction with amplifying devices.

Gas-discharge numerical indicator tubes currently in production may be classified by electrical parameters (firing voltage, operating current), the size of the displayed numerals and their arrangement with respect to the axis of the tube (end or side display), overall dimensions, and the shape of the glass envelope. In multidischarge numerical indicator tubes, several indicators are structurally combined in a single envelope, which reduces the overall dimensions of the unit.

Numerical indicator tubes are characterized by high reliability, long service life (as high as 10⁴ hours), low power consumption (the operating current usually does not exceed several milliamperes for voltages of the order of 100 volts), satisfactory brightness (hundreds of candelas per m²), and high resistance to mechanical shock and other environmental effects. Their principal drawback is their incompatibility with low-voltage devices that incorporate transistors and integrated circuits, caused by the relatively high voltages required for the control of numerical indicator tubes.

In addition to the gas-discharge types, there also exist vacuum numerical indicator tubes, which make use of phenomena other than glow discharge, especially cathode luminescence.