CATHODE-RAY TUBE   video

The cathode-ray tube (CRT) is one of the main elements of an oscilloscope. The tubes are produced with electrostatic and electromagnetic control, where electrostatic or magnetic fields deviate the electron beam respectively. Animation shows the principle scheme of CRT with electrostatic control as well as the motion of the electrons in the beam drawing a sinusoid on the screen of oscilloscope. CRT consists of the glass bulb evacuated to a high vacuum, the cathode (a source of electrons), cathode heater, electrodes for brightness and focus control, several accelerating anodes, the pairs of horizontal and vertical capacitor plates deviating the electron beam, and fluorescing screen. One of anodes, which accelerate the electrons, is placed close to the screen. The high positive voltage is applied to this electrode. Under the action of the applied voltage the electrons are moved  with acceleration from cathode to anode. In the absence of the voltage applied to deviating plates of the capacitor the electron beam will be incident on the screen in the center brightening a point in the fluorescing layer. In oscilloscope the analyzed signal after amplification is applied to vertical deviating plates, while the periodic sawtooth signal is applied to horizontal plates. As a result the electron beam "draws"  the dependence of the investigated signal on time on the screen of the tube. Reaching the right side of the screen the beam has to be returned to an initial point at the left side. Thus, if CRT is not blanked during this retrace, then the beam will leave a track crossing the image of investigated signal. For this reason, during retrace a negative voltage is applied to control electrode situated near to cathode and electrons are locked by such a way at the electron gun. As a result, the electron beam will be discontinuous, as shown in animation.