It is a vacuum tube of a special geometrical shape that converts an electrical signal into a visual one.
In a cathode ray tube electron gun consists of a cathode that produces plenty of electrons, these electrons are accelerated to high velocity and are brought to focus on a fluorescent screen. This results in the applied input signal waveform being displayed visually.
CRT consists of:
1. Glass envelope.
2. Electron gun assembly.
3. Deflection plate assembly.
4. Fluorescent screen.
1. Glass Envelope: It is a conical highly evacuated (empty) glass housing that maintains a vacuum inside and supports the various electrodes. The inner walls of CRT between the neck and screen are usually coated with a conducting material, called aquadag and this coating is electrically connected to the accelerating anode. This coating is used to collect the electrons produced by secondary emission when an electron beam strikes the screen and walls are returned to the anode.
2. Electron Gun Assembly: The arrangement of electrodes that produce a focussed beam of electrons is called the electron gun. The electron gun assembly consists of:
(1) Indirectly heated cathode.
(2) Control grid.
(3) Focussing anode.
(4) Accelerating anode.
The indirectly heated cathode consists of a nickel cylinder coated with an oxide coating and provides plenty of electrons. The focusing anode focuses the electron beam into a sharp pin-point by controlling the positive potential on it. Therefore, the electron gun assembly forms a narrow, accelerated beam of electrons which produces a spot of light when it strikes the screen.
3. Deflection Plate Assembly: The electron beam, after leaving the electron gun, passes through the two pairs of deflection plates.
The deflection of the beam is accomplished by two sets of deflecting plates. They are:
(1) Vertical deflection plates.
(2) Horizontal deflection plates.
The vertical deflection plates are mounted horizontally in the tube and deflect the beam vertically that's why it is called vertical deflection plates or Y-plates.
The horizontal deflection plates are mounted vertically and deflect the beam horizontally that's why it is called horizontal deflection plates or X-plates.
4. Fluorescent Screen: The screen is coated with some fluorescent material such as zinc orthosilicate, zinc oxide, etc, these absorb the energy of the incident electrons. When a high-velocity electron beam strikes the screen, a spot of light is produced at the point of impact. The color of the spot depends upon the nature of the fluorescent material. If zinc orthosilicate is used as the fluorescent material, a green light spot is produced.
NECESSITY OF TIMEBASE VOLTAGE
In order to deflect the beam horizontally proportional to time, a voltage is needed which rises linearly with time. This is known as time-base voltage. It is used to control the position of the light spot on the screen in X-direction.
Since this waveform resembles the teeth of a hacksaw it is also called sawtooth waveform.
When AC voltage (for example, sine wave) is applied to the vertical deflecting plates and no input is applied to the horizontal plates, the spot on the screen moves up and down continuously .e., vertically. A ramp waveform applied to the horizontal deflecting plates of an oscilloscope causes the electron beam to be deflected horizontally across the screen i.e., left to right and right to left. The sine wave waveform is applied to the vertical Reflecting plates and is synchronized with the ramp, the waveform of the sine wave is displayed on screen with respect to time.
HOW WAVEFORM IS DISPLAYED ON CRO
As seen tooth voltage increases from zero to maximum, electron beam moves from left to right. However, when the sawtooth voltage falls rapidly from maximum to zero, the beam quickly moves from right to left. Hence, if the sawtooth voltage is absent, then the beam will move only in the vertical direction. Thus, when a beam travels from left to right, two forces act on it: the horizontal force (due to the saw tooth wave) and the vertical force (due to the y-input signal). So the resultant motion of the electron beam is exactly like the shape of the applied waveform. In this way, the beam draws the shape of the waveform on the screen with respect to time i.e. with respect to the change in saw-tooth voltage.
CONDITIONS FOR STATIONARY WAVEFORMS ON CRO
In order to obtain the stationary waveform on the CRO screen, the horizontal sweep frequency and the vertical input frequency signals must be applied to the corresponding deflecting plates at the same time i.e., they are to be synchronized.
This requires that the frequency of the vertical input signal must be equal to or an exact multiple of the sweep generator signal frequency.
If the vertical input frequency is not exactly equal to or an exact multiple of the sawtooth frequency, the waveforms will not be synchronized and the display "walks" across the screen, i.e., the pattern is not stationary.
The waveforms on the CRO screen move left to right or right to left (not stationary) when the frequency of sweep is not integer multiples of the vertical signal.
When the waveform moves left to right the internal sweep frequency is less than the vertical signal, when waveforms move right to left the internal sweep frequency is greater than the vertical signal frequency. In either case, the stationary waveforms are obtained by varying the variable sweep control. This control sets the sweep signal frequency into integer multiples of the vertical signal.
CONDITIONS FOR FLICKER-FREE WAVEFORMS ON CRO
The waveform observed in a CRO must be free from flicker So, in order to get flicker flicker-free waveform, the sweep frequency generated by the horizontal sweep generator must be greater than the persistence of vision frequency of 16 Hz. So it is better to have a sweep frequency greater than 25 Hz to get a flicker-free waveform.
CRO's flickering is due to the measurement of low-frequency signals (< 30 Hz). The free-running (internal trigger) mode of the sweep generator is not suited to the vertical input signal. In the case of low low-frequency sweep generator flickering does not occur.
The conditions for flicker-free waveforms are :
1. Use low-frequency sweep generators.
2. Use CRO in auto or external triggering mode.
3. Connect the signal to the vertical input and external trigger to the external trigger input.
