# Basic Controls of a CRO

jojo September 24, 2009 10 Comments

### Vertical Deflection System.

Linear Time Base

The function of vertical deflection system is to provide an amplified signal of the proper level to drive the vertical deflection plates without introducing any appreciable distortion into the system.

As we know that electrical signal is delayed by a certain amount of time when transmitted through an electronic circuitry. In CRO, output signal voltage of the vertical amplifier is fed to the vertical plates of CRT and some of its portion is used for triggering the time base generator circuit, whose output is supplied to the horizontal deflection plates through horizontal amplifier. The whole process, which includes generating and shaping of a trigger pulse and starting of a time-base generator and then its amplification, takes time of the order of 100 ns or so. So the input signal of the vertical deflection plates of a CRT is to be delayed by at least the same or little more amount of time to allow the operator to see the leading edge of the signal waveform under study on the screen. For this purpose, delay line circuit is introduced between vertical amplifier and the plates of CRT, as shown in figure.

### Horizontal Deflection System.

External signal is applied to horizontal deflection plates through the horizontal amplifier at the sweep selector switch in EXT position, as shown in figure. The horizontal amplifier, similar to the vertical amplifier, increases the amplitude of the input signal to the level required by the horizontal deflection plates of CRT. When the function of time is required to be displayed on the screen of CRT, INT position of sweep selector switch is used. Before going further we should make ourselves clear first about the linear time base pattern. Assume that we supply an ideal saw-tooth signal voltage to the horizontal deflection plates, keeping vertical deflection plates at zero potential, as shown in figure.

At the starting point A in time, signal voltage is maximum but negative so the spot on the screen of CRO is at the extreme left position. Further at point B in time, signal voltage applied to the horizontal plates is zero so the spot is in the centre position on the screen. Now when voltage increases in + ve direction and becomes maximum just before the point C, the spot on the screen is at the extreme right side of the screen. Just after the point C, next cycle of saw-tooth voltage signal starts and again voltage becomes maximum negative so the spot goes back to the extreme left position of the screen from right position in no time.

From the above discussions we may conclude that

For making idea of time base more clear let us discuss an application. Suppose a sine-wave voltage signal y of time period T is applied to the vertical deflection plates and a saw-tooth voltage signal vh韦德体育投注网是什么,韦德体育app of time period T is applied to horizontal deflection plates, as shown in figure.

At zero time, the spot is at extreme left vertically central position on the screen. Because of zero value of VV and maximum negative values of Vh. At time T/4, the spot is at one-fourth way on the screen in horizontal direction and at maximum positive deflec­tion above the centre line in vertical direction because of maximum positive value of VV. At time T/2, values of both VV and Vh are zero, so the spot is at the central position of the screen. At time 3T/4, the spot is the three-fourth way on the screen in horizontal direction and at the maximum negative deflection in vertical direction. Finally, at the end of time T, the spot is at extreme right vertically central position of the screen and then it moves back to begin a new trace. In this way, sine-wave voltage applied to the vertical deflection system appears on the screen. If the period of sine-wave is reduced to half then two sine-wave cycle appears on the screen.

From the above discussion, it is obvious that the following conditions are to be satisfied in order to have a waveform of the input signal applied to vertical deflection system as a stationary pattern on the screen of CRO.

(i) Both horizontal and vertical signals must start at the same instant.

(ii) Ratio of frequency of horizontal and vertical signals should be a rational or frac­tional number.

In the external position of switch, the trigger is obtained from the external source. In the third position of switch i.e. line, trigger is obtained from the power supply i.e. 230 V and 50 Hz.

### Intensity Control.

The potential of the control grid with respect to cathode is controlled with the help of potentiometer in order to control the intensity of brightness of the spot.

### Focus Control.

In the electron gun of a CRT, middle anode is kept at lower potential with respect to other two anodes and it acts like an electrostatic lens and focal length of this lens can be varied by varying the potential of the middle anode with respect to other two anodes. So focusing of an electron beam is done by varying the potential of middle anode with the help of a potentiometer, as shown in figure. By increasing the positive potential applied to the focusing anode the electron beam can be narrowed and the spot on the screen can be made a pin point.

### Calibration Circuit

Normally an oscillator, which generates a known and fixed voltage in square waveform is fitted in the CRO for calibration purpose.

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• piyu
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April 14, 2013

I’m impress, this basic control of CRO make me 2 understnd hw CRO Functioning

• toheeb
November 21, 2012

韦德体育投注网是什么,韦德体育appwhat are the applications of CRO

• August 7, 2014

It uses for draw graph between current or voltage