Study of Cathode Ray Oscilloscope-CRO

Objective: To understand the operation of the CRO and to learn how to determine the Amplitude Time period and Frequency of a given waveform using Analog CRO

CRO is an electronic device which is capable of giving a visual indication of a signal waveform. With an oscilloscope the waveform of the signal can be studied with respect to amplitude distortion and deviation from the normal. Oscilloscope can also be used for measuring voltage, frequency and phase shift.
Cathode Ray Tube: Cathode Ray Tube is a heart of Oscilloscope providing visual display of the input signals. CRT consists of three basic parts.
1. Electron Gun.
2. Deflecting System.
3. Flouroscent Screen
These essential parts are arranged inside a tunnel shaped glass envelope.
Electron Gun: The function of this is to provide a sharply focused stream of electrons. It mainly consists of an indirectly heated cathode, a control grid, focusing anode and accelerating anode. Control grid is cylinder in shape. It is connected to negative voltage w.r.t to cathode. Focusing and accelerating anodes are at high positive potential. w.r.t anode. Cathode is indirectly heated type & is heated by filament. Plenty of electrons are released from the surface of cathode due to Barium Oxide coating. Control Grid encloses the cathode and controls the number of electrons passing through the tube.

A voltage on the control grid consists the cathode determines the number of electrons freed by heating which are allowed to continue moving towards the face of the tube. The accelerated anode is heated at 9 much higher potential than focusing anode. Because of this reason the accelerating anode accelerates the light beam into high velocity. The beam when strikes the screen produces the spot or visible light. The name electron Gun is used because it fires the electrons like a gun that fires a bullet.
Deflection system: The beam after coming out of the accelerated anode passes through two sets of deflection plates with the tube . The first set is the vertical deflection plate and the second set is horizontal deflection plates. The vertical deflection plates are oriented to deflect the electron beam that moves vertically up and down. The direction of the vertical deflection beam is determined by the voltage polarity applied to the plates. The amount of deflection is set by the magnitude of the applied voltage. The beam is also deflected horizontally left or right by a voltage applied to horizontal plates. The deflecting beam is then further accelerated by a very high voltage applied to the tube.
Fluorescent Screen:
The screen is large inside the face of the tube and is coated with a thin layer of florescent material called Phosphor. On this fluorescent material when high velocity electron beam strikes its converting the energy of the electron the electron beam between into visible light(spots). Hence the name is given as fluorescent screen.

1. POWER ON/OFF /  Push the button switch to supply power to the instrument.
2. X5 / Switch when pushed inwards gives 5 times magnification of the X signal.
3. XY ( used for XY display). /  Switched when pressed cut off the time base and allows access the exit horizontal signal to be fed through CH II
4. CH I/CH II/TRIG I/ TRIG II./  Switch out when selects and triggers CH I and when Pressed selects and triggers CH II.
5. MOD/DUAL /  Switch when selects the dual operation switch
6. ALT/CHOP/ADD / Switch selects alternate or chopped in dual mode. If mode is selected then this switch enables addition or subtraction of the channel i.e. CH-! +- CH II.
7. TIME/DIV / Switch selects the time base speed.
8. AT/NORM / Switch selects AUTO/NORMAL position .Auto is used to get trace when no signal is fed at the input . In NORM the trigger level can be varied from the positive peak to negative peak with level control.
9. LEVEL / Controls the trigger level from the peak to peak amplitude signal.
10. TRIG.INP / Socket provided to feed the external trigger signal in EXT. mode.
11. CAL OUT / Socket provided for the square wave output 200 mv used for probe compensation and checking vertical sensitivity etc.
12. EXT / Switch when pressed allows external triggering signal to be fed from the socket marked TRIG.INP.
13. X-POS / Controls the horizontal position of the trace.
14. VAR / Controls the time speed in between two steps of time/div switch .For calibration put this fully anticlockwise (at cal pos)
15. TV /  Switch when it allows video frequency up to 20 KHz to be locked.
16. + – /  Switch selects the slope of trigger whether positive going or negative.
17. INV CHJ II / Switch when pressed inverts the CH ii.
18. INTENS / Controls brightness of trace.
19. TR /  Controls the alignment of the trace with gratitude (screw driver adjustment).
20. FOCUS: Controls the sharpness of the trace.
21. CT: Switch when pressed starts CT operation.
22. GD/AC /DC: Input coupling switch for each channel. In AC the signal is coupled through the 0.1 MFD capacitor.
23. DC/AC/GD : BNC connectors serve as input connectors for the CH I and CH II channel input connector also serves as the horizontal external signal.
24. CT-IN: To test any components in the circuit, put one test probe in this socket and connect the other test probe in the ground socket.
25. VOLTS /DIV: Switches select the sensitivity of each channel.
26. Y POS I AND II: Controls provided for vertical deflection for each channel.
1. FUSE : 350 mA fuse is provided at the back panel spare fuses are provided inside
the instrument.
2. ZMOD : Banana socket provided for modulating signal input i.e. Z-modulation.
1. Avoid using CRO in high ambient light conditions.
2. Select the location free from Temperature & humidity. It should not be used in dusty environment.
3. Do not operate in a place where mechanical vibrations are more or in a place which generates strong magnetic fields or impulses.
5. Do not increase the brightness of the CRO than that is required.
1. Turn on the power of the CRO.
2. From the Function Generator select the desired frequency and amplitude of the sine wave.
3. The amplitude of the waveform is obtained by noting the number of divisions along the Y-axis in between peak to peak of the waveform (i.e. sine waveform / Triangular waveform /Square waveform) and multiplying with the divisional factor of the amplitude note in volts.
4. Time period is calculated from X-axis.
5. Frequency is obtained by formula F=1/T.
6. This frequency is compared with the frequency applied using function generator.
7. Voltage in the CRO is compared with the voltage applied from function generator.
8. By repeating the above steps we can find frequency and voltages of square wave & triangular waveforms.
Tabular Column:


1. Sinusoidal Waveform:
Amplitude: ____ V
Time Period: ____ Sec
Frequency: ____ Hz
2. Square Waveform:
Amplitude: ____ V
Time Period: ____ Sec
Frequency: ____ Hz
3. Triangular Waveform:
Amplitude:  ____ V
Time Period: ____ Sec
Frequency: ____ Hz
RESULT: The CRO Panel is studied and determined the Amplitude, Time period and Frequency of a given waveform using CRO.


Subramanian MK, currently serving as a workshop instructor at Sakthi Polytechnic College, Erode Tamil Nadu. With a career spanning 25 + years, Subramanian MK has dedicated himself to advancing knowledge in Electronics and Communication Engineering (ECE). His passion for exploring new technologies has led to the development of numerous projects, showcasing expertise in IoT and PCB design.

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