What is Cathode Ray Oscilloscope | Cathode Ray Oscilloscope Block diagram | CRO Tube | CRO Applications

What is Cathode Ray Oscilloscope

Cath­ode ray oscil­lo­scope is an elec­tron­ic device that is wide­ly used in lab­o­ra­to­ries. It has an X‑Y plot­ter. In which the lumi­nous point acts as a ‘sty­lus’. The lumi­nous point moves over the dis­play area when apply­ing input voltages 

Block Diagram Of CRO

  1. Cath­ode Ray Tube 
  2. Ver­ti­cal Amplifier 
  3. Hor­i­zon­tal Amplifier 
  4. Time Base Generator 
  5. Trig­ger Circuit 
  6. Delay Line
  7. Pow­er Supply 

1.Cathode Ray Tube

Cath­ode Ray tube is an impor­tant part of the CRO. It gen­er­ates an elec­tron beam and accel­er­ates the beam to a high veloc­i­ty. This elec­tron beam moves from the elec­tron gun to the screen of the CRT. 

The inside of the screen is coat­ed with a flu­o­res­cent mate­r­i­al. As soon as the elec­tron beam reach­es the screen, a bright light spot appears at that point 

2.Vertical Amplifier

We call this the Y ampli­fi­er. It is used to con­trol the volt­age sig­nal y ampli­tude. It has a cal­i­brat­ed alter­na­tor. You can adjust the ampli­fi­ca­tion of the input sig­nal by mod­i­fy­ing this alter­na­tor. The rate of deflec­tion in votIs / cm is obtained by the cal­i­brat­ed altem­ator switch 

Horizontal Amplifier

This is called the X ampli­fi­er. It gen­er­ates saw tooth volt­age on the time base gen­er­a­tor. This volt­age is applied to the horo­zon­tal ampli­fi­er. This ampli­fi­er is used to con­trol the time rela­tion­ship and the fre­quen­cy of the voltage 

Time Base Generator

The A.C volt­age is applied to the ver­ti­cal deflec­tion plates. Pos­i­tive and neg­a­tive sup­ply of plates is avail­able in suc­ces­sion. Thus the elec­tron­ic spot will be giv­en up and down. Moves on the screen accord­ing to the volt­age. Thus a con­tin­u­ous ver­ti­cal line is obtained. This cre­ates a lin­ear saw tooth wave from 

Trigger Circuit

This cre­ates a trig­ger pulse to start the hor­i­zon­tal sweep. For a steady wave­form to appear on the CRO screen, the ver­ti­cal input sig­nal fre­quen­cy must be equal to the time base gen­er­a­tor frequency 

The trig­ger cir­cuit does this job. The trig­ger cir­cuit pro­duces the trig­ger pulse cor­re­spond­ing to the input wave­form at a select­ed point. This trig­ger pulse is used to start the time base generator 

Delay Line

The lead­ing edge of the input wave­form is used to ini­ti­ate the trig­ger gen­er­a­tor. It also starts the sweep gen­er­a­tor. But sweep can be ini­ti­at­ed only after 0.15ms. But before this 0.15 ms the lead­ing edge of the input sig­nal will pass. 

Thus the wave form does not become a lead­ing edge dis­play. To avoid this the input wave form should be delayed. The delay line is used for this. The Delay line gives a delay of 0.25 ms to the ver­ti­cal plates 

Power Supply

Pow­er sup­ply The pow­er sup­ply includes a pow­er trans­former fil­ter and sta­bi­liz­ers. This includes high volt­age (HV) Selec­tion and low volt­age (LV) sec­tions. The low volt­age sup­ply required for the CR tube is avail­able from pow­er trans­form­ers LT Secondary. 

Sim­i­lar­ly the H.T sup­ply required for the accel­er­at­ing anode is avail­able from the HT Sec­ondary. Thus dif­fer­ent volt­ages in the pow­er sup­ply are applied to the elec­trode and the filament 

Cathode Ray Tube

Cath­ode Ray Tube
  1. Elec­tron Gun Assembly 

Elec­trongun is the name giv­en to the sys­tem of elec­trodes that pro­duce the elec­tron beam. It has an indi­rect­ly heat­ed cath­ode, con­trol grid, focus­ing anode and OLD accel­er­at­ing anodes. The neg­a­tive sup­ply to the con­trol grid depends on the cath­ode. Sim­i­lar­ly pos­sib sup­ply is giv­en to both anodes 

The cath­ode is a nick­el cylin­der. It is coat­ed with oxide around it. Thus pro­duc­ing more elec­trons. The con­trol grid is locat­ed around the cath­ode. It is a met­al cylin­der. There is a small hole in the mid­dle of it. Through this comes the elec­tron beam 

The focus­ing anode trans­mits the elec­tron beam‑m focus to the sharp point. This is because the focus­ing anode is giv­en a pos­i­tive poten­tial. The accel­er­at­ing anode is giv­en a high pos­i­tive poten­tial. So it pulls the elec­trons to its side and con­verts it into a nar­row beam 

Deflecting Plate Assembly

The elec­tron beam leav­ing the elec­tron gun is passed through two set plates inside the tube. One set is called ver­ti­cal plates and the oth­er set is called hor­i­zon­tal plates. The ver­ti­cal plates are arranged hor­i­zon­tal­ly inside the tube. The beam became when the required poten­tial was giv­en to these plates

Moves up and down on the screen. Hor­i­zon­tal plates are arranged ver­ti­cal­ly inside the tube. Giv­en the required poten­tial, the elec­tron beam deflects to the left and right sides of the screen 


The screen is found on the front of the tube. It is coat­ed with fures­cent mate­r­i­al. zinc orthosil­i­cate, zinc oxide etc. are flures­cent mate­ri­als. When elec­tron beams hit the screen, a light spot appears. Its col­or coat­ed with depends on the fluren­cent material 


It is a con­i­cal glass tube that is com­plete­ly vac­u­umed. Its inte­ri­or is vac­u­um and bears dif­fer­ent elec­trodes. This coat­ing is applied to the inner wall of the CRT, ie the area between the strip and the screen, which is lined with aquadag mate­r­i­al — accel­er­at­ing anode. This cre­ates a sec­ondary emis­sion screen where the elec­trons that col­lide with the wall end up caus­ing some acci­dents on the wall 

Because elec­tron bom­bard­ment is hap­pen­ing. To pre­vent this, neg­a­tive poten­tial must be given 


1.Measurement Of Frequency 

This sys­tem uses a lis­sajous pat­tern to com­pare two fre­quen­cies. It uses a sin­soidal oscil­la­tor that can adjust the fre­quen­cy. The out­put of this oscil­la­tor is tak­en and giv­en between a set of deflect­ing plates. have to give. The unknown fre­quen­cy can be mea­sured from the avail­able lis­sajous pattern

Lissajous Patterns
Lis­sajous Patterns

Fv=Number of times tan­gent touch­es top or bottom/Number of times tan­gent touch­es either side 

In this we have to draw two lines. One should be hor­i­zon­tal and the oth­er ver­ti­cal and should not cut any part 

Phase Measurement

Phase measurement
Phase Mea­sure­ment

To mea­sure the phase angle of two sinu­soidal volt­ages of equal ampli­tude and fre­quen­cy we need to give them to the X and Y plates of the cro 

An ellipse is avail­able in which we need to find the val­ue of A, B from this ellipse. We can cal­cu­late the phase angle α by the fol­low­ing expression 


Study of waveforms 

We need to give the sig­nal we want to study to the ver­ti­cal input ter­mi­nal of the CRO. Give the saw tooth wave to the hor­i­zon­tal input. Adjust the con­trols and bring the screen to the cor­rect waveform 

Voltage measurement

Give the sig­nal to the ver­ti­cal deflec­tion plates. A ver­ti­cal line is vis­i­ble on the screen. The height of the ver­ti­cal line depends on the peak to peak mag­ni­tude of the volt­age we have giv­en. Mul­ti­ply this height by the deflec­tion sen­si­tiv­i­ty to get the peak volt­age value 

Current Measurement

The sig­nal must be trans­mit­ted through a known resis­tor. The result­ing volt­age should be mea­sured with CRO. Then find the cur­rent to be mea­sured by ohm’s law 

Time Base Generator 

Time Base Generator
Time Base Generator

When AC volt­age is applied to ver­ti­cal deflec­tion plates, the plates alter­nate­ly become pos­tive and neg­a­tive poten­tial. The result­ing elec­tron beam moves up and down at the fre­quen­cy of the applied volt­age. Now a ver­ti­cal line is vis­i­ble on the screen due to the visu­al stability 

It is nec­es­sary to apply a sweep volt­age or time base volt­age to the hor­i­zon­tal deflec­tion plates to dis­play the wave­form of a giv­en volt­age on the screen 

This sweep volt­age is peri­od­ic. Such a char­ac­ter­is­tic volt­age is called the ramp volt­age. Also this volt­age is called ramp volt­age. Also this volt­age increas­es slow­ly from zero and decreas­es rapid­ly to zero after reach­ing a cer­tain val­ue. Hence it is called saw tooth volt­age. The cir­cuits that make up saw tooth wave­forms are called time base circuits 

Dur­ing sweep time (Ts) the beam moves from left to right on the screen. By increas­ing the ampli­tude of the ramp volt­age, the beam moves to the right side of the screen. Dur­ing retrace or fly­back time (Tr) the beam rotates rapid­ly from right to left of the screen. 

Thus a thin line is vis­i­ble on the screen when turned on. To avoid this, too much neg­a­tive volt­age is applied to the con­trol grid. This blank­outs the elec­tron beam from the elec­tron gun dur­ing retrace time 

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