Cathode Ray Oscilloscope (CRO)

CRO(Cathode Ray Oscilloscope)

CRO is used to display different waveform patterns for measurement of the time period, frequency, peak value, average value.

CRO consists of CRT, different amplifiers, sawtooth  generators alternators delay line etc.

CRO is used in different applications like communication, radar and research applications for comparison of different wave form pattern.

The main part of CRO is CRT(cathode ray tube).

CRT(Cathode Ray Tube)

CRT is working on the principle of thermionic emission that is emission of electron from heated surface (cathode).

• HDP = Horizontal Deflecting Plate

• VDP  = Vertical Deflecting Plate

• D = Time Delay Line

• Electron Gun

• Control Grid

• Pre accelerating anode

• Focus anode

• Post accelerating anode

• Blanking Circuit

• Synchronizer

• Aquadag

• Phosphor coated screen

• HDP

• VDP

Electron Gun

It is the source of electron beam. It consists of heater, control grid and cathode

Intensity or Brightness Control

By controlling the Positive or negative potential of control grid the number of electros entering into the CRT are controlled so that brightness is controlled.

Pre accelerating and accelerating anode potentials are used to increase the velocity of the electrons passing through the CRT.

Focus Anode

For fine focus control small dc potential is applied to X and Y plates which is called astigmatism

For shifting the signal magnitude/amplitude vertical plates voltage are change.

For time shifting of signal we have to change horizontal plate voltage.

Blanking Circuit

Blanking circuit is used to blank-out or stop the electrons entering into the CRO during falling or Flyback/ retrace time of sawtooth signal this done by initiating control grid negative potential.

Time Delay Line

It is used to delay the function generator test signal for the time T_d = t_{f +} t_ic

Where,

t_f = falling time

t_ic = initial charging time.

Synchronizer

It is used to adjust the time period of the sawtooth signal equipment to the test signal time period so that observing smooth and steady waveform on the screen.

Aquadag

Aquadag is graphite coating used for collection of secondary electrons so so that maintaining electrical neutrality with in the CRO

Phosphorus is used for converting heat energy into light energy

 

 

                        Types of CRO

Dual Trace CRO

It consisting of one electron gun one X plate and two Y plate (ch-1  ch-2) by selecting either ch-1 or ch-2 switches one of the signal is displayed on the screen

By pressing Alt or Chop mode buttons two waves applied to  ch-1 and ch-2 are alternatively observed on the screen it consisting of multiplexer with selection signal. By adjusting selection signal frequency is double the sawtooth signal frequency two waveform are alternatively visible on the screen.

Dual Beam CRO

It consists of two or more horizontal or vertical deflecting plate and multiple guns so that two or more waveforms can be simultaneously visible on the screen

This is used in radar applications for comparison of different waveform pattern

High Frequency CRO

It consists of an extra P₁₅ coating and extra post accelerating anode for controlling brightness and faster conversion of heat energy into light energy

Storage CRO

Memory storing element like RAM and ROM are used for storage of different waveform patterns.

                                      Calibration of CRO

CRO is calibrated by applying square or pulse signal of known frequency and peak to peak amplitude/ magnitude. By using this X and Y division (time/division and voltage/division) are calculated from this an unknown signal parameters are calculated.