Cathode Ray Oscilloscope (CRO)

Block diagram of CRO

CRO is an important laboratory instrument used for the measurement and analysis of waveforms. It is a XY plotter which displays an input signal as a function of time. The luminous spot of the electron beam acts as a stylus of XY plotter that moves over the display area in a response to input voltages.  

CRO block diagram

The major blocks of a general purpose CRO consists of the following

1.      Cathode ray tube

2.      Vertical amplifier

3.      Delay line

4.      Time base circuit

5.      Horizontal amplifier

6.      Trigger circuit

7.      Power supply.

The functions of each block are as follows

1.      Cathode Ray Tube
       It is the heart of the oscilloscope. It consists of filament which heated the cathode and electrons are emitted by thermionic emission. The electrons are then passed through a grid which is at negative potential. The potential of the grid is adjusted to control the flow of electrons. There are multiple anodes which are arranged in increasing order of positive potential to accelerate electron beam. When the electrons emitted by the electron gun, strikes the phosphor screen of the CRT, a visual signal is displayed on the CRT.

2. Vertical Amplifier
                               The input signals are amplified by the vertical amplifier. Usually, the vertical amplifier is a wide band amplifier which passes the entire band of frequencies.

3. Delay Line
                       This circuit is used to delay the signal for a period of time in the vertical section of CRT. The input signal is not applied directly to the vertical plates because the part of the signal gets lost, when the delay Time is not used. Therefore, the input signal is delayed by a period of time. This gives time for the sweep to start at the horizontal plates before the signal has reached the vertical plates.

4. Time Base Circuit

                     Time base circuit uses a unijunction transistor, which is used to produce the sweep. The saw tooth voltage produced by the time base circuit is required to deflect the beam in the horizontal section. The spot is deflected by the saw tooth voltage at a constant time dependent rate.

5. Horizontal Amplifier

                 The saw tooth voltage produced by the time base circuit is amplified by the horizontal amplifier before it is applied to horizontal deflection plates.

6. Trigger Circuit
                  The signals which are used to activate the trigger circuit are converted to trigger pulses for the precision sweep operation whose amplitude is uniform. Hence input signal and the sweep frequency can be synchronized.

7. Power supply:
                              The voltages required by CRT, horizontal amplifier and vertical amplifier are provided by the power supply block. High voltage supply is used for CRT and low voltage supply is used for other circuits in the CRO.