Electrical Engineering ⇒ Topic : Applications of Cathode Ray Oscilloscope

Applications of a CRO:-

CROs are widely used in electrical and electronics engineering for troubleshooting and for the measurement of various electrical quantities. Some important common applications are given below.

Measurement of voltage :-The amplitude of a voltage can be measured using a CRO by applying the voltage signal to the vertical deflection plates. As already mentioned, when no voltage is applied to the horizontal plates, only a vertical line will be seen on the screen, whose height gives the amplitude of the voltage to be measured.

        All the oscilloscopes have their screens marked with vertical and horizontal lines in the form of a centimeter graph sheet. To measure the voltage, first the oscilloscope is calibrated using a known voltage in units of volts per cm. Suppose a voltage of 20 V is applied to the vertical plates, and the vertical gain is adjusted till a good deflection is obtained. Suppose the vertical line is one cm in length. Thus a calibration unit of 20 V/cm is obtained. Then, keeping the vertical gain unchanged, the voltage to be measured is applied to the vertical input terminals of the CRO. Let the length of the vertical line be 1 cm. Then the amplitude of the unknown voltage will be given by

                                 unknown voltage = 20 x I V

The magnitude of an unknown current can also be measured using the above procedure.

Study of waveforms:- In addition to the measurement of the amplitudes of voltages and currents, oscilloscopes are widely used to observe wave shapes.For the study of the wave forms, the signal to be studied is applied to the vertical deflection plates and a saw-tooth wave is applied to the horizontal deflection plates. Then, the various controls mentioned in the previous section are adjusted to obtain a sharp and well defined signal waveform on the screen.

Measurement of frequency:- For measuring an unknown frequency of a signal, it has to be applied to the vertical deflection plates, along with a signal of known frequency applied to the horizontal deflection plates. The controls provided are adjusted to get a sharp and well defined waveform of the signal on the screen. From the waveform obtained, it is seen that

the number of loops cut by the horizontal line on the screen gives the frequency of the signal applied to the vertical plates, i.e. f_v, and

the number of loops cut by the vertical line on the screen gives the frequency of the signal applied to the horizontal plates, i.e. f_h which is usually known.

If the magnitude f_v, and f_h are noted from the pattern of the waveform displayed on the screen, knowing the actual value of f_h, the value of the unknown frequency f_v can be calculated

Applications of Cathode Ray Oscilloscope

• In television: In television receiver, a cathode ray tube along with associated sweep circuits is used for the projection of the image. Magnetic deflection is generally used.
• In radar: Radar means ratio detection and ranging and is used to detect and give the position (azimuth angle, elevation angle, and distance) of an object such as aircraft. Information about a detected object within the range of the radar is displayed on a cathode ray tube.
• Measurement of voltage: Cathode ray oscilloscope can be employed for the measurement of voltage (direct or alternating).
• Direct voltage to be measured is applied between a pair of deflection plates and the deflection of the spot is observed. The magnitude of deflection multiplied by the deflection sensitivity gives the value of direct voltage applied. In case of measurement of alternating voltage of sinusoidal waveform, it is applied between a pair of deflection plates and the length of the straight line measured. Knowing the deflection sensitivity, the peak to peak value of applied ac voltage can be determined The R.M.S. value of ac voltage applied will be equal to this peak to peak value divided by 2√2 for sinusoidal waveform.
• Measurement of current: The current to be measured is passed through a known non-inductive resistance and the voltage drop across it is measured by CRO as mentioned above. Current may be calculated. When the currents to be measured are of very small magnitude, the voltage drop across a known non-inductive resistance (small value) is usually amplified by a calibrated amplifier.
• Measurement of current and voltage simultaneously: Double beam CRO is used for measurement of current and voltage simultaneously.
• Study of Lissajous figures: When alternating voltages are applied to the two sets of deflection plates simultaneously, instead of linear sweep voltage, various figures in the form of straight lines or one or more closed loops may be obtained on the screen. These figures are known as Lissajous figures. These figures can be utilized to measure the phase difference and for measurement of frequency