Cathode Ray Oscilloscope Interview Questions and Answers:

1. What is cathode ray oscilloscope (CRO)?

Ans. A CRO is an electronic device with a CRT as its main component and other associated circuits consisting of a power supply unit, a sawtooth wave generator, horizontal and vertical amplifiers.

2. How is CRO superior to ordinary measuring instruments?

Ans. CRO is an electronic device that gives graphical representation of alternating quantities under examination.

The CRO gives very accurate measurements as it is free from the errors introduced by the moving parts. It is also free from damping mechanism and other inertia containing parts.

3. Name main components of a cathode ray tube (CRT).

Ans. The essential components of a CRT are electron gun, focusing and accelerating anodes, horizontal and vertical deflection plates, and evacuated glass envelope with phosphorescent screen.

4. For what vertical and horizontal plates are provided in a CRO ?

Ans. Horizontal and vertical plates are provided between electron gun and screen to deflect the beam according to the input signal.

5. Explain the electron gun assembly in CRT.

Ans. The electron gun assembly consists of an indirectly heated cathode, a control grid surrounding the cathode, a focusing anode and an accelerating anode. The sole function of the electron gun assembly is to provide a focused electron beam which is accelerated towards the phosphor screen.

6. For what electron gun assembly is provided in a CRT?

Ans. The sole function of an electron gun assembly in a CRO is to provide a narrow and sharply focused electron beam which is accelerated towards the phosphor screen.

7. Why is the grid in a CRO provided with a hole in it?

Ans. The hole in a grid of a CRO is provided to allow passage for electrons through it and concentrate the beam of electrons along the axis of the tube.

8. What is aquadag?

Ans. Coating of a conducting material, known as aquadag, is provided over the interior surface of CRT in order to accelerate the electron beam after passing between the deflection plates and to collect the electrons produced by secondary emission when electron beam strikes the screen.

9. Where aquadag coating is used? What is the advantage of this coating?

Ans. Aquadag coating is provided over the interior surface of CRT in order to accelerate the electron beam after passing between the deflection plates and to collect electrons produced by secondary emission when electron beam strikes the screen. The advantage of this coating is that the formation of negative charge on the screen is avoided and state of equilibrium is maintained.

10. How is the electron beam focused to a fine spot on the face of a cathode ray tube?

Ans. Focusing is achieved by various sets of plates forming a sort of electronic lens. In general, there are three sets of plates. Voltages at anode 1 and 2 are kept fixed while it is variable on the third anode. By varying the voltage on the third anode, the spot may be focused.

11. What is meant by deflection sensitivity of CRT?

Ans. The deflection sensitivity of a CRO is defined as the vertical deflection of the beam on the screen per unit deflecting voltage.

12. What is the deflection factor of a CRO?

Ans. The deflection factor of a CRO is the reciprocal of the deflection sensitivity.

13. What is meant by retrace time?

Ans. Retrace time is the time required by the electron beam to return to its original position on a CRT screen after being deflected to the right by a sawtooth waveform.

14. What is sweep time?

Ans. Sweep time is time duration during which the beam is swept from left to right on the screen of a CRT by the linearly increasing sawtooth voltage.

15. What is a linear time base generator?

Ans. A linear time base generator is one that provides an output waveform, a portion of which exhibits a linear variation of voltage or current with time.

16. What is function of time base generator in CRO?

Ans. Most of the CRO applications involve measurement or display of a quantity which varies with respect to time. This requires that the CRT spot moves across the screen with a constant velocity. For this a voltage which varies linearly with time has to be applied to one of deflection plates. This voltage is used to sweep the electron beam across the screen, so it is called a sweep voltage. Because of its shape, it is also sometimes called a sawtooth or ramp voltage. The time base generator is required to provide this sweep, sawtooth or ramp voltage.

17. What is the purpose of triggering circuit in CRO?

Ans. In a CRO, a triggering circuit is provided for synchronising two types of deflections so that horizontal deflection starts at the same point of the input vertical signal each time it sweeps.

18. What is Astigmatism control?

Ans. Astigmatism control is an adjustment that will provide sharp focus over the entire screen.

19. For a CRO, describe astigmatism control.

Ans. Astigmatism is an additional focusing control and is analogous to astigmatism in optical lenses. A beam that is focused at the centre of the screen would be defocused at the edges of the screen because the lengths of the electron paths are different for the centre and the edges. Adjustment of this control gives a sharp focus over the entire screen. This control is affected by varying the potential of deflection plates and accelerating anodes.

20. How the brightness of display in a CRO is controlled?

Ans. The brightness of the glow produced at the screen depends on the number of electrons producing beam. Since the grid controls the electron emission from the cathode, the grid voltage control is a brightness control. The potential of the control grid w.r.t. cathode is controlled with the help of potentiometer so as to control the intensity of brightness of the spot. The grid potential determines the amount of electrons leaving the cathode and thus controls the intensity of beam. A larger number of electrons in the beam causes a brighter spot to appear on the screen. Brightness also depends on beam speed; so for maximum brightness the electrons should be accelerated to the largest possible velocity. However, if the electron velocity is very high while passing through the deflecting plates, the deflecting voltages will have a reduced influence, and the deflection sensitivity will decrease. That is why, postdeflection acceleration is provided i.e., the electrons are accelerated again after they pass between the deflecting plates.

21. What is a Lissajous pattern?

Ans. Lissajous pattern is a pattern that results from applying periodic signals to the deflection plates of a CRO.

22. What is Lissajous pattern? How measurement of frequency can be done with it?

Ans. In a Lissajous pattern ratio of frequency of vertical signal to the frequency of horizontal signal is equal to the ratio of positive Y-peaks to positive X-peaks in that pattern.


Lissajous pattern

Thus by counting the positive Y-peaks and X-peaks on a Lissajous pattern, ratio of frequencies of two voltage signals can be determined. In case of an open Lissajous pattern, free end is treated as half peak.

23. Enlist the applications of CRO.

Ans. The applications of CRO arc given below :

  1. Measurement of voltage and current.
  2. Measurement of phase.
  3. Measurement of frequency.
  4. Comparison of two frequencies.

CRO is a versatile instrument that can be used for display, measurement, waveform analysis and other phenomenon in electrical and electronic circuits.

24. What is a dual beam CRO?

Ans. A dual beam CRO has got two electron beams that are controlled independently. The constructional features are almost the same as in the case of single beam CRO.

25. What is a dual trace oscilloscope?

Ans. Dual trace oscilloscope employs a single electron gun and provides two traces by switching the deflection plates from one input signal to another which implies that Y-channel is time shared by two signals. The display of the input is continuous and simultaneous to the eyes although it is sampled display.

26. What is difference between dual trace and dual beam CRO?

Ans. Dual beam CRO uses two separate electron beams and deflection systems mounted in the same CRT to display two different electron beams simultaneously whereas the dual trace oscilloscope alternately displays two signals on one screen by time sharing one electron gun and deflection system.

27. How is dual trace oscilloscope superior to dual beam oscilloscope?

Ans. Dual beam CROs are not as versatile as the dual trace CROs because the later can operate in a number of modes.

28. What is difference between analog storage oscilloscope and digital storage oscilloscope?

Ans. Digital storage oscilloscope uses digital storage techniques whereas analog storage oscilloscope uses special CRT called the storage tube.

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