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(19) electronic instruments 1


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(19) electronic instruments 1

  1. 1. Topics to be Discussed• CATHODE RAY OSCILLOSCOPE (CRO). (CRO)• Cathode Ray Tube (CRT).• How a CRO Displays Waveforms.• Time-base Voltage.• Block Diagram of a CRO.• APPLICATIONS OF CRO. – Measurement of Frequency. • Lissajous patterns• Measurement of Phase Difference. Difference17-03-13 12:04 PM Electronic Instruments-1 1
  2. 2. CATHODE RAY OSCILLOSCOPE (CRO)• The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accurate time and amplitude measurements of voltage signals over a wide range of frequencies.• Its reliability, stability, and ease of operation make it suitable as a general purpose laboratory instrument.• The most important and versatile electronic instrument (used in Testing).• It is a very fast x - y plotter and is used to display voltage waveforms.• The ‘stylus’ of this ‘plotter’ is a luminous spot which moves on the screen in response to the input voltages. 17-03-13 12:04 PM Electronic Instruments-1 2
  3. 3. • An electronics engineer can “see” what is happening in different parts of a circuit.• The heart of the CRO is a cathode-ray tube shown schematically in Fig. 1.• The rest is the electronic circuitry required to operate the CRT. 17-03-13 12:04 PM Electronic Instruments-1 3
  4. 4. Cathode Ray Tube (CRT)• It consists of : 1. Electron Gun: Produces a sharply focused beam of electrons. 2. Deflection System: Deflects the electron beam, both in horizontal and vertical directions. 3. Fluorescent Screen: Produces a bright spot when the electron beam strikes it. 17-03-13 12:04 PM Electronic Instruments-1 4
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  6. 6.  The cathode ray is a beam of electrons which are emitted by theheated cathode (negative electrode) and accelerated toward thefluorescent screen. The assembly of the cathode, intensity grid, focus grid, andaccelerating anode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control itsintensity and focus between the electron gun and the fluorescentscreen are two pair of metal plates - one oriented to providehorizontal deflection of the beam and one pair oriented to givevertical deflection to the beam. These plates are thus referred to as the horizontal and verticaldeflection plates. The combination of these two deflections allowsthe beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, the phosphor isexcited and light is emitted from that point. This conversion ofelectron energy into light allows us to write with points or lines oflight on an otherwise darkened screen. 17-03-13 12:04 PM Electronic Instruments-1 6
  7. 7.  In the most common use of the oscilloscope thesignal to be studied is first amplified and then appliedto the vertical (deflection) plates to deflect the beamvertically and at the same time a voltage thatincreases linearly with time is applied to the horizontal(deflection) plates thus causing the beam to bedeflected horizontally at a uniform (constant) rate. The signal applied to the vertical plates is thusdisplayed on the screen as a function of time. Thehorizontal axis serves as a uniform time scale. 17-03-13 12:04 PM Electronic Instruments-1 7
  8. 8. CRO Operation17-03-13 12:04 PM Electronic Instruments-1 8
  9. 9.  A simplified block diagram of a typical oscilloscope is shownin Fig. 3. In general, the instrument is operated in the followingmanner. The signal to be displayed is amplified by the verticalamplifier and applied to the vertical deflection plates of theCRT. A portion of the signal in the vertical amplifier is appliedto the sweep trigger as a triggering signal. The sweep trigger then generates a pulse coincident with aselected point in the cycle of the triggering signal. This pulseturns on the sweep generator, initiating the sawtooth waveform. The sawtooth wave is amplified by the horizontal amplifierand applied to the horizontal deflection plates. Usually,additional provisions signal are made for applying an externaltriggering signal or utilizing the 60 Hz line for triggering. Alsothe sweep generator may be bypassed and an external signalapplied directly to the horizontal amplifier. 17-03-13 12:04 PM Electronic Instruments-1 9
  10. 10. How a CRO Displays Waveforms?The bright spot on the screen moves to the leftor to the right when a dc voltage is applied tothe horizontal defection plates.17-03-13 12:04 PM Electronic Instruments-1 10
  11. 11. A solid line trace is obtained on thescreens when we apply ac voltages tothe horizontal deflection plates.17-03-13 12:04 PM Electronic Instruments-1 11
  12. 12. Display of sine wave on CRO17-03-13 12:04 PM Electronic Instruments-1 12
  13. 13. Deflection Sensitivity• It is defined as the vertical deflection of the spot on the screen per unit deflecting voltage applied.• It is designated as S and is measured in m/V.• Deflection factor G, defined as the reciprocal of the deflection sensitivity S.17-03-13 12:04 PM Electronic Instruments-1 13
  14. 14. Time-base Voltage Ideal time- Actual time- Distortedbase voltage base voltage display 1417-03-13 12:04 PM Electronic Instruments-1
  15. 15. Block Diagram of a CRO1. The CRT,2. The vertical deflection system,3. The horizontal defection system, and4. The high-voltage power supply and the low-voltage power supply.17-03-13 12:04 PM Electronic Instruments-1 15
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  18. 18. APPLICATIONS OF CRO• (1) Study of Waveforms : – Dual Trace CRO. – Dual Beam CRO. – Storage CRO.• (2) Measurement of Voltages.• (3) Measurement of Current.• (4) Measurement of Frequency.• (5) Measurement of Phase Difference.17-03-13 12:04 PM Electronic Instruments-1 18
  19. 19. Measurement of Frequency• Example: A sinusoidal voltage is displayed on a CRO. For obtaining a convenient size of the display as shown in figure, the vertical amplifier/attenuator is set at 2 V/cm, and the time-base control at 0.5 ms/cm. Determine the rms value and the frequency of the sine-wave voltage.17-03-13 12:04 PM Electronic Instruments-1 19
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  21. 21. Vp = peak value of the display × deflection sensitivity = 3.5 cm × 2 V/cm =7.0 V 7.0 ∴ rms value = = 4.95 V 2T = one cycle length on x-axis × time-base setting = 4 cm × 0.5 ms/cm = 2.0 ms 1 1 ∴ f = = = 0.5 kHz T 2 ms17-03-13 12:04 PM Electronic Instruments-1 21
  22. 22. Lissajous patterns• A Lissajous pattern is produced on the screen of the CRO when two different sine- wave voltages are simultaneously applied to the two sets of deflection plates.• The type of pattern produced depends on (i) the ratio of the frequencies, and (ii) the relative phase of the two sine-wave voltages. 17-03-13 12:04 PM Electronic Instruments-1 22
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  25. 25. Measurement of Phase Difference Y1 sin θ = Y2 17-03-13 12:04 PM Electronic Instruments-1 25