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Cro(nnm)

CRO

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Cro(nnm)

  1. 1. CRO N. N. Maurya
  2. 2.  Introduction to oscilloscope  Architecture of CRT  Measurement of Oscilloscope  Lissajous Pattern (LP) methods OUTLINE
  3. 3.  The CRO (Cathode Ray Oscilloscope), generally referred to as the oscilloscope or simply “scope” is probably the most versatile measuring instrument  Basically, CRO is a voltmeter with a screen.  Measurement of parameters: ◦ AC or DC voltage ◦ AC or DC current ◦ Time ◦ Phase relationship ◦ Frequency INTRODUCTION TO OSCILLOSCOPE
  4. 4.  Oscilloscope consists of the following parts: ◦ CRT – Cathode Ray Tube (Heart of Instrument) ◦ Vertical amplifier ◦ Horizontal amplifier ◦ Sweep generator ◦ Trigger circuit ◦ Associated power supplies  There are 2 types of oscilloscope: analog and digital  However their principle and basic characteristics are still the same INTRODUCTION TO OSCILLOSCOPE …
  5. 5. INTRODUCTION TO OSCILLOSCOPE … Analog Oscilloscope Digital Oscilloscope
  6. 6. Architecture of CRT • A heated cathode to give off e . electron gun • An anode to accelerate e . • The grid controls brightness on the screen. + If grid is ve, pushes back some e fewer e reach screen Filament e beam Vacuum Electron gun
  7. 7. electron gun • 2 sets of parallel plates • (X-plates & Y-plates). e beam is deflected by them deflection system + filament X-plates Y-plates e beam vacuum Deflection system
  8. 8. electron gun deflection system + filament X-plates Y-plates fluorescent screen e beam vacuum • Coated with fluorescent material Glows where e beam strikes it. Fluorescent screen light spot is formed. spot
  9. 9. Vertical deflection • With no voltage across Y-plates. The light spot rests at centre of screen.
  10. 10. • With a d.c. voltage the light spot is deflected towards +ve Y-plate.
  11. 11. • With a 50 Hz a.c. voltage, the spot makes 50 oscillations per second. appears as a vertical line
  12. 12. • Voltages are applied to the Y-plate via a built-in amplifier. • The gain control adjusts the amplification of the input voltage to give a suitable deflection. Vertical Amplifier Trigger Generator Horizontal Amplifier Horizontal plate Vertical plate Screen Sweep Generator Input signal whose parameter is to be measured
  13. 13. Horizontal deflection Time base circuit varies the voltage across the X-plates. The spot is made to sweep across the screen at a steady speed, then, ‘fly back’ rapidly to the start. Amplitude Time Sudden fall
  14. 14. Waveform display  An a.c. voltage across Y-plates: spot oscillates up & down • Time base across X-plates: spot sweeps across screen + =
  15. 15. MEASUREMENTS OF OSCILLOSCOPE  Voltage Measurements  Period and Frequency Measurements  Phase Measurements or Time Delay
  16. 16.  The vertical scale is calibrated in either volts per division or milivolts per division.  Using the scale setting of the scope and the signal measured off the face of the scope, it can measure peak-to-peak voltage for an ac signal VOLTAGE MEASUREMENT Vp-p = (vertical p-p division) x (volts/div) Vp-p = (no. of vertical division) x (volts/div) OR
  17. 17. VOLTAGE MEASUREMENT … (Volt/Div : 100mV/Div, Time/Div : 0.5ms/Div) a) Voltage Peak-to-Peak Vp-p= (V/Div) x No. of vert. div. = 100 mV/div x (3.8 x 2) = 0.76 V b) Voltage Peak Vp = (V/Div) x No. of vert. div. = 100 mV/div x (3.8) = 0.38 V 3.8 3.8 A B Vp-p Vp T 10 TD 2.5
  18. 18.  PERIOD ◦ Horizontal scale of the scope can be used to measure time in second, milisecond or nanosecond. ◦ The interval of a pulse from start to end is the period of the pulse.  FREQUENCY ◦ The measurement of a repetitive waveform period can be used to calculate the signal frequency. PERIOD AND FREQUENCY MEASUREMENT Period = (horizontal p-p division) x (time/div) F= 1/T
  19. 19. PERIOD AND FREQUENCY MEASUREMENT … 3.8 3.8 A B Vp-p Vp T 10 TD 2.5 (Time/Div : 0.5ms/Div) a) Period, T T = (Time/Div) x (no. div/cycle) = 0.5ms/div x 10 = 5ms b) Frequency, f f = 1/T = 1/5ms = 200 Hz
  20. 20.  The time interval between waveforms is called phase delay.  The phase delay is measured between the midpoint at the start of each waveform PHASE SHIFT (PHASE DIFFERENCE) Phase difference,Ө = (phase difference in division) x (degrees/div)
  21. 21. PHASE SHIFT (PHASE DIFFERENCE) 3.8 3.8 A B Vp-p Vp T 10 TD 2.5 1 cycle = 10 div TD = 2 div Therefore, 1 cycle : 10 div = 360o 1 div = 360o / 10 = 36o 2 div = 2 x 36o = 72o (Time/Div : 0.5ms/Div)
  22. 22. =0 = /4 = /2 =3 /4 = =0 = /4 = /2 =3 /4 = x-amplitude = y-amplitude x-amplitude < y-amplitude Apply 2 sinusoidal signals of SAME frequency to the X-plate (X-input) and Y-plate (Y-input) of a CRO simultaneously.
  23. 23.  FREQUENCY MEASUREMENT ◦ The alternative way of using oscilloscope to measure frequency. ◦ A known reference frequency sine wave is applied to one deflection plate and the unknown sinusoidal signal to the other deflection plate ◦ A Lissajous pattern is produced on the screen according to the frequency ration between the two signal. LISSAJOUS PATTERN fy max. no. of x-intercepts fx max. no. of y-intercepts =
  24. 24. No. of x-intercepts: 3 No. of y-intercepts: 1 fy:fx = 3:1 No. of x-intercepts: 3 No. of y-intercepts: 2 fy:fx = 3:2
  25. 25. Exercise
  26. 26.  PHASE ANGLE MEASUREMENT ◦ Oscilloscope can be used in the X-Y mode to determine the phase angle between two signals. ◦ This useful technique is limited to small frequency. ◦ The formula for phase angle measurement: Where θ = phase angle in degree Y1 = the distance from X-axis to the point where the Lissajous pattern crosses Y-axis Y2 = the maximum vertical distance on the Lissajous X1 = the distance from Y-axis to the point where the Lissajous pattern crosses X-axis X2 = the maximum horizontal distance on the Lissajous LISSAJOUS PATTERN … Sin θ = Y1/Y2 = X1/X2
  27. 27. LISSAJOUS PATTERN … θ- phase angle in degree Yo-Y axis intercept Ym-maximum vertical deflection
  28. 28.  EXAMPLE LISSAJOUS PATTERN … If, in figure above, the distance Yo is 1.8cm and Ym=2.3cm, what is the phase angle?
  29. 29. How to Connect Probe
  30. 30. Dual Trace System (Adding another input channel)
  31. 31. Digital Storage Oscilloscope
  32. 32. Thank You

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