Exp. No #0 OSCILLOSCOPE AND DIGITAL MULTIMETER Date: OBJECTIVE The purpose of the experiment is to understand the operation of cathode ray oscilloscope (CRO) and to become familiar with its usage. Also perform an experiment using function generator to measure amplitude, time period and frequency of the time-varying signals using a calibrated cathode ray oscilloscope and to compare the measured values with that of a digital multimeter. PRELAB 1. In the instruction manual look for product descriptions and specifications that are relevant to the instruments used for this lab. EQUIPMENT USED 30 MHz Dual Channel Cathode Ray Oscilloscope 3 MHz Function Generator 4 ½ digit Digital Multimeter BNC Cables and Probes THEORY Introduction The Cathode Ray Oscilloscope (CRO) provides a visual presentation of any waveform applied to the input terminal. The oscilloscope consists of the following major subsystems. Cathode-ray tube(crt) Vertical amplifier Horizontal amplifier Sweep Generator Trigger circuit Associated power supply It can be employed to measure quantities such as peak voltage, frequency, phase difference, pulse width, delay time, rise time and fall time. Cathode Ray Tube (CRT) The CRT is the heart of the CRO providing visual display of an input signal waveform. A CRT contains four basic parts: An electron gun to provide a stream of electrons. Focusing and accelerating elements to produce a well define beam of electrons. Horizontal and vertical deflecting plates to control the path of the electron beam. An evacuated glass envelope with a phosphorescent screen which glows visibly when struck by electron beam. 2017 2018 Page 1
A cathode containing an oxide coating is heated indirectly by a filament resulting in the release of electrons from the cathode surface. The control grid, which has a negative potential, controls the electron flow from the cathode and thus controls the number of electron directed to the screen. Once the electron passes the control grid, they are focused into a tight beam and accelerated to a higher velocity by focusing and accelerating anodes. The high velocity and well-defined electron beam then passed through two sets of deflection plates. electron gun Y plates cathod anode electron beam fluorescent screen X plates Basic construction of CRT The first set of plates is oriented to deflect the electron beam vertically. The angle of the vertical deflection is determined by the voltage polarity applied to the deflection plates. The electron beam is also being deflected horizontally by a voltage applied to the horizontal deflection plates. The tube sensitivity to deflecting voltages can be expressed in two ways that are deflection factor and deflection sensitivity. The deflected beam is then further accelerated by very high voltages applied to the tube with the beam finally striking a phosphorescent material on the inside face of the tube. The phosphor glows when struck by the energetic electrons. Control Grid: Regulates the number of electrons that reach the anode and hence the brightness of the spot on the screen. Focusing anode: Ensures that electrons leaving the cathode in slightly different directions are focused down to a narrow beam and all arrive at the same spot on the screen. Electron gun: Cathode, control grid, focusing anode, and accelerating anode. Deflecting plates: An electric field between the first pair of plates deflects the electrons horizontally, and an electric field between the second pair deflects them vertically. If no deflecting fields are present, the electrons travel in a straight line from the hole in the accelerating anode to the center of the screen, where they produce a bright spot. In general-purpose oscilloscopes, amplifier circuits are needed to increase the input signal to the voltage levels required to operate the tube because the signals measured using CRO are typically small. There are amplifier sections for both vertical and horizontal deflection of the beam. Vertical Amplifier: amplify the signal at its input prior to the signal being applied to the vertical deflection plates. Horizontal Amplifier: amplify the signal at its input prior to the signal being applied to the horizontal deflection plates. Sweep Generator: develop a voltage at the horizontal deflection plate that increase linearly with time. 2017 2018 Page 2
Operation The four main parts of the oscilloscope CRT are designed to create and direct an electron beam to a screen to form an image. The oscilloscope links to a circuit that directly connects to the vertical deflection plates while the horizontal plates have linearly increasing charge to form a plot of the circuit voltage over time. In an operating cycle, the heater gives electrons in the cathode enough energy to escape. The electrons are attracted to the accelerating anode and pulled through a control grid that regulates the number of electrons in the beam, a focusing anode that controls the width of the beam, and the accelerating anode itself. The vertical and horizontal deflection plates create electric fields that bend the beam of electrons. The electrons finally hit the fluorescent screen, which absorbs the energy from the electron beam and emits it in the form of light to display an image at the end of the glass tube. Front-panel controls 1 4 5 6 7 8 9 12 16 17 13 2 3 10 11 14 15 18 19 1. POWER Pushbutton switch to turn scope ON and OFF. LED indicates POWER ON condition 2. INTENS. Intensity control to adjust brightness of CRT screen. 2017 2018 Page 3
3. FOCUS Focus control to adjust Sharpness of CRT display. 4. TR Trace Rotation Pot. Screw driver adjustment for alignment of trace. 5. X POS Controls Horizontal Position of trace. 6. HOLD OFF Controls Hold OFF time between Sweeps in the ratio 1:10 approx. 7. TIME/DIV Selects Timebase speeds from 0.5us/div to 0.2s/div. 8. AT/NORM. Switch in out position: Automatic Triggering (Trace visible without signal) Switch Pressed: Normal Triggering with Level control. (Trace invisible without signal) 9. LEVEL Adjusts trigger point of the signal from +ve peak to ve peak, if at/norm PB switch is pressed 10. X-MAGx10 Switch when pressed magnifies Trace or Signal 10 times in X-direction. 11. CT Switch when pressed converts the instrument from oscilloscope to Component Tester mode. One test lead is connected to CT socket and the second test lead connected to ground socket. 12. Y-POS. Controls Vertical position of CH.I trace 13. INVERT (CH.I) Switch when pressed, inverts the polarity of CH.I signal. In combination with ADD switch, used for algebraic addition or difference of two channels. 14. CH.I Signal input for CH.I 15. GROUND Separate Ground socket 16. AC/DC/GD Input coupling switches for CH.I AC: Both switches in out position. Signal is capacitively coupled, DC is blocked DC: AC/DC switch when pressed, GD switch in out position. All components (AC & DC) of the signal are passed. GD: GD switch pressed. AC/DC switch may be at any position. Signal is disconnected, input of vertical amplifier is grounded. 17. VOLTS/DIV. CH.I Input attenuator. Selects input sensitivity in mv/div or V/div in 1-2-5 sequence. 18. DUAL Switch in out position: Single Channel separately. Switch pressed: CH.I & CH.II in alternate mode DUAL + ADD switches pressed: CH.I & CH.II in CHOP mode. 19. ADD Only ADD switch is pressed: Algebraic addition or difference of CH.I & CH.II in combination with INVERT switches. FURTHER READING 1. Paul Horowitz and Winfeld Hill, The Art of Electronics, Cambridge University Press, New York, 2nd edition, 1989. 2. The XYZ's of Oscilloscopes, Tektronix, Inc., Beaverton, OR (1992). 3. Patrick C Elliott, The Basics of Digital Multimeters, Ideal Industries, Inc, 2010 2017 2018 Page 4
PRECAUTIONS Do not leave a bright spot on the screen for any length of time. Do not apply signals that exceed the scope s voltage rating. Do not try to make accurate measurements on signals whose frequency is outside the scope s frequency specifications. Be aware that the scope s input circuitry can cause loading effects on the circuitry under test - Use correct probe for the work. PRACTICE PROCEDURE 1. Measurement using CRO 1. Switch on the CRO. Turn the AC-GND-DC to GND. Check if a horizontal trace appears after the CRO warms up. Set the trace centrally in position on the screen. 2. Become accustomed to the operation of the oscilloscope. Move the focus, intensity, and position controls to see the effects produced. Measurement of Frequency 3. Connect the function generator output to one vertical input of the CRO. 4. Set the function generator in sinusoidal mode and adjust the amplitude of the signal so that it just about fills the screen. 5. Set the signal generator dial at any particular frequency and move the dial until you have only a few complete cycles across the CRO face in the horizontal direction. 6. Measure the period T of the signal. To do so, measure (in divisions & subdivisions) the horizontal distance between two successive peaks and multiply this distance by the reading of the TIME/DIV button which is the scale of the time axis. Record your data. 7. This gives the period T of the AC signal; its frequency is then f = 1/T. Measurement of Voltage 8. Use the volts/div selector to convert vertical readings on the oscilloscope into actual voltages. 9. In measuring the voltage, always measure the value from the center of the trace to its peak. This "peak voltage" is half the peak-to-peak voltage, which is the full height of the trace on the CRO screen. Repeat the above procedure of frequency and voltage measurement for continuous signal, triangular wave and square wave also. 2017 2018 Page 5
OBSERVATION a) Measurement of continuous signal Amplitude, A = V/div = div x _ Period for the waveform, T = _ div x ms/div = _ Frequency, f = 1/T = b) Measurement of sinusoidal signal Amplitude, A = V/div = div x _ Period for the waveform, T = _ div x ms/div = _ Frequency, f = 1/T = c) Measurement of triangular signal Amplitude, A = V/div = div x _ Period for the waveform, T = _ div x ms/div = _ Frequency, f = 1/T = 2017 2018 Page 6
d) Measurement of rectangular signal Amplitude, A = V/div = div x _ Period for the waveform, T = _ div x ms/div = _ Frequency, f = 1/T = Table 1 S. No Oscilloscope Make: Frequency Specification: _ Waveform Amplitude Time period Frequency on Function generator dial Calculated frequency, f=1/t 1. Continuous signal 2. Sinusoidal signal 3. Triangular signal 4. Rectangular signal Inference 2017 2018 Page 7
2. Measurements using Digital Multimeter 1. Connect a +4V DC voltage to Oscilloscope and measure the signal with voltage knobs at 1V/div, 2V/div, 5V/div. Also measure the voltage with the DMM in both AC and DC settings. Table 2 Oscilloscope DMM Signal 1V/div 2V/div 5V/div AC Set DC Set + 4V DC - 4V DC Inference 2. Connect a 1 V peak-to-peak sine wave at 1 khz from function generator to oscilloscope. Use the oscilloscope to measure the peak-to-peak voltage of the signal and, hence, calculate the corresponding rms voltage. Now measure with the DMM in AC and DC settings. Table 3 Oscilloscope DMM Signal Vpp (Volts) Vrms (Volts) AC Set DC Set Sine wave 4V, 1 khz Sine wave 4V, 100 khz Sine wave 4V, 1 MHz 2017 2018 Page 8
Inference UNDERSTANDING & LEARNING 2017 2018 Page 9
RESULTS AND CONCLUSION POST LAB INFERENCE 1. What is the range of frequency for which Oscilloscope work satisfactorily? 2. Compare the measurements of Oscilloscope with that of the DMM. 3. What are the measurement range, resolution and accuracy of the DMM you used? 4. Over what frequency range will the DMM operate satisfactorily in AC mode? Prepared by: Name: _ Reg. No.: Experiment Date: Report Submission Date: Submission Delay:... Signature 2017 2018 Page 10