Introduction to the Oscilloscope
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Introduction and Objectives:
The oscilloscope is a useful instrument for studying ac circuits. It is often used to display graphs of voltage versus time.
It is a very fast X-Y plotter and can be used to plot an input signal versus time or another signal. It can also be used to determine the voltage and frequency of an input signal. It consists of a cathode-ray tube (CRT) where a beam of electrons emitted from a filament are accelerated and focused on a fluorescent screen. Deflection plates in the oscilloscope help to move the beam horizontally or vertically.
Equipment Required:
Oscilloscope, signal generator, connecting wires and connectors.
Lab Procedures:
Connect the oscilloscope to the signal generator. An example of an oscilloscope connection is in the picture below, however your oscilloscope may differ based on the make.
Power on the signal generator and oscilloscope. Adjust the signal generator to the desired output frequency. Adjust the oscilloscope controls till you see a suitable display (for example, see the picture below).
Set the signal generator to produce an output as desired using its controls:
The period of the wave displayed on the oscilloscope can be measured by counting the number of divisions required for one wave and then converting this to seconds using the “TIME/DIV” scale setting on the oscilloscope.
Perform your measurements as directed in the sections below. Enter your results in the tables.
For voltage measurements:
The peak voltage is half of the peak to peak voltage. Multiplying the peak voltage by 0.707 gives the rms (root-mean-square) voltage.
Data Tables:
Part 1
Time and Frequency Measurements:
Generator frequency: 90 Hz
No. of divisions / Time sweep/div / Period (s) / Frequency (Hz)[Reciprocal of Period] / Percent difference
(compare with 90 Hz as E1)
[|E2-E1|/(E2+E1)/2] x 100
Calculations:
Generator frequency: 300 Hz
No. of divisions / Time sweep/div / Period (s) / Frequency (Hz)[Reciprocal of Period] / Percent difference
(compare with (300 Hz as E1)
[|E2-E1|/(E2+E1)/2] x 100
Calculations:
Source -Line: 60 Hz
Set the source to “Line” on the oscilloscope and then adjust the signal generator frequency till you get a stationary pattern on the screen. Note the actual graduated marking on the signal generator to obtain this stationary pattern. Calculate the percent error of the marking you find to the accepted value of 60 Hz. usually, the calibrated markings on the signal generator may have inaccuracies and this experiment lets you determine this inaccuracy.
No. of divisions / Time sweep/div / Period (s) / Frequency (Hz) / Percent error(compare with (60 Hz)
[|E-A|/A] x 100
Calculations:
Part 2
Voltage Measurements:
To obtain another reading of the voltage, measure the output voltage of the signal generator with a multimeter. Make sure your multimeter is set to measure an AC signal by pressing on the blue button till a symbol is displayed.
Generator frequency: 90 Hz
Volts/div / No. of divisions (peak to peak) / Peak to peak voltage (V) / Peak voltage(Vo) / rms voltage
(Peak voltage) x (0.707) / Voltage by multimeter / Percent difference
(compare rms voltage with multimeter value)
[|E2-E1|/(E2+E1)/2] x 100
Calculations:
Part 3
Lissajous Figures:
Generator frequency: 60 Hz
When two ac voltages are added such that one is on the x-axis and the other on the y-axis, then patterns called “Lissajous figures” are produced. Depending on the phase and frequency differences between the two signals, different patterns can be seen. If the signals have the same frequency and phase, then a diagonal line is produced. When the signals have the same frequency, but a phase difference of 0°, then an ellipse is produced. If the signals have the same frequency, but a phase difference of 90°, then a circle is produced.
For this part you will need to connect two signal generators to an oscilloscope as shown in the picture:
Oscilloscope settings for producing Lissajous figures:
Use the drawing toolbar in Word to draw the figures you see in the table below:
(In Word 2007: Insert à Shapes à You can use the “Curve,” “line,” “freeform,” “oval” or any other convenient shape tool to draw your figures below).
X- input frequency / Y-input frequency / Sketch of pattern seenAnswers to questions:
1. What is a cathode-ray tube (CRT)?
2. What is the function of deflection plates in an oscilloscope?
3. How can a graph of voltage versus time be obtained on an oscilloscope?
4. What is the difference between peak voltage and rms voltage?
5. What are Lissajous figures? How are they produced on an oscilloscope?
6. How does the number of wave cycles seen on a screen for a fixed input frequency vary with the SWEEP TIME/DIV control setting? Why is this?
7. Explain why a 60 Hz Lissajous figure can be varied from a straight line to a circle.
Conclusions:
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