Orange Coast College Physics 280. Experiment #1. The Oscilloscope

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1 Orange Coast College Physics 280 Experiment #1 The Oscilloscope An oscilloscope is a versatile laboratory instrument that is used to measure voltage signals. In its most basic application it acts as a fast plotter of voltage on the vertical axis and time on the horizontal axis of the screen of the oscilloscope. An oscilloscope works by generating an electron beam that passes between pairs of vertical and horizontal deflection plates; by applying a voltage to these plates the electron beam is deflected from its initial path. The electron beam then strikes a fluorescent screen which is coated with phosphors that emit visible light at the point of electron bombardment. You will become familiar with the operation of an oscilloscope by displaying various voltage signals through the input connector (channel 1). For each of the voltage signal INPUTS, carefully sketch the trace (or profile) of the voltage signal on the corresponding graph-grid provided. Although oscilloscopes do vary in detail and particularly in the location of the controls, they are essentially alike in that all oscilloscopes embody four rather separate, distinct components: 1. Electron gun 2. Vertical deflection mechanism 3. Horizontal deflection mechanism 4. Sweep circuitry and associated synchronization. Controls of most common oscilloscopes: POWER SWITCH: to turn on the oscilloscope. INTENSITY: varies the voltage on the cathode ray tube and thus the intensity of the trace (should be kept low with good visibility) on the screen. FOCUS: varies the electric field in the electrostatic lens to get good focus of the trace. VERTICAL POSITION: moves the trace up or down the oscilloscope screen. HORIZONTAL POSITION: moves the trace to the right or to the left of the screen. VERTICAL INPUT (Channel 1): the terminal for connecting a voltage signal to the vertical deflecting plates through the vertical amplifier. VERTICAL GAIN CONTROL (Volts/Div): controls the height of the voltage signal displayed on the oscilloscope screen. It s like the zoon in or zoom out for the vertical axis. HORIZONTAL GAIN CONTROL (Sec/Div): controls the width of the voltage signal displayed on the screen. Again, it s like a zoom in or zoom out for the horizontal axis. TRIGGERING: controls the starting point (or initial phase) on the screen of the voltage signal. GROUND: terminals used to ground the chassis of the oscilloscope to the ground of other Page 1

2 PART 1. DC Battery 1. With the channel 1 input coupling mode button set on GROUND, center the trace on the screen and draw the trace in the graph-grid below. 2. Connect the DC battery to channel 1 of the oscilloscope by connecting the red lead to the positive terminal of the battery, and the black lead to the negative terminal of the battery. Then UNGROUND channel 1 by switching the channel 1 input coupling mode to DC, and you will observe that the trace will jump to a new position. Draw the trace on this new position on the same graph-grid as the GROUND signal was drawn. What is the measured value of the DC battery voltage? DC battery voltage: 3. Then, reverse the connection of the DC battery polarity to channel 1 (red lead to the negative terminal of the battery, and the black lead to the positive terminal). The trace on the screen of the oscilloscope should jump to a yet new position and draw the trace on the same graph-grid. Label the vertical axis and the horizontal axis of the graph-grid and record the values of the setting for the vertical axis in Volts/Division and for the horizontal axis in Time/Division. What is now the measured value of the DC battery voltage? DC battery voltage: Page 2

3 PART 2. Function Generator 1. Connect the output of the function generator to the input (channel 1) of the oscilloscope. 2. Set the frequency of the function generator to 1000 Hz (or very close to it) and press the corresponding button so the function generator produces a sinusoidal voltage signal. The theoretical value of the frequency is thus 1000 Hz. The DC offset knob of the function generator should be set to zero (pushed in), and the Ramp/Pulse setting knob should be in the calibrated position (fully counterclockwise). 3. Set the channel 1 input coupling mode of the oscilloscope to AC. Play with the Volt/Division knob, the Time/Division knob, and the trigger settings of the oscilloscope until you are able to display a nice trace on the oscilloscope screen. 4. Draw the corresponding trace on the graph-grid provided below. Label the vertical and horizontal axis of the graph-grid as before. Measure the Period of the displayed signal and compute the corresponding (experimentally measured) value of the Frequency of the voltage signal. Also measure the peak-to-peak voltage and the voltage amplitude of the signal. Period: Frequency: Peak-to-Peak Voltage: Voltage Amplitude: 5. How does your measured frequency value (experimental) compare with the frequency setting on the function generator (theoretical)? Calculate the % error. % error: Page 3

4 Explore AC / GND / DC Input Coupling Remove all connections from the oscilloscope, and set it for internal trigger on channel 1, in AUTO mode. Set the oscilloscope channel 1 input coupling mode on Ground, and adjust the trace to lie on the central horizontal grid line. Connect the output of the function generator to channel 1 of the oscilloscope, with the function generator in the sinusoidal wave setting and the DC offset knob pushed in (as before). Change the oscilloscope channel 1 input coupling mode selector to DC, and adjust the function generator amplitude to give a voltage signal trace of about 2 divisions on the screen. Now turn on the function generator s DC offset (by pulling the knob out), and adjust this knob so that the sine wave is displaced relative to the ground. (Don t adjust it so far that the sine wave distorts). Now switch the oscilloscope input coupling mode back to AC and describe the result on the trace on the screen. State in your own words the function of the AC/DC coupling mode on the oscilloscope. Also explain the function of the Ground coupling mode. Is the peak-to-peak amplitude of the observed trace the same for both the AC and DC input coupling modes? It should be, if it is doing what we want it to do. Measure the peak-topeak amplitudes for both the AC and DC input coupling modes (frequency = 1000 Hz): Now change the frequency setting on the function generator to about 3 Hz. Do you still measure the same peak-to-peak voltage signal amplitudes for both AC and DC input settings on the scope? Yes or no? (frequency = 3 Hz) The trouble is that at low frequencies the oscilloscope in AC coupling mode basically can t decide whether it is looking at an AC signal (which it should pass) or a very slowly varying DC signal (which it should block). What happens for a 10 Hz signal? Are you safe if the frequency of the voltage signal is above 50 Hz? Yes or no? What you are observing is an example of a high pass filter, which we will study later. Page 4

5 PART 3. Sound Signals A. Connect the microphone to the channel 1 input of the oscilloscope. Create a (pure-tone = single frequency) sound by tapping the tuning fork attached to the wooden box provided. The microphone converts the sound signal into a voltage signal that we can observe on the oscilloscope screen. Freeze the signal on the oscilloscope screen by pressing the RUN/STOP button on the oscilloscope. Carefully draw on the graph-grid the trace of the voltage signal. Label the axes and compute the frequency of the pure-tone sound. How does it compare to the frequency value written on the tuning fork itself? Period: Frequency (experiment): Frequency (theory): % error: Page 5

6 B. Use the microphone to display your voice or whistle signal on the screen of the oscilloscope. Carefully draw on the graph-grid the trace displayed on the oscilloscope. Label the axes and compute the frequency of your voice signal. Period: Frequency (experiment): Page 6