Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS EECS 100/43 Lab 2 – Function Generator and Oscilloscope 1. Objective In this lab you learn how to use the oscilloscope and function generator 2. Equipment a. Breadboard b. Wire cutters c. Wires d. Oscilloscope e. Function Generator f. 1k resistor x 2 h. Various connectors (banana plugs-to-alligator clips) for connecting breadboard to power supply and for multimeter connections. 3. Theory a. The HP33120A Function Generator The front panel of your function generator is shown in Figure 1. This instrument outputs a time-varying periodic voltage signal (the OUTPUT connector, do not use the sync connector, refer to figure 2). By pushing the appropriate buttons on the front panel, the user can specify various characteristics of the signal. Figure 1. Front panel of your function generator (Ref: Agilent Function Generator User’s Guide #33120-90006)Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS Figure 2. Make sure you use BLACK BNC input cables. Connect them to the OUTPUT terminal as shown above. Do not use the SYNC connector The main characteristics that you will be concerned with in this class are: • Shape: sine, square, or triangle waves. • Frequency: inverse of the period of the signal; units are cycles per second (Hz) • Vpp: peak to peak Voltage value of the signal • DC Offset: constant voltage added to the signal to increase or decrease its mean or average level. In a schematic, this would be a DC voltage source in series with the oscillating voltage source. Figure 3 below illustrates a couple of the parameters above.Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS Figure 3. Sine wave Vpp and DC offset When the function generator is turned on, it outputs a sine wave at 1 kHz with amplitude of 100 mVPP (figure 4). Figure 4. Function generator has been turned on You must specify the characteristics of the signal you need. For example, to set the frequency of the signal: 1. Enable the frequency modify mode by pressing the Freq button.Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS 2. Enter the value of the desired frequency by pressing the Enter Number button and entering the appropriate numbers on pads labeled with green numbers, or by using the wheel and the left and right arrows to move the tens place. (To cancel the number mode, press Shift and Cancel.) 3. Set the units to the desired value by using the arrow keys (up or down) on the right side of the front panel. IMPORTANT NOTE: There is an internal resistor 50 ohms in series with the oscillating voltage source inside the function generator, refer to figure 5. Figure 5. The internal load resistor in your function generator Thus, if you connect the function generator to an external resistor RL, it will form a voltage divider with the 50 ohms resistor, refer to figure 6.Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS Figure 6. External resistor forming a voltage divider Hence the voltage seen at the output of the instrument is: The purpose of the internal resistance is to have impedance matching (especially important for high frequency circuits). In RF electronics, resistances of 50 ohms are very common. Therefore if RL = 50 ohm, we have: The front panel meter assumes RL = 50 ohms. As we saw above, a 50 ohm load leads to a voltage divider with a gain of ½, so the instrument compensates for this by raising vint to twice what the display shows. In other words, if you set the instrument to produce a 5 V sine wave, it actually produces a 10 V sine wave on vint and relies on the external voltage divider to reduce the signal by a factor of two. We are not going to change the default setting of this instrument, so just remember that you are getting twice the voltage displayed on the function generator at the output terminal. That’s all for the function generator. Lets get to the crux of this lab – the oscilloscope.Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS b. Oscilloscope Note: This section is mostly a paraphrase of [1]. It might also be useful to go through the Prelab as you read this section. Nature moves in the form of a sine wave, be it an ocean wave, earthquake, sonic boom, explosion, sound through air or the natural frequency of a body in motion. Even light – part particle, part wave – has a fundamental frequency which can be observed as color. Sensors can convert these forces into electrical signals that you can observe and study with an oscilloscope. You will learn an example of a sensor – the Strain Gauge – in a later lab. For now, we will learn how to use an oscilloscope1. Oscilloscopes enable scientists, engineers, technicians, educators and others to “see” events that change over time. They are indispensable tools for anyone designing, manufacturing or repairing electronic equipment. Oscilloscopes are used by everyone from physicists to television repair technicians. An automotive engineer uses an oscilloscope to measure engine vibrations. A medical researcher uses an oscilloscope to measure brain waves. The possibilities are endless. i. Basic concepts behind an oscilloscope What is an oscilloscope? An oscilloscope is basically a graph-displaying device – it draws the graph of an electrical signal. In most applications, the graph shows how signals change over time: the vertical (Y) axis represents voltage and the horizontal (X) axis represents time. The intensity or brightness of the signal is sometimes called the Z-axis (refer to figure 7). Figure 7. X, Y and Z components of a waveform 1 An oscilloscope takes sometime to get used to. Just remember a simple rule: oscilloscopes do not generate waveforms (except for a simple test signal), they measure waveforms.Summer 2007 Lab 2 EE100/EE43 University of California, Berkeley Department of EECS This simple graph can tell you many things about a signal such as: • The time and voltage values of a signal • The frequency of an oscillating signal • Whether or not a malfunctioning component is distorting the signal • How much of a signal is direct current (DC) or alternating current (AC) What kind of signals can you measure with an oscilloscope?
View Full Document
Unlocking...