Exercise 24A Additional Waveform Measurements

Transcription

1 NAME Exercise 24A Additional Waveform Measurements Purpose The purpose of this exercise is to demonstrate frequency and period measurements of common electronic signals. Objectives After completing this exercise, you should be able to: 1. To measure frequency and period using a frequency counter. 2. To measure frequency and period using an oscilloscope. 3. To use a function generator to create analog and digital waveforms of various frequencies. 4. To distinguish the differences between analog and digital waveforms. Lab Preparation Review Sections 14.3 and 15.2 of Fundamentals of Electronics: DC/AC Circuits. Materials Discussion 1 trainer with hookup wires as needed 1 Function generator w/leads 1 Frequency counter with RF cable 1 Oscilloscope w/probe (1) 1 1-kΩ resistor Electronic signals can be divided into two broad categories--analog and digital. An analog signal changes smoothly from one value to the next with respect to time (a continuous change in amplitude occurs). Figure 24A.1 (a) shows a variety of signals found in electronic circuits. All are considered analog in nature except for the square wave. A digital signal (Figure 24A.1 (b)) abruptly changes from one level to the next with no intermediate values. Thus, amplitude changes occur in steps. Another characterization for electronic signals is something you have already been studying, that is, electronic signals can be classified as AC (alternating current) or DC (direct current) signals. You have already learned that DC is current flowing in one direction only, while AC is bidirectional current flow. If we assume that the dashed lines in Figure 24A.1 (a) are the zero reference points, then all signals represented in the figure are AC. The waveform in 24A.1 (b) is a DC signal because that waveform does not cross zero.

2 In this lab you will investigate several pieces of test equipment (oscilloscope, function generator, and frequency counter) that are regularly used by technicians to repair and test electronic circuits. You will use these pieces of equipment to measure both the frequency and period of several types of electronic signals. (a) (b) Figure 24A.1

3 Procedure Complete each of the following steps in the order given. A. Waveform Analysis: Figure 24A.2 1. Construct the circuit shown in Figure 24A.2 using the function generator as the source. Use the breadboard of the analog trainer. 2. Most function generators have at least three waveform choices--sine, square, and triangle. 3. Set the function generator to: a. sine wave function b. frequency of 1 khz c. output amplitude midway d. DC offset set to 0V (some generators may not have one) 4. There are three basic sets of controls for an oscilloscope. a. vertical controls for each channel b. horizontal time base settings c. triggering controls 5. Set the oscilloscope to: a. Vertical: 1V/Div., DC coupled-channel 1 b. Vertical Mode: CH 1 c. Sweep Time: 0.5mS/Div. (AS A STARTING POINT) d. Triggering Mode: AUTO e. Trigger Slope: Negative (-) f. Trigger Source: CH 1 6. Position the traces on the scope as follows: a. Channel 1 - center horizontal line

4 7. Connect the ground lead of the CH 1 scope probe to the bottom of R Connect CH 1 probe (X1 position) of the scope to the top of R If a sine wave is not visible on the scope ask your instructor for assistance. If you have a sine wave, adjust the amplitude control for a 2 Vpp signal. 10. Is a sine wave analog or digital? 11. Switch the output of the function generator to produce a square wave. 12. Is a square wave analog or digital? 13. Switch the output of the function generator to produce a triangle wave. 14. Is a triangle wave analog or digital? 15. Describe the characteristics of the wave that made you choose your answers for steps 10, 12, and 14. B. Frequency and Period Analysis: 16. Switch the output of the function generator to produce a square wave again, use this output for steps (Frequency still at 1 khz.) 17. To measure the frequency of the waveform on the scope count the number of centimeters (divisions) that the full wave covers. A full waveform (one cycle) includes both one high and one low value. The number of divisions is: div 18. Multiply the number of divisions by the setting of the horizontal time base control. TIME = # of div X time per div TIME = X = 19. Calculate the frequency of the waveform by dividing the time into 1 (f = 1/T). Frequency = Hz 20. Connect the frequency counter across R 1.

5 21. Set the counter to the period mode. Record the period of the waveform. sec 22. Set the counter to the frequency mode. Record the frequency of the waveform. Hz 23. Are the values of steps 18 compared to 21 and 19 compared to 22 close to each other? YES/NO 24. Set the function generator to the frequencies listed in the table and then repeat steps 17 through 22 and complete the table. Counter (Period) Counter (Frequency) Scope (Period) Scope Calculation (Frequency) 250 Hz 10 khz 25 khz 100 khz Questions 1. Using complete sentences, describe the difference between an analog and a digital signal.