ENGR43Lab9

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ENGR 43 Lab Activity Student Guide LAB 9 - Intro to RC and L/R Filters Student Name: __Mathew Adug, Thi Pham ______ Overview One common application of RC and L/R filters is audio speaker crossover networks. This lab activity will demonstrate the performance of the crossover and provide additional practice with oscilloscope measurements. Before Starting This Activity This activity builds on the prior lab activities using the oscilloscope. You should have completed labs 3 and 4 before starting this activity. Learning Outcomes For Activity Relevant knowledge (K), skill (S), or attitude (A) student learning outcomes K1. Define peak-to-peak voltage, period, and frequency for AC waveforms K2. Describe the function of the vertical, horizontal, and triggering sections of an oscilloscope S1. Measure peak-to-peak voltages, period, and frequency of AC waveforms S2. Measure rise and fall times in RC and L/R exponential decay circuits. S3. Compile data into a test report. A1. Recognize the significance of the oscilloscope as a primary tool for the technician. Getting Started Lab Activity and Deliverables: It should take students approximately 2 hours to complete the lab activity, and 1 hour of homework time to complete the lab report. Equipment & Supplies Item Quantity 24-ohm speakers 2 Mini-clip and alligator clip leads As req. BNC test leads 1 1x/10x scope probes 3 2-way crossover network board 1 9.4 Ω test loads 2 DMM 1 Special Safety Requirements The sound produced by the audio speakers can be potentially damaging to your hearing if the speakers are very close to your ears. Do not place your ears close to the speakers unless you ensure that the audio levels are low. Lab Preparation Verify that your lab station has a function generator and an oscilloscope; and that all are plugged in and powered up. Lab 9 – Intro to RC and L/R filters © 2012 ENGR 43 1

ENGR 43 Lab Activity Student Guide Introduction Audio crossover networks use the energy- storage characteristics of capacitors and inductors to selectively block (attenuate) signals based on their frequency. When a capacitor is connected in series with a load, high-frequency signals can pass to the load because the capacitor can charge and discharge rapidly. When an inductor is connected in series with a load, the inductor opposes changes in current, and the faster the change in current, the greater the opposition. This allows low frequencies to pass while blocking the high frequencies. The transition point between passing and blocking frequencies is known as the crossover frequency. The values of resistance, capacitance, and inductance determine the crossover frequency. Task #1 – Listen to the effect of the crossover network Follow the steps below to attempt to determine the crossover frequencies by listening to the speaker outputs. 1. Place two speakers face down on the test bench with the metal connection lugs facing you. Find the “+” and “–” markings near the lugs. Connect the speakers and crossover network to the function generator as shown in figure 1. Figure 1 2. Set the function generator for a sine wave output and set the amplitude for a moderate output from the speakers. Use the rotary control to sweep the frequency from 100 Hz to 20 kHz. Listen to the output from the speakers. You should be able to hear the low frequency sine waves from the “woofer,” and as you increase the frequency, the output will shift to the “tweeter.” 3. Adjust the frequency until the sound is evenly split between the two speakers. It will be difficult to determine “by ear” exactly where the “crossover point” will be. Make your best determination. Crossover frequency = _1kHz 4. Switch the function generator to a 1 kHz square wave. You should hear a low tone (the “fundamental”) from the woofer, and a higher tone (the “harmonics’) from the tweeter. 5. Experiment with other wave forms (triangle, ramp). Have your lab partner select a wave form and try to identify it by listening to the sound it produces. Trade places and let your lab partner try. Lab 9 – Intro to RC and L/R filters © 2012 ENGR 43 2

ENGR 43 Lab Activity Student Guide Task #2 – Measure the frequency response 1. Remove the connections from the speakers and substitute the two 9.4 Ω test load resistors in place of the speakers, as shown in figure 2. Figure 2 2. Connect the oscilloscope as follows: a. Channel 1 probe to the function generator output (probe to red, ground to black). b. Connect channel 2 probe to the woofer output (“W” on the crossover board), and the probe ground to the circuit ground. c. Connect channel 3 probe to the tweeter output (“T” on the crossover board), and the probe ground to the circuit ground. d. Remember to set the attenuation of the probe and oscilloscope to 1X attenuation. Using the Autoset button helps with the setup. 3. Set the function generator: Amplitude (VhiZ setting): 1 Vp-p, Waveform: sinewave, Frequency: 100 Hz. Measure the amplitude of the function generator output on channel 1 of the oscilloscope. Adjust the amplitude of the function generator (to compensate for loading effect) until the oscilloscope measures 1Vp-p. What output voltage is shown on the function generator display? __5.6V_____ 4. Measure the peak-to-peak voltage at the woofer output (channel 2). It should closely match the input voltage. Enter this value in Table 1 of the data summary spreadsheet. 5. Measure the peak-to-peak voltage at the tweeter output (channel 3). Enter this value in Table 1 of the data summary spreadsheet. 6. Repeat the measurements for the frequencies listed in table 1. As you adjust the frequency, check the voltage on oscilloscope channel 1 and re-adjust the function generator output for 1 Vp-p as measured on the oscilloscope, if necessary. 7. Examine the Bode plot (another name for a frequency response graph). Determine the crossover frequency, sometimes noted as f c . f c = __7_ kHz How does this compare with the crossover frequency in step 3 in Task #1? This is 7 times higher than the crossover frequency in step 3. Lab 9 – Intro to RC and L/R filters © 2012 ENGR 43 3

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ENGR 43 Lab Activity Student Guide Part 3: Measure the step response charge / discharge times 1. Look at the channel 2 scope probe connection to the woofer output. Set the function generator for a 1 kHz square wave output. With the channel 2 volts/div set to 100 mV, adjust the function generator amplitude for a channel 2 height of 5 divisions, as shown in figure 3. Measure the time for the waveform to rise to 63% (one time constant) of the full height (approximately 3.1 divisions). Figure 3 Enter your measured rise time in Table 2 of the spreadsheet. 2. Look at the channel 3 scope probe connection to the tweeter output. Adjust the function generator amplitude for a channel 3 height of 5 divisions, as shown in figure 4. Note that the negative-going half of the waveform will extend beyond the bottom of the display. Measure the time for the waveform to fall by 63% (one time constant) of the full height (approximately 3.1 divisions). Figure 4 3. Enter your measured fall time in Table 2 of the spreadsheet. 4. Enter the resistance of the load (9.4 ohms) in Table 2. The spreadsheet will calculate the values of the capacitor and inductor for the crossover. 5. Disconnect the crossover board from your test circuit. Use the capacitance measurement function on the DMM to measure the capacitance of the tweeter capacitor. Enter this value on your spreadsheet. Deliverable(s) Save your completed Lab 5 Activity Guide and Performance Report (which starts on the next page of this document). Print the spreadsheet and attach with your performance report. Lab 9 – Intro to RC and L/R filters © 2012 ENGR 43 4

ENGR 43 Lab Activity Student Guide Lab 5 – Intro to RC and L/R Filters Student Name: _____Mathew Adug, Thi Pham ____________ Note: Print and turn in the performance report pages along with the lab activity procedure pages. Scope Measurements Why is listening to the speaker outputs an inaccurate method for determining the crossover frequency? There is uncertainty in human’s perception of sound that may lead to the inaccuracy. Why was it helpful to replace the speakers with load resistors for the measurements? By using the resistors, it is easier to get the measurements, while using the speakers, it is harder to estimate using our ears. The capacitor and inductor values on the crossover board were selected for use with 8Ω speakers (close to our 9.4Ω load resistors). However, our speakers are rated at 24Ω impedance. How does this effect the crossover frequency in Task #1, when you were trying to determine the crossover frequency by listening to the speakers? Hint: crossover frequency is where R=1/(2πfC) and R=2πfL. Z = X_C+X_L = 1/(2πfC) + 2πfL As we increase the resistance while replacing the speakers with the resistors with higher value, the frequency response in capacitor would decrease, while that of the inductor increase. Both would compensate each other. Therefore, it would not affect the crossover frequency. Lab 9 – Intro to RC and L/R filters © 2012 ENGR 43 5

ENGR 43 Lab Activity Student Guide What difficulties did you have in measuring the decay time of the tweeter output in step 2 of Task #3? Can you describe this in terms of how the tweeter filter reacts to the harmonics of the input signal? Why doesn’t the woofer output present the same problem? In measuring the decay time of the tweeter output in step 2 of Task#3, the tweeter filter make the signal unstable, so we needed to stabilize the signal to measure, while the woofer is more stable due to having the inductor to stabilize the signal. Suppose you wanted the crossover frequency to be two times your measured frequency. If the speaker load resistance is unchanged, what values of capacitor and inductor would you use? (The rise and fall times would be one half of your measured values). fall time = R * C or C = (fall time) / R rise time = L / R or L = (rise time) * R If the load resistance is unchanged, to fall time/rise time would be one half, C2 = 1/2C, L2 = 1/2L Lab 9 – Intro to RC and L/R filters © 2012 ENGR 43 6

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