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Lab Activity
Student Guide
LAB 5 - Intro to RC and L/R Filters
Student Name: ___________________________________________________
Getting Started
Overview
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.
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.
Equipment & Supplies
Item
24-ohm speakers
Mini-clip and alligator clip
leads
BNC test leads
1x/10x scope probes
2-way crossover network
board
9.4 Ω test loads
24-ohm speakers
DMM
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
Quantity
2
As req.
1
2
1
2
2
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.
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
Lab Preparation
S2. Measure rise and fall times in RC and
L/R exponential decay circuits.
Verify that your lab station has a function
generator and an oscilloscope; and that all
are plugged in and powered up.
S3. Compile data into a test report.
A1. Recognize the significance of the
oscilloscope as a primary tool for the
technician.
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Lab Activity
Student Guide
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.”
Introduction
Audio crossover networks use the energystorage 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.
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 = __________Hz
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.
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
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Lab Activity
Student Guide
Task #2 – Measure the frequency
response
on o-scope channel 1 and re-adjust the
function generator output for 1 Vp-p as
measured on the o-scope, if necessary.
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.
6. Move the channel 2 scope probe to the
tweeter output (“T” on the crossover
board). Repeat all the frequency
measurements and enter the data in table
1.
7. Examine the Bode plot (another name
for a frequency response graph).
Determine the crossover frequency,
sometimes noted as fc.
fc = ___________ Hz
How does this compare with the
crossover frequency in step 3 in Task
#1?
________________________________
Figure 2
2. Connect the o-scope channel 1 probe to
the function generator output (probe to
red, ground to black). Connect channel 2
probe to the woofer output (“W” on the
crossover board), and the probe ground
to the circuit ground
________________________________
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 o-scope.
Adjust the amplitude of the function
generator (to compensate for loading
effect) until the o-scope measures 1Vpp. What output voltage is shown on the
function generator display?
_____________________________
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. Repeat the measurements for the
frequencies listed in table 1. As you
adjust the frequency, check the voltage
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Lab Activity
Student Guide
Part 3: Measure the step response
charge / discharge times
1. Move the channel 2 scope probe back 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 4
Enter your measured fall time in Table 2
of the spreadsheet.
3. 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.
4. 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.
Figure 3
Enter your measured rise time in Table 2
of the spreadsheet.
2. Move the channel 2 scope probe to the
tweeter output. Adjust the function
generator amplitude for a channel 2
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).
Lab 5 – Intro to RC and L/R filters
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Deliverable(s)
Save your completed Lab 5 Activity Guide
and Performance Report (which starts on the
next page of this document) in your Lab
Activity Binder. Print the spreadsheet and
attach with your performance report.
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Lab Activity
Student Guide
Lab 5 – Intro to RC and L/R Filters
Student Name: ___________________________________________________
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?
Why was it helpful to replace the speakers with load resistors for the measurements?
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).
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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?
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).
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