Download Frequency Selective Circuits

Salt Lake Community College
Electrical Engineering Department
EE2210
Frequency Selective Circuits Lab
Revision 3/20/2006
Latest Revision 8/3/2010 by Lee Brinton and James Quebbeman
Abstract
Depending on configuration, basic circuit components can be used to
allow or reject voltage signals depending on frequency.
Objectives
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Understand the design of RC high and low pass filters
Determine a cutoff frequency
Test a filter using two input signals
Equipment
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2 Function Generators
Oscilloscope
Matlab
Proto-board
10nF capacitor (nominal)
10kΩ Potentiometer
Discussion
As we have learned, circuits containing resistance, capacitance and/or
inductance respond differently to signals with different frequencies present. In
this lab we will mathematically describe this frequency dependence with the use
of the circuit transfer function and demonstrate the selective nature of circuits
using a simple low pass and high pass circuit.
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Pre-Lab
1. Transfer functions and cutoff frequency
a. Determine the magnitude of the transfer function for Figure 1 and
Figure 2. Identify which figure is high-pass and which is low-pass.
b. Determine the cutoff frequency of each circuit in terms of R and C.
c. Using a 10nF capacitor, choose an appropriate value of resistance to
provide a cutoff frequency of 2 kHz.
d. Use Matlab to plot the function:
𝑉𝑖 = sin 2,000𝜋𝑡 + sin 20,000𝜋𝑡.
Print the figure and include it in your lab book.
Experiment
1. Reality interrupts:
Because the capacitors available are never exact (they may be as much
as ±30% tolerance) the values calculated for R in the pre-lab will not put the
cutoff frequency at 2 kHz anymore.
a. Measure the capacitance of the capacitor you are using and record it.
b. Recalculate the value you need for R and record it.
2. Transfer Functions
a. Connect the circuit as shown in Figure 1 using the value of R
calculated in experiment part 1. A potentiometer can be used to
represent R.
b. Connect channel 1 of the oscilloscope to display the input voltage.
Adjust the amplitude of the signal generator and the volts/division
setting on the scope to display an 8VPP signal across the full vertical
range of the scope.
c. Connect channel 2 to display VO.
d. Measure the amplitude of the output voltage for input frequency values
of 200 Hz, 500 Hz, 1 kHz, 2 kHz, 5 kHz, 10 kHz and 20 kHz. Calculate
the ratio of the amplitude of the input voltage to the amplitude of the
output voltage for each measurement. Plot these ratios versus
frequency.
e. Redo steps 2.a-d using Figure 2.
f. Characterize each circuit as low pass or high pass.
3. Frequency Selection
We will now demonstrate how these circuits can be used to filter separate
frequency components of a single signal.
a. Adjust one signal generator to produce a frequency of 1 kHz. Adjust
the amplitude to 8VPP. Adjust the vertical volts/division setting on the
oscilloscope so the signal sweeps full range vertically. This generator
will be Vf1 in Figure 3.
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b. Use a plug adaptor to make the
other signal generator a floating
source. Adjust it to produce a signal
with a 10 kHz frequency at 8VPP. This
generator will be Vf2 in Figure 3.
c. Connect both signal generators to the
circuit in figure 1 as shown in Figure 3
using the value of resistance
calculated in Pre-Lab part 1.c.
d. Connect the oscilloscope channel 1
probe between Vf1 and Vf2
(Remember: the black leads of these
scopes should always be connected to
ground). Connect channel 2 to Node
A (this measures VA, which can get as
large as 16VPP, so you will need to change the Volts/div for channel 2
accordingly). Connect channel 3 to show VO. Capture the image from
the scope (if you don’t know how, the instructor can show you) and
attach the printout in your notebook; then compare it with your Matlab
generated plot.
e. Describe the differences in the combined input signal at node A and
the output voltage, VO.
f. Switch the capacitor with the resistor so that channel 3 now measures
across the resistor. Repeat steps 3.c-3.e with this new configuration.
Conclude
Before you put everything away, call your lab instructor over to check you
off. Write a conclusion in your notebook. Discuss the agreement of
measurements and calculations. If you are concerned about disagreements,
make some % error calculations. Usually your errors are smaller than they at first
appear. Also remember that no measuring instrument is perfect and neither are
parts.
Mention any problems that you encountered in this lab and how you overcame
them.
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