Download Preliminary Work

ECE 313
DESIGN, SIMULATION, CONSTRUCTION, AND DEBUGGING A
CIRCUIT
Objective: This experiment is intended to teach several principles that will be used in most of
the laboratory experiments that follow. These principles include the following.
1.
2.
3.
4.
5.
Drawing a schematic in an acceptable design format
DC analysis by hand calculations
Using SPICE simulation to predict circuit performance
Constructing a breadboard circuit
Characterizing and debugging a circuit
Preliminary Work
Please complete the preliminary work before coming to lab. This will greatly reduce the amount
of time spent in the lab and will allow you to get more meaningful help from the TAs.
1. Design a bandpass filter with a pass-band with a lower frequency cut-off of fL=1 kHz and an
upper frequency cut-off around fH=100 kHz. The design should have two capacitors. The
j
impedance of a capacitor is Z cap 
. With low frequency the impedance gets large and
C
behaves like an open circuit. With high frequency the impedance gets small and it behaves
like a short circuit.
a. For the low pass part of the filter the capacitor needs to drop the output voltage to
zero when the frequency is high. At high frequency the capacitor behaves as a short
so it should be in parallel with the output resistance.
b. For the high pass part of the filter the capacitor needs to drop the output voltage to
zero when the frequency is low. At low frequency the capacitor behaves as an open
so it should be in series the source resistance.
c. Figure 1 shows the corresponding circuit for a simple passive bandpass filter. Figure
2 shows the corresponding schematic in which the input and output voltages are
nodes and all of the elements are referenced to ground.
Figure 1
Figure 2
d. The exact solution would be fairly complicated so we want to make some
approximations to make the problem easier to design.
i. At the high frequency corner (f=100kHz) we want the magnitude of impedance
1
of CHP to be much less than R1. This means that
 R1 when f==100kHz
 C HP
1
or C HP 
.
2 10 5 R1
ii. At the low frequency corner (f=1kHz) we want the magnitude of impedance of
1
CLP to be much larger than R2. This means that
 R1 when f==100kHz
 C LP
1
or C LP 
.
2 10 3 R 2
iii. The simplified calculation for determining the lower frequency corners is to
find the Thevenin resistance seen by the capacitor and use the equation
1
. With these two approximations made above we solve for the
f 
2 R C
lower corner frequency with a single capacitor CLP   . The resulting
Thevenin resistance seen by the capacitor is the two resistors in series or
1
R1+R2. The resulting equation is f L  10 3 
.
2  R1  R 2C HP
iv. With the two approximations made above we solve for the upper corner
frequency with a single capacitor CHP  0 . The resulting Thevenin
R1 R 2
resistance seen by the capacitor is the two resistors in parallel or
R1  R 2
1
R1+R2. The resulting equation is f H  10 5 
.
 R1 R2 
2 
 C HP
 R1  R 2 
e. The resulting design has 2 equations and 4 unknowns so there are an infinite number
of solutions that will work. This is typical for a design problem. You want R2 to
large and R1 to be small so that the midband voltage is not too small. Since you have
more resistors than capacitors in your lab kit you will just be picking values for C LP
and CHP and calculating the values for R1 and R2. A summary of the design
equations and constrains are the listed below. You can solve for the two equation
numerically using your calculator or solve for R1 from the first equation and plug it
into the second equation.
1
R1  R 2 
3
10  2  C HP 
R1 * R 2
1

5
R1  R 2 10  2  C LP 
1
R1 
2 10 5 C HP
1
R 2 
2 10 3 C LP
R 2  R1
f. Use values for R1, R2, CLP, and CHP that are available in your lab kit. You don’t need
to get the corner frequencies exact. Try to get 0.8kHz < fL < 1.2 kHz and 80kHz < fL
< 120 kHz. (This is a 20% accuracy.)
2. Evaluate your design.
a. Calculate the upper and lower frequency corners for your design using the resistors
and capacitors from your lab kit. They should be close to fL=1kHz and fH=100kHz
but they don’t need to be exact.
b. Calculate the midband gain of your design? (Since the design does not have any
amplification, the midband is actually an attenuated signal, but we are going to call it
midband gain anyways.). Provide the midband gain both in decibels and linear units.
c. Include a BODE plot for your design.
Laboratory Work
1. Simulation: PSPICE can be used to perform lots of different types of circuit simulations. In
this class we will primarily be performing (1) DC analysis, (2) AC Sweep, and (3)
Transients.
a. Perform an AC sweep of the bandpass filter that you designed in PSPICE.
i. You will need to use an AC voltage source. This part in PSPICE is called
VAC.
ii. What is the simulated midband gain? Fix any large discrepancies between
your preliminary analysis and the simulation. Discuss any remaining
discrepancies.
iii. Provide a frequency plot using PSPICE. This plot should be similar to your
Bode plot from the Preliminary section so the voltage needs to be in dB and
the frequency needs to be in log scale. Discuss the differences between your
calculations and the simulated response.
iv. What are the corner frequencies of your design? To find the corner
frequencies use the cursors. Place one cursor at your midband point and place
the other cursor where the amplitude is 3dB less, then read off the frequency
corner. (This approach is only valid if the two corner frequencies are
sufficient distance apart.)
b. Perform a transient analysis with a sine wave input. The sine wave transient input in
PSCICE is VSIN. Use a sine wave with a frequency of the lower corner (f=1kHz).
Compare the source and the output signal. To get a signal that looks smooth you
might need to set the ‘Step Ceiling’ on the ‘Transient’ pop up. This is the signal that
you will be reproducing using your oscilloscope later in the lab. You can change the
frequency and see the change in the signal attenuation. You only need to include the
one plot at f=1kHz in your lab report.
c. Perform a transient analysis with a square wave input. We want to learn what the low
pass and the high pass filtering does to the square wave.
i. Use a square wave input. The PSPICE part is VPULSE.
ii. First of all we want to just look at the effect of the low pass filtering.
1. Remove the high pass capacitor (the one in series with the source).
2. Adjust the period (PER) and the pulse width (PW=0.5*PER) until you
can see what the low pass filtering does to the square wave. Start with
a square wave with a frequency of 100 kHz (PW=5u and PER=10u).
Be sure to explain in your lab report what you learned.
iii. Now we want to just look at the effect of the high pass filtering.
1. Remove the low pass capacitor (the one in parallel with the load
resistance) and add back in the high pass capacitor (the one in series
with the source).
2. Start with a frequency of f=100kHz (PW=0.05u, PER=0.1u). Be sure
to simulate a long enough time until the square wave settles down to
have an average value of 0.
3. Now simulate the square wave with frequencies down to the lower
corner f=1kHz (PW=0.5m, PER=1m). Be sure to explain in your lab
report what you learned.
4. Now simulate the square wave with frequencies down to an even
lower frequency, f=100Hz (PW=5m, PER=10m). Be sure to explain
in your lab report what you learned.
2. Build your bandpass circuit on your breadboard.
3. Measure the operation of your circuit. The oscilloscope is the tool that you should be using
to debug your circuit.
a. Measure the midband gain and compare it to the simulated value.
i. Set the frequency of the function generator to the midband frequency
(f=10kHz).
ii. Measure the source and the load voltages simultaneously using your
oscilloscope.
iii. The midband gain in the ratio of the two amplitudes.
b. Measure the frequency response of your circuit.
i. Adjust the frequency of the function generator until you find the lower
frequency corner of your circuit. This is where the ‘gain’ is 3 dB lower than it
was at the midband frequency. Since your measurement is probably in linear
units remember that 3 dB is 10-3/20 = 0.707.
ii. Adjust the frequency of the function generator until you find the upper
frequency corner of your circuit.
iii. Compare your measured frequency corners to the simulated values.
iv. Take more measurements until you have enough to create a Bode plot
c. Measure the source current.
i. Measure the voltage across the resistor that you placed in series with the
source.
1. This is more difficult than it may seem because the oscilloscope is
probably tied to earth ground.
2. Place the two scope probes on the two sides of the resistor.
3. Use the subtract option built into the oscilloscope to find the voltage
across the resistor.
ii. Divide the voltage by the resistance to determine the current.
d. Measure the transient operation of your circuit.
i. Change the function generator to a square wave with a DC offset so that the
low voltage value is 0V.
ii. Measure the response of your circuit and compare it to your simulated results.
Be sure to include at least the following in your lab report.
 Your preliminary analysis
 Your schematics from PSPICE
 Your PSPICE simulations
 Your measurements
 Discussions of what you learned throughout the lab
 A summary section at the end of the report. This summary section should include what
you learned from the lab as a whole.