Download Preliminary Work

ECE 313
DOCUMENTING, CONSTRUCTING, 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:
1.
2.
3.
4.
5.
Drawing a schematic in an acceptable design format
DC analysis by hand calculation
Using a SPICE simulation to predict circuit performance
Constructing a breadboard circuit
Characterizing and debugging a circuit
Preliminary Work
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 the 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 simplified calculation for determining the upper and lower frequency corners is
to find the Thevenin resistance seen by each capacitor and use the equation
1
1
. The high pass corner would be f L  10 3 
and
f 
2 R C
2  R1  50  R 2C HP
1
the low pass corner would be f H  10 5 
.
2  R1  50 // R 2C LP
e. The resulting design has 2 equations and 4 unknowns so there are an infinite number
of solutions that we work. This is typical for a design problem. You typically want
R2 to large and R1 to be small so that the midband voltage is not too small. This is
accomplished by making CLP < CHP. Since you have more resistors than capacitors in
your lab kit you will just be picking values for CLP and CHP and calculating the values
for R1 and R2.
f. Use values for R1, R2, CLP, and CHP that are available in your lab kit.
2. Evaluate your design.
a. Calculate the upper and lower frequency corners for your design. 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 diffences 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. The transient is similar to what you should see on an
oscilloscope. There are two types of sources that we will use in this class (1)
sinusoidal wave, and (2) square wave.
i. Sinusoidal Wave Simulation
1. Replace your AC voltage source (VAC) with a sinusoidal source that
allows transient analysis called VSIN.
2. Set the amplitude (VAMPL) to 1 and the frequency (FREQ) to your
midband region. You also need to set VOFF=0, TD=0, and DF=0.
3. Place markers on both your voltage source and your output.
4. Perform a Transient Analysis.
a. Set your Final Time to be able to see around 5 oscillations.
b. Set your step to be small enough to see a smooth sinusoidal
wave. (The Print Step is what is displayed on your plot and
Step Ceiling is the maximum step size for the calculation so set
Step Ceiling equal to Print Step.)
5. Change your frequency to now be the lower corner (FREQ=1k).
6. Change your frequency to now be the lower corner (FREQ=100k).
ii. Square Wave Simulation
1. With a sinusoidal voltage source the low pass filtering and the high
pass filter are similar and are just reductions in the amplitude of the
wave. With the square wave simulation we are going to look at the
low pass and high pass filtering separate.
2. Perform a low pass filter simulation.
a. Remove the capacitor in series with the source resistance.
b. Replace the VSIN with a VPULSE part.
c. Set V1=0, V2=1, TR=1n, TD=1n, PER=your desired period,
PW=0.5*PER. These settings will produce a waveform that
goes from zero to 1V with a period of PER. This wave will
have an average voltage 0.5V.
d. Perform a transient analysis.
e. Adjust the period of the VPULSE until you can explain what
the low pass filtering does to the signal. Make sure to include a
frequency much greater than the upper corner frequency and
one much lower than the upper corner frequency.
3. Perform a high pass filter simulation.
a. Put the capacitor in series with the source resistance back into
your circuit and remove the capacitor in parallel with your load
resistor.
b. Perform a transient analysis.
c. Adjust the period of the VPULSE until you can explain what
the low pass filtering does to the signal. Make sure to include a
frequency much greater than the upper corner frequency and
one much lower than the upper corner frequency.
2. Build your bandpass circuit on your breadboard.
a. Connect your circuit to the function generator.
b. Connect your oscilloscope on both the function generator and the load resistor (R2).
3. Measure the AC operation of your circuit.
a. Set the function generator to be sinusoidal with amplitude of 1V. (This would be a
peak to peak voltage of 2Vpp.)
b. Measure the midband gain and compare it to the simulated value.
c. 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.
d. Adjust the frequency of the function generator until you find the upper frequency
corner of your circuit.
e. Compare your measured frequency corners to the simulated values.
f. Take enough measurement points to create a Bode plot. Compare your measured
frequency plot to the simulated one.
4. Measure the transient operation of your circuit.
a. Change the function generator to a square wave with a DC offset so that the low
voltage value is 0V.
b. Measure the response of your circuit and compare it to your simulated results.