ENGR 223
Project I – Digital Noise Reduction
Due: 4/23/15
Introduction
In most electronic circuits, noise is a large concern. This is especially true for digital circuits. Noise
is considered a random signal that alters the desired signal. If noise levels are large enough they can
cause incorrect data to be processed or electronics to shut down.
In this project we will look at power supply noise. This is noise that appears on the DC power
supply that powers our digital electronics. Switching noise is the most common source of power
supply noise. This is noise that is generated when the digital signal switches from logic high to logic
low or vice versa.
While the actual circuit that we are testing is very complex, measurements have been taken to create
a simple model of the system. This model is shown in Figure 1. There are two major portions of
this model. One part of the model represents the digital system and how it switches. This is shown
on the right side of the figure in the box labeled “Model of our Digital Chip.” Q1 is a NPN transistor
that operates as a switch. When VF is high (5V), Q1 is on and acts as a short circuit, so R2 and R3
are in parallel. When VF is low (0V), Q1 is off and acts as an open circuit, only R2 draws current.
This models the changing current in the digital system as it switches. The Q1 is switched at a
frequency of 2KHz.
The other part of the system is the line that supplies power from the power source to the chip. This
is modeled as a RLC circuit. It is comprised of Lx, Rx, and C1. VS is the supply voltage and VCC
is the voltage supplied to our digital chip.
V1
5
VS
Lx
Rx
.01
100
Model of our Digital Chip. Q1 is just a switch
VCC
C1
R2
R3
3.3k
1.5k
C
10n
Model of power line to chip
Q1
B
2N3904
E
R4
4.7k
VF
V2
Function Generator
PULSE(0 5 .1m 1p 1p .25m .5m)
5m
Figure.tran
1: Schematic
for modeling power supply noise in a digital system
We know this system has a noise problem. Our goal is to identify the source of the noise, and then
try to reduce the noise. The ultimate goal is we would like a solid 5VDC value to appear at VCC.
This goal is not perfectly attainable. You will have to make decisions and compromises to get as
close as possible.
When the project is complete, each group will present their findings. Then we, as a class will
discuss and decide what solution we would use to fix the problem. This is not a competition
between groups. There is no bonus for having your solution picked. We will be mimicking the
actual design process that goes on in industry.
Farrell
1
03/27/15
ENGR 223
Project I – Digital Noise Reduction
Due: 4/23/15
Task 1: Milestones/Schedule
The first task is to create a schedule and plan. The schedule mileposts should show what is going to
be accomplished each week and who is responsible for what tasks. In doing this you will be able to
track whether you are ahead or behind on the project.
The results of task 1 need to be typed into a 1 or 2 page document and approved by your instructor.
Task 2: System Analysis
The first requirements are to analyze the system by hand. By hand I mean using Laplace transforms
and Matlab to determine what VCC(t) is when R3 is switched in and out of the circuit. We then need
to determine what causes this noise. We know that it is caused by the switching, but that is not
something we can eliminate. So what parts of our power supply line cause the noise when switching
occurs? Prove this using hand calculations and Matlab of course. At this point you may wish to
simulate using LTSpice. However I don't want any Ltspice simulation being done until you have a
Matlab plot to compare to.
You will be given either a capacitor (10nF) or inductor (10mH) and resistor (100Ω) to place in the
line to reduce the switching noise. Not every group will be given the same component to fix the
noise with. If you get the inductor and resistor those two components must be used as a single series
impedance, i.e. the resistor and inductor must be placed in series with each other like in Figure 1.
Investigate how to fix the circuit with your given element. Analyze your ideas mathematically in
Matlab. Once you have a good Matlab simulation and are happy with the results, you should move
to the simulation phase.
Now simulate both the original and your version in LTSpice. Verify that Matlab is correct both
before and after the fix.
Remember the goal is 5VDC signal applied to the chip.
Task 3: Construction
Construct on a breadboard the original circuit and your fixed circuit. Collect data that supports your
Matlab and LTSpice information. You may not start construction until you have Matlab and LTspice
information to verify with construction.
Task 4: Presentation
You must now present your findings to the class. Your presentation should be 10-15 minutes
including questions. Remember the goal is to find a signal that most closely matches the 5VDC
requirement. Your presentation should focus on the cause of the noise and your fix to the problem.
Remember other groups are going to propose a different fix, you want to make a case for why your
fix helps. Don't lose site of the fact that you have to present you data and explain what the data
shows or confirms. After all the presentations are done, we will make a decision as a class as to how
we want to fix our broken system.
Farrell
2
03/27/15
ENGR 223
Project I – Digital Noise Reduction
Due: 4/23/15
Tips for all parts
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•
•
•
•
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Analyze circuit in S-domain. Use R, L, and C as variables, not values. This should help you
see what component is making VCC move, and hopefully give you an idea of how to reduce
noise.
Matlab is capable of performing Laplace and Inverse Laplace transforms.
http://www.egr.msu.edu/~aviyente/LT_matlab.pdf
Q1 is a 2N3904 transistor. Look up the data sheet to see its pinout. The pins are labeled in
Figure 1. If Q1 is connected backwards you may blow it up.
The function generator does not factor into your analysis steps, neither does Q1. They are
only necessary to create the real world switching affect.
Before you start constructing circuits, you must have done a design by hand. You must have
expected results to verify once you begin testing your real circuit. Verify your calculations
with the instructor before beginning circuit construction.
Presentation during Lab Week 4.
Farrell
3
03/27/15