EE 3305 Oscillator Design
Objective
The objective of this experiment is to give the student experience in designing simple
oscillator circuits, to explore the relationship between amplifiers, active filters and
oscillators and to experience the effects of “real world” components on theoretical
designs.
Equipment Requirements
Oscilloscope
Operational Amplifier (Bench Stock)
1/4 Watt Resistors (Bench Stock)
Capacitors (Component Kit + Bench Stock + Special Order)
Inductors (Component Kit + Bench Stock + Special Order)
Discussion
Most analog circuit designers will tell you that the quickest way to build an oscillator is
to set out to build an amplifier and as sure as night follows day, it will oscillate. If your
intent is to deliberately create a circuit that oscillates, the trick is to decide up front what
the output signal will look like in terms of wave shape, frequency and stability. In order
to do that you need to understand what an oscillator is and why it does what it does.
One way to describe an oscillator is a circuit that will spontaneously generate an AC
output given only a DC input. This seems on the surface to be impossible but in reality
its an example of what happens in real life when you try to divide by zero. Take a look at
Figure 1. You see the classic block diagram for a feedback amplifier system. What were
looking for is a system that will produce a non-zero output, even when the input isn’t
there.
Input
Output
Ideal
Amplifier
Frequency
Selective
Network
Figure 1.
EE 3305
Michigan Tech University
Electrical and Computer Engineering Department
EE 3305 Oscillator Design
Figure 2 shows the same basic system with a little more mathematical rigor thrown in.
Vin + β Vout
Vin
Σ
Vout
A(f)
β Vout
Β(f)
Figure 2, System Block Diagram
The transfer function is given by: Vout = A(f) [Vin + β(f) Vout] which can be solved
for Vout :
Vout  Vin
A( f )
1  A( f ) B( f )
Equation 1.1
Notice that our design criteria is for Vout to be non-zero in spite of the fact that Vin = 0.
the only way that can happen is if the circuit satisfies the so-called Barkhausen criteria.
If A(f)B(f) =1, the denominator is forced to zero and drives the output to be large even
when Vin is very small. Vin is never truly zero. There will always be stray noise or
some small transient spike that will set the oscillations into motion. One way to think of
this functional diagram is to create an amplifier with gain (A(f)) that has a frequency
selective feedback path (B(f))
Design Problem
Design, simulate, construct, and test a circuit that displays the following characteristics:
System Output: 500 mVp, 200kHz ±5%, Total Harmonic Distortion 10%, sinusoidal
wave form.
System Input: ±10V DC, system may be operated single ended at the discretion of the
design team.
EE 3305
Michigan Tech University
Electrical and Computer Engineering Department
EE 3305 Oscillator Design
The circuit will be integrated into a system with a variable input impedance. The input
impedance can be set to HiZ mode with Zinput = at least 1M Ohm. The input impedance
can also be set to 50 Ohms. Your oscillator must be able to function with both load
impedances
Deliverables
The research and design phase of this project will begin immediately. Circuit design and
P-Spice simulation will be accomplished during the first week of the project. The
functional circuit must be ready for performance tests at the beginning of the second
week. The system design report will be one week after the performance testing
concludes. Thorough documentation is vital to the success of this project.
Your documentation must present a thorough mathematical description of the system and
its predicted behavior. You may use computer simulations to augment your design
efforts but they can not be used to replace a thorough theoretical description of the
circuit. Verify the functionality of your design by comparing the predicted values to the
circuit’s observed behavior and the system specifications. Explain any discrepancies.
Estimate the cost per thousand units to manufacture your device.
Resources
You should be able construct your design with the basic components that have been given
to you in your component kit and the bench stock in your lab. Your design may require
other resistance, capacitance, or inductance values. If so, request them through your
TA. Your TA will vet all component requests and will be the only point of contact to the
component supplies under Mr. Miller’s control. Most standard values for resistors and
capacitors are readily available. Inductors however, may require some resourcefulness
on the part of the designer.
Guidance
There are a wide variety of approaches to this design problem. You may choose to create
your oscillator using a discrete device or devices, an integrated circuit like the 741 Op
Amp, or you may choose to design a microprocessor based system using the Motorola
HC11 microcontroller. Each choice has consequences in terms of system complexity,
cost, and manufacturability. Be prepared to justify your decisions.
EE 3305
Michigan Tech University
Electrical and Computer Engineering Department