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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