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MIT 16 01 - Study References

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Massachusetts Institute of Technology Department of Aeronautics and Astronautics Cambridge, MA 02139 16.03/16.04 Unified Engineering III, IV Spring 2004 Systems Problem 11 Name: Due Date: 4/29/04 Time Spent (min) SP11 Announcements:Unified Engineering Systems Problem 11 21st April 2004 Objectives After completing this lab, you will 1. Gain an appreciation for the amplitude modulation process, which we will study later in the term. 2. Be able to assemble small circuits out of standard components. 3. Have some experience using an oscilloscope. Due Date All students will complete the lab by Wednesday, April 28. Each student will complete the lab exercise individually or in a group of two. If you work in a group of two, each student should do half the assembly. Whether you work alone or in a group, each student should report individually on testing the circuit. You will assemble and test the circuit in a two-hour block of time. Our best estimate is that some students will complete the assembly in as little as an hour; some may take up to three hours. In no event should you take more than three hours. If, after three hours, your circuit is not complete, you may cease work on assembling the circuit, and proceed to take data, using one of the circuits we have already assembled. You should turn in the data plots you take when testing the circuit, as well as your circuit. Please out your initials on the circuit board with a Sharpie, and staple the circuit (in a baggie) to your lab before handing in. If you work in a team of two, staple the two labs together, along with the circuit. Make sure that you put your name on each page that you turn in. Please turn in only the cover page, and the observed waveforms. You will be graded on how well you have assembled the circuit, and on your observations of the circuit in operation. 1Components Each kit contains the following components: • 1 Datak 12-617B protoboard 1 LMC555 CMOS timer • • 1 8-pin IC socket 8 • 11 resistors: 470 Ω (1); 3.3 kΩ (1); 1 kΩ(3); 4.7 kΩ (4); and 10 kΩ (2) • 4 capacitors: 100 pF (2); 0.1 µF (1); and 1 µF (1) • 1 9V battery connector 2 PN2222A NPN transistors • 1 1N914BCT diode • • 1 3-pin pin strip connector • 1 2-pin pin strip connector The components are shown in Figure 1. The resistors are color-coded. To learn about resistor color coding, see, for example, http://www.elexp.com/t resist.htm. The color codes for the resistors in the kit are given in the table below: Resistor Value Color Code 470 Ω yellow-violet-brown 4.7 kΩ yellow-violet-red 3.3 kΩ orange-orange-red 1 kΩ brown-black-red 10 kΩ brown-black-orange The capacitor markings are typically a three-digit number. For example, the 100 pF capacitor is marked “101,” meaning 10 × 101 pF. The 1 µF capacitor is marked “105,” and the 0.1 µF capacitor is marked “104.” In addition to the above discrete components, you will need solder and some wire, which will be available in the lab. In the lab, you will check out tools (a soldering iron, wire cutters, and needle nose pliers). 2Figure 1: Circuit Components 3R3 1 kΩR4 1 kΩR74.7 kΩR54.7 kΩR64.7 kΩC31 µFR13.3 kΩR2470 ΩR1010 kΩC4100 pFQ1PN2222AQ2PN2222AR114.7 kΩ+Vs+Vse1e2e3e4e5e6P1P2C20.1 µFLMC55512345678+VsC1100 pFR81 kΩR910 kΩw(t)D11N914BCTFigure 2: Amplitude Modulation Circuit Theory of Operation The circuit diagram is shown in Figure 2. The circuit has two main parts: the oscillator (in the lower right of the circuit diagram), and the modulation and signal conditioning (in the upper part of the circuit diagram). The oscillator is based on the LMC555 timing chip. (See http://www.national.com/ds/LM/LMC555.pdf for a description.) The circuit configuration used here is a standard circuit used to produce a square-wave oscillator. The frequency of oscillation is proportional to 1/(R1 C1). We have selected R1 and C1 to produce a frequency of oscillation of about 1 MHz. The output signal, w(t), at pin 3 is then a 1 MHz square wave, which switches between roughly 0 V and 9 V. The audio input is connected through the pin strip connector P1. Resistors R3 and R4 act to average the left and right audio channels, so that the average audio signal e1 can be sent as a monaural signal. (AM radio is not designed to transmit stereo.) Resistors R5 and R6 and capacitor C3 act as a high-pass filter on the audio input. The corner frequency of the high-pass is such that essentially all audio frequencies are passed through. More 4importantly, R5, R6, and C3 provide a voltage offset to the audio signal. R5 and R6 act ap-proximately as a voltage divided leg, so that the d.c. voltage at the node is approximately 4.5 volts. The capacitor C3 has infinite impedance at d.c. frequencies, so that the circuit to the left does not affect the voltage divider. There is, however, some current out of e1 through the transistor, so the voltage divider is not perfect. In any event, the signal e2 is approximately the audio signal e1, plus a d.c. offset of about 4.5 volts. The transistor Q1 and the resistor R7 are an “emitter follower” amplifier. The emitter is the leg of the transistor connected to R7 at node e3. In this configuration, with the transistor collector connected to +VS = 9 volts, and the transistor base connected to the node e2 with voltage greater than 0 volts and less than 9 volts, the transistors acts as a sort of valve, which allows just enough current to flow from the collector to the emitter to ensure that the emitter and base voltages are the same. (In fact, the emitter voltage will be about 0.6 volts less than the base voltage.) Very little current flows through the base of the transistor. Hence, the emitter follower acts as a buffer, preventing the circuit to the right of Q1 from having much influence on the circuit to the left of Q1. The resistor R8 and the diode D1 act as a chopper. When w(t) is high (close to 9 volts), the voltage e3(t) will be lower, and the diode will be reversed biased, so that almost no current will flow through it. As a result, the voltage e4(t) will be the same as e3(t). When w(t) is low (close to 0 volts), the voltage e3(t) will be higher, and the diode will be forward biased. In this state, the diode allows as much current to flow as is necessary to make the voltage across the diode close to zero. (In fact, the voltage drop across the diode will be very close to 0.6 volts.) As a result, the


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MIT 16 01 - Study References

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