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AM+transmission+of+iPod+audio - EECS 70LB AM Transmission...

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EECS 70LB AM Transmission of iPod Audio Objective: To build a transmission system in which an audio signal from iPod (or equivalent audio device) is AM modulated, demodulated, and then played through a speaker. I. Reading Alexander and Sadiku, pp. 657-659, 836-838. Haykin, Communication Systems ,4 th ed., pp. 88-93. Scherz, pp. 384, 429, 617-619, 624-625, 627. II. Components Note: Students are expected to bring the following components to the lab : (1) 741 op-amp (1) 1N4148 Si diode (2) 1.0 kΩ resistor (1) Variable resistor (e.g., pot) (1) 0.1 μF capacitor (1) 1 μF capacitor (1) Portable audio device. Any portable audio device with a 3.5 mm mini headphone jack, which includes most MP3 and CD players, handheld game devices, and laptops, will do. The following components will be provided: (1) 3.5 mm mini to BNC female adapter (1) 10 Ω resistor (1) 80 kΩ resistor (68+12k ) resistor (1) 220 μF capacitor (1) 2.2 μF capacitor (1) 10 μF capacitor (1) 1 nF capacitor (1) 47 nF capacitor (1) LM386N low voltage audio power amplifier (1) 741 op-amp (1) Miniature PM speaker III. Background Amplitude modulation/demodulation Amplitude modulation (AM) of a message signal was discussed in lab 5. In this lab, we are going to AM modulate an audio signal, demodulate it, and then listen to it. An audio signal m(t) contains frequency components within the range 20 Hz to 20 kHz (humans can only hear sounds with frequencies within that range). If we want to transmit m(t) efficiently over air without any modulation, we would need an incredibly long antenna. We can use a practical antenna size if we modulate the signal. In modulation, the frequencies of m(t) are shifted to higher frequencies in the MHz range. AM modulation is done by multiplying m(t) with a carrier signal, c(t) , where A c is carrier amplitude and f c is the carrier frequency. This carrier frequency becomes new the center frequency of m(t) . When you tune the frequency of an AM station on a radio, you’re selecting the carrier frequency of the station to which you want to listen, such as 570 kHz, 980 kHz, and 1070 kHz. The modulated signal, s(t) is obtained by multiplying a weighted m(t) along with an offset by a sinusoidal carrier signal c(t) , Multiplying the signals, we obtain
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where k a is the modulation index or amplitude sensitivity. A block diagram of the modulation process is shown below: Fig. 9.3.1. General block diagram of the AM modulation and transmission process In this lab, we will use a function generator to do the modulation for us. Our function generators have an input for m(t) . The modulated signal s(t) is transmitted over air and then received by a receiver (e.g., your car radio) and demodulated so that m(t) is recovered. A block diagram of the receiver/demodulator is shown in Fig. 9.3.2. The detailed circuit schematics for modulation/demodulation are shown below in the prelab. Fig. 9.3.2. General block diagram of the AM demodulation
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AM+transmission+of+iPod+audio - EECS 70LB AM Transmission...

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