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# lec23 - 6.003 Signals and Systems Modulation December 3...

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6.003: Signals and Systems Modulation December 3, 2009

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Communications Systems Today we will look at applications of signals and systems in com- munication systems. Example: Transmit voice via telephone wires (copper) mic amp telephone wire amp speaker Works well: basis of local land-based telephones.
Wireless Communication In cellular communication systems, signals are transmitted via elec- tromagnetic (E/M) waves. mic amp E/M wave amp speaker For eﬃcient transmission and reception, antenna length should be one-quarter of the wavelength. Telephone-quality speech contains frequencies from 200 to 3000 Hz. How long should the antenna be?

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Check Yourself For eﬃcient transmission and reception, the antenna length should be comparable to one-quarter of the wavelength. Telephone-quality speech contains frequencies between 200 Hz and 3000 Hz. How long should the antenna be? 0. 4 m 1. 40 m 2. 400 m 3. 4 km 4. 40 km 5. 400 km
Check Yourself The wavelength is λ = c f so the lowest frequencies ( 200 Hz) produce the longest wavelengths λ = c f = 3 × 10 8 m/s 200 Hz = 1 . 5 × 10 6 m = 1500 km . Therefore the antenna length should be on the order of λ 4 = 1500 km 4 400 km 250 miles .

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Check Yourself For eﬃcient transmission and reception, the antenna length should be comparable to one-quarter of the wavelength. Telephone-quality speech contains frequencies between 200 Hz and 3000 Hz. How long should the antenna be? 5 0. 4 m 1. 40 m 2. 400 m 3. 4 km 4. 40 km 5. 400 km
Check Yourself What frequency E/M wave is well matched to an antenna with a length of 4 cm (about 1.5 inches)? 0. 200 kHz 1. 2 MHz 2. 20 MHz 3. 200 MHz 4. 2 GHz 5. 20 GHz

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Check Yourself A quarter-wavelength should be 4 cm, λ 4 4 cm so the frequency is f = c λ 3 × 10 8 m/s 16 cm 2 GHz . Modern cell phones use frequencies near 2 GHz.
Check Yourself What frequency E/M wave is well matched to an antenna with a length of 4 cm (about 1.5 inches)? 4. 0. 200 kHz 1. 2 MHz 2. 20 MHz 3. 200 MHz 4. 2 GHz 5. 20 GHz

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Wireless Communication Speech is not well matched to the wireless medium. Many applications require the use of signals that are not well matched to the required media. signal
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lec23 - 6.003 Signals and Systems Modulation December 3...

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