chapter_3_part_2 - Chapter 3 Analog Signal Transmission and...

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Chapter 3: Analog Signal Transmission and Reception Part II Dr. Barış Atakan
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Conventional AM: Power Content of Signal Conventional AM is similar to DSB-SC when m(t) is substituted with 1+am n (t) DSB-SC signal: Conventional AM signal: Remember: the power content of DSB-SC signal was derived as Hence, if we derive the power P m for the signal (1+am n (t)), we can find the power content of conventional AM signal
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Conventional AM: Power Content of Signal The power P m of the signal [1+am n (t)], It is assumed that the average of m n (t) is zero . This is a reasonable assumption for many signals including audio signals. Therefore, for conventional AM
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Conventional AM: Power Content of Signal The first component is due to the existence of the carrier and this component does not carry any information The second component is the information carrying component . Note that the second component is usually much smaller than the first component (|m n (t)|<1, and for signals with large dynamic range, Pm n << 1) This shows that the conventional AM systems are far less power efficient compared with DSB-SC systems The advantage of conventional AM is that it is easily demodulated.
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Conventional AM: Demodulation Conventional AM signal: t t m all for 0 ) ( 1 t t m all for 0 ) ( 1 t t m all for 1 ) ( If 1+m(t)>0 for all t, the envelop of the modulated signal is proportional to m(t) Otherwise, it is not always proportional to the m(t)
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Conventional AM: Demodulation For such a demodulation, the envelop detector circuit is used Ideally , the output of the envelope detector is of the form where g 1 represents a dc component and g 2 is a gain factor due to the signal demodulator. The simplicity of the demodulator has made conventional DSB AM a practical choice for AM radio broadcasting.
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Conventional AM: Demodulation The time constant RC must be selected so as to follow the variations in the envelope of the carrier-modulated signal In such a case, the capacitor discharges slowly through the resistor and, thus, the output of the envelope detector closely follows the message signal.
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Single Sideband AM: Background We showed that a DSB-SC AM signal required a channel bandwidth of B c = 2W Hz for transmission, where W is the bandwidth of the baseband signal However, the two sidebands are redundant. Next, we demonstrate that the transmission of either sideband is sufficient to
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