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Published byTimothy Kearney Modified over 2 years ago

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Chapter 2: Digital Modulation Introduction –Electronic communications is the transmission, reception and the processing of the information with the use of electronic circuits –Information knowledge or intelligence that is communicated ( transmitted or received ) –Digital modulation the transmittal of digitally modulated analog signals (carrier) between two points or more –Digital radio Relatively high-frequency analog carriers modulated by relatively low-frequency digital signals (can be carried by free space)

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If the information signal is digital and the amplitude (V) is varied proportional to the information signal amplitude shift keying (ASK) If the information signal is digital and the frequency (f) is varied proportional to the information signal frequency shift keying (FSK) If the information signal is digital and the phase of the carrier is varied proportional to the information signal phase shift keying (PSK)

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Digital modulation is good for the use in both cable and wireless system such as –Low-speed voice band data communication modem –High-speed data transmission system such as DSL (digital subscriber line) –Digital microwave and satellite communication systems –Cellular telephone Personal communications systems (PCS)

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Information theory study the efficient use of bandwidth to propagate information through electronic communication systems –Information capacity represent the number of independent symbols (usually binary digit or bit) that can be carried through a system in a given unit of time (bit rate bps) –Hartleys Law –Shannon limit for information capacity

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Amplitude-shift keying (ASK) the binary information signal directly modulates the amplitude of an analog carrier there are only two output amplitudes possible mathematically written as

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Frequency-shift keying (FSK) mathematically written as when the binary input change logic from 1 to logic 0, the output frequency change from a mark (f m ) to a space frequency (f s ) as shown in picture

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Continuous-Phase Frequency-Shift Keying (CP-FSK) is normal binary FSK except the mark and space frequencies are synchronized (synchronous does not have to be equal, but there is a precise time relationship between the two frequency ) with CP-FSK, the mark and space frequencies are selected such that they are separated from the center frequency by an exact multiple of one-half the bit rate (f m and f s =n*(f b /2)) in discontinuity FSK, when the input changes, there is an abrupt phase discontinuity in analog signal. So, the demodulator has trouble following the shift frequency causing error CP-FSK has better bit-error performance but expensive to implement

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Binary Phase-Shift Keying two phases are for carrier. One phase represents logic 1 and the other represents logic 0 the phase of the output carrier shifts between two angles that are separated by 180 degree Quadrature –Amplitude Modulation (QAM) is a form of digital modulation similar to PSK but digital information is contained in both the amplitude and phase of the transmitted carrier

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