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# Lecture 12

## on Mar 16, 2014

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## Lecture 12Presentation Transcript

• DATA COMMUNICATIONS & NETWORKING LECTURE-12 Course Instructor : Sehrish Rafiq Department Of Computer Science University Of Peshawar
• LECTURE OVERVIEW  Types of Link/channel  Low Pass channel  Band Pass channel  Analog to analog modulation  AM  FM  PM
• LOW PASS CHANNEL 0 f1 Amplitude Frequency
• BAND PASS CHANNEL f1 f2 Amplitude Frequency
• TYPES OF LINK/CHANNEL  A channel or link is either low-pass or band-pass.  Low-Pass Channel  A low-pass channel has a width with frequencies between 0 and f.  Band-Pass Channel  A band-pass channel has a bandwidth with frequencies between f1 and f2.
• ANALOG TO ANALOG MODULATION  Modulation of an analog signal or analog-to- analog conversion is the representation of analog information by an analog signal.  Modulation is needed if the medium has a band nature or if only band-pass bandwidth is available to us.  E.g. radio station
• ANALOG-TO- ANALOG MODULATION
• TYPES OF ANALOG TO ANALOGTYPES OF ANALOG TO ANALOG MODULATIONMODULATION  AM  FM  PM
• AM(AMPLITUDE MODULATION)  In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal.  The frequency and phase of the carrier remains the same.  Only the amplitude changes to follow variations in the information.  The modulating signal becomes an envelope to the carrier.
• AM(AMPLITUDE MODULATION)
• AM BANDWIDTH  The band width of an AM Signal is equal to twice the bandwidth of the modulating signal and covers a range centered on the carrier frequency.  BWt = 2 x BWm.
• AM BANDWIDTH
• BAND WIDTH NEEDS OF A RADIO STATION USING AM  The bandwidth of an audio signal is usually 5 KHz.  Each AM radio station needs a minimum bandwidth of 10KHz.  AM stations are allowed carrier frequencies anywhere between 530 and 1700 KHz(1.7 MHz).  Each station’s carrier frequency must be separated by at least 10 KHz(one AM bandwidth) to avoid interference.
• BAND WIDTH NEEDS OF A RADIO STATION USING AM
• NOTE  FCC Federal Communications Commission(FCC) allows 10KHz for each AM station.
• Example 1Example 1 We have an audio signal with a bandwidth of 4 KHz. What is the bandwidth needed if we modulate the signal using AM? Ignore FCC regulations. SolutionSolution An AM signal requires twice the bandwidth of the original signal: BW = 2 x 4 KHz = 8 KHz
• FREQUENCY MODULATION (FM)  In FM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level(amplitude) of the modulating signal.  The peak amplitude and the phase of the carrier signal remains constant.  As the amplitude of the information signal changes, the frequency of the carrier changes correspondingly.
• FREQUENCY MODULATION
• BANDWIDTH REQUIREMENTS FOR FM  The bandwidth of an FM signal is equal to 10 times the bandwidth of the modulating signal.  Like AM bandwidth, FM also covers a range centered on the carrier frequency.
• FM BANDWIDTH
• BAND WIDTH NEEDS OF A RADIO STATION USING FM  The bandwidth of an audio signal(speech and music ) broadcast in stereo is almost 15 KHz.  Each FM radio station therefore needs a minimum bandwidth of 150 KHz.  The FCC allows 200 KHz(0.2 MHz) for each station to provide some room for guard bands.  FM stations are allowed carrier frequencies anywhere between 88 and 108 MHz.  FCC requires that in a given area only alternate bandwidth allocations may be used.
• FM BANDWIDTH ALLOCATION
• PHASE MODULATION  In PM transmission, the phase of the carrier signal is modulated to follow the changing voltage level(amplitude) of the modulating signal.  The peak amplitude and frequency of the carrier signal remain constant.  As the amplitude of the information signal changes, the phase of the carrier changes correspondingly.
• PM SIGNAL
• ENOUGH FOR TODAY!!!