Lecture 12
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This document provides an overview of different types of links/channels and analog-to-analog modulation techniques. It discusses low-pass channels that have frequencies between 0 and f1, and band-pass channels between f1 and f2. It also describes amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). For AM, it notes the carrier amplitude varies while frequency and phase remain constant. For FM, the carrier frequency varies while amplitude and phase remain constant. It provides examples of bandwidth needs for radio stations using AM and FM.
Lecture 12
- 1. DATA COMMUNICATIONS & NETWORKING LECTURE-12 Course Instructor : Sehrish Rafiq Department Of Computer Science University Of Peshawar
- 2. LECTURE OVERVIEW Types of Link/channel Low Pass channel Band Pass channel Analog to analog modulation AM FM PM
- 3. LOW PASS CHANNEL 0 f1 Amplitude Frequency
- 4. BAND PASS CHANNEL f1 f2 Amplitude Frequency
- 5. 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.
- 6. 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
- 8. TYPES OF ANALOG TO ANALOGTYPES OF ANALOG TO ANALOG MODULATIONMODULATION AM FM PM
- 9. 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.
- 11. 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.
- 12. AM BANDWIDTH
- 13. 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.
- 14. BAND WIDTH NEEDS OF A RADIO STATION USING AM
- 15. NOTE FCC Federal Communications Commission(FCC) allows 10KHz for each AM station.
- 16. 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
- 17. 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.
- 19. 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.
- 20. FM BANDWIDTH
- 21. 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.
- 23. 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.
- 24. PM SIGNAL