dc-lec-17 (analog to analog)

8/3/2019 DC-Lec-17 (Analog to Analog)

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DATA

COMMUNICATION

Lecture-17


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Recap of Lecture 15 & 16

Digital-to Analog ConversionBit Rate and Baud Rate

Carrier SignalsASK FSK PSK QAM


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Overview of Lecture 17

Analog-to Analog Conversion

Amplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)


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Analog To Analog Conversion

Representation of Analog information (Voice) by an Analog signal.For Example: Radio communication (in whichvoice is generated and sent across the air usingradio waves or micro waves.)


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Analog To Analog

Conversion

As Human voice (Analog data) is not in a formatthat can be sent in air, (its frequency is not in thatform so that it can be sent separately) it must be sentwith the help of carrier signal (higher frequency than the original data signal frequency generated by

transmitter itself).This technique can also be called as modulation of Analog signal.(as a result of modulation theconverted signal will have much higher frequency

then the original signal)


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Analog To Analog Conversion


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Analog To Analog Conversion Methods


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Amplitude Modulation (AM)

Amplitude of carrier signal ischanged/varied according to theamplitude of modulating signal(originalsignal).

Frequency and phase of the carrier remainthe same


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Amplitude Modulation (AM)


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AM Bandwidth

Bandwidth of AM signal (modulated signal) = 2 *

bandwidth of modulating signal (original signal)

Significant spectrum of AM audio = 5 KHz10 KHz bandwidth for an AM station


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AM Bandwidth


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AM Band Allocation


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AM Band Allocation

Actual signal (modulating signal) is of 5 KHz, so therefore itsmodulated signal will be of 10KHz.Or we can say that every AM station will use 10 KHz frequency fortransmission.FCC (Federal Communication Commission) is the Regulatory Agencbased in USA, which regulates all the frequency issues (allotsfrequency to every AM station), it allows only 10KHz for each AMstation.Every AM station frequency is changed from another AM station by10 KHz frequency e.g. 1st AM station has 1100 KHz so next AM station will have 1110KHz frequency.


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Example

We have an audio signal with a BW of 4 KHz. What isthe BW needed, if we modulate the signal using AM?

Solution:AM signal requires twice the BW of original signal

BW = 2 * 4 KHz = 8 KHz


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Frequency Modulation (FM)

Frequency of carrier signal is changed according

to the amplitude of modulating signal.

Amplitude and Phase of the carrier signal remainconstant


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Frequency Modulation (FM)


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FM Bandwidth

Bandwidth of FM signal (modulated signal) =10 * bandwidth of modulating signal

Significant spectrum of FM audio = 15 KHzMinimum 150 KHz bandwidth


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FM Bandwidth


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FM Band Allocation


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FM Band Allocation

The Gap between two stations will be 200KHz (according to FCC)Every station will have 200 KHz Bandwidth, so the total Gapbetween two FM stations will be 400 KHz

(100 KHz+200Khz+100Khz) .This Gap is Used to Prevent overlapping two signals and Extra200KHz is placed for more safety from interference.According to FCC we can only use 88MHz to 108MHz for FM

stations, so there will be total 100 FM stations, but as we placed100KHz free space, so now there will be only 50 FM stations.


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Example

We have an Audio signal with a BW of 4 MHz.What is the BW needed if we modulate the signalusing FM?

Solution:

BW = 10 * 4 MHz = 40 MHz


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Phase modulation (PM)

Simpler hardware requirements

Phase is modulated with the amplitude

Amplitude & Frequency of the carrier signal remain

constant.Note: Every other aspect of PM is same to FM.


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Summary

Analog-to Analog ConversionAmplitude ModulationFrequency ModulationPhase Modulation


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Suggested Reading

Section5.4,

Data Communications and Networking 4th Edition by Behrouz A. Forouzan

dc-lec-17 (analog to analog)

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