ii. review on analog modulation-1

7/24/2019 II. Review on Analog Modulation-1

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Broadcast Engineeringand AcousticsI I . REVIEW ON ANALOG MODULATION


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Modulation

The process of modifying the property of a higfrequency electromagnetic wave in accordancsignal.


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Baseband and Carrier Signal

The baseband signal is the information signal. Also cthe modulating signal.

The carrier signal is the information being modulate

the signal that is transmitted.


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Different types of modulation

A/A A/D D/A D/D

AM

FM

PM


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A/A A/D D/A D/D

PAM

PDM

PPM

PCM


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A/A A/D D/A D/D

ASK

FSK

PSK


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A/A A/D D/A D/D

Unipolar

PolarBipolar


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mplitude Modulation

In amplitude modulation, the amplitude of a carrier varied by the modulating signal.


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Equations of M Signal

Modulating Signal Carrier Signal

Amplitude Modulated Signal


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Instantaneous value of the modulatsignalThe instantaneous values of modulating signal and carrier signal can brepresented as given below.

em = Em sin(wmt)

Where: em = instantaneous amplitude

Em = maximum amplitude

wm = 2(pi)fm = angular frequency

f = frequency of the modulating signal


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Instantaneous value of the carrier si

ec = Ec sin(wct)

Where ec = instantaneous amplitude

Ec = maximum amplitude

wc = 2(pi)fc angular frequency

fc = frequency of the carrier signal


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Instantaneous value of amplitudemodulated signal

EAM = Ec + emEAM = Ec + Em sin(wmt)

eAM = Eam sin

eAM = Eam sin(wct)eAM = [Ec + Em sin(wm t)]sin(wct)


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Modulation Index and Percentage of

Modulation

Modulation index is a number lying between 0 and 1, and iratio of the amplitude of the modulating frequency to the aof the carrier frequency. It is very often expressed as a percand is called the percentage of modulation.

Overmodulation refers to conditions where in the modulatiis greater that 1, causing distortion to the amplitude modulwave.


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Modulation Index

m = Em / Ec

m = modulation index

Em = amplitude of modulating frequencyEc = amplitude of carrier frequency


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Percentage of Modulation

%m = (Em / Ec ) * 100%

m = percentage of modulation

Em = amplitude of modulating frequencyEc = amplitude of carrier frequency


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Frequency Spectrum of the M Wave

The modulated carrier has new signals at differentfrequencies, called side frequencies or sidebands.

The sidebands occur in the frequency spectrum dire

above and below the carrier frequency.


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Upper and Lower Sidebands

fUSB

= fc

+ fm

fLSB = fc fm

where:

fUSB = upper sideband frequency

fLSB =lower sideband frequency

fc =carrier frequency

fm =modulating frequency


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`


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Bandwidth of M Wave

Bandwidth is a measure of the width of a range offrequencies, measure in hertz.

The bandwidth in am can be measured by subtractin

lowest frequency of the signal from the highest freqof the signal.


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B = fUSB fLSBB = (fc + fm ) (fc - fm )

B = 2fm

where

B = bandwidth


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ngle Modulation

Is a modulation technique which varies the phase anhigh-frequency carrier signal in proportion with theinstantaneous amplitude of a modulating signal.


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Forms of ngle Modulation

Frequency Modulation (FM)

- is an angle modulation method which the frequencycarrier signal is varied in proportion with the amplitude of tmodulating signal.

Phase Modulation (PM)

- is an angle modulation method which the phase of asignal is varied in proportion with the amplitude of the modsignal.


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General Waveforms


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Frequency Modulation


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Frequency Deviation

Is used in radio to describe the maximum instantaneous diffbetween an FM modulated frequency and the nominal carrfrequency.


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Modulation Index

Modulation is generally expressed in terms of the modulatioThe modulation index is the ratio of the frequency deviationmodulating frequency.

Also called as the phase deviation.

m = f / fm


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Percent modulation

It is the modulation index, expressed as a percentag

% m = (f / fm) * 100%


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Deviation Ratio

The ratio of the maximum frequency deviation to the maximmodulating frequency.


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Frequency Spectrum of an FM

Wave

In AM wave, it is easy to tell what frequencies are prthe modulated wave. Unfortunately, the situation is complicated, mathematically for FM.

In FM, there is an infinite number of side bands.


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Bessel Functions

Are canonical solutions to Bessels differential equat

To describe and FM wave, the only solution involvesof Bessel Functions


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Bandwidth Requirements

The bandwidth of an FM wave is given by the followequation, formulated through Carsons Bandwidth R

BW = 2(m+1) fm

BWFM = 2(f + fm)


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Example

Find the bandwidth, given the following parameters

1. f = 75 kHz f m = 50 Hz

2. f = 75 KHz f m = 500 Hz

3. f = 75 KHz f m = 5 kHz


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Uses of Frequency Modulation

Service Type Frequency Assignment Channel Bandwidth Maximum Deviation

Commercial FM 88.0 to 108.0 MHz 200 KHz (+/-) 75 KHz

radio Broadcast

Television sound 4.5 MHz above picture 100 KHz (+/-) 25 KHz mono

carrier frequency (+/-) 50 KHz stereo

Public safety: 50 MHz 20 KHz (+/-) 5 KHz

police, fire, 122 to 174 MHz

ambulance, transpo.,

government, etc.

Amateur band radio 216 to 470 MHz 15 KHz (+/-) 3 KHz

Wireless mics andThe same as commercial FM broadcast, but limited in power less than 1

wireless telephone

Military Intermingled with public safety and extending to microwave frequenci

AM FM

Stands for AM stands for Amplitude Modulation FM stands for Frequency Modulation

h d f d f f ll d h d d d l d h d


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OriginAM method of audio transmission was first successfully carried out in the mid

1870s.

FM radio was developed in the United states

Armstrong.

Modulating differences

In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in

amplitude by the signal that is to be transmitted. The frequency and phase

remain the same.

In FM, a radio wave known as the "carrier" o

frequency by the signal that is to be transmit

remain the same.

Pros and cons

AM has poorer sound quality compared with FM, but is cheaper and can be

transmitted over long distances. It has a lower bandwidth so it can have more

stations available in any frequency range.

FM is less prone to interference than AM. Ho

by physical barriers. FM has better sound qu

Frequency Range

AM radio ranges from 535 to 1705 KHz (OR) Up to 1200 bits per second. FM radio ranges in a higher spectrum from 8

bits per second.

Bandwidth Requirements

Twice the highest modulating frequency. In AM radio broadcasting, the

modulating signal has bandwidth of 15kHz, and hence the bandwidth of an

amplitude-modulated signal is 30kHz.

Twice the sum of the modulating signal freq

deviation. If the frequency deviation is 75kH

frequency is 15kHz, the bandwidth required

Complexity

Transmitter and receiver are simple but syncronization is needed in case ofSSBSC AM carrier.

Transmitter and receiver are more complex ahas to be converted and detected from corre

frequencies.(i.e. voltage to frequency and fre

has to be done).

Noise

AM is more susceptible to noise because noise affects amplitude, which is

where information is "stored" in an AM signal.

FM is less susceptible to noise because infor

transmitted through varying the frequency, a

ii. review on analog modulation-1

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