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Lecture No.5 & 6

Data Communications System & Networks

By


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The AM is susceptible to noise, asnoise adds up to the amplitude of the

wave and distorts the information

content of the signal. This is not the

case with FM. In FM the amplitude

of the carrier remain the same, but

the frequency is varied in accordancewith the modulating signal.

)( tSinmtSinVv mfccFM [[ !

where

vFM = instantaneous voltage of the FMwave

Vc = Peak amplitude of carrier voltage

[c = angular velocity of carrier = 2Tfc[m= angular velocity of modulating

signal = 2Tfm

mf= modulation factor


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The degree of modulation in FM is measured by modulation factor, and is given as

Where

fd = Frequency Deviation

fm = Modulating frequency

m

d

f

f

fm !


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In AM wave there are only two sidebands, but

in FM we have practically infinite number of

sidebands, because FM wave goes through

numerous frequency shifts produced by

modulating signal. The frequency spectrum of a

typical FM wave is shown below

It is clear that to transmit such a broad spectrum

wave, we need huge bandwidth. But fortunately,

many higher order sidebands contain an

insignificant amount of energy and can therefore

be disregarded. Any sideband with amplitude less

than 1% of the carrier is considered as

insignificant and therefore, left out.


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The above figure shows a voltage divider bias, BJT amplifier, with varactor tuned modulator circuit for FM

generation. A varactor diode is a semiconductor diode that has been specifically manufactured to have its depletion

region optimized for variable capacitance effect under reverse bias condition.

The figure above is actually a Colpitts Oscillatorconfiguration. The oscillator is used to produce the carrier frequency, while

the audio signal is used to change the frequency when applied to the varactor diode. The audio signal actually varies the

junction capacitance of the varactor diode; hence the resonant frequency of the tank circuit changes with the audio signal.Thus FM signal is produced at the output.


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As clear form its name, in Phase Modulation the phase of the carrier signal is varied in accordance with

the modulating signal.

The resulting waveform can be mathematically expressed as

where all the parameters are the same as defined earlier, and

Jm = Modulation Index for PM.

tSintSinVv mmccpm [[


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Phase Modulation Frequency Modulation

The carrier deviation is proportional to

the rate of change of modulating signal as

well as amplitude change of that.

The amount of carrier deviation is directly

proportional to the amplitude of the

modulating signal. The rate of carrier

deviation is same as frequency of the

modulating signal.


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AUDIO INPUTAUDIO INPUTAUDIO INPUTFREQUENCYFREQUENCYFREQUENCY

MODULATORMODULATORMODULATOR

HIGH PASSHIGH PASSHIGH PASS

FILTERFILTERFILTER

PHASEPHASEPHASE

MODULATIONMODULATIONMODULATION

AUDIO INPUTAUDIO INPUT PHASEMODULATOR

LOW PASSFILTER

FREQUENCYMODULATION


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PURPOSE

Frequency modulation was originally developed to cope with Undesirable noise

which completed with the with the desired signal When amplitude modulation wasused.

Most noise appeared as an additional amplitude modulation on the signal.

When frequency modulating a carrier, information is placed on the carrier by varying

its frequency while holding its amplitude fixed upon being received. Variations in

amplitude are eliminated Prior to demodulation without affecting the information

content contained in the frequency variations, thereby eliminating any noise which

may appear as an amplitude modulation of the carrier.


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Many of the advantages obtained with wideband FM, such as Noise

reduction, are not available with narrowband FM. Why, Would one want to

use narrowband FM rather than AM?

One reason is that with narrowband FM (as well as with wideband FM) the

power content as the carrier frequency decreases as the Modulation

increase so that we have the desirable situation of putting the power where

the information is.

ADVANTAGES


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The resting frequency is the frequency of an

FM carrier when no modulating signal is

present. The presence of a modulating signal

will cause the carrier to shift away from the

resting frequency.

Resting frequency

lecture5 & 6_freq modulation_done

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