FM report sa acoustis

8/7/2019 FM report sa acoustis

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e=Em sin(wt + )

Unmodulated wave


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FM- is a system in which the amplitude of the

modulated carrier is made constant, where as

its frequency is varied in accordance with

variation of the modulating signal

the information signal varies the frequency of the

carrier.e = Em sin(wct + mfsin(wmt))

f = fc(1+kEmcoswmt)

Where K =proportionality constant


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The modulation index m of an FM signal is the ratio of thefrequency deviation to the modulating frequencyfm

mf= /fm (modulation index for FM)The modulation index determines the number of significant

pairs of sidebands in an FM signal.

The amplitudes of the carrier and sidebands vary with themodulation index and can be calculated with amathematical procedure known as the Besselfunctions.


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The amount by which the carrier frequency is varied

from its modulated value is called deviation and

is made proportion to the instaneous value of themodulating voltege

The amount of frequency change from the carrier

center frequency is called the maximumdeviation.

= kEmfc


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Sample problems

Find the carrier and modulating frequency, the

modulating index, and the maximum deviation ofthe FM wave represented by the voltage equation

e=12sin(6x108t+5sin1250t)


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Since it is possible to obtain frequency modulationfrom phase modulation by the ofArmstong system

The FM produced by PM is called indirect FM.

In PM, the phase shift of the carrier is varied by theamplitude of the modulating signal.

the deviation is proportional to the amplitude of themodulating signal.


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Both FM and PM are types of angle modulations

Maximum frequency deviation in a PM signal occurswhere the rate of change of the modulating signalamplitude is greatest, which is at its zero-crossingpoints.

Frequency deviation does not occur at the output of aphase modulator unless the modulating signalamplitude varies.


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The amount of frequency deviation produced by aphase modulator increases with the

modulating frequency.

To produce true FM from a PM signal, theamplitude of the modulating signal must be

decreased with frequency so that frequencydeviation does not change with modulatingfrequency.


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In PM, a low-pass filter on the modulating signal

compensates for increased frequency

deviation at the higher modulating frequencies.

Frequency modulation produces pairs of

sidebands spaced from the carrier in multiplesof the modulating frequency.


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The carrier or sideband amplitudes are zero at some

modulation indexes.

The bandwidth of an FM signal is proportional to the

modulation index.

There are two ways to calculate the bandwidth of an FMsignal.

a. BW = 2Nfm max

b. BW = 2(fdmax + fm max)


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The primary advantage of FM over AM is its immunity tonoise.

Noise is short-duration amplitude variations caused bylightning, motors, auto ignitions, power transients, andother sources.

Limiter circuits in FM receivers clip off noise signals.

Another benefit of FM over AM is the capture effect thatallows the strongest signal on a frequency to dominatewithout interference from the other signal.


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A third benefit of FM over AM is greater transmitter

efficiency since class C amplifiers may be used.

A majordisadvantage of FM is that its bandwidth is wider

than the bandwidth of AM.

The spectrum space taken up by an FM signal may belimited by carefully controlling the deviation ratio.


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Another disadvantage of FM is that the circuits to

produce and demodulate it are usually more

complex and expensive than AM circuits.

Noise occurs primarily at high frequencies;

therefore, noise interferes more with highmodulating frequencies.


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Interference from high-frequency noise can be minimizedby boosting the amplitude of high-frequency

modulating signals prior to modulation.T

his is calledpre-emphasis.

Pre-emphasis is accomplished by passing the modulatingsignal through an RCnetwork that linearly boosts the

amplitude of frequencies above 2122 Hz in proportionto frequency. This increases the signal-to-noise ratio atthe higher frequencies.


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The effect of pre-emphasis is corrected for, in an

FM receiver by de-emphasizing the higher

frequencies by passing them through an RC

low-pass filter.

networks have a time constant of 75 s and a cutofffrequency of 2122 Hz.

FM report sa acoustis

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