chapter 4 frequency modulation

Chapter 4Chapter 4FREQUENCY MODULATIONFREQUENCY MODULATION

INTRODUCTIONINTRODUCTION 3 properties of an analog signal can be 3 properties of an analog signal can be

modulated by information signal:modulated by information signal:

o Amplitude - - -> produce AMAmplitude - - -> produce AMo Frequency - - - > produce FMFrequency - - - > produce FMo Phase - - - > produce PMPhase - - - > produce PM

FM & PM are forms of FM & PM are forms of angle modulationangle modulation and often referred as frequency modulation.and often referred as frequency modulation.

FM is considered to be superior to AM.FM is considered to be superior to AM. Transmission efficiency:Transmission efficiency:

AM use linear amplifier to produced the final RF signal.AM use linear amplifier to produced the final RF signal. FM has constant carrier amplitude so it is not FM has constant carrier amplitude so it is not

necessary to use linear amplifier.necessary to use linear amplifier. Fidelity (capture effect):Fidelity (capture effect):

The stronger signal will be capture and eliminate the The stronger signal will be capture and eliminate the weaker.weaker.

In AM, the weaker signal can be heard in the In AM, the weaker signal can be heard in the background.background.

Noise immunity (noise reduction):Noise immunity (noise reduction): Constant carrier amplitude.Constant carrier amplitude. FM receiver have limiter circuitFM receiver have limiter circuit

FM VS AMFM VS AM

Disadvantages of FMDisadvantages of FM Use too much spectrum space.Use too much spectrum space. Requiring a wider bandwidthRequiring a wider bandwidth

Reduce modulation index to minimize BW but Reduce modulation index to minimize BW but in FM although we reduced the modulation in FM although we reduced the modulation index, BW is still larger.index, BW is still larger.

typically used at high frequencies (VHF,UHF typically used at high frequencies (VHF,UHF & microwave frequencies& microwave frequencies

More complex circuitryMore complex circuitry

Amplitude of the modulated carrier is held constant and Amplitude of the modulated carrier is held constant and either the phase or the time derivative of the phase of the either the phase or the time derivative of the phase of the carrier is varied linearly with the message signal m(t).carrier is varied linearly with the message signal m(t).

General angle-modulated signal is given byGeneral angle-modulated signal is given by

In angle modulation, In angle modulation, (t) (t) is prescribed as being a function is prescribed as being a function of the modulating signalof the modulating signal

If If vvmm(t) (t) is the modulating signal, angle modulation is is the modulating signal, angle modulation is expressed asexpressed as

wherewhere

( ) ( )mt F v t

( ) sin( )

2m m m

m m

v t V t

f

ANGLE MODULATIONANGLE MODULATION

ttVtm cc cos

FM OR PM ?FM OR PM ?

Both must occur whenever either form of angle Both must occur whenever either form of angle modulation is performed.modulation is performed.

FMFM PMPMInstantaneous frequencyInstantaneous frequency of of the carrier is varied from the carrier is varied from its reference value by an its reference value by an amount proportional to the amount proportional to the modulating signal modulating signal amplitudeamplitude

Freq. carrier - - - > directly Freq. carrier - - - > directly variedvaried

Phase carrier - - -> Phase carrier - - -> indirectly variedindirectly varied

Phase anglePhase angle of the carrier of the carrier is varied from its reference is varied from its reference value by an amount value by an amount proportional to the proportional to the modulating signal modulating signal amplitudeamplitude

Phase carrier - - - > directly Phase carrier - - - > directly variedvaried

Freq. carrier - - -> Freq. carrier - - -> indirectly variedindirectly varied

Figure 4.1 : Frequency deviation

∆f ∆f

fc-∆f fc fc+∆f f

-Vm 0 +Vm

vm(t) = Vm cos 2πfmt

2∆f

Instantaneous frequency deviationInstantaneous frequency deviation Instantaneous change in the frequency of the Instantaneous change in the frequency of the

carrier and is defined as the first time derivative of carrier and is defined as the first time derivative of the instantaneous phase deviationthe instantaneous phase deviation

Instantaneous frequencyInstantaneous frequency the precise frequency of the carrier at any given the precise frequency of the carrier at any given

instant of time and is defined as the first time instant of time and is defined as the first time derivative of the instantaneous phasederivative of the instantaneous phase

instantaneous frequency deviation '( ) rad/s

'( ) rad/s cycleor Hz

2 rad/cycle s

t

t

instantaneous frequency ( ) ( )

'( ) rad/s

i c

c

dt t t

dtt

MATHEMATICAL MATHEMATICAL ANALYSISANALYSIS

Substituting 2Substituting 2ffcc for for cc gives gives

Frequency modulation is angle modulation in Frequency modulation is angle modulation in which the instantaneous frequency deviation, which the instantaneous frequency deviation, ’(t), is proportional to the amplitude of the ’(t), is proportional to the amplitude of the modulating signal, and the instantaneous modulating signal, and the instantaneous phase deviation is proportional to the phase deviation is proportional to the integral of the modulating signal voltage.integral of the modulating signal voltage.

instantaneous frequency ( )

rad cyclesand ( ) 2 '( ) 2 '( ) rad/s

cycle s

i

i c c

f t

t f t f t

DEVIATION SENSITIVITYDEVIATION SENSITIVITY

For modulating signal For modulating signal vvmm(t),(t), the frequency the frequency modulation aremodulation are

frequency modulation frequency modulation = = ’(t) = k’(t) = kffvvmm(t)(t) rad/s rad/s

where where kkff are constant and are the deviation are constant and are the deviation sensitivities of the frequency modulator.sensitivities of the frequency modulator.

Deviation sensitivities are the output-versus-input Deviation sensitivities are the output-versus-input transfer function for the modulators, which gave transfer function for the modulators, which gave the relationship between what output parameter the relationship between what output parameter changes in respect to specified changes in the input changes in respect to specified changes in the input signal.signal.

frequency modulator,frequency modulator, rad/s

VfkV

FREQUENCY MODULATION FREQUENCY MODULATION (FM)(FM)

Variation Variation of of dd/dt /dt producesproduces Frequency ModulationFrequency Modulation

Frequency modulation implies that Frequency modulation implies that dd/dt/dt is proportional to the is proportional to the modulating signal.modulating signal.

This yields This yields ( ) sin ( )

sin '( )

sin ( )

sin sin ( )

sin cos ( )

FM c c

c c

c c f m

c c f m m

f mc c m

m

v t V t t

V t t dt

V t k v t dt

V t k V t dt

k VV t t

Example 4.1Example 4.1

( ) cos ( )

( ) cos

for PM

( ) cos ( )

cos cos( )

c c

m m m

PM c c p m

c c p m m

v t V t t

v t V t

v t V t k v t

V t k V t

for FM

( ) cos ( )

cos cos( )

cos cos( )

cos sin( )

FM c c f m

c c f m m

c c f m m

f mc c m

m

v t V t k v t dt

V t k V t dt

V t k V t dt

k VV t t

Derive the FM signal using both cosine wave Derive the FM signal using both cosine wave signal.signal.

Figure 4.2: Phase and Frequency modulation ; (a) carrier signal (b) modulating signal (c) frequency modulated wave (d) phase modulated wave

FM WAVEFORMFM WAVEFORM

Carrier amplitude remains constant Carrier amplitude remains constant Carrier frequency is changed by the modulating Carrier frequency is changed by the modulating

signal.signal. amplitude of the information signal varies, the carrier amplitude of the information signal varies, the carrier

frequency shift proportionately.frequency shift proportionately. modulating signal amplitude increases, the carrier frequency modulating signal amplitude increases, the carrier frequency

increases.increases. modulating signal amplitude varies, the carrier frequency modulating signal amplitude varies, the carrier frequency

varies below and above it normal center or resting, varies below and above it normal center or resting, frequency with no modulation.frequency with no modulation.

The amount of the change in carrier frequency The amount of the change in carrier frequency produced by the modulating signal known as produced by the modulating signal known as frequency deviation ffrequency deviation fdd..

Maximum frequency deviation occurs at the Maximum frequency deviation occurs at the maximum amplitude of the modulating signal.maximum amplitude of the modulating signal.

The frequency of the modulating signal determines The frequency of the modulating signal determines the frequency deviation ratethe frequency deviation rate

MODULATION INDEXMODULATION INDEX

Directly proportional to the amplitude of the Directly proportional to the amplitude of the modulating signal and inversely proportional to the modulating signal and inversely proportional to the frequency of the modulating signalfrequency of the modulating signal

Ratio of the frequency deviation and the modulating Ratio of the frequency deviation and the modulating frequencyfrequency

FM equation :FM equation : as modulation index :as modulation index :

Example:Example: Determine the modulation index for FM signal with Determine the modulation index for FM signal with

modulating frequency is 10KHz deviated by ±10kHz.modulating frequency is 10KHz deviated by ±10kHz. Answer : (20KHz/10KHz) = 2 .0 (unitless)Answer : (20KHz/10KHz) = 2 .0 (unitless)

The total frequency change, 10kHz x 2 is called the The total frequency change, 10kHz x 2 is called the carrier carrier swingswing

( ) sin cos ( )FM c c mv t V t t

f m c

m m

k V f

f

Example:Example: a simple transmitter with an assigned rest frequency a simple transmitter with an assigned rest frequency

of 100MHz deviated by a ±25kHz, the carrier changes of 100MHz deviated by a ±25kHz, the carrier changes frequency with modulation between the limits of frequency with modulation between the limits of 99.975MHz and 100.025MHz99.975MHz and 100.025MHz

The total frequency change, 25kHz x 2 is called the The total frequency change, 25kHz x 2 is called the carrier swingcarrier swing

Table 1 display the transmission band that use FM Table 1 display the transmission band that use FM and the legal frequency deviation limit for each and the legal frequency deviation limit for each categorycategory

Deviation limits are based on the quality of the Deviation limits are based on the quality of the intended transmissions, wider deviation results in intended transmissions, wider deviation results in higher fidelityhigher fidelity

The frequency deviation is a useful parameter for The frequency deviation is a useful parameter for determining the bandwidth of the FM-signalsdetermining the bandwidth of the FM-signals

Specifications for transmission of FM signal

Table 1 display the transmission band that use FM and the legal Table 1 display the transmission band that use FM and the legal

frequency deviation limit for each categoryfrequency deviation limit for each category

PERCENT MODULATIONPERCENT MODULATION Simply the ratio of the frequency deviation Simply the ratio of the frequency deviation

actually produced to the maximum frequency actually produced to the maximum frequency deviation allowed by law stated in percent formdeviation allowed by law stated in percent form

For For exampleexample if a given modulating signal if a given modulating signal produces ±50kHz frequency deviation, and the produces ±50kHz frequency deviation, and the law stated that maximum frequency deviation law stated that maximum frequency deviation allowed is ±75kHz, thenallowed is ±75kHz, then

max

% modulation actualf

f

50% modulation = 100 67%

75

kHz

kHz

A 1 MHz carrier freq with a measured A 1 MHz carrier freq with a measured sensitivity of 3 kHz/V is modulated with a 2 sensitivity of 3 kHz/V is modulated with a 2 V, 4 kHz sinusoid. DetermineV, 4 kHz sinusoid. Determine

1. the max freq deviation of the carrier1. the max freq deviation of the carrier2. the modulation index2. the modulation index3. the modulation index if the modulation 3. the modulation index if the modulation

voltage is doubledvoltage is doubled4. the modulation index for 4. the modulation index for

vvmm(t)=2cos[2π(8kHz)t)]V(t)=2cos[2π(8kHz)t)]V5. express the FM signal mathematically for a 5. express the FM signal mathematically for a

cosine carrier & the cosine-modulating signal cosine carrier & the cosine-modulating signal of part 4. Carrier amplitude is 10Vof part 4. Carrier amplitude is 10V


FM RADIO FREQUENCYFM RADIO FREQUENCY

Commercial radio FM band, 88MHz – Commercial radio FM band, 88MHz – 108MHz108MHz

Each station allotted to a frequency Each station allotted to a frequency deviation of ±75kHz (150 carrier deviation of ±75kHz (150 carrier swing) and 25kHz of guard band swing) and 25kHz of guard band added above and below the carrier added above and below the carrier frequency swingfrequency swing

Total bandwidth is 200kHzTotal bandwidth is 200kHz Therefore, maximum of 100 stations Therefore, maximum of 100 stations

can be made availablecan be made available

FREQUENCY FREQUENCY ANALYSIS OF FM ANALYSIS OF FM

WAVESWAVES

Tabulated value for Bessel Function for the first kind of the nth order

BESSEL TABLEBESSEL TABLE

,

The first column gives the modulation , while the first row The first column gives the modulation , while the first row gives the Bessel function. gives the Bessel function.

The remaining columns indicate the amplitudes of the The remaining columns indicate the amplitudes of the carrier and the various pairs of sidebands. carrier and the various pairs of sidebands.

Sidebands with relative magnitude of less than 0.001 have Sidebands with relative magnitude of less than 0.001 have been eliminated. been eliminated.

Some of the carrier and sideband amplitudes have negative Some of the carrier and sideband amplitudes have negative signs. This means that the signal represented by that signs. This means that the signal represented by that amplitude is simply shifted in phase 180amplitude is simply shifted in phase 180 (phase inversion). (phase inversion).

The spectrum of a FM signal varies considerably in The spectrum of a FM signal varies considerably in bandwidth depending upon the value of the modulation bandwidth depending upon the value of the modulation index. The higher the modulation index, the wider the index. The higher the modulation index, the wider the bandwidth of the FM signal. bandwidth of the FM signal.

With the increase in the modulation index, the carrier With the increase in the modulation index, the carrier amplitude decreases while the amplitude of the various amplitude decreases while the amplitude of the various sidebands increases. With some values of modulation index, sidebands increases. With some values of modulation index, the carrier can disappear completely.the carrier can disappear completely.

Bessel Function, Jn(m) vs m

Property - 1:Property - 1:For For nn even, even,

we have we have JJnn(() = J) = J-n-n(()) For For nn odd, odd,

we have we have JJnn(() = (-1) J) = (-1) J-n-n(()) Thus,Thus,

JJnn(() = (-1)) = (-1)nn J J-n-n ( ())

Property - 2:Property - 2:For small values of the modulation index For small values of the modulation index , , we havewe have

JJ00(() ) 1 1

JJ11(() ) /2/2

JJ33(() ) 0 0 for for n > 2n > 2

Property - 3:

2 ( ) 1nn

J

PROPERTIES OF BESSEL PROPERTIES OF BESSEL FUNCTIONFUNCTION

AMPLITUDE SPECTRUMAMPLITUDE SPECTRUM

Amplitude spectrum of different value of

FM BANDWIDTHFM BANDWIDTH The total BW of an FM signal can be determined by The total BW of an FM signal can be determined by

knowing the modulation index and Bessel function.knowing the modulation index and Bessel function.

N = number of significant sidebandsN = number of significant sidebands

ffm m = modulating signal frequency (Hz)= modulating signal frequency (Hz)

Another way to determine the BW is use Carson’s Another way to determine the BW is use Carson’s rulerule

This rule recognizes only the power in the most This rule recognizes only the power in the most significant sidebands with amplitude greater than significant sidebands with amplitude greater than 2% of the carrier.2% of the carrier.

NfBW m2

Example 4.3 Example 4.3

Calculate the bandwidth occupied by a FM Calculate the bandwidth occupied by a FM signal with a modulation index of 2 and a signal with a modulation index of 2 and a highest modulating frequency of 2.5 kHz. highest modulating frequency of 2.5 kHz. Determine bandwidth with table of Bessel Determine bandwidth with table of Bessel functions. functions.

Referring to the table, this produces 4 Referring to the table, this produces 4 significant pairs of sidebands.significant pairs of sidebands.

2 4 2.5

20kHz

BW

CARSON’S RULECARSON’S RULE

ffd (max) d (max) = max. frequency deviation = max. frequency deviationffm (max) m (max) = max. modulating frequency = max. modulating frequency

Carson’s rule always give a lower BW calculated with Carson’s rule always give a lower BW calculated with the formula BW = 2fthe formula BW = 2fmmN.N.

Consider only the power in the most significant Consider only the power in the most significant sidebands whose amplitudes are greater than 1% sidebands whose amplitudes are greater than 1% of the carrier.of the carrier.

Rule for the transmission bandwidth of an FM Rule for the transmission bandwidth of an FM signal generated by a single of frequency signal generated by a single of frequency ffmm as as follows:follows:

][2 (max)(max) md ffBW

12 2 2 (1 )

or = 2 1

T m

m

B BW f f f

f


For an FM modulator with a modulation For an FM modulator with a modulation index index = 1 = 1, a modulating signal , a modulating signal

vvmm(t) = V(t) = Vmmsin(2sin(2ππ1000t) and 1000t) and unmodulated carrierunmodulated carrier

vvcc(t) = 10sin(2(t) = 10sin(2ππ500kt), determine500kt), determinea)a) Number of sets of significant sidebandNumber of sets of significant sidebandb)b) Their amplitudeTheir amplitudec)c) Then draw the frequency spectrum Then draw the frequency spectrum

showing their relative amplitudesshowing their relative amplitudes


For an FM modulator with a peak freq For an FM modulator with a peak freq deviation deviation ΔΔf = 10kHz, a modulating signal f = 10kHz, a modulating signal freq ffreq fmm= 10kHz, V= 10kHz, Vc c =10V and 500kHz =10V and 500kHz carrier, determinecarrier, determine

a)a) Actual minimum bandwidth from the Actual minimum bandwidth from the Bessel function tableBessel function table

b)b) Approximate minimum bandwidth using Approximate minimum bandwidth using Carson’s ruleCarson’s rule

c)c) Plot the output freq spectrum for the Plot the output freq spectrum for the Bessel approximationBessel approximation

DEVIATION RATIO (DR)DEVIATION RATIO (DR)

Minimum bandwidth is greatest when Minimum bandwidth is greatest when maximum freq deviation is obtained with maximum freq deviation is obtained with the maximum modulating signal frequencythe maximum modulating signal frequency

Worst case modulation index and is equal Worst case modulation index and is equal to the maximum peak frequency deviation to the maximum peak frequency deviation divided by the maximum modulating divided by the maximum modulating signal frequencysignal frequency

Worst case modulation index produces the Worst case modulation index produces the widest output frequency spectrumwidest output frequency spectrum

Mathematically,Mathematically,max

(max)

max peak freq deviationDR

max mod signal freq m

f

f


• Determine the deviation ratio and Determine the deviation ratio and bandwidth for the worst case (widest bandwidth for the worst case (widest bandwidth) modulation index for an FM bandwidth) modulation index for an FM broadcast band transmitter with a broadcast band transmitter with a maximum frequency deviation of 75kHz and maximum frequency deviation of 75kHz and a maximum modulating signal frequency of a maximum modulating signal frequency of 15kHz15kHz

• Determine the deviation ratio and maximum Determine the deviation ratio and maximum bandwidth for an equal modulation index bandwidth for an equal modulation index with only half the peak frequency deviation with only half the peak frequency deviation and modulating signal frequencyand modulating signal frequency

The power in an angle-modulated signal is The power in an angle-modulated signal is easily computed easily computed

P = VP = VCC22/2R W/2R W

Thus the power contained in the FM signal is Thus the power contained in the FM signal is independent of the message signal. This is an independent of the message signal. This is an important difference between FM and AM. important difference between FM and AM.

The time-average power of an FM signal may The time-average power of an FM signal may also be obtained from also be obtained from ( ) cos(2 ( ))FM c cv t V f t t

POWER IN ANGLE-POWER IN ANGLE-MODULATED SIGNALMODULATED SIGNAL


An FM signal is given as An FM signal is given as vvFMFM(t)=12cos[(6π10(t)=12cos[(6π1066t) + 5sin(2π x t) + 5sin(2π x 1250t)] V. Determine1250t)] V. Determinea.a. freq of the carrier signalfreq of the carrier signal

b.b. freq of the modulating signalfreq of the modulating signal

c.c. modulation indexmodulation index

d.d. freq deviationfreq deviation

e.e. power dissipated in 10 ohm resistor.power dissipated in 10 ohm resistor.


Determine the unmodulated carrier Determine the unmodulated carrier power for the FM modulator given that power for the FM modulator given that ==1, V1, Vcc=10 V, R = 50 Ω. Then, determine =10 V, R = 50 Ω. Then, determine the total power in the angle-modulated the total power in the angle-modulated wave.wave.

Solution: Solution:

not exactly equal because values in not exactly equal because values in Bessel Bessel table have been rounded off.table have been rounded off.


An FM signal expressed asAn FM signal expressed asis measured in a 50 ohm antenna. Determine the is measured in a 50 ohm antenna. Determine the following :-following :-

a.a. total powertotal powerb.b. modulation indexmodulation indexc.c. peak freq deviationpeak freq deviationd.d. modulation sensitivity if 200 mV is required to modulation sensitivity if 200 mV is required to

achieve part cachieve part ce.e. amplitude spectrumamplitude spectrumf.f. bandwidth (99%) and approximate bandwidth by bandwidth (99%) and approximate bandwidth by

Carson’s ruleCarson’s ruleg.g. power in the smallest sideband of the 99% BWpower in the smallest sideband of the 99% BWh.h. total information powertotal information power

)102sin5.0102cos(1000)( 47 tttvFM


An FM signal with 5W carrier power An FM signal with 5W carrier power is fluctuating at the rate of 10000 is fluctuating at the rate of 10000 times per second from 99.96 MHz to times per second from 99.96 MHz to 100.04 MHz. Find100.04 MHz. Finda.a. carrier freqcarrier freq

b.b. carrier swingcarrier swing

c.c. freq deviationfreq deviation

d.d. modulation indexmodulation index

e.e. power spectrumpower spectrum


In an FM transmitter, the freq is changing In an FM transmitter, the freq is changing between 100 MHz to 99.98 MHz, 400 times between 100 MHz to 99.98 MHz, 400 times per seconds. The amplitude of the FM signal is per seconds. The amplitude of the FM signal is 5 V, determine :-5 V, determine :-

1.1. carrier and modulating freqcarrier and modulating freq

2.2. carrier freq swingcarrier freq swing

3.3. amplitude spectrumamplitude spectrum

4.4. bandwidth by using Bessel Table and bandwidth by using Bessel Table and Carson’s ruleCarson’s rule

5.5. average power at the transmitter if the average power at the transmitter if the modulator carrier power is 5 W.modulator carrier power is 5 W.

FM SIGNAL GENERATIONFM SIGNAL GENERATION

They are two basic methods They are two basic methods of generating frequency-of generating frequency-Modulated signals:Modulated signals:Direct MethodDirect Method Indirect MethodIndirect Method

DIRECT FMDIRECT FM In a direct FM system the instantaneous frequency is In a direct FM system the instantaneous frequency is

directly varied with the information signal. To vary the directly varied with the information signal. To vary the frequency of the carrier is to use an Oscillator whose frequency of the carrier is to use an Oscillator whose resonant frequency is determined by components that resonant frequency is determined by components that can be varied. The oscillator frequency is thus changed can be varied. The oscillator frequency is thus changed by the modulating signal amplitude. by the modulating signal amplitude.

• For example, an electronic Oscillator has an output For example, an electronic Oscillator has an output frequency that depends on energy-storage devices. frequency that depends on energy-storage devices. There are a wide variety of oscillators whose There are a wide variety of oscillators whose frequencies depend on a particular capacitor value. By frequencies depend on a particular capacitor value. By varying the capacitor value, the frequency of oscillation varying the capacitor value, the frequency of oscillation varies. If the capacitor variations are controlled by varies. If the capacitor variations are controlled by vvmm(t), (t), the result is an FM waveformthe result is an FM waveform

( )i c f mf f k v t

INDIRECT FMINDIRECT FM

Angle modulation includes frequency modulation Angle modulation includes frequency modulation FM and phase modulation PM.FM and phase modulation PM.

FM and PM are interrelated; one cannot change FM and PM are interrelated; one cannot change without the other changing. The information signal without the other changing. The information signal frequency also deviates the carrier frequency in PM.frequency also deviates the carrier frequency in PM.

Phase modulation produces frequency modulation. Phase modulation produces frequency modulation. Since the amount of phase shift is varying, the effect Since the amount of phase shift is varying, the effect is that, as if the frequency is changed.is that, as if the frequency is changed.

Since FM is produced by PM , the later is referred Since FM is produced by PM , the later is referred to as indirect FM. to as indirect FM.

The information signal is first integrated and then The information signal is first integrated and then used to phase modulate a crystal-controlled used to phase modulate a crystal-controlled oscillator, which provides frequency stability.oscillator, which provides frequency stability.

NOISE AND PHASE SHIFTNOISE AND PHASE SHIFT

The noise amplitude added to an FM The noise amplitude added to an FM signal introduces a small frequency signal introduces a small frequency variation or phase shift, which variation or phase shift, which changes or distorts the signal.changes or distorts the signal.

Noise to signal ratio N/S Noise to signal ratio N/S

Signal to noise ration S/NSignal to noise ration S/N

deviationallowedMaximum

noisebyproduceddeviationFrequency

S

N

SNN

S 1

INTERFERENCEINTERFERENCE

A major benefit of FM is that interfering signals on the A major benefit of FM is that interfering signals on the same frequency will be effectively rejected. same frequency will be effectively rejected.

If the signal of one is more than twice the amplitude of If the signal of one is more than twice the amplitude of the other, the stronger signal will "capture" the the other, the stronger signal will "capture" the channel and will totally eliminate the weaker, channel and will totally eliminate the weaker, interfering signal. interfering signal.

This is known as the This is known as the capture effect capture effect in FM. in FM. In FM, the capture effect allows the stronger signal to In FM, the capture effect allows the stronger signal to

dominate while the weaker signal is eliminated.dominate while the weaker signal is eliminated. However, when the strengths of the two FM signals However, when the strengths of the two FM signals

begin to be nearly the same, the capture effect may begin to be nearly the same, the capture effect may cause the signals to alternate incause the signals to alternate in their domination of the their domination of the frequency.frequency.

Despite the fact that FM has superior noise Despite the fact that FM has superior noise rejection qualities, noise still interferes with an rejection qualities, noise still interferes with an FM signal. This is particularly true for the high-FM signal. This is particularly true for the high-frequency components in the modulating signal. frequency components in the modulating signal.

Since noise is primarily sharp spikes of energy, it Since noise is primarily sharp spikes of energy, it contains a considerable number of harmonics and contains a considerable number of harmonics and other high-frequency components. other high-frequency components.

These high frequencies can at times be larger in These high frequencies can at times be larger in amplitude than the high-frequency content of the amplitude than the high-frequency content of the modulating signal. modulating signal.

This causes a form of frequency distortion that This causes a form of frequency distortion that can make the signal unintelligible.can make the signal unintelligible.

To overcome this problem Most FM system use a To overcome this problem Most FM system use a technique known as Pre-emphasis and De-technique known as Pre-emphasis and De-emphasis.emphasis.

chapter 4 frequency modulation

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