Frequency Modulation PPT

(1)

(2)

What is Angle

What is Angle Modulation?

Modulation?



 In angle modulation, information isIn angle modulation, information is embedded in the

embedded in the angle angle of the carrier.of the carrier.



 We define the angle of a modulated carrierWe define the angle of a modulated carrier by the argument of...

by the argument of...

s

(3)

Phasor Form



 In the complex plane we haveIn the complex plane we have

t=1 t=1 t=0 t=0 t=3 t=3

Phasor rotates with nonuniform speed

(4)

Angular Velocity



 Since phase changes nonuniformly vs.Since phase changes nonuniformly vs. time, we can define a rate of change

time, we can define a rate of change



 This This is is what what we we know know as as frequencyfrequency

  _ii  d d   ii((t t )) dt dt s s t   t  AA_cc ccooss 22  f f _cct t   _cc   _ii   t t            d d   ii dt dt  22  f f cc

(5)

Instantaneous Frequency



 We are used to signals with constantWe are used to signals with constant

carrier frequency. There are cases where

carrier frequency itself changes with time.



 We can therefor talk aboutWe can therefor talk about instantaneous instantaneous

frequency

frequency defined asdefined as

f f _ii

  

t t  11 2 2  d d   _ii

  

t t dt dt

(6)

Examples of Inst. Freq.



 Consider an AM signalConsider an AM signal



 Here, the instantaneous frequency is theHere, the instantaneous frequency is the frequency itself, which is constant

frequency itself, which is constant

s s t   t 

 

11 kmkm((t t ))



ccooss 22  f f _cct t    _cc   _ii  t t            d d   ii dt dt  22  f f cc

(7)

Impressing a message on

the angle of carrier



 There are two ways to form a an angleThere are two ways to form a an angle modulated signal.

modulated signal.

–

– Embed it in the phase of the carrier Embed it in the phase of the carrier

Phase Modulation(PM)

–

– Embed it in the Embed it in the frequency of the carrier frequency of the carrier

Frequency

(8)

Phase Modulation(PM)



 In PM, carrier angle changes linearly withIn PM, carrier angle changes linearly with the message

the message



 WhereWhere

–

– 2πf 2πf _c_c=angle of unmodulated carrier =angle of unmodulated carrier

–

– kk_p_p=phase sensitivity in radians/volt=phase sensitivity in radians/volt

s

(9)

Frequency Modulation



 In FM, it is the instantaneous frequencyIn FM, it is the instantaneous frequency that varies linearly with message

that varies linearly with message

amplitude, i.e.

f

(10)

FM Signal



 We saw that I.F. is the derivative of theWe saw that I.F. is the derivative of the phase phase   Therefore,Therefore, f f _ii

  

t t  11 2 2  d d   _ii

  

t t dt dt   _ii  t t  22  f f _cct t  22  k k _f_f m m t   t 0 0 t t



s s t

  

t  AA_cc cocoss 22  f f _cct t  22  k k _f_f mm((t t ))dt dt 0 0 t t



           

(11)

FM for Tone Signals



 Consider a sinusoidal messageConsider a sinusoidal message



 The instantaneous frequencyThe instantaneous frequency

corresponding to its FM version is

m m((t t ))  AA_m_m ccosos 2



2  f f _m_mt t f f _ii  t t  f f _cc  k k _f_fmm((t t ))  f f _cc resting resting frequen frequencycy  k k _f_fAA_m_m ccooss 2



2  f f _m_mt t



(12)

Illustrating FM

0 0 00..0011 00..0022 0..00033 00..0044 00..0055 00..0066 00..0077 0..00088 00..0099 00..11 -1 -1 -0.8 -0.8 -0.6 -0.6 -0.4 -0.4 -0.2 -0.2 0 0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1 1 FM FM message message Inst.frequency Inst.frequency

Moves with the

Message amplitude

(13)

Frequency Deviation



 Inst. frequency has upper and lowerInst. frequency has upper and lower bounds given by bounds given by f f _ii  t t  f f _cc   f f ccooss 22___f_f m mt t



 where where 

 f f  frequencyfrequency deviationdeviation k k _f_f A A_m_m then then f f _ii _max_max  f f _cc    f f f f _ii _mi_min_n  f f _cc   f f

(14)

FM Modulation index



 The equivalent of AM modulation index isThe equivalent of AM modulation index is which is also called

which is also called deviation ratio deviation ratio . It. It

quantifies how much carrier frequency

swings relative to message bandwidth



   f f W W

ba

ba seb seband and

or or  f f f f _m_m tone tone

(15)

Example:carrier swing



 A 100 MHz FM carrier is modulated by anA 100 MHz FM carrier is modulated by an audio tone causing 20 KHz frequency

audio tone causing 20 KHz frequency

deviation.

deviation. Determine Determine the the carrier carrier siwngsiwng

and highest and lowest carrier frequencies



 f f  2020 K KHz Hz fr

freeqquencyuency swing swing  22 f f  4040 K KHz Hz fr

freeqquencyuency rangerange:: f

f _high_high 100100 M MHz Hz  2020 K KHz Hz  100.02 M100.02 MHz Hz f

(16)

Example: deviation ratio



 What is the modulation index (or deviationWhat is the modulation index (or deviation ratio) of an FM signal with carrier swing of

ratio) of an FM signal with carrier swing of

150 KHz when the modulating signal is 15

KHz? KHz?   f f  150150 2 2  7575 K KHz Hz     f f f f _m_m  75 75 1 155  55

(17)

Myth of FM



 Deriving FM bandwidth is a lot moreDeriving FM bandwidth is a lot more involved than AM

involved than AM



 FM was initially thought to be a bandwidthFM was initially thought to be a bandwidth efficient communication because it was

efficient communication because it was

thought that FM bandwidth is simply 2

thought that FM bandwidth is simply 2 ff



 By keeping frequency deviation low, weBy keeping frequency deviation low, we can use arbitrary small bandwidth

(18)

FM bandwidth



 Deriving FM bandwidth is a lot moreDeriving FM bandwidth is a lot more involved than AM and it can barely be

involved than AM and it can barely be

derived for sinusoidal message



 There is a graphical way to illustrate FMThere is a graphical way to illustrate FM bandwidth

(19)

Piece-wise approximation of

baseband



 Look at the following representationLook at the following representation

1/2W 1/2W Baseband bandwidth Baseband bandwidth =W =W

(20)

Corresponding FM signal



 FM version of the above is an RF pulse forFM version of the above is an RF pulse for each square pulse.

each square pulse.



 The frequency of the kth RF pulse at t=tThe frequency of the kth RF pulse at t=t_k_k isis given by the height of the pulse. i.e.

given by the height of the pulse. i.e.

f

(21)

Range of frequencies?



 We have a bunch of RF pulses each at aWe have a bunch of RF pulses each at a different frequency.

different frequency.



 Inst.freq corresponding to square pulsesInst.freq corresponding to square pulses lie in the following range

lie in the following range

f

f _{ii m}_max_ax  f f _cc  k k _f_fmm_m_max_ax f

f _{ii m}_min_in  f f _cc  k k _f_fmm_m_min_in

m

(22)

A look at the spectrum



 We will have a series of RF pulses each atWe will have a series of RF pulses each at a different frequency. The collective

a different frequency. The collective

spectrum is a bunch of sincs

f f highest highest lowest lowest 4W 4W

(23)

So what is the

So what is the bandwidth?

bandwidth?



 Measure the width from the first upperMeasure the width from the first upper zero crossing of the highest term to the

zero crossing of the highest term to the

first lower zero crossing of the lowest

term term f f highest highest lowest lowest

(24)

Closer look



 The highest sinc is located at fThe highest sinc is located at f_c_c+k+k_f_fmm_p_p



 Each sinc is 1/2W wide. Therefore, theirEach sinc is 1/2W wide. Therefore, their zero crossing point is always 2W above

zero crossing point is always 2W above

the center of the sinc.

f

2W

(25)

Range of frequenices



 Above range liesAbove range lies <f <f_c_c-k-k_f_fmm_p_p-2W,f-2W,f_c_c+k+k_f_fmm_p_p+2W>+2W> f f highest highest lowest lowest

(26)

FM bandwidth



 The range just defined is one expressionThe range just defined is one expression for FM bandwidth. There are many more!

for FM bandwidth. There are many more!

B

B_FM_FM=4W+2k=4W+2k_f_fmm_p_p



 UsingUsing =∆f/W with ∆f=k=∆f/W with ∆f=k_f_fmm_p_p B

(27)

Carson’s Rule



 A popular expression for FM bandwidth isA popular expression for FM bandwidth is

Carson’s rule. It is a bit smaller than what Carson’s rule. It is a bit smaller than what

we just saw

B

(28)

Commercial FM



 Commercial FM broadcasting uses theCommercial FM broadcasting uses the following parameters

following parameters

–

– Baseband;15KHzBaseband;15KHz

–

– Deviation ratio:5Deviation ratio:5

–

– Peak freq. Deviation=75KHzPeak freq. Deviation=75KHz

B

(29)

Wideband vs. narrowband

FM



 NBFM is defined by the conditionNBFM is defined by the condition

–

– ∆f<<W ∆f<<W BB_FM_FM=2W=2W

–

– This is just like AM. No advantage hereThis is just like AM. No advantage here



 WBFM is defined by the conditionWBFM is defined by the condition

–

– ∆f>>W ∆f>>W BB_FM_FM=2 ∆f =2 ∆f

–

(30)

Boundary between narrowband and

wideband FM



 This distinction is controlled byThis distinction is controlled by

–

– If If >1 --> WBFM>1 --> WBFM

–

– If If <1-->NBFM<1-->NBFM



 Needless to say there is no point for goingNeedless to say there is no point for going with NBFM because the signal looks and

with NBFM because the signal looks and

sounds more like AM

(31)

Commercial FM spectrum



 The FM landscape looks like thisThe FM landscape looks like this

FM station B

FM station A

FM station A _{FM station C}_{FM station C}

25KHz guardband 25KHz guardband 150 KHz 150 KHz 200 KHz 200 KHz carrier carrier

(32)

FM stereo:multiplexing



 First, two channels are created; (left+right)First, two channels are created; (left+right) and (left-right)

and (left-right)



 Left+right is useable by monauralLeft+right is useable by monaural receivers receivers -Left channel Left channel Right channel Right channel + + + + + + mono mono

(33)

Subcarrier modulation



 The mono signal is left alone but theThe mono signal is left alone but the

difference channel is amplitude modulated

with a 38 KHz carrier with a 38 KHz carrier Left channel Left channel Right channel Right channel + + + + + + mono mono DSB-SC DSB-SC f f _sc_sc=38 kHz=38 kHz + + fsc= fsc= 38KHz 38KHz freq freq divider divider Composite baseband Composite baseband

(34)

-Stereo signal

Stereo signal



 Composite baseband signal is thenComposite baseband signal is then frequency modulated frequency modulated Left channel Left channel Right channel Right channel + + + + + + mono mono DSB-SC DSB-SC f f _sc_sc=38 kHz=38 kHz + + fsc= fsc= 38KHz 38KHz freq freq divider divider Composite baseband Composite baseband FM FM transmitter transmitter

(35)

-Stereo spectrum

Stereo spectrum



 Baseband spectrum holds all theBaseband spectrum holds all the

information. It consists of composite

baseband, pilot tone and DSB-SC

spectrum spectrum 38 KHz 38 KHz 19 KHz 19 KHz 15 KHz 15 KHz Left+right Left+right DSB-SC DSB-SC

(36)

Stereo receiver



 First, FM is stripped, i.e. demodulatedFirst, FM is stripped, i.e. demodulated



 Second, composite baseband is lowpassSecond, composite baseband is lowpass filtered to recover the left+right and in

filtered to recover the left+right and in

parallel amplitude demodulated to recover

the left-right signal

38 KHz 38 KHz 19 KHz 19 KHz 15 KHz 15 KHz Left+right Left+right DSB-SC DSB-SC

(37)

Receiver diagram

FM FM receiver receiver lowpass lowpass filter(15K) filter(15K) bandpass bandpass at 38KHz at 38KHz X lowepass X lowepass VCO VCO Divide 2 Divide 2 X lowpass X lowpass + + + + -+ + + + + + Left+right

Left+right _left_left

right right PLL PLL coherent detector coherent detector 38 KHz 38 KHz 19 KHz 19 KHz 15 KHz 15 KHz

(38)

Subsidiary communication

authorization(SCA)



 It It is is possible possible to to transmit transmit “special“special

programming” ,e.g. commercial

programming” ,e.g. commercial-free-free

music for banks, department stores etc.

embedded in the regular FM programming



 Such programming is frequencySuch programming is frequency

multiplexed on the FM signal with a 67

KHz carrier and

(39)

SCA spectrum

38 KHz 38 KHz 19 KHz 19 KHz 15 KHz 15 KHz Left+right Left+right DSB-SC DSB-SC 59.5 59.5 67 67 74.5 74.5 f(KHz)f(KHz) SCA signal SCA signal

(40)

FM receiver



 FM receiver is similar to the superhetFM receiver is similar to the superhet layout layout RF RF mixer mixer LO LO limiter

limiter Discrimi-

Discrimi-nator

nator deemphasisdeemphasis

AF power AF power amp amp IF IF

(41)

Frequency demodulation



 Remember that message in an FM signalRemember that message in an FM signal is in the instantaneous frequency or

is in the instantaneous frequency or

equivalently derivative of carrier angle

s s t

  

t  AA_cc cocoss 22  f f _cct t  22  k k _f_f mm((t t ))dt dt 0 0 t t



             s s t

  

t  AA_cc



2 2 f f _cc  22  k k _f_fm m t

  

t



ssiinn 22  f f _cct t  22  k k _f_f mm((t t ))dt dt   t t



          Do envelope detection on s’(t) Do envelope detection on s’(t)

(42)

Receiver components:RF

amplifier



 AM may skip RF amp but FM requires itAM may skip RF amp but FM requires it



 FM receivers are called upon to work withFM receivers are called upon to work with weak signals (~1

weak signals (~1 V or less as compared toV or less as compared to

30

30 V for AM)V for AM)



 An RF section is needed to bring up theAn RF section is needed to bring up the signal to at least 10 to 20

(43)

Limiter



 A limiter is a circuit whose output isA limiter is a circuit whose output is constant for

constant for all all input input amplitudes amplitudes above above aa

threshold



 Limiter’s function in an FM receiver is toLimiter’s function in an FM receiver is to remove unwanted amplitude variations of

remove unwanted amplitude variations of

the FM signal

Limiter

(44)

Limiting and sensitivity



 A limiter needs about 1V of signal, calledA limiter needs about 1V of signal, called

quieting

quieting oror threshold threshold voltage, voltage, to to beginbegin

limiting



 When enough signal arrives at theWhen enough signal arrives at the

receiver to start limiting action, the set

quiets, i.e. background noise disappears



 Sensitivity is the min. RF signal toSensitivity is the min. RF signal to

produce a specified level of quieting,

normally

(45)

Sensitivity example



 An FM receiver provides a voltage gain ofAn FM receiver provides a voltage gain of 200,000(106dB) prior to its limiter. The

200,000(106dB) prior to its limiter. The limiter’s quieting voltage is 200 mV.

limiter’s quieting voltage is 200 mV. WhatWhat

is the

is the receiver’s sensitivity?receiver’s sensitivity?



 What we are really asking is the requiredWhat we are really asking is the required

signal at RF’s input to produce

signal at RF’s input to produce 200 mV at200 mV at

the output

200 mV/200,000= 1

(46)

Discriminator



 The heart of FM is this relationshipThe heart of FM is this relationship



 What we need is a device that linearlyWhat we need is a device that linearly follows inst. frequency

follows inst. frequency

f f _i_i(t)=f (t)=f _c_c+k +k _f_fm(t)m(t) Disc.output Disc.output f f Deviation limits Deviation limits +75 KHz +75 KHz -75 KHz -75 KHz f f _carrier_carrier f

f _carrier_carrieris at the IF frequencyis at the IF frequency

Of 10.7 MHz

(47)

Examples of discriminators



 Slope detector - simple LC tank circuitSlope detector - simple LC tank circuit

operated at its most linear response curve

This setup turns an FM signal

into an AM into an AM f f _c_c f f _o_o output output f f

(48)

Phase-Locked Loop



 PLL’s are increasingly used as FMPLL’s are increasingly used as FM

demodulators and appear at IF output

Phase Phase comparator comparator Lowpass Lowpass filter filter VCO VCO fin

fin Error Error signalsignal

f

f _vco_vco VCO inputVCO input

Control signal:constant

When f

When f _in_in=f =f _vco_vco

Output proportional to

Difference between f

(49)

PLL states



 Free-runningFree-running

–

– If the input and VCO frequency are too far apart,If the input and VCO frequency are too far apart,

PLL free-runs



 CaptureCapture

–

– Once Once VCO VCO closes in closes in on on the the input frequency, PLLinput frequency, PLL

is said to be in the tracking or capture mode



 Locked or trackingLocked or tracking

–

– Can stay locked over a wider range than wasCan stay locked over a wider range than was

necessary for capture

(50)

PLL example



 VCO free-runs at 10 MHZ. VCO does notVCO free-runs at 10 MHZ. VCO does not change frequency until the input is within

change frequency until the input is within

50 KHZ.



 In the tracking mode, VCO follows theIn the tracking mode, VCO follows the input to ±200 KHz of 10 MHz before

input to ±200 KHz of 10 MHz before losinglosing

lock. What is the lock and capture range?

–

– Capture range= 2x50KHz=100 KHzCapture range= 2x50KHz=100 KHz

–

(51)

Advantages of PLL



 If there is a carrier center frequency or LOIf there is a carrier center frequency or LO frequency drift, conventional detectors

frequency drift, conventional detectors

will be untuned



 PLL, on the other hand, can correct itself.PLL, on the other hand, can correct itself.

PLL’s need no tuned circuits PLL’s need no tuned circuits

f f _c_c f f _o_o output output f f If f

If f _c_c drifts detector has no way of drifts detector has no way of

correcting itself

Slope detector

(52)

Zero crossing detector

Hard Hard limiter limiter Zero Zero Crossing Crossing detector detector Multi-vibrator vibrator Averaging Averaging circuot circuot FM Output FM Output FM input FM input Hard limiter Hard limiter ZC detector ZC detector multiV multiV more frequent more frequent ZC’s means ZC’s means

higher inst freq

in turn means in turn means Larger message Larger message amplitudes amplitudes Averaging circuit Averaging circuit

(53)

NOISE IN ANALOG

MODULATION

AMPLITUDE MODULATION AMPLITUDE MODULATION

(54)

Receiver Model



 The objective here is to establish aThe objective here is to establish a

relationship between input and and output

SNR of an AM receiver SNR of an AM receiver BPF detector BPF detector Noise n(t) Noise n(t) Modulated signal s(t)l Modulated signal s(t)l output output filter filter f f _c_c -f -f _c_c B B_T_T=2W=2W

(55)

Establishing a reference

SNR



 Define “channel” SNR measured atDefine “channel” SNR measured at receiver input

receiver input

(SNR)

(SNR)_c_c=avg. power of modulated signal/=avg. power of modulated signal/

avg. noise power in the message bandwidth

(56)

Noise in DSB-SC Receiver



 Tuner plus coherent detectionTuner plus coherent detection

BPF LPF BPF LPF DSB-SC DSB-SC n(t) n(t) _Cos(2πfct)_Cos(2πfct) x(t) v(t) x(t) v(t) s(t) s(t) s s t

  

t  AA_ccmm((t t ) c) cooss 2



2  f f _cct t



 ss22

  

t t  avg avg .. pow power er  AA_cc22  mm22((t t ))  // 22  AA_cc22 P P // 22 P

(57)

Receiver input SNR



 Also defined as channel SNR:Also defined as channel SNR:

((SNSNRR))_cc  AAcc 2 2 P P // 22 W WN N _o_o noi

noisse power in te power in the message bandwidthhe message bandwidth

 AAcc 2 2 P P 2 2WWN N _o_o W W -W -W No/2 No/2

Flat noise spectrum:white noise

Noise power=hatched area

(58)

Output SNR



 Carrying signal and noise through the restCarrying signal and noise through the rest of the receiver, it can be shown that

of the receiver, it can be shown that

output SNR comes out to be equal to the

input. Hence



 Therefore, any reduction in input SNR isTherefore, any reduction in input SNR is linearly reflected in the output

linearly reflected in the output

SNR SNR

 



_o_o SNR SNR

 



_cc  11

(59)

(SNR)

_o

for DSB-AM



 Following a similar approach,Following a similar approach,



 Best case is achieved for 100%Best case is achieved for 100% modulation index which, for tone

modulation index which, for tone

modulation, is only 1/3 modulation, is only 1/3 SNR SNR

 



_o_o SNR SNR

 



_cc  k k 22 P P 1 1  k k 22 P P 11 k

k :: AM AM mmodulatiodulation indexon index P

(60)

DSB-AM and DSB-SC noise

performance



 An AM system using envelope detectionAn AM system using envelope detection needs 3 times as much power to achieve

needs 3 times as much power to achieve

the same output SNR as a suppressed

carrier AM with coherent detection



 This is a result similar to power efficiencyThis is a result similar to power efficiency of the two schemes

(61)

Threshold effect-AM



 In DSB-AM (not DSB-SC) there is aIn DSB-AM (not DSB-SC) there is a phenomenon called

phenomenon called threshold effect threshold effect



 This means that there is a massive drop inThis means that there is a massive drop in output SNR if input SNR drops below a

output SNR if input SNR drops below a

threshold



 For DSB-AM with envelope detection, thisFor DSB-AM with envelope detection, this threshold is about 6.6 dB

(62)

NOISE IN ANALOG

MODULATION

FREQUENCY MODULATION FREQUENCY MODULATION

(63)

Receiver model



 Noisy FM signal at BPF’s output isNoisy FM signal at BPF’s output is

BFP Limiter BFP Limiter FMFM detector detector LPF LPF (W) (W) n(t) n(t) FM FM s(t) s(t) x x t   t  s s t   t  nn((t t ))  A

A_cc ccooss 2



2  f f _cct t   t   t



 r r ((t t ) c) cooss 2



2  f f _cct t   t   t



noise noise where where   t   t 



mm((t t ))dt dt

(64)

Phasor model



 We can see the effect of noise graphicallyWe can see the effect of noise graphically

reference



(t)(t) (t)(t)

The angle FM detector will extract



(65)

Small noise



 For small noise, it can be approximatedFor small noise, it can be approximated that the noise inflicted phase error is

that the noise inflicted phase error is =[r⁄Ac]Sin(

=[r⁄Ac]Sin(



 So the angle available to the FM detectorSo the angle available to the FM detector is

is ++



 FM Detector computes the derivative ofFM Detector computes the derivative of this angle. It will then follow that...

(66)

FM SNR for tone modulation



 Skipping Skipping further further detail, detail, we we can can show show thatthat for tone modulation, we have the following

for tone modulation, we have the following

ratio



 SNR rises as power of 2 of bandwidth; e.g.SNR rises as power of 2 of bandwidth; e.g. doubling deviation ratio quadruples the

doubling deviation ratio quadruples the

SNR SNR SN SNRR

 



_o_o SN SNRR

 



_cc  3 3 2 2   2 2 Bandwidth-SNR exchange Bandwidth-SNR exchange

(67)

Comparison with AM



 In DSB-SC the ratio was 1 regardless.In DSB-SC the ratio was 1 regardless.



 For commercial FM,For commercial FM, =5. Therefore,=5. Therefore, (SNR)

(SNR)_o_o/(SNR)/(SNR)_c_c=(1.5)x25=37.5=(1.5)x25=37.5



(68)

Capture effect in FM



 An FM receiver locks on to the stronger ofAn FM receiver locks on to the stronger of two received signals of the same

two received signals of the same

frequency and suppresses the weaker one



 Capture ratio is the necessaryCapture ratio is the necessary

difference(in dB) between the two signals

for capture effect to go into action



(69)

Normalized transmission

bandwidth



 With all these bandwidths numbers, it isWith all these bandwidths numbers, it is good to have a normalized quantity.

good to have a normalized quantity.



 DefineDefine

normalized bandwidth=B

normalized bandwidth=B_n_n=B=B_T_T/W/W

Where W is the baseband bandwidth

(70)

Examples of B

_n



 For AM:For AM:

B B_n_n=B=B_T_T/W=2W/W=2/W=2W/W=2   For FMFor FM B B_n_n=B=B_T_T/W~2/W~2 to 3to 3 

 ForFor =5 in commercial FM, this is a very=5 in commercial FM, this is a very large expenditure in bandwidth which is

large expenditure in bandwidth which is

rewarded in increased SNR

(71)

Noise/bandwidth summary



 AM-envelope detectionAM-envelope detection

SNR SNR

 



_o_o    22 2 2    22

 

SNRSNR



cc B B_n_n  22

(72)

Noise/bandwidth summary



 DSB-SC/coherent detectionDSB-SC/coherent detection

(SNR) (SNR)_o_o=(SNR)=(SNR)_c_c B B_n_n=2=2   SSBSSB (SNR) (SNR)_o_o=(SNR)=(SNR)_c_c B B_n_n=1=1

(73)

Noise/bandwidth summary



 FM-tone modulation andFM-tone modulation and =5=5 (SNR) (SNR)_o_o=1.5=1.5 22_(SNR)_(SNR) c c=37.5 (SNR)=37.5 (SNR)cc B B_n_n~16 for~16 for =5=5

(74)

Preemphasis and

deemphasis



 High pitched sounds are generally ofHigh pitched sounds are generally of lower amplitude than bass. In FM lower

lower amplitude than bass. In FM lower

amplitudes means lower frequency

deviation hence lower SNR.



 Preemphasis is a technique where highPreemphasis is a technique where high frequency components are amplified

frequency components are amplified

before modulation



 Deemphasis network returns theDeemphasis network returns the baseband to its original form

(75)

Pre/deemphasis response



 Flat up to ~500Hz, rises from 500-15000 HzFlat up to ~500Hz, rises from 500-15000 Hz

500 500 Hz Hz 2120 2120 Hz Hz 15KHz15KHz -17dB -17dB 17dB 17dB +3dB +3dB -3dB -3dB preemphasis preemphasis deemphasis deemphasis Deemphasis circuit Deemphasis circuit

Is between the detector

And the audio amplifier

(76)

Suggested homework

  3.413.41   5.35.3   5.75.7