.Amplitude Modulation and Problems in AM

Amplitude Modulation

1. Types of communication
system

3. Types of communication system
1)Analogcommunicationsystem
•Itisdesignedtotransmitanaloginformation
usinganalogmodulationschemessuchas
amplitudemodulationandanglemodulation.
2)Digitalcommunicationsystem
•Itisdesignedtotransmitdigitalinformation
usingdigitalmodulationschemessuchasPCM,
DM,DPCMetc.
3)Hybridcommunicationsystem
•Itisdesignedtousedigitalmodulation
schemesfortransmittingsampledand
quantizedvalueofanalogsignals.(Both)

4. Definition of Modulation:
•Modulationistheprocessbywhichsome
characteristicsofacarrierwaveisvariedin
accordancewiththeinstantaneousvalueofa
modulatingsignalormessagesignal.
•Theresultantwaveiscalledmodulatedwaveor
radiowave.

5. Amplitude Modulation

6. Definition
•Amplitudemodulationmaybedefinedasa
systeminwhichtheamplitudeofthe
carrierwavechangesinaccordancewith
theinstantaneousvalueofthemodulating
signal.
•Carriersignalfrequencyandphaseremain
constant.

7. Definition
•Letthecarrierandmodulating(message)
signalbe

11. Am<Ac.
Forthis,ma<1is
oneoftheconditions
forproperAM
transmission.

13. ECE 457 Spring 2005

15. Frequency spectrum of AM signal

16. 0 500100015002000250030003500400045005000
0
0.5
1
1.5
Frequency (Hz)
|M(f)| 0 500100015002000250030003500400045005000
0
0.5
1
1.5
Frequency (Hz)
|Y(f)| Frequency spectrum of AM signal

17. Microwave and optical Lab
7.5 GHZ spectrum analyzer

18. 1.05 GHZ spectrum analyzer –Communication Lab

19. Graphical representation of AM wave.

22. Phase representation of AM with carrier.
•ItistheeasyrepresentationofAM,whereAcisthecarrier
wavephasor(takenasreferencephasor).
•Thetwosidebandshavingafrequencyof(ω
c+ω
m)and(ω
c-ω
m)
arerepresentedbytwophasorsrotatinginoppositedirections
withangularfrequencyofωm.
•Thenetorresultantphasoriss(t).Itdependsontheposition
ofthesidebandphasorandcarrierwavephasor.

23. Modulation Index
•Therearethreedegrees(depth)ofmodulation
dependingupontheamplitudeofthemessage
signalrelativetocarrieramplitude.Theyare
(i)Undermodulation
(ii)Overmodulation
(iii)Criticalmodulation

24. Under modulation ( ma <1 )
•In this case the modulation index ma <1 , ie. Am <Ac.
•It may be observed that the envelope is not reaching the zero
amplitude axis of the AM wave form.
•So the baseband signal may be fully recovered from the
envelope of AM wave.

25. Over modulation (ma > 1)
•Inthiscase,theamplitudeofabasebandsignalexceedsmaximum
carrieramplitudeAm>Ac.
•Thereforecertainportionoftheenvelopeofthemodulated
signalcrossesthezeroaxis.
•Sobothpositiveandnegativeextensionsofmodulatingsignalare
cancelledorclippedout

26. Critical modulation (ma = 1 )
•In this case the modulation index ma = 1 i.e., Am = Ac.
•Here envelope of the modulated signal just reaches the zero
amplitude axis.
•In this case also the modulated signal can be recovered by using
envelope detector without any distortion.
Themodulatingsignalispreserved
intheenvelopeofamplitude
modulatedsignalonlyifAm<Ac.

27. Trapezoidal modulation patterns

28. Power relations in AM

29. Power relations in AM
•Thecarriercomponentofthemodulatedwavehasthesame
amplitudeastheun-modulatedcarrier.
•Thatis,theamplitudeofthecarrierisunchanged.
•Themodulatedwavecontainsextraenergyinthetwosideband
components.
•Therefore,themodulatedwavecontainsmorepowerthanthe
carrierhadbeforemodulationtookplace.

30. Power relations in AM -contd…
•The amplitude of the sidebands depends
on the modulation index Am/Ac.
•The total power in the modulated wave
will depend on the modulation index.
•The total power in the modulated wave
will be

31. Power relations in AM -contd…
•Whereallthreevoltagesare(rms)
values(1√2peak)andRisthe
resistance(e.g.,antennaresistance),in
whichthepowerisdissipated.
•The unmodulated carrier power
component is,

32. Power relations in AM -contd…
•The power components due to side
bands are,

33. Power relations in AM -contd…

34. Current Relation in AM
I
t= total or modulated current
I
c= carrier current

35. Power Efficiency of AM wave
•It can be defined as the ratio of power
in sidebands to total power.
•Because sidebands only contain the
useful information ( modulating or
message signal).

36. Power Efficiency of AM wave contd…

37. Multiple-Tone Amplitude Modulation

38. Power Content in Multiple-Tone Amplitude
Modulation
•Amultitoneamplitudemodulationisa
typeofmodulationinwhichthe
modulatingsignalconsistsofmorethan
onefrequencycomponents.
•LetusconsiderthatacarriersignalAc
cosωctismodulatedbyamodulating
signalVm(t).

40. WWWW-World Wide Wireless Web

41. Power Content in Multiple-Tone Amplitude
Modulation contd…

42. Power Content in Multiple-Tone Amplitude
Modulation contd…

43. Power Content in Multiple-Tone Amplitude
Modulation contd…

44. Power Content in Multiple-Tone Amplitude
Modulation contd…

46. ECE 457 Spring 2005

48. MainPointsofAM

49. 6.TheefficiencyofAMwavedependsonmodulationindex
onlyanddoesnotdependoneitherfrequenciesof
modulatingsignalorcarrier.
7.ThemaximumefficiencyofAMoccurswhenand
Maximumefficiencyis33.33%.
8.ThemaindisadvantageofAMiswastageofcarrierpower.
9.AnothermajordisadvantageofAMiswastageofbandwidth
whichistwiceofmessagesignalbandwidth.
10.ThemainadvantageofAMisthat,sinceacoherent
referenceisnotneededfordemodulationaslongas,the
demodulatorbecomessimpleandinexpensive.
11.Inmanyapplications,suchascommercialbroadcast,AM
issufficienttojustifyitsuse.
MainPointsofAM

50. 1.Amplitudemodulationiswastefuloftransmittedpower.
Thetransmissionofthecarrierwaverepresentsawasteof
power.
2.Amplitude modulation is wasteful of channel bandwidth
•Asfarasthetransmissionofinformationisconcerned,
onlyonesidebandisnecessary,andthecommunication
channelthereforeneedstoprovideonlythesame
bandwidthasthemessagesignal.
•ButAMrequiresatransmissionbandwidthequalto
twicethemessagesignalbandwidth.
Limitationsof AM

51. •clc;
•clear all;
•close all;
•Ac=2; %carrier amplitude
•fc=0.5; %carrier frequency
•Am=.5; %message signal amplitude
•fm=.05; %message signal frequency
•Fs=100; %sampling rate/frequency
•ma=1;%Amplitude Sensitivity
•t=[0:0.1:50];%defining the time range & disseminating it into samples
•ct=Ac*cos(2*pi*fc*t); %defining the carrier signal wave
•mt=Am*cos(2*pi*fm*t); %defining the message signal
•AM=ct.*(1+ma*mt); %Amplitude Modulated wave, according to the standard definition
•subplot(3,1,1);%plotting the message signal wave
•plot(mt);
•ylabel('Message signal');
•subplot(3,1,2); %plotting the carrier signal wave
•plot(ct);
•ylabel('carrier');
•subplot(3,1,3); %plotting the amplitude modulated wave
•plot(AM);
•ylabel('AM signal');
MATLAB

52. WiMAX

53. Worldwide Interoperability for
Microwave Access (Wimax)

54. Problems 1

55. Problems 1

59. Problems 2

60. Problems 2

76. Ref: https://slidetodoc.com/analog-communications-dr-m-venu-gopala-rao-a-2/