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ANALOG MODULATION
CHAPTER 2
Modulation is the process of having an information signal modify a carrier signal in some way
Modulation is operation performed at the transmitter to achieveefficient and reliable information transmission
Modulation
MODULATION PROCESS
Baseband signal
Modulated signal
Carrier
Amplitude Modulation is the process of changing the amplitude of a relatively high frequency carrier signal in accordance with the amplitude of the modulating signal (Information).
It is a low quality form of modulation and often used for commercial broadcasting of both audio and video signals.
AM Modulators are nonlinear devices with 2 inputs and 1 output; a single, high frequency of carrier signal of constant-amplitude carrier signal and the low frequency information signal.
The Output of AM Modulator is called Modulated Wave and the shape of the Modulated Wave is called AM Envelope.
AMPLITUDE MODULATION
Types of AM1) Double sideband full carrier (DSBFC)
- Contains USB, LSB and Carrier
2) Double sideband suppressed carrier (DSBSC)- Contains only USB & LSB- A circuit that produces DSBSC is Balanced
modulator
3) Single sideband (SSB) - Contains either LSB or USB- Produce efficient system in term orpower consumption and bandwidth
AM WaveformGenerally.
Carrier Signal => VCsin(2πfct)Modulating Signal => Vmsin(2πfmt)Modulated Wave => Vam(t)
For Double sideband full carrier (DSBFC) AM waveform consists of:
DC voltageThe carrier frequency fcLower side frequency (fc - fm)Upper side frequency (fc + fm)
Known as AM envelope
It is an indicator to describe the amount of amplitude change (modulation) present in an AM waveform
Modulation Index
m = modulation coefficient (unitless)Vm = peak change in the amplitude of the output waveform voltage
Vc = peak change in the amplitude of the unmodulated carrier voltage
Percent modulation:
c
m
VVm =
%100%100 ´=´=c
m
VVmM
minmaxminmax
VVVV
cVmV
m +
−==
Voltage DistributionAn unmodulated carrier (carrier signal) is described by the following equation :-
Vc (t) = Ec sin (2πfct)
The Amplitude of the AM Wave varies proportional to the amplitude of the modulation signal, and the maximum of the modulated wave equal to Ec + Em.
Thus the amplitude of the modulated wave can be expressed as :-
Vam(t) =[Ec + Emsin(2πfmt)] sin (2πfct)
Ec + Emsin(2πfmt) Amplitude of modulated wave.
Em= Peak Change in the Amplitude of Envelope
fm= Frequency of Modulating signal
Voltage ModulationSince Em = mEc and by developing the equation for modulated wave, the final equation of the modulated wave can be expressed in term of its Carrier Component and Side Frequencies Component (usf & lsf):-
Where Ecsin(2πfct) carrier signal (V)
upper side frequency signal (V)
lower side frequency signal (V)
])(2cos[2
])(2cos[2
)2sin( tffmEtffmEtfEV mcc
mcc
ccam −++−= πππ
])(2cos[2
tffmEmc
c +− π
])(2cos[2
tffmEmc
c −+ π
Given the first input to AM Modulator is 500 kHz Carrier signal with Amplitude of 20V. The second input to AM Modulator is the 10kHz modulating signal which cause a change in output signal of ± 7.5 Vp. Determine the following :-
a. USF & LSFb. Modulation Index or Coefficient, Mc. Peak Amplitude of modulated carrierd. Upper & Lower side frequency voltagee. Maximum & Minimum Amplitude of the
envelope, Vmax and Vminf. Expression of Modulated Waveg. Output Spectrum & Envelope
The total transmitted power is the sum
of the carrier power and the power in
the sidebands.
Carrier power :
Sideband power:
AM Power Distribution
48
222cc
LSBUSBPm
RVmPP ===
RVP c
c 2
2
=
2
2c
LSBUSBSBPmPPP =+=
÷÷⎠
⎞⎜⎜⎝
⎛+=
+=++=
21
2
c
SBc
LSBUSBctotal
mP
PPpPPP
The efficiency of the AM in term of power consumption is
Thus, at optimum operation (m = 100%), only 33% of power is usedto carry information
From previous equation, total current flow in AM is
22
2
+==
a
a
T
SB
mm
PPη
AM Power Distribution
IT=Ic√(1+m2)
2
1. An AM Transmitter has a carrier power output of 50W.
Determine the total power that produced 80% modulation.
2. For a modulation index or coefficient of 0.4 and carrier power of 400W, determine the following:a. Total sideband powerb. Total transmitted power
3. For AM DSBFC wave with an unmodulated carrier voltage, Vc = 10 Vp , a load resistance of 10 Ω and modulation index of 1, determine the following :
a. Power of the carrier, and sideband frequencies
(Plsf & Pusf)b. Total Power of sideband, PT
c. Draw Power Spectrum
m for Complex SignalAs most of the signals are complex and can be
represented by combination of various sine waves, m can be determined by
Thus, total power for this complex signal is
......23
22
21 +++== mmmmm eff
]2
1[2eff
cT
mPP +=
For AM DSBFC transmitter with an unmodulated carrier Power, Pc = 100 W is modulated simultaneously with 3 other modulating signals with coefficient of modulation m1 = 0.2, m2 = 0.4, m3 = 0.5, determine the following :-
a. Total Modulation Index or Coefficientb. Upper and Lower sideband powerc. Total transmitted power
DSBFC has two drawbacks; it waste power and bandwidth
Power sent as the carrier contains no information and each sideband carries the same information independently
The DSBSC is introduced to eliminate carrier hence improve powerefficiency
It is a technique where it is transmitting both the sidebands without the carrier (the carrier is being suppressed)
Although, the power is improved, the bandwidth remain unchanged,that is BW = 2B = 2 fmax
DSBSC
SSBFC
Carrier signal is transmitted at full powerOnly one of the sidebands is transmittedRequire only half as much bandwidth as conventional AMHowever, this type of single sideband, the information-carrying portion still utilize small percentage from the total power transmitted.
SSBSCIn this system, the carrier signal is totally suppressed and one of the sideband removed
The sideband power makes up 100% of the total transmitted power
As the results of SSBSC, the transmitted waveform is not an envelope, it is simply a sine-wave which frequency is either
fc+fm or fc-fmdepending on which sideband to be
transmitted
The suppressed carrier is further improved by sending only one sideband
This not only uses less power but also only half of the bandwidth and it is called single sideband suppressed carrier (SSBSC)
There are two possible of SSBSCthe lower sideband VLSB = Vm cos (wc-wm)tthe upper sideband VUSB = Vm cos (wc+wm)t
SSBSC
As both DSB and standard AM waste a lot of power and occupy large bandwidth, SSB is adopted
SSB is a process of transmitting one of the sidebands of the standard AM by suppressing the carrier and one of the sidebands (only transmits upper or lower sideband of AM)
Reduces bandwidth by factor of 2
SSBSC
Filter method
Phase-Shift Method
SSB Transmitter
Advantage & Disadvantage of SSB Vs DSB
ADVANTAGE OF SSB
SSB Amplitude Modulation is widely used by military or radio amateurs in high-frequency communication. Occupy one half of the spectrum space.Efficient in terms of Power UsageLess Noise on the signal
DISADVANTAGE OF SSB
When no information or modulating signal is present, no RF signal is transmitted.Most information signals transmitted by SSB are not pure sine waves.A voice signal will create a complex SSB signal.Expensive
Advantage & Disadvantage of SSB Vs DSB
ADVANTAGE OF DSB
Efficient in terms of Power UsageLow cost
DISADVANTAGE OF DSB
Signal is rarely used because the signal is difficult to recover at the receiver.Waste power and BW
AM ApplicationThe AM SSB is used in telephone systems and 2 way radio and also in Military communication.
The AM DSB is used in FM and TV Broadcasting
Advantages and Disadvantages of AMAdvantages:
simple with proven reliabilitylow cost
Disadvantages:
wastage of power as most of the transmitted power are in the carrier component which does not contain information. When ma=1, 2/3 of the power is wastedLarge BWNoisyInefficient
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