AM

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