Amplitute modulation

MODULATION

1. What is modulation?

• Modulation is the process of putting information

onto a high frequency carrier for transmission

(frequency translation).

1. What is modulation?

• Once this information is received, the low frequency

information must be removed from the high frequency

carrier. This process is known as “ Demodulation”.

2. What are the reasons for modulation?

1. Frequency division multiplexing (To support multiple

transmissions via a single channel)

To avoid interference

2. What are the reasons for modulation?

f

M1(f)

0

f

M2(f)

0

f

M(f)

0 f1 f2

Multiplexed signal

+

2. Practicality of Antennas

Transmitting very low frequencies require antennas with

miles in wavelength

3.What are the Different of Modulation Methods?

1. Analogue modulation- The modulating signal and carrier both are analogue signals

Examples: Amplitude Modulation (AM) , Frequency Modulation (FM) , Phase Modulation (PM)

2. Pulse modulation- The modulating signal is an analogue signal but Carrier is a train of pulses

Examples : Pulse amplitude modulation (PAM), Pulse

width modulation (PWM), Pulse position modulation

(PPM)

3. What are the Different of Modulation Methods?

3. Digital to Analogue modulation- The modulating signal is a digital signal , but the carrier is an analogue signal.

Examples: Amplitude Shift Keying (ASK), FSK, Phase Shift Keying (PSK)

4. Digital modulation -

Examples: Pulse Code Modulation, Delta Modulation,Adaptive Delta Modulation

3.What are the Different of Modulation Methods?

ANALOG AND DIGITALANALOG AND DIGITAL

Analog-to-analog conversion is the representation of Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask analog information by an analog signal. One may ask why we need to modulate an analog signal; it is why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is bandpass in nature or if only a bandpass channel is available to us. available to us.

Amplitude ModulationFrequency ModulationPhase Modulation

Topics discussed in this section:Topics discussed in this section:

Figure Types of analog-to-analog modulation

Figure Amplitude modulation

The total bandwidth required for AM can be determined

from the bandwidth of the audio signal: BAM = 2B.

Note

Figure AM band allocation

The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.

Note

Figure Frequency modulation

Figure FM band allocation

Figure Phase modulation

The total bandwidth required for PM can be determined from the bandwidth

and maximum amplitude of the modulating signal:

BPM = 2(1 + β)B.

Note

4. What are the Basic Types of Analogue Modulation Methods ?

Consider the carrier signal below:

sc(t ) = Ac(t) cos( 2fc t + )

1. Changing of the carrier amplitude Ac(t) produces

Amplitude Modulation signal (AM)

2. Changing of the carrier frequency fc produces

Frequency Modulation signal (FM)

3. Changing of the carrier phase produces

Phase Modulation signal (PM)

4. What are the Basic Types of Analogue Modulation Methods ?

Analogue Modulation Methods

5. What are the different Forms of Amplitude

Modulation ?

1. Conventional Amplitude Modulation (DSB-LC) (Alternatively known as Full AM or Double

Sideband with Large carrier (DSB-LC) modulation

2. Double Side Band Suppressed Carrier (DSB-SC) modulation

3. Single Sideband (SSB) modulation

4. Vestigial Sideband (VSB) modulation

5. What are the different Forms of Amplitude

Modulation ?

Conventional Amplitude Modulation (Full AM)

6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)

1 The carrier signal is

ccccc ftAts 2 where)cos()(

2 In the same way, a modulating signal (information

signal) can also be expressed as

tAts mmm cos)(

6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)

3 The amplitude-modulated wave can be expressed as

)cos()()( ttsAts cmc

)cos()cos()( ttAAts cmmc

4 By substitution

c

m

A

Am

5 The modulation index.

6 Therefore The full AM signal may be written as

)cos())cos(1()( ttmAts cmc

)]cos()[cos(2/1coscos BABABA

tmA

tmA

tAts mcc

mcc

cc )cos(2

)cos(2

)(cos)(

7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth

fC fc+fmfc-fm

2fm

7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth

8. Draw Frequency Spectrum for a complex input signal with AM

8. Draw Frequency Spectrum for a complex input signal with AM

fcfc-fm fc+fm

The frequency spectrum of AM waveform contains three parts:

1. A component at the carrier frequency fc

2. An upper side band (USB), whose highest frequency component is at fc+fm

3. A lower side band (LSB), whose highest frequency component is at fc-fm

The bandwidth of the modulated waveform is twice the information signal bandwidth.

Frequency Spectrum of an AM signal

• Because of the two side bands in the frequency spectrum its

often called Double Sideband with Large Carrier.(DSB-

LC)

• The information in the base band (information) signal is

duplicated in the LSB and USB and the carrier conveys no

information.

ExampleExample

We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM?

ExampleExample

We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM?

SolutionSolution

An AM signal requires twice the bandwidth of the original signal:

BW = 2 x 5 KHz = 10 KHz

AM Radio Band

Modulation Index (m)

• m is merely defined as a parameter, which determines the

amount of modulation.

• What is the degree of modulation required to establish a

desirable AM communication link?

Answer is to maintain m<1.0 (m<100%).

• This is important for successful retrieval of the original

transmitted information at the receiver end.

9. What is the significance of modulation index ?

Modulation Index (m)9. What is the significance of modulation index ?

• If the amplitude of the modulating signal is higher than the

carrier amplitude, which in turn implies the modulation

index . This will cause severe distortion to the

modulated signal.

%)100(0.1m

Power distribution in full AM10. Calculate the power efficiency of AM signals

• The ratio of useful power, power efficiency :

2

2

2

2

22/1

2/

m

m

m

m

powertotal

powersidebands

• In terms of power efficiency, for m=1 modulation, only

33% power efficiency is achieved which tells us that only

one-third of the transmitted power carries the useful

information.

10. Calculate the power efficiency of AM signals

• The carrier component in full AM or DSB-LC does not convey any

information. Hence it may be removed or suppressed during the

modulation process to attain higher power efficiency.

• The trade off of achieving a higher power efficiency using DSB-SC

is at the expense of requiring a complex and expensive receiver due

to the absence of carrier in order to maintain transmitter/receiver

synchronization.

Double Side Band Suppressed Carrier (DSB-SC) Modulation

1 Consider the carrier

ccccc ftAts 2 where)cos()( 2 modulated by a single sinusoidal signal

mmmm ftAts 2 wherecos)( m 3 The modulated signal is simply the product of these two

LSB

mccm

USB

mccm

mcmc

mmcc

tAA

tAA

BABABA

ttAA

tAtAts

)cos(2

)cos(2

)cos()cos(2

1coscos since

)cos()cos(

)cos()cos()(

11. Derive the Frequency Spectrum for Double Sideband Suppressed Carrier Modulation (DSB-SC)

tAts mmm cos)(

tAts ccc cos)(

)cos()cos()( tAtAts mmcc X

fcfc-fm fc+fm

Frequency Spectrum of a DSB-SC AM Signal

• All the transmitted power is contained in the two sidebands

(no carrier present).

• The bandwidth is twice the modulating signal bandwidth.

• USB displays the positive components of sm(t) and LSB

displays the negative components of sm(t).

Generation and Detection of DSB-SC

• The simplest method of generating a DSB-SC signal is

merely to filter out the carrier portion of a full AM (or

DSB-LC) waveform.

• Given carrier reference, modulation and demodulation

(detection) can be implemented using product devices or

balanced modulators.

BALANCED MODULATOR

AM Modulator 1

AM Modulator 2

Carrier

Sm(t)

Sm(t)

-Sm(t)

Accos(ct)

Accos(ct)

S2(t)

S1(t)

S(t)

DSB-SC

• The two modulators are identical except for the sign

reversal of the input to one of them. Thus,

)cos())cos(1()(1 ttmAts cmc

)cos())cos(1()(2 ttmAts cmc

)cos()cos(2

)()()( 21

ttmA

tststs

cmc

DSB-SC Signal s(t)

Local Oscillator

LPFX

Cosct

v(t) vo(t)

COHERENT (SYNCHRONOUS) DETECTOR OR

DSB-SC (PRODUCT DETECTOR)

• Since the carrier is suppressed the envelope no longer

represents the modulating signal and hence envelope

detector which is of the non-coherent type cannot be used.

2cos)

)cos()( since

)2cos()cos()cos(

2

2cos1)cos(2

)(cos)cos(2

)cos()cos()cos(2)cos()()(

2

d by LPF)erm(removeUnwanted t

cmm

mmm

cmmmm

cmm

cmcc

m

ccmcc

t)((ts(t)s

tAts

ttAtA

ttA

ttAA

A

tttmAttstv

• It is necessary to have synchronization in both frequency

and phase between the transmitter (modulator) & receiver

(demodulator), when DSB-SC modulation ,which is of the

coherent type, is used.

Both phase and frequency must be known to demodulate

DSB-SC waveforms.

LACK OF PHASE SYNCHRONISATION

Let the received DSB-SC signal be

ccmSCDSB Attsts cos)()(if is unknown,

ttsA

tttsA

ttstv

cmc

ccmc

cSCDSB

2coscos)(2

coscos)(

cos)()(

Output of LPF

cos)(2

)( tsA

tv mc

o

But we want just

)(2

)( tsA

tv mc

o

Due to lack of phase synchronization, we will see that the

wanted signal at the output of LPF will be attenuated by an

amount of cos.

In other words, phase error causes an attenuation of the

output signal proportional to the cosine of the phase error.

The worst scenario is when =/2, which will give rise to

zero or no output at the output of the LPF.

LACK OF FREQUENCY SYNCHRONISATION

Suppose that the local oscillator is not stable at fc but at fc+f, then

tttsA

tttsA

ttstv

cmc

ccmc

cSCDSB

2coscos)(2

coscos)(

cos)()(

Output of LPF

ttsA

tv mc

o cos)(2

)(

Thus, the recovered baseband information signal will vary

sinusoidal according to cos t

This problem can be overcome by adding an extra

synchronization circuitry which is required to detect and

t and by providing the carrier signal to the receiver.

A synchronizer is introduced to curb the synchronization

problem exhibited in a coherent system.

Let the baseband signal be

tAts mmm cos)( Received DSB-SC signal

ttsAts cmc cos)()(

( )2 PLL BPF 2

SYNCHRONISER

Mathematical analysis of the synchronizer is shown below:

ttttAA

ttttAA

ttAA

ttAAts

mcmccmmc

cmcmmc

cmmc

cmmc

2cos2

12cos

2

12cos2cos1

4

2cos2cos2cos2cos14

2cos12cos14

coscos)(

22

22

22

22222

Output of BPF t

AAc

mc 2cos4

22

Output of frequency divider tk ccos

where k is a constant of proportionality.

DISADVANTAGE OF USING COHERENT SYSTEMS

• The frequency and phase of the local oscillator signal must

be very precise which is very difficult to achieve.

It requires additional circuitry such as synchronizer circuit

and hence the cost is higher.

Single-Sideband Modulation

How to generate SSB signal?

• Generate DSB-SC signal

• Band-pass filter to pass only one of the sideband

and suppress the other.

For the generation of an SSB modulated signal

to be possible, the message spectrum must have

an energy gap centered at the origin.

Single Side Band Modulation (SSB)

• Example of signal with -300 Hz ~ 300 Hz energy gap

Voice : A band of 300 to 3100 Hz gives goodarticulation

• Also required for SSB modulation is a highly selective filter

• Vestigial Sideband Modulation

Instead of transmitting only one sideband as SSB, VSB modulation transmits a partially suppressed sideband and a vestige of the other sideband.

Vestigial Side Band Modulation (VSB)

Comparison of Amplitude Modulation methods

Full AM (or DSB-LC)- Sidebands are transmitted in full with the carrier.- Simple to demodulate / detect- Poor power efficiency- Wide bandwidth ( twice the bandwidth of the information

signal)- Used in commercial AM radio broadcasting, one

transmitter and many receivers.

Comparison of Amplitude Modulation methods

DSB-SC- Less transmitted power than full AM and all the transmitted

power is useful.- Requires a coherent carrier at the receiver; This results in

increased complexity in the detector(i.e. synchroniser)- Suited for point to point communication involving one

transmitter and one receiver which would justify the use of increased receiver complexity.

Comparison of Amplitude Modulation methods

SSB- Good bandwidth utilization (message signal bandwidth =

modulated signal bandwidth)- Good power efficiency- Demodulation is harder as compares to full AM; Exact

filter design and coherent demodulation are required- Preferred in long distance transmission of voice signals

Comparison of Amplitude Modulation methods

VSB- Offers a compromise between SSB and DSB-SC- VSB is standard for transmission of TV and similar signals- Bandwidth saving can be significant if modulating signals

are of large bandwidth as in TV and wide band data signals.

• For example with TV the bandwidth of the modulating signal can extend up to 5.5MHz; with full AM the bandwidth required is 11MHz

Comparison of Amplitude Modulation methods