• 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”.

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

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

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 ?

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)(

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

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.

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 ?

• 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

• 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