This document discusses amplitude modulation (AM) as a type of modulation used to transmit information signals. Modulation involves varying a high frequency carrier signal by an information signal in order to transmit the information signal over long distances. In AM, the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating or information signal. This creates two new sideband frequencies above and below the carrier frequency equal to the modulation frequency. The carrier and sidebands together make up the modulated signal. Only a portion of the transmitted power is present in the sidebands containing the information, while the rest is wasted in the carrier.

2. INTRODUCTION
What is communication
Transmitter Channel Receiver
A communication system
Process of conveying message or information
Transmitter modifies the message signal into a form which is
suitable for transmission over channel which is achieved by
MODULATION
Information
source

3. WHAT IS MODULATION
MODULATION is the basic requirement for transmitting
the message signal through free space
 It is the process of transmission of information signal
(low frequency audio signal) using a high frequency
carrier signal

4. WHY MODULATION?
Carrying one signal to another : uses carrier (having high
frequency , smaller wavelength)
Modulated signal is transmitted
Problems with transmitting baseband signal/
Need of modulation
Height of transmitting and receiving antenna
Noise and interference from other sources at low
frequencies: Multiplexing
Narrow banding

5. PRACTICABILITY OF ANTENNAS
h=λ/4, for efficient transmission.
For f=30 Hz => h= 2500 km
f=3kHz => h= 25 km
f=3MHz => h= 25 m
Thus as
Frequency increases height of the antenna
decreases

6. MODULATION
Defined as
“ The process by which some characteristics of a
signal called carrier varied in accordance with the
instantaneous value of another signal called
modulating signal “
 The information bearing signal is called modulating
signal
The signal resulting from process of modulation is
known as modulated signal

7. TYPES OF MODULATION
Continuous wave Modulation : carrier is continuous in
nature (usually sinusoidal) (AM,FM,PM)
Pulse Modulation :Carrier is pulse type waveform

8. TYPES OF MODULATION
 Sine wave (carrier) described by 3 parameters:
amplitude, frequency and phase.
 Let carrier signal be:
v(t) = A sin (ωt + φ)
So can have
 – Amplitude modulation (AM)
 – Frequency modulation (FM)
 – Phase modulation (PM)
Frequency and phase combined are known as
Angle Modulation

9. AMPLITUDE MODULATION
The amplitude of the carrier is changed in
accordance with the instantaneous value of
modulating signal
Carrier : c(t) = Vc cos (2πfct + φ)
modulating signal v(t) = Vm cos (2πfmt)
Information is contained in the envelop

10. AMPLITUDE MODULATION

11. AMPLITUDE MODULATION
Modulated signal:
v(t) = Vc cos (2πfct) {1 + m cos (2πfm t)}
 Vc = unmodulated peak carrier amnplitude
fc= carrier frequency
fm = modulation frequency
m = modulation index (“degree” of modulation)
m must be between 0 and 1
If m > 1 get overmodulation (bad …distortion)

12. VARYRING MODULATION INDEX
m =Vmax – Vmin / Vmax + Vmin

13. AMPLITUDE SPECTRUM
Modulation produces two new components called
sidebands, at frequencies above and below the
carrier
The spacing in frequency between carrier and
sidebands is equal to fm (the modulating frequency)
Bandwidth requirement : 2 fm

14. AMPLITUDE SPECTRUM

15. WHERE DO THE SIDEBANDS COME FROM
Expand v(t) = Vc cos (2π fct) {1 + m cos (2π fm t)}
Using trig identities to get:
v(t) = Vc cos (2π fct)
+0.5m Vc cos (2π [fc- fm ]t)
+0.5m Vc cos (2π [fc+ fm ]t)
 This expression consists of 3 sine waves at
frequencies of carrier (fc), lower sideband (fc-fm)
and upper sideband (fc+fm). fc= carrier frequency

16. EFFICIENCY
 For a fully modulated carrier (m=1), 2/3 of
the power is in the carrier, the rest in the
sidebands (33.33% efficient )
 Total power Pt = Pc (1 + m2 /2)
 Carrier Power (Pc) = Vc
2 / 2
 Side band Power =Plsb=Pusb= m2 Pc / 4
 Information in side band : Power gets wasted in
carrier
AM is bandwidth inefficient (2 fm)
Gets effected due to noise

17. VARIATIONS OF AM
 Double Sideband with carrier (AM): This is the most
widely used type of AM modulation. In fact, all radio
channels in the AM band use this type of modulation.
Double Sideband Suppressed Carrier (DSBSC): This
is the same as the AM modulation above but without the
carrier.
Single Sideband (SSB): In this modulation, only half of
the signal of the DSBSC is used.
Vestigial Sideband (VSB): This is a modification of the
SSB to ease the generation and reception of the signal.