basics of amplitude modulation

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.