Amplitude Modulation

1. 1. What is modulation?
• Modulation is the process of putting information
onto a high frequency carrier for transmission
(frequency translation).

2. • Once this information is received, the low frequency
information must be removed from the high frequency
carrier. This process is known as “ Demodulation”.

4. 2. What are the reasons for
modulation?
1. Frequency division multiplexing (To support multiple
transmissions via a single channel)
To avoid interference

5. M1(f)
M(f) Multiplexed
f
signal
0
+
M2(f)
0 f1 f2
f
f
0

6. 2. Practicality of Antennas
Transmitting very low frequencies require antennas with
miles in wavelength

7. ANALOG AND DIGITAL
Analog-to-analog conversion is the representation of
analog information by an analog signal. One may ask
why we need to modulate an analog signal; it is
already analog. Modulation is needed if the medium is
bandpass in nature or if only a bandpass channel is
available to us.
Topics discussed in this section:
Amplitude Modulation
Frequency Modulation
Phase Modulation

8. Figure Types of analog-to-analog modulation

9. Figure Amplitude modulation

10. Note
The total bandwidth required for AM
can be determined
from the bandwidth of the audio
signal: BAM = 2B.

11. Figure AM band allocation

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

13. 4. What are the Basic Types of
Analogue Modulation Methods ?
Consider the carrier signal below:
sc(t ) = Ac(t) cos( 2πfc t + θ )
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 θ produces
Phase Modulation signal (PM)

14. 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

15. Conventional Amplitude Modulation
(Full AM)

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

17. 6. Derive the Frequency Spectrum for Full-AM
Modulation (DSB-LC)
1 The carrier signal is
sc (t ) = Ac cos(ω c t ) where ω c = 2πf c
2 In the same way, a modulating signal (information
signal) can also be expressed as
sm (t ) = Am cos ωm t

18. 3 The amplitude-modulated wave can be expressed as
s (t ) = [ Ac + sm (t )] cos(ω c t )
4 By substitution
s (t ) = [ Ac + Am cos(ω mt )] cos(ω c t )
5 The modulation index.
Am
m =
Ac

19. 6 Therefore The full AM signal may be
written as
s (t ) = Ac (1 + m cos(ω mt )) cos(ω c t )
cos A cos B = 1 / 2[cos( A + B ) + cos( A − B )]
mAc mAc
s (t ) = Ac (cos ω ct ) + cos(ω c + ω m )t + cos(ω c − ω m )t
2 2

20. 7. Draw the Frequency Spectrum of the above AM
signal and calculate the Bandwidth
fc-fm fC fc+fm
2fm

21. 8. Draw Frequency Spectrum for a complex input
signal with AM
fc-fm fc fc+fm

22. Frequency Spectrum of an AM signal
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.

23. • 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.

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

26. • If the amplitude of the modulating signal is higher than the
carrier amplitude, which in turn implies the modulation
index m ≥ 1.0(100%)
. This will cause severe distortion to the
modulated signal.

27. 10. Calculate the power efficiency of AM signals
• The ratio of useful power, power efficiency :
sidebands power m2 / 2 m2
= =
total power 1 + m / 2 2 + m2
2
• 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.