Types of Amplitude Modulation
• Double Sideband with full carrier (AM DSBFC): 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.
Advantages
1.Simplicity: AM is a simple modulation technique, and
the required equipment for transmission and reception is
relatively simple and inexpensive.
2.Efficient use of bandwidth: AM signals occupy less
bandwidth than other modulation techniques such as
frequency modulation (FM).
3.Compatibility: AM is compatible with existing AM
radio receivers, which are widely available and cheap.
4.Long-range transmission: AM signals can travel long
distances without significant loss of signal strength.
Disadvantages
1.Poor signal quality: AM signals are susceptible to noise and
interference, which can degrade signal quality.
2.Limited frequency response: AM signals have limited frequency
response and cannot transmit high-frequency signals such as music
with high fidelity.
3.Inefficient power usage: AM transmitters consume a lot of power to
transmit signals, and much of the transmitted power is wasted as
carrier signal, reducing the overall efficiency.
4.Vulnerability to fading: AM signals are vulnerable to fading, which
can occur when the signal passes through buildings or other obstacles.
Application
1.AM Radio Broadcasting: AM is widely used for broadcasting audio
signals such as news, music, and talk shows over long distances. AM
radio stations use the medium wave (MW) and long wave (LW)
frequency bands for broadcasting.
2.Two-Way Radio Communications: AM is also used in two-way radio
communications such as citizen band (CB) radios and aviation
communication systems.
3.Radar Systems: AM is used in radar systems to transmit and receive
signals for detecting and tracking objects such as airplanes, ships, and
satellites.
4.Wireless Power Transmission: AM can be used to transmit power
wirelessly over short distances, such as in wireless charging systems
for electronic devices.
Application
5. Industrial Control Systems: AM can be used in industrial control
systems to transmit signals for controlling machinery and
equipment.
6. Medical Imaging: AM is used in medical imaging devices such as
MRI machines to transmit and receive signals for creating images of
the body.
7. Telemetry Systems: AM is used in telemetry systems for
transmitting data from remote sensors and measurement devices.
8. Navigation Systems: AM is used in navigation systems such as
LORAN-C for transmitting timing signals that can be used to
determine the location of a receiver.
What is Angle Modulation?
v c (t) V sin(2 π f c t phase)
Angle modulation is a variation
of one of these two
parameters.
Angle Modulation?
Frequency modulation In the frequency of the carrier signal is
modulated to follow the changing voltage level (amplitude) of the
modulating signal. The peak amplitude and phase of
the carrier signal remain constant.
Phase modulation In the phase of the carrier signal is modulated to
follow the changing voltage level (amplitude) of the modulating
signal. The peak amplitude and frequency of
the carrier signal remain constant.
Understanding Phase vs. Frequency
To understand the difference
between phase and frequency,
a signal can be thought of
V using a phasor diagram. The
phase
distance from the center is the
signal's amplitude. The angle
from the positive horizontal
axis is the phase.
Understanding Phase vs. Frequency
The change in the phase over
time (the phase velocity) is
the signal's frequency.
Understanding Phase vs. Frequency
PM is the same as FM with one difference. In FM,
the instantaneous change in the carrier frequency is
proportional to the amplitude of the modulating
signal; in PM the instantaneous change in the carrier
frequency is proportional to the derivative of the
amplitude of the modulating signal.
Understanding Phase vs. Frequency
Phase
Frequency
t
Understanding Phase vs. Frequency
Frequency modulation
Understanding Phase vs. Frequency
Phase modulation
Calculating Total Power
Frequency Modulation
Envelope V
The envelope, meaning the difference between
the maximum and minimum of the carrier, is
constant in an FM signal. That's why FM is
called a constant envelope signal. The power
Power V 2
of an FM signal is shown at right. It does not
depend upon the modulating signal or the
amount of deviation
Calculating Total Power
v c (t) V sin(2 π f c t ( t ))
Total Power V 2
The total power of an FM signal is simply V2. Therefore, the total
power of an FM signal is the power of the carrier. This is
regardless of the information or the deviation ratio
Varying the Frequency
Information
Signal VCO
Frequency
Modulation
The voltage-controlled oscillator (VCO) is a device whose output
frequency changes with the amplitude of the modulating signal. The
amount of change, called its deviation constant, is dependent upon
its design.
Frequency Modulation
frequency of the modulated carrier increases as the signal
amplitude increases but decreases as the signal amplitude
decreases. It is at its highest frequency (point H) when the
signal amplitude is at its maximum positive value and is at
its lowest frequency (point L) when signal amplitude has
maximum negative value.
When signal amplitude is zero, the carrier frequency is at
its normal frequency f (also called resting or centre
0
frequency.)
Frequency Modulation
The amount of frequency deviation (or shift or variation)
depends on the amplitude (loudness) of the audio signal.
Louder the sound, greater the frequency deviation and vice-
versa.
The rate of frequency deviation depends on the signal
frequency.
FM Modulation
- In frequency modulation the angle θ(t) is varied linearly with the
integral of message signal m(t) as:
t
(t ) c t k f m( ) d
where kf is the frequency deviation.
- Thus the frequency modulated signal is defined as:
t
S FM (t ) Ac cos c t k f m( ) d
19
Frequency Modulation
This change or shift either above or below the resting
frequency is called frequency deviation
The total variation in frequency from the lowest to the
highest is called carrier swing (CS).
Frequency Modulation
A maximum frequency deviation of 75 kHz is allowed for
commercial FM broadcast stations in the 88 to 168 MHz VHF band.
Hence, FM channel width is 2*75 = 150 kHz. Allowing a 25 kHz
guard band on either side, the channel width becomes = 2(75 + 25) =
200 kHz .
This guard band is meant to prevent interference between adjacent
channels. However, a maximum frequency deviation of 25 kHz is
allowed in the sound portion of the TV broadcast.
In FM, the highest audio frequency transmitted is 15 kHz.
Frequency Modulation
Frequency Modulation
Center
Frequency
An FM signal has its energy spread over an infinite number of
spectral components. It's center frequency is the average
center of the energy.
Frequency Modulation
Deviation
The deviation is the maximum frequency change from the
center frequency.
Frequency Modulation
Excursion
The excursion is the difference between the maximum and
minimum frequency changes. This is also called the maximum
deviation or total deviation.
Frequency Modulation
Modulation Index
Frequency Modulation
Deviation Ratio
It is the worst-case modulation index in which maximum permitted frequency deviation and
maximum permitted audio frequency are used.
Frequency Modulation
Percent Modulation
the ratio of actual frequency deviation to the maximum allowed frequency deviation
What is the modulation index of an FM carrier
having a carrier swing of 100
kHz and a modulating signal of 5 kHz ?
