electronics and telecommunications: Amplitude modulation

1. U N I T I V
AMPLITUDE
MODULATION

2. CONCEPT OF AMPLITUDE
• The amplitude or peak amplitude of a wave is a measure of how big
its oscillation is.
• The distance from the top of one peak to the bottom of another is
called peak-to-peak amplitude.

3. INTRODUCTION OF A GENERAL
COMMUNICATION SYSTEM
• Message needs to be encoded first in system understandable
bits. i. e. 0s & 1s.
• Message is low frequency signal.
• Message is to be modulated using carrier frequency.
• Modulated signal is to be transmitted using channel.
channel

4. RADIO TRANSMITTER

5. RADIO RECEIVER

6. CONCEPT OF AMPLITUDE
MODULATION
• In order that a radio signal can carry audio
or other information for broadcasting or for
two way radio communication, it must be
modulated or changed in some way.
• Although there are a number of ways in
which a radio signal may be modulated,
one of the easiest, and one of the first
methods to be used was to change its
amplitude in line with variations of the
sound.
• The amplitude of the signal is changed in
line with the instantaneous intensity of the
sound. In this way the radio frequency
signal has a representation of the sound
wave superimposed in it.
• In view of the way the basic signal
"carries" the sound or modulation, the
radio frequency signal is often termed the
"carrier".

7. AM MODULATION INDEX
• Modulation indices are described for various forms of
modulation. The amplitude modulation, AM, modulation index
can be defined as the measure of extent of amplitude
variation about an un-modulated carrier.
• Modulation Index - The ratio between the amplitudes
between the amplitudes of the modulating signal and
carrier, expressed by the equation:
Where:
A is the carrier amplitude.
M is the modulation amplitude and is the peak change in the RF
amplitude from its un-modulated value.

8. POWER RELATION IN AM
• Power in a transmitter is
important, but the most
important power measurement
is that of the portion that
transmits the information
• AM carriers remain unchanged
with modulation and therefore
are wasteful
• Power in an AM transmitter is
calculated according to the
formula at the right
Pt  Pc 1
m2
2







9. GENERATION OF AM
• Transmitters that transmit AM signals are known as AM transmitters.
These transmitters are used in medium wave (MW) and short wave
(SW) frequency bands for AM broadcast. The MW band has
frequencies between 550 KHz and 1650 KHz, and the SW band has
frequencies ranging from 3 MHz to 30 MHz. The two types of AM
transmitters that are used based on their transmitting powers are:
• High Level
• Low Level
• High level transmitters use high level modulation, and low level
transmitters use low level modulation.
• The choice between the two modulation schemes depends on the
transmitting power of the AM transmitter. In broadcast
transmitters, where the transmitting power may be of the order of
kilowatts, high level modulation is employed.
• In low power transmitters, where only a few watts of transmitting
power are required , low level modulation is used.

10. GENERATION OF AM
• Figure shows the block diagram of high-level AM transmitter.
• In high-level transmission, the powers of the carrier and modulating signals
are amplified before applying them to the modulator stage

11. GENERATION OF AM
• The various sections of the HIGH LEVEL AM
TRANSMITTER are:
• Carrier oscillator
• Buffer amplifier
• Frequency multiplier
• Power amplifier
• Audio chain
• Modulated class C power amplifier
DETAIL DESCRIPTION

12. AM DEMODULATION
• When demodulating a signal, two basic steps may be considered:
• Create baseband signal: The main element of AM demodulation is to
create the baseband signal. This can be achieved in a number of ways
- one of the easiest is to use a simple diode and rectify the signal. This
leaves elements of the original RF signal. When other forms of
demodulation are used, they too leave some elements of an RF signal.
• Filter: The filtering removes any unwanted high frequency elements
from the demodulation process. The audio can then be presented to
further stages for audio amplification, etc.

13. AM DEMODULATION
• Filter: The filtering removes any unwanted high
frequency elements from the demodulation process. The
audio can then be presented to further stages for audio
amplification, etc.

14. SINGLE-SIDEBAND AM (SSB)
• The two sidebands of an AM signal
are mirror images of one another
• As a result, one of the sidebands
is redundant
• Using single-sideband suppressed-
carrier transmission results in
reduced bandwidth and therefore
twice as many signals may be
transmitted in the same spectrum
allotment
• Typically, a 3dB improvement in
signal-to-noise ratio is achieved as a
result of SSBSC

15. ADVANTAGES OF SSB OVER AM
• Amplitude modulation of a carrier results in a transmitted signal consisting of the
carrier, plus an 'upper sideband' and a 'lower sideband', spaced above and below the
carrier frequency by the frequency of the modulation.
• The bandwidth of the whole signal is double the modulation frequency.
• Also, the power in the carrier is constant, and power must be added in order to
radiate the sidebands.
• All the receiver needs in order to extract the information from the signal is one
complete sideband, and knowledge of the frequency and phase of the carrier.
• Economically, the carrier is wasted power, and the other sideband is wasted power
and wasted spectrum.
• If we can filter away one of the sidebands before transmission, then you save
half of the occupied spectrum, and the receiver has everything it needs to
decode the signal.
• If you can also filter away the carrier ... or at least knock it way down ... before
transmission, you can save a lot of power and use it for the remaining sideband, which
extends your range for a given amount of power. The receiver still has everything it
needs, as long as it can pick up a sniff of carrier ... enough to derive the carrier
frequency and phase.

16. BALANCED MODULATOR
• There's a special type of modulator called a "balanced modulator" which
gives no carrier output when there is no modulating signal fed to it.
• When a signal is fed to the balanced modulator, its output is
proportional to the level of the signal being fed, with one special
attribute.
• If the signal being fed to it is "positive," the output of the modulator's carrier
is at 0 degrees phase, but if the signal being fed to it is "negative," the
output of the carrier is at 180 degrees phase.

17. GENERATION OF SSB
• There are two methods used for SSB Transmission.
• Filter Method
• Phase Shift Method

18. GENERATION OF SSB
• Filter Method

19. GENERATION OF SSB FILTER METHOD

20. GENERATION OF SSB
PHASE SHIFT METHOD
The phasing method of SSB generation uses a phase shift technique that
causes one of the side bands to be canceled out. A block diagram of a
phasing type SSB generator
It uses two balanced modulators instead of one. The
balanced modulators effectively eliminate the carrier.
The carrier oscillator is applied directly to the upper
balanced modulator along with the audio modulating
signal.
Then both the carrier and modulating signal are
shifted in phase by 90o and applied to the second,
lower, balanced modulator.
The two balanced modulator output are then added
together algebraically. The phase shifting action
causes one side band to be canceled out when the
two balanced modulator outputs are combined