Communications Systems 2
Amplitude Modulation
Outline
 Modulation in Communication Systems
 Definition of Amplitude Modulation
 Amplitude Modulation
 Amplitude Modulation – The Math
 Power Distribution in Carrier
Modulation in Communication Systems
 Communication systems transmit a modulating signal or modulating waveform.
 The modulating waveform is derived from the information source.
 The modulating waveform has wave properties of amplitude, frequency, and phase.
 Communication systems also comprise a carrier signal or carrier waveform.
 This carrier waveform is generated by an oscillator.
 The carrier waveform also ‘carries’ the modulating waveform
 The ‘carriage’ is from the transmitter to the receiver.
 In executing the carrying function, the carrier signal’s properties is varied in proportion
to the value of the modulating signal/modulating waveform.
 This variation of property is called signal modulation.
Modulating Signal
Information Source
This carrying function is realized via
the frequency translation being
executed by the carrier signal.
Carrier Signal
Oscillator
Amplitude Modulation
 In amplitude modulation, the concerned property of the carrier signal is the Amplitude
Carrier signal amplitude is used to represent the information in the modulating waveform.
 In amplitude modulation, the carrier signal/waveform has a constant frequency.
 An important aspect of amplitude modulation (AM) is mixing.
This is the combination of signals having different frequencies.
Mixing results in other signals having various frequencies.
 Frequencies arising due to mixing combine with the carrier to form a modulated signal
High frequency modulated signals can be propagated in a wireless channel by antennas.
 Modulated signals can be received and information extracted via amplitude demodulation.
Amplitude Modulation
 Amplitude Modulation (AM) has the benefit of simplicity at the transmitter and receiver.
Its simplicity has propelled the wide adoption of AM technique in communication systems.
 AM varies carrier signal amplitude to represent information in modulating signal.
 This modulation results in a waveform with different sidebands.
AM variants can emerge based on the transmitted sidebands.
 Examples of AM variants are double sideband full carrier AM (DSBFC-AM)
DSBFC-AM transmits the entire modulated signal at full power
DSBFC-AM is used in broadcasting and aviation (aircraft to tower) communications.
 Another form is the single sideband (SSB) AM.
SSB is used in military services and in marine and citizens band radio i.e., amateur radio.
Amplitude Modulation
 AM has other variants in which the carrier is partially or completely removed.
In these cases, the sidebands are present.
 These variants are used in stereo broadcasting, analog video and analog color television. .
 An understanding of the AM signal techniques provides a good basis to understand
other methods used in modern communication systems.
Amplitude Modulation
 DSBFC AM
The upper sideband and lower sideband of the modulated signal are transmitted.
These sidebands are transmitted alongside the carrier frequency.
 Modulating signal is added to the upper sideband and lower sideband of the carrier.
 The output signal has twice the bandwidth of the modulating signal.
This doubling of bandwidth implies that DSBFC –AM is not spectrally efficient
and is considered a disadvantage
A main benefit of DSBFC AM is its simplicity.
The need to enhance the spectral efficiency has motivated the SSB AM and
the vestigial sideband AM (VSB AM) forms of amplitude modulation
SSB AM and VSB AM are more complex to implement in comparison to DSBFC AM.
Amplitude Modulation
 DSBFC AM
The upper sideband and lower sideband of the modulated signal are transmitted.
These sidebands are transmitted alongside the carrier frequency.
 Modulating signal is added to the upper sideband and lower sideband of the carrier.
 The output signal has twice the bandwidth of the modulating signal.
This doubling of bandwidth implies that DSBFC –AM is not spectrally efficient
and is considered a disadvantage
A main benefit of DSBFC AM is its simplicity.
The need to enhance the spectral efficiency has motivated the SSB AM and
the vestigial sideband AM (VSB AM) forms of amplitude modulation
SSB AM and VSB AM are more complex to implement in comparison to DSBFC AM.
Carrier Signal
Different Modulating Signal
AM Modulated Signal Output
Amplitude Modulation
 DSBFC AM
The upper sideband and lower sideband of the modulated signal are transmitted.
These sidebands are transmitted alongside the carrier frequency.
 Modulating signal is added to the upper sideband and lower sideband of the carrier.
 The output signal has twice the bandwidth of the modulating signal.
This doubling of bandwidth implies that DSBFC –AM is not spectrally efficient
and is considered a disadvantage
A main benefit of DSBFC AM is its simplicity.
The need to enhance the spectral efficiency has motivated the SSB AM and
the vestigial sideband AM (VSB AM) forms of amplitude modulation
SSB AM and VSB AM are more complex to implement in comparison to DSBFC AM.
Amplitude Modulation
 SSB FC AM – Single sideband full carrier amplitude modulation
A single sideband of the modulated signal is transmitted alongside the carrier frequency.
These sidebands are transmitted alongside the carrier frequency.
 DSB SC AM – Double sideband suppressed carrier amplitude modulation
 In this case, the carrier signal is not transmitted alongside the double sidebands.
Circuits producing this behavior are called Balanced Modulators.
and is considered a disadvantage
 The DSBSC signal is an intermediate signal between the DSBFC signal in realizing the
SSB signal.
Amplitude Modulation
 AM involves a change in the amplitude of the carrier signal/carrier waveform.
 However, it is important to consider if there is a limit to the maximum amplitude.
Maximum amplitude refers to that of the modulating/information signal.
 A consistent and continuous increase in the modulating signal amplitude results in
a case of overmodulation.
Amplitude Modulation
 The occurrence of overmodulation can be evaluated via the modulation index
 Modulation Index or Modulation Factor , 𝒎 can be given as:
Maximum amplitude refers to that of the modulating/information signal.
Amplitude Modulation
Amplitude Modulation
 If the value of A falls to zero as a result of the increase in amplitude of information signal.
 The modulation index is 100%.
 Carrier amplitude is varied between zero and double its unmodulated value.
 Modulated carrier approach double its unmodulated value and falls to zero for a time interval.
 This results in a gap called sideband splatter.
Amplitude Modulation - The Maths
Amplitude Modulation - The Maths
Amplitude Modulation - The Maths
Amplitude Modulation - The Maths
Amplitude Modulation - The Maths
Amplitude Modulation - The Maths
Power Distribution in Carrier
 A carrier signal/waveform can be modulated by a modulating signal which is pure sine wave.
 The upper sideband and lower sideband frequencies at 100% modulation
 are one-half the carrier amplitude.
Power Distribution in Carrier
Power Distribution in Carrier
Power Distribution in Carrier
Power Distribution in Carrier
Power Distribution in Carrier
Power Distribution in Carrier