COMMUNICATION SYSTEM EECB353 Chapter 2 Part I AMPLITUDE MODULATION

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COMMUNICATION SYSTEM EECB353Chapter 2 Part I
AMPLITUDE MODULATION

• Anas Bin Muhamad Bostamam
• Dept of Electronics Communication Engineering
• Universiti Tenaga Nasional
• http//metalab.uniten.edu.my/shafinaz

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Continuous-Wave (CW) Modulation

• Contents
• Amplitude Modulation (AM)
• Coefficient of Modulation Percent of Modulation
• AM Power Distribution
• AM Modulator Circuit
• AM Demodulator
• Rectifier Detector
• Envelope Detector
• Superheterodyne Receiver
• Double Sideband Suppressed Carrier (DSBSC)
• Single Sideband (SSB) System
• AM Single Sideband Full Carrier (SSBFC)
• AM Single Sideband Suppressed Carrier (SSBSC)
• AM Single Sideband Reduced Carrier (SSBRC)
• AM Independent Sideband
• AM Vestigal Sideband
• Advantage of SSB Transmission
• Disadvantage of SSB Transmission
• Reference

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Introduction to CW

• CW is the basis of analog communication system.
  There are two types of CW modulation namely
• Amplitude Modulation (AM) amplitude of carrier
  varied according to message signal.
• Angle Modulation instantaneous frequency or
  phase of carrier varied according to message
  signal.

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Introduction to CW

• Information signals are transported between Tx
  and Rx over some form of transmission medium.
• However, the original information signals are
  seldom in a form that is suitable for
  transmission.
• ?They must be transformed from their original
  form into a form that is more suitable for
  transmission.

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Introduction to CW

• The process of impressing low frequency
  information signals onto a high frequency carrier
  signal is called modulation.
• Demodulation is the reverse process where the
  received signal is transformed to their original
  form.
• Amplitude Modulation (AM) is the process of
  changing the amplitude of a relatively high
  frequency carrier signal in proportion with the
  instantaneous value of the modulating signal.

• High frequency carrier signal is also termed the
  radio-frequency (RF) signal because it is at a
  high-enough freq to be transmitted through free
  space as a radio wave.
• Low frequency information signal is also term
  such as modulating signal, intelligence, audio
  signal.

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Amplitude Modulation Envelope

• Most commonly used is AM double-sideband full
  carrier (DSBFC), or sometimes called conventional
  AM or AM.

• NB
• Repetition rate of the envelope frequency of
  the modulating signal.
• Shape of the envelope shape of the modulating
  signal

AM Envelope Generation
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AM Envelope

• Example An AM signal can be produced by using
  instantaneous amplitude of the information signal
  (the baseband or modulating signal) to vary the
  peak amplitude of a higher-frequency signal.
• Figure (a) shows a 1kHz sinewave, which combined
  with the 10kHz signal shown in Figure (b) to
  produce the AM signal in Figure (c).
• If the peaks of the individual waveform of the
  modulating signal are joined, the resulting
  envelope resembles the original modulating
  signal.
• It repeats at the modulating frequency and the
  shape of each half (i.e positive or negative)
  is the same as that of the modulating signal.

(a) Modulating Signal
(b) Carrier Signal
(c) Modulated Signal
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AM Frequency Spectrum and Bandwidth

• Output envelope contains of dc voltage, carrier
  frequency, the sum (fc fm) and difference (fc
  fm) frequencies.
• Bandwidth, B difference between highest USB and
  lowest LSB
• i.e B 2fm(max).
• Figure below shows the frequency spectrum for an
  AM waveform

Carrier
Lower sideband (LSB)
Upper sideband (USB)
Amplitude
Lower Side Frequencies (LSF)
Upper Side Frequencies (USF)
fc
fc- fm(max)
fc fm(max)
Bandwidth, B
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AM Frequency Spectrum- Sidebands
(c)
(a)
(b)
(d)
freq
The AM wave is the algebraic sum of the carrier
and upper and lower sideband sine waves. (a)
Intelligence or modulating signal. (b) Lower
sideband. (c ) Carrier. (d ) Upper sideband. (e )
Composite AM wave.
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Example 1

• For an AM DSBFC modulator with a carrier
  frequency, fc 100 kHz and a maximum modulating
  signal frequency, fm(max) 6 kHz, find
• Frequency limit for upper and lower sideband.
• Bandwidth.
• Upper and lower side frequencies produced when
  the modulating signal is a single frequency 4 kHz
  tone.
• Draw the output frequency spectrum.
• A standard AM broadcast station is allowed to
  transmit modulating frequencies up to 5 kHz. If
  the AM station is transmitting on a frequency of
  980 kHz, what are sideband frequencies and total
  bandwidth?

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Coefficient Percent of Modulation

• Coefficient of Modulation, m (or modulation index
  or modulation factor or degree or depth of
  modulation), is a term used to describe the
  amount of amplitude change present in the AM
  waveform.
• Mathematically, the modulation coefficient,
• where
• m modulation coefficient (unitless)
• Em peak change in the amplitude of output
  
• waveform voltage (volts)
• Ec peak amplitude of the unmodulated
• carrier voltage (volts)

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Derivation

• Percentage of Modulation can be derived as
  follows
• then,
• and

• where Vmax Ec Em
• Vmin Ec - Em

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Derivation

• The peak change in the amplitude of the output
  wave, Em sum of voltages from upper and lower
  side frequencies i.e Em Eusf Elsf and Eusf
  Elsf , then
• where Eusf peak amplitude of upper side
  frequencies
• Elsf peak amplitude of lower side
  frequencies

Note If M 100, when Em Ec and Vmin 0V.
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Coefficient Percent of Modulation

• Percent Modulation,M is m stated as a percentage
  i.e
• When Em Ec, i.e m 1, there is no distortion
  at the output signal.
• When Em 0, i.e m 0, we have the original,
  unmodulated carrier.
• When m gt 1, overmodulation is said to be present.
• Generally, amplitude of message signal should be
  less than amplitude of carrier signal to avoid
  overmodulation.

or

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Coefficient Percent of Modulation
(a) modulating signal
(b) unmodulated signal
(c) 50 modulated wave (m 0.5)
(d) 100 modulated wave (m 1.0)

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Coefficient Percent of Modulation

• Overmodulation and Distortion
• The modulation index should be a number between 0
  and 1.
• If the amplitude of the modulating voltage is
  higher than the carrier voltage, m will be
  greater than 1, causing distortion.
• If the distortion is great enough, the
  intelligence signal becomes unintelligible.

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Coefficient Percent of Modulation

• Overmodulation and Distortion
• Distortion of voice transmissions produces
  garbled, harsh, or unnatural sounds in the
  speaker.
• Distortion of video signals produces a scrambled
  and inaccurate picture on a TV screen.

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Coefficient Percent of Modulation
Figure Distortion of the envelope caused by
overmodulation where the modulating signal
amplitude Vm is greater than the carrier signal
Vc.
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Example 2

• For the AM waveform shown, determine
• Peak amplitude of the upper and lower side
  frequencies.
• Peak amplitude of the unmodulated carrier.
• Peak change in the amplitude of the envelope.
• Coefficient of modulation.
• Percent modulation.

Vmax 18Vp
Vmin 2Vp
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Vmin 3Vp
Vmax 10Vp

• A 2MHz carrier signal is modulated with a 20 kHz
  modulating signal to produce an AM waveform as in
  Figure above. Determine
• Upper and lower side frequencies 2 marks
• Coefficient of modulation and percent modulation
  2 marks
• Peak amplitude of the modulated carrier and the
  upper/lower side peak voltage 2 marks