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COMMUNICATION SYSTEM EECB353 Chapter 2 Part I AMPLITUDE MODULATION

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Title: COMMUNICATION SYSTEM EECB353 Chapter 2 Part I AMPLITUDE MODULATION


1
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

2
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

3
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.

4
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.

5
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.

6
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
7
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
8
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
9
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.
10
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?

11
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)

12
Derivation
  • Percentage of Modulation can be derived as
    follows
  • then,
  • and
  • where Vmax Ec Em
  • Vmin Ec - Em

13
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.
14
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

15
Coefficient Percent of Modulation
(a) modulating signal
(b) unmodulated signal
(c) 50 modulated wave (m 0.5)
(d) 100 modulated wave (m 1.0)

16
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.

17
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.

18
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.
19
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
20
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
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