Communication Systems
Lecture 2
Analog Modulation
Dr. Adnan Ismail Al-Sulaifanie
Department of Electrical and Computer Engineering
College of Engineering
University of Duhok
2020 - 2021
Outline
I RF spectrum.
I Baseband vs. Bandpass signals.
I Definition and objectives of analog modulation.
I Types of Analog Modulations.
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References
1. Lathi, Modern digital and analog communication systems, chapters 1 & 4
2. Stremler, Introduction to communication systems, chapter 5
3. Carlson, Communication Systems, pages 141 - 178
4. Haykin, Digital Communication systems, pages 88 - 106
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Baseband signals
I Analog signal generated by message source or digital signals generated
through A/D conversion of analog signal are referred to as baseband signals.
I Example: human voice, TV picture, email message, data.
I Non-electric message should be converted to electrical signal by means of
input transducer.
I The generated signal called baseband signal.
I Baseband describes signals and systems whose range of frequencies is
measured from 0 to a maximum bandwidth or highest signal frequency.
I Examples:
• Voice: Telephone 0 – 3.5 KHz, CD 0 – 22.05 KHz
• Analog video: Analog TV 4.5 MHz, TV channel is 0 – 6 MHz.
• Digital video: depends on the size, movement, frames per second, etc.
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I Baseband signals are not suitable for direct transmission over a given channel
because:
1. Each channel operates on specific range of frequencies.
2. Requires large size antenna. Efficient antenna size is at least 1/4th of
signal wavelength (f = 3 kHz ⇒ λ = 100, 000 m)
3. Direct transmission of multiple baseband signals simultaneously over the
same channel is not possible.
I When baseband signal and channel frequency band do not match, baseband
signal must be moved to the right channel frequency bandwidth.
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RF Spectrum
https://www.rfpage.com/
what-are-radio-frequency-bands-and-its-uses/
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Modulation
I Modulation is the process of varying one or more properties of a periodic
waveform (carrier signal) with a modulating signal that typically contains
information to be transmitted.
I The message signal m(t) is transmitted through the communication channel by
impressing it on a carrier signal of the form:
c(t) = Ac cos(2πfc t + Φc )
I Message signal modulates the carrier signal in either amplitude, frequency, or
phase.
I There are three type of analog modulation: amplitude Modulation, frequency
modulation, and phase modulation
I Modulation converts the message signal m(t) from lowpass to bandpass
around the center of carrier frequency (fc ).
I Modulator is a device that performs modulation.
I Demodulator is a device that performs demodulation, the inverse of modulation.
I Modem (from modulator-demodulator) can perform both operations.
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Objectives of Modulation
I The aim of analog modulation is to transfer an analog baseband (or lowpass)
signal over an analog bandpass channel at a different frequency.
I Objectives of modulation:
1. Spectrum of the transmitted signal will match the characteristics of the
channel.
2. Enable simultaneous transmission of different baseband signals by means
of frequency division multiplexing.
3. Expand bandwidth of transmitted signal to increase its noise immunity over
noisy channel.
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Frequency-shifting Property I
m(t) ⇐⇒ M(f )
m(t) ∗ ej2πfc t ⇐⇒ M(f − fc )
1 [ j2πfc t ]
m(t) cos(2πfc t) = m(t) ∗ e + e−j2πfc t
2
1 [ j2πfc t −j2πfc t
] 1[ ]
m(t) ∗ e +e ⇐⇒ M(f − fc ) + M(f + fc )
2 2
I This shows that the multiplication of a signal m(t) by a sinusoid of frequency fc
shifts the spectrum M(f) by ±fc .
I Multiplication of a sinusoid cos(2πfc t) by m(t) amounts to modulates the
amplitude of sinusoid signal.
I This type of modulation is known as amplitude modulation.
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I The sinusoid c(t) = Ac cos(2πfc t) is called a carrier signal.
I m(t) is modulating signal.
I s(t) = m(t) cos(2πfc t) is modulated signal.
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Trigonometric Functions I
sin(x ∓ y ) = sin x cos y ∓ cos x sin y
cos(x ∓ y ) = cos x cos y ± sin x sin y
1[ ]
cos x cos y = cos(x − y ) + cos(x + y )
2
1[ ]
sin x sin y = cos(x − y ) − cos(x + y )
2
1[ ]
sin x cos y = sin(x + y ) + sin(x − y )
2
1[ ]
cos x sin y = sin(x + y ) − sin(x − y )
2
1 + cos 2x
cos2 x =
2
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Trigonometric Functions II
1 − cos 2x
sin2 x =
2
cos x cos (y + ϕ) =
1[ ] 1[ ]
cos(x − y ) cos ϕ + sin(x − y ) sin ϕ + cos(x + y ) cos ϕ − sin(x + y ) sin ϕ
2 2
Homework: expands the following
sin x sin (y + ϕ) =
sin x cos (y + ϕ) =
cos x sin (y + ϕ) =
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