Lecture 03-Digital modulation techniques

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Lecture# 03
Modulation Techniques
Dr. Shahzad Ashraf
Associate Professor
About Modulation
 A process of encoding information in a transmitted signal
 It involves varying one or more characteristics of a high-frequency carrier wave in
accordance with the information signal, which is typically a lower frequency signal
containing the data or message to be transmitted
 Modulation takes two inputs, a carrier and a signal
Why is modulation needed?
o Efficient transmission over long distances
--Higher frequency carrier waves can travel longer distances with less
attenuation compared to lower frequency signals.
o Multiple channel transmission
--Modulation allows multiple signals to be transmitted simultaneously over a
single medium using techniques like frequency-division multiplexing (FDM)
o Improved signal quality
--Modulation can reduce noise and interference, improving the quality of the
received signal

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Digital modulation
• The process of suppressing low frequency information signal on a high
frequency carrier signal is called DM. OR a method of altering the
properties of a high frequency carrier (amplitude, frequency, phase) in
accordance with a low frequency information signal.
• The modulation is always depends on two signals that is high frequency
carrier and low frequency information. Thus, resultant wave achieved
known as modulated signal.
• It can be stated as
Information (modulating signal) + carrier = modulated signal
Digital modulation
• Why does the modulation is indispensable?
because the carrier wave usually appears as a sine wave and the information signal may be
of any type (either analog or digital)
• Converting digital or analog information to a waveform suitable for
transmission over given medium.
• The modulation can be expressed as
• When working with digital data, altering parameters is referred to as keying
such as ASK, PSK, FSK.

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Digital modulation
Categories of the modulation
In either type of modulation (analog or digital), carrier is of analog format.
Digital modulation
Categories of the modulation
• Why not digital carrier?
The only difference between both categories is type of information (modulating) signal.
i.e. analog modulation = analog information signal
digital modulation = digital information signal
Therefore. outcome of a modulated wave is a change in the high frequency carrier's properties.
Analog modulation: If the variation in the parameter of the carrier is
continuous in accordance to the input analog signal the modulation technique is
termed as analog modulation scheme.
Digital modulation: If the variation in the parameter of the carrier is discrete
then it is termed as digital modulation technique. In digital modulation , an
analog carrier signal is modulated by a discrete signal. The modulating wave
consists of binary data and the carrier is sinusoidal wave.
Digital modulation techniques: Three main digital modulation techniques are
discussed as.
a. Amplitude Shift Keying (ASK) b. Frequency Shift Keying (FSK)
c. Phase Shift Keying (PSK)

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Digital modulation techniques
a. Amplitude Shift Keying (ASK)
o It is a simplest digital modulation technique where digital data is converted into analog signal.
o The amplitude of carrier signal varies with respect to amplitude of message signal.
o Bit 1 is transmitted by a signal of one particular amplitude, to transmit 0,only change the amplitude
while keeping the frequency constant.
a. Amplitude Shift Keying (ASK)
The outcome:
o The information signal carries the discrete bit, only two voltage levels occur at input (either +1V or -1V)
o Thus, only two voltage levels at output are available (either [Ac cos(ωct)] or OV)
o because of these two output voltage levels, the carrier is either in ON or OFF state.
o Therefore, ASK is also known as ON-OFF Keying (OOK)
Applications
o Broadcasting of signal
o Lazer intensity modulation in optical fiber communication

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b. Frequency Shift Keying (FSK)
o Only change in frequency is occurred in response to the information, a binary information signal
directly modulates frequency of an analog carrier.
o The modulated signal 1, comes up with high frequency while 0 with low frequency
o Only change in frequency is occurred in response to the information, a binary information signal

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The outcome:
o The carrier wave continuously shifted by an amount specified as Δf.
o when logic 1 input is applied, the carrier wave modulated itself to a frequency known as mark
frequency (fm).
o when logic 0 input is applied, the carrier wave modulated itself to a frequency known as space
frequency (fc).
o The overall spectrum of FSK will be observed as
c. Phase Shift Keying (PSK)
o In this technique, the phase of the sinusoidal carrier to indicated information is being changed. Phase
in this context is the starting angle at which the sinusoidal starts.
o One phase change encodes 0 while another phase change encodes 1.To transmit 0,we shift the phase
of the sinusoid by 180. Thus, as the input digital signal changes state (i.e. from 1 to 0 or 0 to 1), the
phase of the output carrier shifts between two phases (that are separated by 180o).
o It is one of a simplest form of Binary phase shift keying (BPSK)

Lecture 03-Digital modulation techniques

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