1- Amplitude Modulation

‫عبدهللا احمد طارق‬

& Demodulation

Q1) Define AM and draw its spectrum?

Ans/ Amplitude Modulation (AM) is a modulation technique where the

amplitude of a high-frequency carrier wave is varied in proportion to
the instantaneous amplitude of the baseband (message) signal while
keeping the frequency and phase constant.
Mathematically, an AM signal is represented as:

Where:

• AC = Amplitude of the carrier wave

• Ka = Amplitude sensitivity (modulation index factor)
• m(t) = Baseband (message) signal
• fC = Carrier frequency

The spectrum :
Q2) Draw the phasor representation of an amplitude modulated
wave?
Ans/

Q3) Give the significance of modulation index ?

Ans/
The modulation index (ka or m ) is a critical parameter in Amplitude
Modulation (AM) that determines the quality, efficiency, and fidelity of the
transmitted signal. Here’s why it matters:

kₐ = Aₘ/Aₐ
where:

• Aₘ = peak amplitude of the modulating (message) signal

• Aₐ = peak amplitude of the carrier signal

It has three operational ranges:

 • Under modulation (kₐ < 1)
• Critical modulation (kₐ = 1)
• Over modulation (kₐ > 1)

2. Signal Quality and Power Efficiency

The modulation index directly affects:

• Signal-to-noise ratio (SNR)

• Power distribution between carrier and sidebands
• Transmission efficiency

At kₐ = 1 (100% modulation):

• Maximum useful power is transmitted in the sidebands

• Power efficiency reaches 33.3% (theoretical maximum for AM)
• No distortion occurs
3. Practical Implications

For broadcast applications:

• Typical kₐ values range from 0.7 to 0.9

• This range provides a good balance between:
o Signal strength
o Power efficiency
o Distortion avoidance
4. Effects of Improper Modulation Index

Under modulation (kₐ < 1):

• Results in weak signal transmission

• Poor utilization of transmitter power
• Reduced signal-to-noise ratio

Over modulation (kₐ > 1):

• Causes envelope distortion

• Leads to phase reversals
• Requires complex demodulation techniques

5. Technical Considerations

• The modulation index determines the relative amplitudes of the

sidebands
• It affects the bandwidth requirements (BW = 2fₘ, where fₘ is the
message frequency)
• Impacts the design of both transmitters and receivers

Q4) What are the different degrees of modulation?

Ans/
In amplitude modulation (AM), the degree of modulation is quantified
by the modulation index (m), which indicates the extent of amplitude
variation in the carrier signal due to the modulating signal. It's
calculated as:

Peak amplitude of modulating signal

𝑚=
𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑢𝑛𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑒𝑑 𝐶𝑎𝑟𝑟𝑖𝑒𝑟

The degrees of modulation are categorized as:

• Under-modulation: Occurs when m<1m < 1m<1. The carrier's

amplitude varies less than its unmodulated amplitude, resulting
in a weaker transmitted signal.
• 100% Modulation: Occurs when m=1m = 1m=1. The carrier's amplitude
varies exactly up to its unmodulated amplitude, achieving optimal
transmission without distortion.
• Over-modulation: Occurs when m>1m > 1m>1. The carrier's amplitude
variation exceeds its unmodulated amplitude, leading to signal
distortion and potential loss of information.

Q5) What are the different degrees of modulation?

Ans/
A square law modulator utilizes the nonlinear region of a device's
characteristic curve to generate an AM signal. Its limitations include:

• Limited Linearity: The modulator operates effectively only within a

restricted range of input signal amplitudes. Exceeding this range
results in significant distortion due to the nonlinear behavior of the
device.
• Inefficiency at Higher Frequencies: Square law modulators are less
effective at higher frequencies, making them unsuitable for
applications requiring high-frequency modulation.
• Generation of Unwanted Harmonics: The nonlinear operation can produce
unwanted harmonic components, necessitating additional filtering to
isolate the desired AM signal.
• Poor Noise Performance: Devices like diodes, when used in square law
modulators, exhibit poor noise performance, which can degrade the
quality of the modulated signal.
Q6) Compare linear and nonlinear modulators?

Ans/
Linear Modulators:

• Produce exact scaled versions of input

• Preserve frequency relationships
• Used in DSB-SC and SSB generation
• More complex circuitry
Nonlinear Modulators:

• Generate new frequency components

• Enable simple AM generation
• Introduce harmonic distortion
• Simpler circuits (e.g., diode modulators)

Q7) What are the different degrees of modulation?

Ans/
Base Modulation:

• Circuit: Modulating signal applied to transistor base.

• Advantage: Simple implementation (1–2 components).
• Disadvantage: Nonlinearity at high modulation indices (kₐ > 0.7).
Emitter Modulation:

• Circuit: Modulating signal injected at emitter.

• Advantage: Improved linearity (kₐ up to 0.9 achievable).
• Disadvantage: Requires higher-power audio amplifier.
Q8) Explain how AM wave is detected?
Ans/
Detecting an AM wave involves extracting the original modulating signal from
the modulated carrier. Common methods include:

• Envelope Detection: Utilizes a diode to rectify the AM signal, followed

by a low-pass filter to remove the high-frequency components, leaving
the original modulating signal.
• Product Detection: Multiplies the AM signal with a locally generated
carrier signal synchronized with the transmitted carrier, then passes
the result through a low-pass filter to retrieve the modulating signal.

Q9) Define detection process?

Ans/
The process of extracting the original information signal from the modulated
carrier wave while maintaining its essential characteristics (amplitude,
frequency, or phase content).

Q10) What are the different types of distortions that occur in an

envelop detector? How can they be eliminated??

Ans/
An envelope detector can experience distortions such as:

• Diagonal Clipping: Occurs when the RC time constant of the detector

circuit is too small, causing the detector to be unable to follow the
envelope of the modulated signal accurately.
• Negative Peak Clipping: Happens when the modulation index exceeds 100%,
leading to the carrier's amplitude dropping below zero and resulting in
distortion.

Elimination Methods:

• Adjusting the RC Time Constant: Ensuring the RC time constant is

appropriately chosen allows the detector to accurately follow the
envelope without introducing distortion.
• Preventing Over-Modulation: Maintaining the modulation index at or
below 100% prevents negative peak clipping and ensures the integrity of
the detected signal.
Q11) What is the condition of for over modulation?
Ans/
Over modulation occurs when:

𝐴𝑚
𝐾𝑎 = >1
𝐴𝐶

Effects:

• Envelope crosses zero, causing phase reversals.

• Distortion in the demodulated signal

Q12) Define modulation & demodulation?

Ans/
Modulation is the process of varying a carrier signal's properties—such as
amplitude, frequency, or phase—in accordance with the information signal.
This allows the information to be effectively transmitted over long distances
and through different transmission mediums. For instance, in amplitude
modulation (AM), the amplitude of the carrier wave is varied in proportion to
the information signal.

Demodulation is the reverse process, where the received modulated signal is

converted back to its original form. This involves extracting the information
signal from the modulated carrier wave at the receiver end, enabling the
retrieval and interpretation of the transmitted data.

Q13) What are the different types of linear modulation techniques?

Ans/
Linear modulation techniques include:

• Amplitude Modulation (AM): Varying the carrier signal's amplitude in

proportion to the message signal.
• Double Sideband Suppressed Carrier (DSB-SC): Transmitting both
sidebands without the carrier.
• Single Sideband (SSB): Transmitting only one sideband, reducing
bandwidth usage.
• Vestigial Sideband (VSB): Transmitting one full sideband and a portion
of the other, commonly used in television broadcasting.
Q14) What are the different types of linear modulation technique?
Ans/
A carrier wave generator produces a stable sinusoidal signal at a specific
frequency, serving as the carrier for modulation. It typically employs
oscillators, such as crystal oscillators, to ensure frequency stability and
low phase noise, essential for effective modulation and transmission.

Q15) Explain the working of the modulator circuit?

Ans/
A modulator circuit combines the information signal with the carrier wave,
altering the carrier's properties (amplitude, frequency, or phase) to encode
the information. For instance, in an AM modulator, the modulating signal
varies the carrier's amplitude. Various circuit designs, including switching
modulators and balanced modulators, achieve this by mixing the signals and
filtering the desired output

2- AM-DSB-SC Modulation
& Demodulation
Q1) What are the two ways of generating DSB_SC?
Ans/
Double Sideband Suppressed Carrier (DSB-SC) signals can be generated
using:

• Balanced Modulator: This circuit combines the message signal with

the carrier, effectively suppressing the carrier component and
producing the DSB-SC signal.
• Ring Modulator: Utilizing diodes arranged in a ring
configuration, this modulator multiplies the carrier and message
signals, resulting in a DSB-SC output.
Q2) What are the applications of balanced modulator?
Ans/
• DSB-SC generation in communication systems.
• Single Sideband (SSB) modulation when combined with filtering.
• Frequency translation in mixers and demodulators.
• Analog multiplier circuits in signal processing.

Q3) What are the advantages of suppressing the carrier?

Ans/
Suppressing the carrier in modulation offers:

• Power Efficiency: More power is allocated to the sidebands, which

carry the actual information.
• Bandwidth Efficiency: Reduces the bandwidth requirement, allowing
more channels within the same frequency spectrum.

Q4) What are the advantages of balanced modulator?

Ans/
Suppressing the carrier in modulation offers:

• Power Efficiency: More power is allocated to the sidebands, which

carry the actual information.
• Bandwidth Efficiency: Reduces the bandwidth requirement, allowing
more channels within the same frequency spectrum.
Q5) What are the advantages of Ring modulator?
Ans/
1. Excellent Carrier Suppression
• The ring modulator effectively cancels the carrier signal due to its
symmetrical diode configuration.
• This results in a pure DSB-SC output, where only the sidebands
remain.
2. High-Frequency Operation
• The diode-based design allows efficient operation at radio
frequencies (RF).
• Suitable for frequency mixing, up-conversion, and down-conversion in
communication systems.
3. No Need for Transformers (in Some Designs)
• Unlike some balanced modulators, certain ring modulator designs do
not require bulky transformers, making them more compact.
4. Good Balance Between Sidebands
• Produces symmetrical sidebands with minimal distortion.
• Useful in applications requiring precise frequency translation.
5. Simple and Robust Construction
• Uses only four diodes and a few passive components, making it cost-
effective and reliable.
• Less prone to drift compared to active modulator circuits.
6. Wideband Operation
• Can handle a broad range of modulating frequencies, making it
versatile for different applications.
7. Low Power Consumption
• Since it uses passive diodes, it does not require external biasing,
reducing power consumption.
8. Applications in Analog Multiplication
• Can function as an analog multiplier for signal processing tasks.

Q6) Write the expression for the output voltage of a balanced

modulator?

Ans/
The output voltage Vout of a balanced modulator is given by:

𝑉𝑜𝑢𝑡 = 𝐴𝑚 𝐴𝑐 cos[(𝑤𝑐 + 𝑤𝑚 )𝑡] + 𝐴𝑚 𝐴𝑐 cos [(𝑤𝑐 − 𝑤𝑚 )𝑡]

Where:

• 𝐴𝑚 = Amplitude of the message signal.

• 𝐴𝑐 = Amplitude of the carrier signal.
• 𝑤𝑚 = Angular frequency of the message signal.
• 𝑤𝑐 = Angular frequency of the carrier signal.

This expression represents the sum of the upper and lower sidebands,
characteristic of a DSB-SC signal.

Q7) Explain the working of balanced modulator and Ring Modulator using
diodes?

Ans/
Balanced Modulator: Employs nonlinear components, such as diodes, to
mix the carrier and message signals. By arranging the circuit to
cancel out the carrier component, it outputs a signal containing only
the sidebands, effectively producing a DSB-SC signal.

Ring Modulator: Consists of four diodes configured in a ring. The

carrier and message signals are applied to this diode ring, resulting
in the multiplication of the two signals. This process suppresses both
the carrier and the original message signal frequencies, outputting a
signal that contains only the sum and difference frequencies
(sidebands), characteristic of DSB-SC modulation.
 ‫عبدهللا احمد طارق‬

3- AM-SSB-SC Modulation
& Demodulation

Q1) What are the different methods to generate SSB-SC signal?

Ans/
Single Sideband Suppressed Carrier (SSB-SC) signals can be generated using
two primary methods:

• Phase Discrimination Method: This technique involves creating two

versions of the message signal, each shifted by 90 degrees in phase.
These phase-shifted signals are then used to modulate two carriers that
are also 90 degrees out of phase with each other. By combining the
appropriately modulated signals, either the upper or lower sideband can
be selected, resulting in an SSB-SC signal.

• Frequency Discrimination Method: This approach utilizes frequency-

selective filters to extract one of the sidebands from a double
sideband signal, effectively suppressing the carrier and the unwanted
sideband, thereby producing an SSB-SC signal.

Q2) What is the advantage of SSB-SC over DSB-SC?

Ans/

1. Bandwidth Efficiency:
• SSB-SC transmits only one sideband (USB or LSB), occupying half the
bandwidth of DSB-SC (which sends both sidebands).
 • For a baseband signal of bandwidth 𝐵 Hz:
o DSB-SC bandwidth = 2𝐵 Hz
o SSB-SC bandwidth = 𝐵 Hz
• This allows more channels in the same frequency spectrum, improving
spectral utilization.
2. Power Efficiency:

• SSB-SC suppresses the carrier (like DSB-SC) and eliminates one

sideband, meaning all transmitted power is concentrated in the
useful sideband.
• In DSB-SC, power is wasted in transmitting two identical sidebands.
• Result: SSB-SC delivers more power efficiency for the same
transmission power.

3. Reduced Noise & Interference:

• Since SSB-SC uses half the bandwidth, it picks up less thermal noise
(noise is proportional to bandwidth).
• Also, fewer interfering signals enter the receiver due to narrower
bandwidth.

4. Longer Transmission Range:

• With all power focused into a single sideband, SSB-SC can achieve
better signal-to-noise ratio (SNR) at the receiver compared to DSB-SC.
• This allows for longer communication range in applications like radio,
military, and maritime communications.

5. Less Fading in Multipath Environments:

• Narrower bandwidth means less susceptibility to frequency-selective

fading (a problem in DSB due to wider bandwidth).

Q3) Explain Phase Shift method for SSB generation.?

Ans/
The Phase Shift method for SSB generation involves the following steps:

• Phase Shifting: The baseband message signal is split into two identical
signals. One signal is passed through a 90-degree phase shift network,
resulting in two signals that are 90 degrees out of phase with each
other.
• Balanced Modulation: Each of these phase-shifted signals is then used
to modulate a carrier wave. This is typically achieved using balanced
modulators, which suppress the carrier component and produce double
sideband signals.
 • Combination: By appropriately combining the outputs of these
modulators, either the upper or lower sideband can be selected,
resulting in an SSB-SC signal.

Mathematically:

• Upper Sideband (USB) Cancellation:

𝑉𝑜𝑢𝑡 = 𝑚(𝑡) cos(𝑤𝑐 𝑡) − 𝑚

̂ (𝑡) sin(𝑤𝑐 𝑡)

• Lower Sideband (LSB) Cancellation:

𝑉𝑜𝑢𝑡 = 𝑚(𝑡) cos(𝑤𝑐 𝑡) + 𝑚

̂ (𝑡) sin(𝑤𝑐 𝑡)

Where 𝑚
̂ is the Hilbert transform of m(t)

Q4) Why SSB is not used for broadcasting?

Ans/
1- Complexity: SSB generation and demodulation are more complex compared
to standard Amplitude Modulation (AM). They require precise frequency
and phase synchronization, making the equipment more intricate and
costly.
2- Receiver Requirements: SSB signals cannot be demodulated using simple
envelope detectors. Specialized receivers capable of coherent detection
are necessary, limiting the accessibility for general broadcasting
audiences.
Q5) Give the circuit for synchronous detector?
Ans/
Q6) What are the uses of synchronous or coherent detector?
Ans/
Synchronous, or coherent, detectors are advanced signal demodulation systems
that recover modulated signals by maintaining precise phase alignment with
the carrier wave. Unlike simple envelope detectors that only track amplitude,
synchronous detectors multiply the received signal with a locally generated
carrier that is phase-locked to the original. This method significantly
enhances signal fidelity and noise rejection. Key applications include:

1. Weak Signal Recovery in Noisy Environments:

• Deep-Space Communications: NASA space probes use synchronous detection

to recover signals buried in cosmic noise. By phase-locking onto faint
carriers using Phase-Locked Loops (PLLs), even weak telemetry data can
be accurately retrieved.
• Quantum Systems: In superconducting qubit readout systems, coherent
detection is used to identify tiny signal changes in the nanovolt range
with high sensitivity.

2. Modern Digital Communication Systems:

• QAM and PSK Demodulation: Quadrature Amplitude Modulation (QAM),

Binary Phase Shift Keying (BPSK), and Quadrature Phase Shift
Keying (QPSK) require coherent detection to accurately separate
in-phase (I) and quadrature (Q) components without distortion or
crosstalk.
• FSK and OFDM Systems: Frequency Shift Keying (FSK) and Orthogonal
Frequency Division Multiplexing (OFDM) also rely on coherent
detection for symbol synchronization and phase tracking.

3. Medical and Scientific Measurement:

• Magnetic Resonance Imaging (MRI): Synchronous detection isolates the

weak RF signals emitted by hydrogen atoms in tissue, improving image
clarity by rejecting thermal and electronic noise.
• Laser Doppler Vibrometry: Measures nanometer-scale mechanical
vibrations by coherently mixing the reflected laser beam with a
reference, enabling high-precision displacement measurements.

4. Radar, Sonar, and Non-Destructive Testing:

• Pulse-Doppler Radar: Coherent integration of return pulses

enables velocity measurement of moving targets and effective
clutter rejection.
 • Ultrasonic Testing: Detects internal material flaws by analyzing
the phase of reflected ultrasonic waves, increasing detection
sensitivity.

5. Software-Defined and Legacy Radio Systems:

• Software-Defined Radio (SDR): Emulates synchronous detection

digitally, enabling clear demodulation of AM and other legacy
signals even when the carrier is suppressed or distorted.
• DSB-SC and SSB Demodulation: Coherent detection is essential for
demodulating Double Sideband Suppressed Carrier (DSB-SC) and Single
Sideband (SSB) signals, where the carrier is missing or reduced.

Q7) Give the block diagram of synchronous detector?

Ans/

Q8) Why the name synchronous detector?

Ans/
The wide-ranging uses of the synchronous detector stem from its unique
ability to recover signals that would be lost to noise, distortion, or
suppression using ordinary detection methods. This circuit doesn’t just
“listen” to a signal—it locks in with it, faithfully tracking the carrier’s
phase and frequency to extract the original message with surgical precision.
This makes it invaluable in environments where clarity is everything.
In deep-space communications, where signals arrive fainter than a whisper
drowned in cosmic noise, synchronous detectors serve as lifelines—locking
onto distant carriers and resurrecting data that conventional detectors would
simply miss. In the digital age, they’re the backbone of modern modulation
schemes like QAM, PSK, and FSK, where information is intricately woven into
both amplitude and phase. Without synchronous detection, decoding these
signals would be like trying to understand speech without hearing tone or
rhythm.

Their impact stretches into the realms of science and medicine as well. MRI
machines, for instance, rely on coherent detection to isolate the tiny radio
signals emitted by hydrogen atoms, creating detailed images from a sea of
biological noise. Instruments like laser Doppler vibrometers and ultrasonic
testers use the same principle—detecting minute phase shifts to measure
microscopic vibrations or uncover hidden flaws in solid materials.

Even in the world of software-defined radio, where analog hardware gives way
to digital algorithms, the concept remains unchanged. The carrier is
reconstructed in code, and the same synchronous principle allows ancient
signals to be heard clearly once more. Wherever the goal is faithful signal
recovery—be it from deep space, within the human body, or through layers of
digital complexity—the synchronous detector proves itself not just useful,
but essential.
 4- Frequency Modulation
& Demodulation

Q1) Define FM & PM ?

Ans/
Frequency Modulation (FM):
Frequency Modulation (FM) is a type of angle modulation where the frequency
of the carrier signal is varied in proportion to the instantaneous amplitude
of the message signal, while the amplitude and phase of the carrier remain
constant. It is commonly used in analog radio broadcasting and communication
systems due to its resilience to noise.
Phase Modulation (PM):
Phase Modulation (PM) is a form of angle modulation in which the phase of the
carrier signal is varied directly in proportion to the instantaneous
amplitude of the message signal. The frequency and amplitude of the carrier
remain unchanged. PM is widely used in digital communication systems, such as
phase shift keying (PSK) techniques.

Q2) What are the advantages of Angle modulation over amplitude

modulation ?

Ans/
The superiority of angle modulation encompassing both frequency modulation
(FM) and phase modulation (PM)—over amplitude modulation (AM) lies in its
robustness to noise, superior signal fidelity, and efficient power
utilization, especially under real world transmission conditions where
reliability often matters more than simplicity.
Unlike AM, where information is encoded in the amplitude of the carrier and
thus directly exposed to every form of noise that alters signal strength—such
as thermal fluctuations, atmospheric interference, and impulse noise—angle
modulation shifts the burden of information to frequency or phase, which are
far more immune to such disturbances. In practical terms, this means that FM
and PM signals can traverse noisy or cluttered environments (like urban
canyons, broadcast airwaves, or even space) with far less degradation in
intelligibility or quality.
The most iconic demonstration of this is in FM radio, where listeners enjoy
static-free music even while driving through tunnels or under power lines—
conditions that would cause severe distortion in AM. This noise resistance is
not just anecdotal: angle modulation's constant-amplitude nature allows the
use of nonlinear power amplifiers, which are far more energy-efficient than
the linear amplifiers required for AM systems. This makes FM ideal for high
power transmitters, such as those in commercial broadcasting and long-range
communications.

Moreover, angle modulation supports greater bandwidth and dynamic range,

allowing for higher fidelity transmission of complex analog signals like
voice and music. In scientific and military contexts, PM is favored in secure
or high-precision applications, where the phase of the signal can encode
intricate patterns resistant to eavesdropping or jamming.
Another key advantage is the compatibility of angle modulation with modern
digital systems. Schemes like FSK (frequency shift keying) and PSK (phase
shift keying)—which form the foundation of Wi-Fi, Bluetooth, 4G, and
satellite links—inherit the same noise rejection and efficiency properties
from their analog ancestors.

While AM retains its value in simplicity and low-cost receiver design,

particularly in legacy systems, the long-term shift toward angle modulation
in both analog and digital domains reflects a broader engineering truth: when
clarity, reliability, and performance are paramount, modulating the angle not
the amplitude is the smarter choice.

Q3) What is the relationship between PM and FM ?

Ans/
The relationship between Phase Modulation (PM) and Frequency
Modulation (FM) is both mathematical and conceptual, rooted in their
shared identity as forms of angle modulation. In angle modulation, the
information is encoded not in the amplitude of the carrier, but in its
angular properties—specifically the instantaneous phase.

At first glance, FM and PM appear distinct: FM varies the frequency of

the carrier in response to the message signal, while PM varies its
phase. However, they are deeply interconnected, to the point that one
can be derived from the other through simple signal processing
operations—specifically, integration and differentiation.

Mathematically, if we denote the message signal as m(t)m(t)m(t), then:

• In FM, the instantaneous frequency deviation is proportional to

the message signal:

𝑡
𝑆𝐹𝑀 (t) = A𝐶 cos[2π 𝑓𝐶 t + 2πk𝑓 ∫ m(τ)dτ]
0
 • In PM, the instantaneous phase deviation is directly proportional
to the message signal:

𝑆𝑃𝑀 (𝑡 ) = 𝐴𝐶 𝑐𝑜𝑠[2𝜋 𝑓𝐶 𝑡 + 𝑘𝑃 𝑚(𝑡)]

This reveals a profound symmetry: PM is the derivative of FM, and FM is the

integral of PM. In other words:
• Applying PM to the derivative of m(t)m(t)m(t) yields an FM signal.

• Applying FM to the integral of m(t)m(t)m(t) yields a PM signal.

This equivalence is not merely theoretical it’s exploited in practical
systems. In many communication architectures, a transmitter might implement
FM using a PM modulator, or vice versa, by simply integrating or
differentiating the baseband signal prior to modulation. This flexibility is
especially valuable in digital signal processing, where modulation is
implemented algorithmically and the boundary between PM and FM becomes fluid.
Despite their interconvertibility, FM and PM behave differently in response
to various signal properties. For example, PM is more sensitive to high-
frequency components in the message, as those cause rapid phase changes. FM,
on the other hand, responds to the magnitude of the signal, regardless of how
quickly it changes, making it better suited for voice and audio applications
where large excursions are more common than fast transitions.
In essence, PM and FM are two perspectives on the same underlying modulation
space, each emphasizing a different aspect of the carrier’s angular behavior.
Their relationship underscores the elegance of angle modulation theory, and
their interchangeability offers system designers a powerful toolkit for
tailoring performance, robustness, and implementation cost across a wide
range of analog and digital communication systems.

Q4) What are the advantages of Angle modulation over amplitude

modulation ?
 5- Frequency Division Multiplexing
& DeMultiplexing (with DSB-SC)

Q2) What is Multiplexing ?

Ans/
Multiplexing is a technique used in communications to combine multiple
signals into one signal over a shared medium. The main goal of multiplexing
is to maximize the use of the available bandwidth and improve the efficiency
of the communication system. It enables multiple data streams to be
transmitted simultaneously over a single communication channel, thereby
improving the overall system throughput. This concept is widely used in
various fields of communication, including telecommunication, radio
broadcasting, and computer networks.

Q3) What are the different types of Multiplexing techniques ?

Ans/
The main types of multiplexing techniques are:
1. Time Division Multiplexing (TDM): Divides the available bandwidth into
time slots, with each signal being assigned a specific time slot to
transmit. This ensures that signals do not interfere with each other as
they are transmitted one after another.
2. Frequency Division Multiplexing (FDM): Allocates separate frequency
bands to different signals. Each signal is modulated onto a different
frequency, allowing simultaneous transmission without interference.

3. Wavelength Division Multiplexing (WDM): Similar to FDM, but used in

optical fiber communication. Multiple signals are transmitted over
different wavelengths (or colors) of light, allowing for high-capacity
data transmission.

4. Code Division Multiplexing (CDM): Each signal is encoded with a unique

code and transmitted over the same frequency band. The receiver uses
the corresponding code to decode the signal, allowing multiple signals
to occupy the same frequency without interference.

These techniques allow multiple signals to share the same communication

medium, improving bandwidth efficiency and system capacity.

Q4) What are the advantages and disadvantages of FDM ?

Ans/
Advantages of Frequency Division Multiplexing (FDM):
1. Simultaneous Transmission: FDM allows multiple signals to be
transmitted at the same time, using different frequency bands, which
makes it efficient for real-time communication systems like radio,
television, and satellite transmission.

2. Simple and Efficient for Analog Signals: FDM works well for analog
signals, such as audio and video, where continuous transmission is
needed without the need for complex encoding or compression.
 3. Independent Signal Processing: Each signal is modulated onto a separate
frequency, so they can be processed independently at the receiver. This
makes it easier to isolate and demodulate individual signals.
4. Low Latency: Since signals are transmitted simultaneously, FDM
generally offers lower latency compared to other multiplexing
techniques, especially when dealing with continuous data streams.

Disadvantages of Frequency Division Multiplexing (FDM):

1. Bandwidth Waste: FDM requires guard bands between frequency channels to
avoid interference. These guard bands consume bandwidth that could
otherwise be used for transmitting data, leading to inefficiencies,
especially in systems with high bandwidth requirements.
2. Complexity in Signal Separation: At the receiver end, FDM systems need
filters and demodulators that can accurately separate each signal from
the others. This adds complexity to the system, particularly when many
signals are involved.
3. Interference and Crosstalk: If the frequency bands are not sufficiently
spaced or if there are hardware imperfections, signals can interfere
with each other, causing crosstalk and degraded performance.

4. Limited Scalability: The number of signals that can be transmitted

using FDM is limited by the available bandwidth of the communication
medium. As more signals are added, the frequency bands become narrower,
which can reduce the quality and efficiency of transmission.

5. Fixed Bandwidth Allocation: Each signal in FDM is allocated a fixed

frequency range, which may not be ideal if the signal's data rate
fluctuates. If a signal requires more bandwidth than allocated, it may
not be able to use the channel efficiently.

Q5) What is the difficult part in FDM ?

Ans/
The most difficult part of Frequency Division Multiplexing (FDM) lies in the
separation and management of frequency bands to ensure that signals do not
interfere with each other. Several factors contribute to this challenge:
1. Guard Bands: FDM requires the use of guard bands, which are small
frequency gaps between adjacent channels to prevent interference or
crosstalk between signals. While these guard bands help prevent
overlap, they waste valuable bandwidth, making the system less
 efficient. The proper design and allocation of guard bands can be
complex, especially when the available spectrum is limited.

2. Precise Frequency Allocation: Each signal needs to be assigned a

specific frequency band, and these bands must be carefully managed. If
the frequency bands are not allocated correctly, there can be overlap
between adjacent signals, leading to interference and degradation of
signal quality.
3. Signal Interference and Crosstalk: Even with guard bands, signals can
still interfere with each other if the frequency separation is not
perfect, or if there's equipment malfunction. This is especially
problematic in systems with high channel density or in environments
with fluctuating signal characteristics.

4. Complex Hardware Requirements: FDM systems require complex hardware,

such as filters and demodulators, to separate and decode each signal at
the receiver. These components must be highly accurate to ensure that
the signals are processed correctly without introducing distortion or
loss of information.
5. Fixed Bandwidth Allocation: Each signal in FDM is allocated a fixed
bandwidth, and if a signal fluctuates in data rate (for example, if it
requires more bandwidth than originally allocated), it can cause
inefficiencies. This is a limitation in dynamic or variable data rate
environments, where the signal's data rate might not align with the
fixed bandwidth allocations.
Overall, the most difficult part of FDM involves efficiently managing
frequency space, ensuring that each signal is correctly separated, and
maintaining the integrity of the system in the face of possible interference
and hardware complexity.

Q6) What is the overall bandwidth if N number of signals are

multiplexed ?

Ans/
The overall bandwidth required for multiplexing N signals using Frequency
Division Multiplexing (FDM) can be calculated by considering the bandwidth of
each individual signal and the guard bands that are needed to prevent
interference.

Formula:

𝑇𝑜𝑡𝑎𝑙 𝐵𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ = 𝑁 × 𝐵𝑠𝑖𝑔𝑛𝑎𝑙 + (𝑁 − 1) × 𝐵𝑔𝑢𝑎𝑟𝑑

Where:

• N is the number of signals being multiplexed.

• 𝐵𝑠𝑖𝑔𝑛𝑎𝑙 is the bandwidth of each individual signal.

• 𝐵𝑔𝑢𝑎𝑟𝑑 is the width of the guard band between adjacent signals.

Q7) Why AM SSB-SC is preferred for FDM ?

Ans/
AM SSB-SC (Amplitude Modulation Single Sideband Suppressed Carrier) is
preferred for Frequency Division Multiplexing (FDM) due to several advantages
it offers in terms of bandwidth efficiency, power efficiency, and overall
system performance. Here’s why it's commonly used in FDM systems:
1. Bandwidth Efficiency:

• Traditional AM (Amplitude Modulation) requires a bandwidth that is

twice the frequency of the modulating signal
( 𝐵𝐴𝑀 = 2𝐵𝑀𝑒𝑠𝑠𝑎𝑔𝑒 ).

• AM SSB-SC, on the other hand, eliminates one of the sidebands (the

carrier and the opposite sideband), leaving only one sideband. This
reduces the bandwidth requirement to just the bandwidth of the message
signal itself, 𝐵𝑆𝑆𝐵−𝑆𝐶 = 𝐵𝑀𝑒𝑠𝑠𝑎𝑔𝑒 .

• By using SSB-SC modulation in FDM, the available spectrum can be

utilized more efficiently, allowing more signals to be transmitted in
the same frequency range, thus maximizing the use of bandwidth.

2. Power Efficiency:
• In conventional AM, a significant amount of power is wasted in
transmitting the carrier signal, which carries no information.
• In SSB-SC, the carrier is suppressed, and only the sideband is
transmitted. This results in more efficient power usage because the
energy that would have been used to transmit the carrier is now
available for transmitting the information in the sideband.
 • This makes SSB-SC more power-efficient, especially when there are many
signals multiplexed in a system.

3. Reduced Interference:
• Since SSB-SC transmits only one sideband, it minimizes the chance of
interference between signals, which is particularly useful in crowded
frequency bands, as is typical in FDM systems.

• Additionally, the suppression of the carrier reduces the likelihood of

causing unwanted interference with nearby systems.

4. Lower Occupation of Frequency Spectrum:

• By using SSB-SC, the signal's frequency spectrum is halved compared to
conventional AM. This allows more signals to fit into the same overall
bandwidth in an FDM system.

• This is particularly important in systems with limited bandwidth, such

as radio communication and satellite transmission, where efficient use
of frequency resources is crucial.
5. Simplification of Demodulation:

• The demodulation of SSB-SC signals can be done without the need for a
carrier signal, making the receiver design simpler and more cost-
effective.
• In an FDM system, where multiple signals are transmitted
simultaneously, using SSB-SC modulation simplifies the overall system
because each signal can be received and demodulated independently
without needing to regenerate the carrier.
6. Improved Signal-to-Noise Ratio (SNR):

• The elimination of the carrier in SSB-SC modulation means that there is

less power to be overcome by noise in the receiver. The receiver can
focus on the sideband, which contains the actual information, leading
to better signal-to-noise ratio (SNR).

• In FDM systems with multiple multiplexed signals, maintaining a high

SNR for each signal is crucial for reliable communication, and SSB-SC
helps achieve that.
Conclusion:

AM SSB-SC is preferred for FDM because it offers better bandwidth efficiency,

lower power consumption, reduced interference, and simplified demodulation
compared to traditional AM. These benefits make it a more practical and
efficient choice for multiplexing multiple signals in communication systems.
Q8) Why AM SSB-SC is preferred for FDM ?
Ans/
Demultiplexing is the process of separating a combined signal that contains
multiple individual data streams into its original, distinct signals. It is
the inverse of multiplexing, which combines several signals for simultaneous
transmission over a shared communication medium. In demultiplexing, the
receiver identifies and extracts each component signal from the multiplexed
input based on specific characteristics such as timing, frequency, code, or
wavelength, depending on the type of multiplexing used.

This operation is essential for ensuring that each recipient receives only
the data intended for it. In practical terms, a demultiplexer functions by
analyzing the composite signal and selectively routing each part to its
respective output channel. This allows complex communication systems to
efficiently transmit large volumes of data while maintaining clarity and
separation between individual streams.

Demultiplexing plays a critical role in both digital and analog communication

systems, enabling effective resource utilization, minimizing interference,
and ensuring accurate data delivery. It supports a wide range of
applications, from telecommunications and broadcasting to computer networks
and satellite systems, where the integrity and proper distribution of
transmitted information are paramount.