ECC405: Principles of Communication Engineering

Module No.2: Amplitude Modulation and Demodulation

2.1A) Basic concepts

Transmission of information by communication systems over large distances is quite a feat of
human ingenuity. The communication system uses a very clever technique called Modulation.
Hence, modulation is defined as the process of superimposing a low-frequency signal on a high-
frequency carrier signal( like radio waves or light pulses). This is a method of converting
information to signal for transferring the digital signals over an electronic medium. The
frequency transmission in modulation goes from low to high. For example:- a TV without a
setup box- in this case TV operates through the operator. The other example is that we all use
microphones to record the audio clip the voice used in audio is human voice which is in the
form of Analog signals, the computer converts the human voice into the digital signal and vice-
versa.
Modulation and demodulation are components of modems that convert analog signals to digital
signals and vice versa. The process of converting digital signal to analog signal is known as
modulation, and the reverse of modulation is known as demodulation. Analogue signals are
continuous, whereas digital signals are discrete, or change abruptly.

The working of modulation is the method of superimposing high frequency over low
frequency. The circuit we used in the modulation is the modulator. In modulation, they have
carrier signals and modulated signals. In carrier signal, the signal contains no information but
they have a phase, amplitude, and frequency. In modulated signal, it is the combination of
Carrier signal and modulation.

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 2.1B) Need for modulation
The primary purpose of modulation in a communication system is to generate a modulated signal
which is well suited to the characteristics of transmission medium. The reason why low
frequency signals cannot be transmitted over long distances through space is listed below:
1. Short Operating Range – When a wave has a large frequency, the energy associated with it
will also be large. Thus low frequency signals have less power that does not enable them to
travel over long distances.
2. Poor Radiation Efficiency – The radiation efficiency becomes very poor for low frequency
signals.
3. Mutual Interference – If all audio frequencies are send continuously from different sources,
they would all get mixed up and cause erroneous interference in air. If modulation is done, each
signal will occupy different frequency levels and can be transmitted simultaneously without any
error.
4. Huge Antenna Requirement – For a effective signal transmission, the sending and receiving
antenna should be at least 1/4th of the wave length of the signal. Thus, for small frequencies,
the antenna will have kilometres of length. But if the signal has the range of MegaHertz
frequency, then the antenna size would be less. The carrier wave cannot be used alone for
transmission purposes. Since its amplitude, frequency, and phase angle are constant with
respect to some preference. Additionally, antennas tuned to higher frequencies often exhibit
better radiation patterns, improving signal strength and coverage.

Let’s take 20 kHz as an example:

Using the formula (λ = c/f, where c is the speed of light in
free space and f is the frequency):
λ = (3 x 10^8 m/s) / (20 x 10^3 Hz) = 15,000 meters = 15 km
Imagine an antenna stretching over 15 kilometers! That’s obviously impractical for
communication purposes.
This is where modulation comes in as a lifesaver. You proposed using a 100MHz carrier
wave to carry the 20 kHz speech signal.
We want to transmit audio with a frequency range of 20 Hz to 20 kHz. Instead of directly
transmitting this baseband signal, we employ modulation with a carrier frequency of 100
MHz.
Using the formula λ = c/f, where c is the speed of light in free space (3 x 10^8 m/s) and f is
the carrier frequency:
λ = (3 x 10^8 m/s) / (100 x 10^6 Hz) = 3 meters
As compare to 15 km ,3 meters of antenna is actually possible .

5. Efficient transmission over long distances:

Low-frequency audio or data signals have limited transmission range due to attenuation and
interference. Modulating them onto a high-frequency carrier wave creates a signal with improved
propagation characteristics, allowing for efficient transmission over longer distances.

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 6. To overcome hardware system limitations.
7. To reduce the interference, noise & distortions made when we transmit the signals with nearly
same frequency in the audio frequency range (20-20k) Hz.
8. To multiplex the more number of signals (Multiple signal transmission)
Modulation allows multiple transmitters to share the same medium (like airwaves) without
interference. Each transmitter uses a different carrier frequency for its modulated signal,
enabling simultaneous transmission and reception of numerous signals.
Modulation facilitates multiplexing, where multiple data streams are combined onto a single
carrier wave for efficient transmission. This is used in technologies like cable TV and DSL
internet, radio, television, and mobile phone networks where different channels or data
packets share the same medium.
.9. To narrow banding the signal.
10. To reduce the complexity of the transmission system.
11. To increase the bandwidth of the signal.
12. Noise reduction: Certain modulation techniques, like FM, are relatively resistant to noise and
interference. This helps ensure the integrity of the transmitted information even in noisy
environments.
13. Enhanced signal control: Modulation allows for manipulating the properties of the signal,
like its bandwidth and power, to cater to specific transmission requirements. This can be
crucial for optimizing signal strength, reducing interference, and ensuring compatibility with
different communication systems.

Advantage of Modulation:
Based on need of modulation below are the listed advantages :
• Increased Transmission Range: By riding on high-frequency carrier waves, low-frequency
signals travel further due to less attenuation and interference.
• Multiplexing: Multiple transmitters share the same medium without interference by using
different carrier frequencies, enabling simultaneous broadcasts and data streams.
• Reduced Antenna Size: High-frequency carrier waves enable using shorter antennas for
transmitters and receivers, making devices more compact and portable.
• Improved Noise Resistance: Certain modulation techniques like FM inherently suppress noise
and interference, leading to clearer signals and reduced data errors.
• Enhanced Signal Control: Modulation allows adjusting signal properties like bandwidth and
power to optimize transmission based on distance, coverage, and compatibility needs.
• Diverse Applications: Modulation enables various communication technologies like radio,
television, mobile phones, satellite communication, wireless internet, and more.
• Technologically Enabling: Modulation underpins continuous advancements in communication,
increasing data rates, expanding coverage areas, and improving signal quality.

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