Understanding Modulation & Demodulation in communication system (Why?)

Communication is the process of transmitting information.

Baseband communication: Communication that directly transmit the baseband signal without any modulation is known as baseband communication.

The term baseband is used to designate the band of frequencies of the signal delivered by the source or the input transducer.( The transducer is a general term used for representing a device which can convert physical signal into electrical signal or electrical signal into physical signal). In baseband communication, baseband signals are transmitted without modulation because they have sizeable power at low frequencies. They can’t be transmitted over a radio link but are suitable for transmission over a pair of wires, coaxial cables or optical fibers. 

Carrier communication: Communication that uses modulation to shift the frequency spectrum of a signal is known as carrier communication.  

Modulation: The process of changing some characteristics (amplitude, frequency or phase) of a carrier wave in accordance with the intensity of the signal is known as modulation.

 Why modulation?:

Antenna height and radiation property is related to transmitted signal frequency.   Theory shows that in order to transmit a wave effectively or efficiently, the length of the transmitting antenna should be at least approximately equal to or greater than the (1/10)* wavelength of the wave . Actually different antennas height are related differently with wavelength. For low frequency the wavelength ( ƛ = v/f) becomes very high. So physical length of antenna becomes impractically large.

At low frequency radiation is poor and signal gets highly attenuated.  Also the energy of a wave depends upon its frequency. So the greater the frequency, the greater the distance of transmission. Also at low frequency signals bandwidth is low.

In nature the generated signals (like sound range 20 Hz - 20 kHz) have closely similar frequency bands. So the efficiency of the signal can also decrease by their interference.

Therefore for low frequency baseband signals, it is hard to transmit at large distance . All these problem can be overcome by modulating  low frequency signal into a much higher frequency signal in term of carrier wave with additional facilities :

          1. Allows multiplexing
           2. Allows adjustments in bandwidth
           3. Avoids mixing of signals
           4. Improve quality of reception and SNR
           5. Reduce height of the antenna
           6. Increase the range of communication

Demodulation:- The process of recovering the message signal from the modulated signal is known as demodulation or detection.

Why Demodulation?: 

The modulated wave has large frequency and consists of carrier and sideband frequencies. If the modulated wave is directly fed to the operating device, it wouldn’t at all able to respond to such a high frequencies. So we must separate the higher carrier frequency. In the channel, it is also possible to add some noise to the modulated wave which should be rejected. That is why demodulation is needed.

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Semiconductor Diode: its advantages and disadvantages

What is Diode?

If we connect a p-type material with a n-type material such that the crystal structure remains continuous at the boundary, a pn-junction is formed; which is known as semiconductor diode. Every semiconductor device has at least on pn-junction. 

Above definition is given according to structural properties. But according to characteristics, we may define diode or ideal diode as a two terminal device which:

• Conducts with zero resistance when forward bias.
• Appears as an infinite resistance when reverse bias.

What is Real Diode?

A real diode may be defined as a two terminal device which neither conducts with zero resistance when forward biased nor appears as an infinite resistance when reverse biased.

Advantages of Diode:

•     They are smaller, more rugged and have a longer life.
•     They are simpler and inherently cheaper.
•      They require no filament power. So they produce less heat than the equivalent vacuum-tubes. 

Disadvantages of Diode:

• They are extremely heat sensitive.
• They can handle small current and low inverse voltage.
• They can handle small current and low inverse voltage.
• They can’t be an overload even for a short period of time. 
• A slight overload can permanently damage the diode.

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Integrated Circuit (IC): Exploring its Advantages and Disadvantages

What is IC?

An Integrated circuit is one in which circuit elements such as transistors, diodes, resistors, capacitors etc. and their inter connections are automatically part of a small semiconductor chip to perform the complete electronic function.

Advantages of IC: 

• The circuit layout is greatly simplified and increased reliability due to lesser number of connections. 

•  Extremely small in size, light in weight, and cheap in cost.
•  Low power requirements.
• Greater ability to operate at extreme values of temperature. 

Disadvantages of IC:

1.The size of ICs is so small that you need a microscope to see the inter connections of components.

2. There is a lack of flexibility because individual components can’t be removed or replaced.

3. It is not possible to produce High Power ICs greater than 10W.

4. When a component in a IC goes out of order, the whole IC has to be replaced by new one.

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A Communication system and its limitations

Communication System: 

Communication system is the system designed to transmit information. The components of a communication system are as follows:-

Fig: Block diagram of a general communication system

Source: The source originates a message such as human voice. If  the message is nonelectrical, it must be converted by an input transducer into an electrical waveform referred to as the baseband signal or message signal.  

Transmitter: The transmitter modifies the baseband signal for efficient transmission.

Channel: The channel is a medium such as wire, co-axial cable, waveguide, optical fiber or a radio link through  which the transmitter output is sent.

 

Receiver: The receiver reprocesses the signal received from the channel by undoing the signal modifications made at the transmitter and the channel. The receiver output is fed to the output transducer which converts the electrical signal to its original form- the message. 

Destination: The destination is the unit to which the message is communicated.

Limitations of a Communication System: 

In a communication system the efficient transmission depends on bandwidth of the channel, signal power and instrumental requirements. Shanon’s shows that the rate of transmission without error is given as     

           C = B log₂ ( 1 + SNR )       bit/s

Where  B = channel bandwidth,   SNR = signal to noise ratio = signal power / noise power

Bandwidth of a channel is the range of frequencies that it can transmit with reasonable fidelity. So the increase in bandwidth means the increase in number of frequencies. As a result the transmission speed also increase. 

SNR is defined as the ratio of signal power to noise power. As we have no control on noise, increasing the signal power we able to increase SNR. That means reducing the noise effect.

But we can’t increase the signal power and channel bandwidth unlimitedly. Because they are exchangeable. Which describes that increase in bandwidth causes decrease in signal power and vice versa.

From shanon equation we see that if there is no error/noise signal, the rate of transmission will be infinite. Physical meaning of this is that we will able to transmit the signal in any distance without error or loss of energy. In practical, establishing such a system and generation of wave have some difficulties. Therefore we have some fundamental limitations in our communication system.

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