Modulation is a technique which is used to enhance the capability of an information signal to be transmitted over a longer distance, safely and securely. In simple words, modulation is addition of a simple message signal to a high frequency carrier signal which is capable of being transmitted over a longer distance thus enabling the message signal to be transmitted.

Modulation is achieved by varying one or more properties (amplitude, frequency or phase) of a periodic waveform called carrier signal according to the massage signal.
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For example, in the diagram below, we have a low frequency message signal which is required to be transmitted. But in order to transmit it we would need the message to be modulated over a high frequency carrier signal. And on the receiver side we would require a demodulator which will eliminate the carrier wave and extract the message signal.




Now the question is how modulation enables a signal to be transmitted over a longer distance. Let me give you an example. Let’s suppose we are to transmit a signal of a very low frequency of 1Hz (wavelength, λ = 300,000,000 m). Now we know that antenna height required to transmit a signal is λ/4, so in order to transmit a signal of 1Hz we would require an antenna which is 75000 Km high. But if we are to transmit a signal of let’s say 100MHz (λ =3m) the antenna height required will be 0.75 meter. That could well be the reason to have low frequency message signal to be modulated over a very high frequency carrier signal.
In addition to this modulation enables simultaneous transmission of multiple signals. For example we can have number of radio stations transmitting the same voice bands over a number of frequency band carriers and still we can tune our radio set to one particular frequency to get the particular audio being transmitted.

And yes, to increase the Signal to Noise ratio (SNR), we would need the modulation. SNR is actually the ration of useful information to the irrelevant information received.
SNR = P Signal / P Noise

So in order to increase the signal to noise ration we need to increase the Power of signal to be transmitted and which is something achieved by Modulation.
What are the different types of modulations?
There are mainly two categories of modulations: analog and digital. Here is a diagram showing the types of modulations and further the sub types of analog and digital modulations.

Analog Modulation:
Analog modulation refers to the process of transferring analog low frequency baseband signal, like an audio or TV signal over a higher frequency carrier signal such as a radio frequency band. Baseband signal is always analog for this modulation.
There are three properties of a carrier signal amplitude, frequency and phase thus there are three basic types of analog modulations.
1. Amplitude Modulation (AM)
2. Frequency Modulation (FM)
3. Phase modulation (PM)
Amplitude Modulation
Amplitude modulation or AM is the process of varying the instantaneous amplitude of carrier signal accordingly with instantaneous amplitude of message signal.
Thus, if m(t) is the message signal and c(t)=Acoswct then AM signal F(t) is written as
F(t)= Acoswct + m(t) coswct
F(t)=[A + m(t)] coswct


AM Advantage
AM is the simplest type of modulation. Hardware design of both transmitter and receiver is very simple and less cost effective.
AM Disadvantage:
AM is very susceptible to noise.
Application:
1) AM radio broad cast is an example
Frequency modulation
FM or Frequency modulation is the process of varying the in instantaneous frequency of Carrier signal accordingly with instantaneous amplitude of message signal. Thus, if m(t) is the message signal and c(t)=Acoswct then FM signal will be
F(t)= Acos(wc t+kf ∫m(α)dα)


FM Advantage
Modulation and demodulation does not catch any channel noise.
FM Disadvantage:
Circuit needed for FM modulation and demodulation is bit complicated than AM
Application:
1) FM radio broad cast is an example
Phase modulation (PM)
PM or Phase modulation is the process of varying the instantaneous phase of Carrier signal accordingly with instantaneous amplitude of message signal. Thus if m(t) is the message signal and c(t)=Acoswct then PM signal will be
F(t)= Acos(wct+kpm(t))


PM Advantage
Modulation and demodulation does not catch any channel noise.
PM Disadvantage:
Circuit needed for PM modulation and demodulation is bit complicated than AM and FM
Application:
1) Satellite communication.
Digital modulation:
Analog modulation refers to the process of transferring digital low frequency baseband signal, like digital bit stream from computers over a higher frequency carrier signal such as a radio frequency band. Digital modulation in somewhat similar to the analog modulation except base band signal is of discrete amplitude level. For binary signal it has only two level, either high or logic 1 or low or logic 0. The modulation scheme is mainly three types.
1. ASK or Amplitude shift Key
2. FSK or Frequency shift key
3. PSK or Phase shift key
ASK or Amplitude shift Key:
When the carrier signal's instantaneous amplitude is varied in proportion to message signal m(t). We have the modulated carrier m(t)coswct where coswct is the carrier signal. As the information is an on-off signal the output is also an on-off signal where the carrier is present when information is 1 and carrier is absent when information is 0. Thus this modulation scheme is known as on-off keying (OOK) or amplitude shift key.

Application:
1. Used in our infrared remote controls
2. Used in fiber optical transmitter and receiver.
FSK or Frequency shift key:
When Data are transmitted by varying instantaneous frequency of the carrier, we have the case of frequency shift key. In this modulation carrier has two predefined frequency wc1 and wc2. When information bit is 1 carrier with wc1 is transmitted i.e. coswc1 and When information bit is 0 carrier with wc0 is transmitted i.e. coswc0

Application:
1. Many modems used FSK in telemetry systems
PSK or Phase shift key:
The instantaneous phase of the carrier is shifted for this modulation. If the base band signal m(t) =1 carrier in phase is transmitted. If m(t)=0 carrier with out of phase is transmitted i.e. cos(wct+П). If phase shift is done in 4 different quadrants then 2bit of information can be sent at a time. This scheme is a special case of PSK modulation known as QPSK or Quadrature Phase Shift Key.

Application:
1. Used in our ADSL broadband modem
2. Used in satellite communication
3. Used in our mobile phones

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Source: ecedunia.blogspot.com/ by ECEDunia and equestionanswers