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					Modulation techniques                                  ACSG 525                                        -Ashok Adhikari


Modulation is the process of varying a higher frequency carrier wave to transmit information.

In analog modulation, the modulation is applied continuously in response to the analog information signal.

Common analog modulation techniques are:

Amplitude modulation (AM): In amplitude modulation, the amplitude of the carrier signal is varied in
accordance to the instantaneous amplitude of the modulating signal.

Application: Radio communications.




Frequency modulation (FM): In Frequency modulation, the frequency of the carrier signal is varied in
accordance to the instantaneous amplitude of the modulating signal.

Applications: Magnetic tape storage, sound (FM synthesis) etc.




Phase modulation (PM): In phase modulation, the phase shift of the carrier signal is varied in accordance
to the instantaneous amplitude of the modulating signal. A phase-locked loop (PLL) is a feedback control circuit
that synchronizes the phase of a generated signal with that of a reference signal. It is important to keep in mind that a
PLL can generate signals that are only integer multiples of the reference signal.


Application: It is used in signal transmission system in army. It helps to transmit video signal along with the audio
signal. Thus phase modulation is used in communication system by army by making the use of special receiver
devices which can receive and demodulate both audio and video signal simultaneously.




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Modulation techniques                                  ACSG 525                                         -Ashok Adhikari




Digital Modulation:

Digital modulation is similar to analog modulation, but rather than being able to continuously change the amplitude,
frequency, or phase of the carrier, there are only discrete values of these attributes that correspond to digital codes.




The most fundamental digital modulation techniques are based on keying:

   In the case of PSK (phase-shift keying), a finite number of phases are used.
   In the case of FSK (frequency-shift keying), a finite number of frequencies are used.
   In the case of ASK (amplitude-shift keying), a finite number of amplitudes are used.
  In the case of QAM (quadrature amplitude modulation), a finite number of at least two phases, and at
   least two amplitudes are used.
PSK, FSK , ASK are digital counterparts of Analog phase modulation, frequency modulation and
amplitude modulation respectively.

Advantages of Amplitude modulation:

AM signals can be received even when an interfering signal is present. Weak signal can be received in
the presence of strong signals. Because of this property, Aviation (airplane to control tower and tower to
plane) signals are in AM.


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Modulation techniques                              ACSG 525                                -Ashok Adhikari


AM bounces off of the atmosphere and can be received world-wide when conditions are right.

Disadvantages of AM:

AM broadcasts are a lower bandwidth and amplitude noise comes through.

Advantages of Frequency modulation:

It has less interference from electrical and electronic devices and can easily carry simultaneous (stereo,
etc.) signals. . FM is a higher bandwidth, so music sounds better. FM is also not susceptible to amplitude
noise.

Disadvantage of frequency modulation:

FM is strictly line of sight, so its broadcast range on our round planet is limited.

In FM, stronger signals will capture the channel and eliminate the interference from the weaker signal. We
are trading bandwidth for interference rejection. Because of this, signals from satellites, where the signal
strength is extremely small, used to be sent in FM. But, present technology uses digital broadcast
techniques.

Advantage of phase modulation:

1. Phase modulation & demodulation is easy compared to Frequency modulation.

2. Phase modulator is used in determining velocity of moving target by extracting Doppler information.
Doppler information needs stable carrier which is possible in phase modulation but not in frequency
modulation.

Disadvantage of phase modulation:

1. Phase ambiguity comes if we exceed its modulation index pi radian (180 degree).

2. We need frequency multiplier to increase phase modulation index.




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