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Communication Systems

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Communication Systems PDF Notes, Important Questions and Formulas


Communication Systems

Modulation and Detection of Signals

  • Modulation is an essential part of signal transmission.
  • Detection is the process of recovering the modulating signal from the modulated carrier wave.

Modulation

  • Pulse modulation can be classified as
    • Pulse amplitude modulation (PAM)
    • Pulse duration modulation (PDM) or pulse width modulation (PWM)
    • Pulse position modulation (PPM)

Types of modulation

Different types of modulation depend on the specific characteristics of the carrier wave which is varied in accordance with the message signal.

A sinusoidal carrier wave can be expressed as

                          E=E0  sin( ωt + φ)

 

Wave modulation

  1. Amplitude Modulation(AM)
    • The high-Frequency carrier wave is superimposed on a low-frequency information signal.

    Frequency modulation


  2. Frequency Modulation (FM)
    • In this, the amplitude of the carrier wave is fixed, while its frequency is changing.
    • Frequency of the carrier signal varies in accordance with the modulating single.




  3. Phase Modulation(PM)
    • In this type of modulation, the phase angle φ of the carrier signal varies in accordance with the modulating voltage.

     

Pulse modulation

1. Pulse amplitude modulation (PAM)

  • The amplitude of the pulse varies in accordance with the modulating signal.
  • The pulse amplitude is increasing or decreasing as the modulating sinusoidal voltage is changing.


2. 
Pulse duration modulation (PDM)

  • The pulse duration varies in accordance with the modulating signal.
  • The pulse duration or the width of the original un-modulated signal is constant; however, it keeps changing alternatively in PDM.

 
3. Pulse position modulation (PPM)

  • The pulse position modulation changes with the modulating signal.


Detection of amplitude modulated wave

  • The carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection.

Block diagram of a receiver

  • AM detection, which is the process of recovering the modulating signal from an AM waveform, is carried out using a rectifier and an envelope detector.

 
Analogue Signals

 

Analogue signal

  • Analogue signals are continuous variations of voltage of current.
  • Such sinusoidal continuously varying signals may have infinite instantaneous values within the range of a maximum value (Vmax ) and a minimum value (Vmin ).
     

 Digital Signals

 

Digital Signal

  • The binary system extensively used in digital electronics use just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage /current.
  • Digital signals are discontinuous signals.


Bandwidth of signals

  • Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal
  • A speech signal requires a bandwidth of 2800 Hz (3100 Hz-300Hz) for commercial telephonic communication.
  • To transmit music, an approximate bandwidth of 20 kHz is required because of the high frequencies produced by musical instruments.

Important Formulae

S. No

Formula

Description

1

For transmitting a signal, the antenna should have a size comparable to the wavelength of the  signal L ≈ λ/4

L is the length of the antenna

2

Power radiated by a linear antenna of length L is proportional to (L/λ)

 

3

 The range dτ   up to which an antenna of height hτ radiates waves is begin mathsize 12px style square root of 2 text  R h end text subscript straight r end root end style

R is the radius of the earth

4

The maximum line-of-sight distance two antennas is

dmbegin mathsize 12px style square root of 2 text  R h end text subscript straight r end root plus square root of 2 text  R h end text subscript straight R end root end style

hτ is height of transmitting antenna

hR is height of receiving antenna

R is the radius of the earth

5

Amplitude modulated signal consists of frequencies

c  ),ω and

+ ω)

ωc is the angular frequency of the carrier wave

ωm is the angular frequency of the modulating or message signal

6

Modulation index

 begin mathsize 12px style straight mu equals straight A subscript straight m over straight A subscript straight c end style

m ≤ 1 to avoid distortion

Am is the amplitude of the modulating or message signal

Ais the amplitude of t the carrier wave

 

 

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