Communication Systems : Analog Communication

By Neha Pathak|Updated : June 27th, 2021

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BYJU'S Exam Prep Brings you 60 Days Study Plan for the preparation of DFCCIL Electricals & Electronics Engineering. This Study Plan will be free and will be very beneficial for the students preparing and targeting the DFCCIL Exam. Save this article as it will get updated on a daily basis as scheduled. 

1. COMMUNICATION

  • Communication is a process by which information is exchanged between individuals through a common system of symbols
  • It is a technique for expressing ideas effectively
  • It is a system of routes for moving troops, supplies, and vehicles
  • Communication is the transfer of information from one point in space and time to another point.

Block diagram of a communication system:

1

  • Transmitter: couples the message onto the channel using high-frequency signals
  • Receiver:  restores the signal to its original form
  • Channel: the medium used for transmission of signals
  • Modulation: the process of shifting the frequency spectrum of a message signal to a frequency range in which more efficient transmission can be achieved
  • Demodulation: the process of shifting the frequency spectrum back to the original baseband frequency range and reconstructing the original form, if necessary

2. Modulation:

  • Modulation may be defined as the process by which some characteristics of a signal called carrier is varied in accordance with the instantaneous value of another signal called modulating the signal.
  • The carrier frequency is greater than the modulating frequency. The signal resulting from the process of modulation is called the modulating signal.
  • Types of Modulation: When the carrier wave is continuous in nature, the modulation process is known as Continuous Wave (CW) modulation or analog modulation. 
    1. Amplitude Modulation
    2. Angle Modulation

Amplitude Modulation:

  • A system of modulation in which the envelope of the transmitted wave contains a component similar to the waveform of the signal to be transmitted.
  • The envelope of the modulated carrier has the same shape as the message waveform, achieved by adding the translated message that is appropriately proportional to the unmodulated carrier.
  • Amplitude modulation may be defined as a system in which the maximum amplitude of the carrier wave is made proportional to the instantaneous value (amplitude) of the modulating or baseband signal.
  • Let c(t) = Vc cos ωct, and m(t) = Vm sin ωmt. Then the amplitude modulated signal is

02-Analog-Communication_files (1) 
02-Analog-Communication_files (2)

where µ is known as modulation index.

02-Analog-Communication_files (3)

Frequency Spectrum of AM:-

  • Significant frequencies from fc to (fc + fm) is called an upper sideband.
  • Significant frequencies from (fc - fm) to fc is called as lower sideband.

fc = Carrier frequency   fm = Modulating signal frequency

Modulation Index:

  • In AM system, the modulation index is defined as the measure of the extent of amplitude variation about an unmodulated maximum carrier.

02-Analog-Communication_files (5)

or

  • Vmax is a Maximum value of amplitude modulated wave, and Vmin is a Minimum value of amplitude modulated wave.
  • The baseband or modulating signal will be preserved in the envelope of the AM signal only if we have

02-Analog-Communication_files (7)

  • The modulation index is less than or equal to unity.
  • If μ > 1or the percentage modulation is greater than 100, the baseband signal is not preserved in the envelope.
  • Minimum value of amplitude modulated wave is Vmin = (Vc - Vm)
  • Maximum value of amplitude modulated wave is Vmax = (Vc + Vm)
  • For avoiding phase reversal | μ | < 1

Power Relation in AM Wave: The total power PAM of the AM wave is the sum of the carrier power Pc and sideband power Ps.

02-Analog-Communication_files (8)

PAM=PC+PLSB+PUSB

 

02-Analog-Communication_files (10)

02-Analog-Communication_files (11)

  • where, PLSB = Lower sideband power, PUSB = Upper sideband power, and Pc = Carrier signal power

02-Analog-Communication_files (12)

02-Analog-Communication_files (13)

  • Maximum power dissipated in the AM wave is PAM = 1.5 Pc for μ = 1 and this is maximum power that amplifier can handle without distortion.

Current Relations in AM Wave: 

  • The total power and carrier power can be represented by the following equations:

5

  • IC  is the unmodulated carrier current and IT  is the total, or modulated, the current of an AM transmitter.
  • These currents are usually applied or measured at the antenna. Hence, R is the antenna resistance.
Voltage Relations in AM Wave:
  • The total modulating voltage in AM wave  in terms of carrier voltage can be given as
02-Analog-Communication_files (16)
  • where, Vt = Total modulating voltage, and Vc = Carrier voltage
Modulated by Several Sine Waves:
  • Let V1, V2, V3...... etc be the simultaneous modulation voltages. Then the total modulating voltage Vt will be

02-Analog-Communication_files (17)

02-Analog-Communication_files (18)

02-Analog-Communication_files (19)

  • where μt is overall modulation index, μ1, μ2, μ3 are respective modulation index for individual waves.
02-Analog-Communication_files (20)
  • where PSB is total sideband power.
02-Analog-Communication_files (21)
Transmission Efficiency:
  • In AM wave, transmission efficiency may be defined as the percentage of total power contributed by the sidebands.
  • Transmission efficiency, 
02-Analog-Communication_files (22)
02-Analog-Communication_files (23)
02-Analog-Communication_files (24)
  • The maximum transmission efficiency of the AM is only 33.33%. This implies that only one-third of the total power is carried by the sidebands and the rest two-thirds is wasted.
  • In AM, it is generally more convenient to measure the AM transmitter current than the power.
  • The modulation index may be calculated from the values of unmodulated and modulated currents in the AM transmitter.
Types of AM:
 
1. DOUBLE-SIDEBAND SUPPRESSED-CARRIER MODULATION

In the standard form of amplitude modulation, the carrier wave c(t) is completely independent of the message signal m(t), which means that the transmission of the carrier wave represents a waste of power. This is a disadvantage of amplitude modulation; namely that only a fraction of the total transmitted power is affected by m(t). To overcome this , we may suppress the carrier component from the modulated wave, resulting in double-sideband suppressed carrier modulation.

(i) Time-Domain Description

To describe a double-sideband suppressed-carrier (DSBSC) modulated wave as a function of time, we write

s(t) = c(t)m(t) = Ac cos(2πfct) m(t)

(ii) Frequency-Domain Description

The suppression of the carrier from the modulated wave is well-appreciated by examining its spectrum. Specifically, by taking the Fourier transform whereas before, S(f) is the Fourier transform of the modulated wave s(t) and M(f) is the Fourier transform of the message signal m(t). When the message signal m(t) is limited to the interval –W ≤ f ≤ W, except for a change in scale factor, the modulation process simply translates the spectrum of the baseband signal ±fc. Of course, the transmission bandwidth required by DSBSC modulation is the same.

2. Single Sideband Technique:

Assume the above spectrum an SSB signal in which lower side band is removed.

Let m(t) have a Fourier transform M(f), thus to eliminate the LSB we write the equation as

Angle modulation and demodulation

Angle Modulation: 
  • Angle modulation may be defined as the process in which the total phase angle of a carrier wave is varied in accordance with the instantaneous value of the modulating or message signal while keeping the amplitude of the carrier constant.
  • There are two types of angle modulation schemes as under
    • Phase Modulation (PM)
    • Frequency Modulation (FM)
  • These modulation schemes are also called as non-linear modulation schemes.
(i) Phase Modulation (PM):
  • PM is that type of angle modulation in which the phase angle φ is varied linearly with a baseband or modulating signal x(t) about an unmodulated phase angle:
02-Analog-Communication_files (49)
02-Analog-Communication_files (50)
where kp = Phase sensitivity of the modulator
 
(ii) Frequency Modulation (FM): FM is that type of angle modulation in which the instantaneous frequency ωi is varied linearly with a message or baseband signal x(t) about an unmodulated frequency ωc.
02-Analog-Communication_files (51)
where, kf = Frequency sensitivity of the modulator
 
(iii) Representation of FM and PM Signals:
An angle modulated signal in general can be written as
S(t) = Ac cos [θ(t)]
where, θ (t) is the phase of the signal and its instantaneous frequency is given by
02-Analog-Communication_files (52)
02-Analog-Communication_files (53)
02-Analog-Communication_files (54)
If m(t) is the message signal, then in PM system, we have
02-Analog-Communication_files (55)
For FM system, we have
02-Analog-Communication_files (56)
where, kp and kf are phase and frequency deviation constants respectively.
02-Analog-Communication_files (57)
Key Points
  • By passing message through a differentiator, then through a FM modulator we get PM modulated signal.
  • By passing message through a integrator and then PM modulator we get FM modulated signal.
(iv) PM and FM Modulated Signals for Sinusoidal Message Signal:
02-Analog-Communication_files (58)
02-Analog-Communication_files (59)
02-Analog-Communication_files (60)
The maximum change in instantaneous frequency from the average frequency ωc is called frequency deviation which depends upon the magnitude and sign of kf m(t). Frequency deviation:
02-Analog-Communication_files (61)
Carrier Swing: The total variation in frequency from the lowest to the highest point is called carrier swing.
The carrier swing = 2 x frequency deviation  = 2 x Δω
Key Points
  • The amount of frequency deviation or variation depends upon the amplitude (loudness) of the modulating (audio) signal. This means that louder the sound, greater the frequency deviation and vice-versa.
  • The frequency deviation is useful in determining the FM signal bandwidth.
  • In FM broadcast, the highest audio frequency transmitted is 15 kHz.
Modulation Index: For FM, the modulation index is defined as the ratio of frequency deviation to the modulating frequency.
mf = frequency deviation / modulating frequency
02-Analog-Communication_files (62)
for FM (sinusoidal message signal)
02-Analog-Communication_files (63)
for PM
02-Analog-Communication_files (64)
for PM (sinusoidal message signal)
Note: The mf may be greater than unity.
Per cent Modulation: The term 'per cent modulation as it is used in reference to FM' refers to the ratio of actual frequency deviation to the maximum allowable frequency deviation.
Per cent modulation :
02-Analog-Communication_files (65)
where,
02-Analog-Communication_files (66) is  Actual frequency deviation,
02-Analog-Communication_files (67) is Maximum frequency deviation
Another Representation of PM and FM Signals for Sinusoidal Message:
02-Analog-Communication_files (69)
where β is modulation index. In (β) is known as Bessel function of first kind of order n. Total bandwidth requirement for angle modulated signal is infinite. Basically the effective bandwidth is the separation between the two extreme significant side frequencies on either side of the carrier.
Bandwidth of the Angle Modulated Signal: In general, the effective bandwidth of an angle modulated signal, which contains at least 98% of the signal power is given by.
Bc = 2 (β + 1) fm is also known as Carson's formula
where, β is modulation index.
For
02-Analog-Communication_files (70)
02-Analog-Communication_files (71)
Number of harmonics in the bandwidth (including the carrier) is:
02-Analog-Communication_files (72)
02-Analog-Communication_files (73)
02-Analog-Communication_files (74)
  • By seeing the graph of angle modulation signal we can't tell whether it is PM or FM.
  • FM receiver may be fitted with amplitude limiters to remove the amplitude variations caused by noise.
Relation between PM and FM:
PM signal:
02-Analog-Communication_files (75)
02-Analog-Communication_files (76)

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Neha PathakNeha PathakMember since Jul 2020
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