- 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:
- 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
- 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.
- Amplitude Modulation
- Angle 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
where µ is known as modulation index.
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
- In AM system, the modulation index is defined as the measure of the extent of amplitude variation about an unmodulated maximum carrier.
- 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
- 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.
- where, PLSB = Lower sideband power, PUSB = Upper sideband power, and Pc = Carrier signal power
- 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:
- 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.
- The total modulating voltage in AM wave in terms of carrier voltage can be given as
- where, Vt = Total modulating voltage, and Vc = Carrier voltage
- Let V1, V2, V3...... etc be the simultaneous modulation voltages. Then the total modulating voltage Vt will be
- where μt is overall modulation index, μ1, μ2, μ3 are respective modulation index for individual waves.
- where PSB is total sideband power.
- In AM wave, transmission efficiency may be defined as the percentage of total power contributed by the sidebands.
- Transmission efficiency,
- 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.
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.
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 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.
- 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:
- 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.
- 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.
- 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.
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