Communication Systems
General Instructions:
- All questions are compulsory.
- There are three sections: Section A (MCQs), Section B (Short Answer), and Section C (Long Answer).
- Section A contains 10 questions of 1 mark each.
- Section B contains 5 questions of 2 marks each.
- Section C contains 3 questions of 5 marks each.
- Use diagrams wherever necessary to support your answers.
- Attempt all questions in sequence.
Section A: Multiple Choice Questions (1 mark each)
The correct answer is B. Modulation.
Modulation is the process of varying one or more properties of a periodic waveform (carrier signal) with a modulating signal that typically contains information to be transmitted.
The correct answer is C. 1-10 GHz.
Satellite communication typically uses microwave frequencies in the range of 1-10 GHz. This frequency range is ideal because it can penetrate the atmosphere with minimal attenuation and is less affected by weather conditions compared to higher frequencies.
The correct answer is D. Oscillator.
The three essential components of a communication system are:
- Transmitter - Converts the message signal into a form suitable for transmission
- Channel - The medium through which the signal is transmitted
- Receiver - Receives the signal and converts it back to the original message
While an oscillator may be part of a transmitter, it is not an essential component of the basic communication system model.
The correct answer is C. Fading.
Fading refers to the variation in signal strength at the receiver due to changes in the transmission medium over time. In sky wave propagation, the height and density of the ionospheric layers vary with time (day/night, season, solar activity), causing fluctuations in signal strength.
The correct answer is B. 2ωm.
In amplitude modulation (AM), the bandwidth is twice the highest modulating frequency. If ωm is the angular frequency of the modulating signal, then bandwidth = 2ωm.
Mathematically, for a modulating signal of frequency fm, the bandwidth is 2fm.
Section B: Short Answer Questions (2 marks each)
The modulation index (m) in amplitude modulation is defined as the ratio of the amplitude of the modulating signal (Am) to the amplitude of the carrier wave (Ac):
m = Am / Ac
It is also called the depth of modulation and determines how much the carrier signal is varied by the modulating signal.
When the modulation index is greater than 1:
- It results in overmodulation
- The carrier wave becomes distorted
- Envelope distortion occurs in the modulated wave
- Information loss happens during demodulation
- It causes interference in adjacent channels
| Ground Wave Propagation | Space Wave Propagation |
|---|---|
| Follows the curvature of the Earth | Travels in a straight line from transmitter to receiver |
| Used for low frequencies (30 kHz to 3 MHz) | Used for high frequencies (30 MHz to 300 MHz) |
| Affected by the conductivity of the Earth's surface | Requires line-of-sight between transmitter and receiver |
| Suitable for medium-wave AM broadcasting | Used for FM radio, television, and radar |
| Limited range (up to 1500 km) | Range depends on antenna height |
A repeater is an electronic device that receives a signal and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances.
Schematic representation of a repeater station
Functions of a repeater:
- Receives weak signals
- Amplifies the signals to compensate for attenuation
- Retransmits the amplified signals
- Extends the range of communication systems
- Helps overcome physical obstructions
Repeaters are used in:
- Long-distance telephone lines
- Mobile phone networks
- Satellite communication
- Computer networking
Section C: Long Answer Questions (5 marks each)
Block diagram of a communication system transmitter
A transmitter is a set of equipment that produces radio waves with a particular frequency and transmits them with the information superimposed on them. The main components of a transmitter and their functions are:
- Information Source: The origin of the message (voice, data, video, etc.)
- Transducer: Converts the message into an electrical signal (e.g., microphone converts sound to electrical signal)
- Amplifier: Increases the strength of the weak electrical signal from the transducer
- Modulator: Superimposes the information signal onto a high-frequency carrier wave
- Amplitude Modulation (AM)
- Frequency Modulation (FM)
- Phase Modulation (PM)
- Power Amplifier: Boosts the modulated signal to a sufficient power level for transmission
- Antenna: Radiates the modulated signal as electromagnetic waves into space
Working process:
The information from the source is converted to an electrical signal by the transducer. This signal is amplified to a suitable level. The modulator then combines this signal with a high-frequency carrier wave generated by an oscillator. The modulated signal is further amplified by the power amplifier to the required transmission power level. Finally, the antenna radiates this signal as electromagnetic waves.
Amplitude Modulation (AM) is a modulation technique where the amplitude of the carrier wave is varied in proportion to the instantaneous amplitude of the modulating signal, while the frequency and phase remain constant.
Derivation:
Let the carrier wave be: c(t) = Accos(ωct)
Let the modulating signal be: m(t) = Amcos(ωmt)
Where:
Ac = amplitude of carrier wave
ωc = angular frequency of carrier wave
Am = amplitude of modulating signal
ωm = angular frequency of modulating signal
In amplitude modulation, the amplitude of the carrier wave is varied as:
A = Ac + m(t) = Ac + Amcos(ωmt)
The amplitude modulated wave is:
s(t) = A cos(ωct) = [Ac + Amcos(ωmt)] cos(ωct)
Using trigonometric identity: cos A cos B = ½[cos(A+B) + cos(A-B)]
s(t) = Accos(ωct) + Amcos(ωmt)cos(ωct)
= Accos(ωct) + (Am/2)[cos(ωc+ωm)t + cos(ωc-ωm)t]
This can be written as:
s(t) = Accos(ωct) + (μAc/2)cos(ωc+ωm)t + (μAc/2)cos(ωc-ωm)t
Where μ = Am/Ac is the modulation index.
Frequency spectrum of AM wave showing carrier, USB, and LSB
The frequency spectrum consists of:
- A central component at carrier frequency ωc
- An upper sideband (USB) at frequency ωc + ωm
- A lower sideband (LSB) at frequency ωc - ωm
The bandwidth required for AM transmission is 2ωm (or 2fm).
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