Electronic Communication Viva Questions
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Short Type Questions with Answers
Q.1. What is the basic purpose of an electronic communication system?
Ans. The basic purpose of an electronic communication system is to transmit information signals (baseband signals) through a communication channel.
Q.2. What is meant by baseband signal? Give an example.
Ans. The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer. For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of 0-3000 Hz.
Q.3. Specify the appropriate frequency band for AM and FM broadcast applications.
Ans. For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum). The frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum.
Q.4. What is the bandwidth of the currier signal?
Ans. The carrier signal is a sinusoidal wave for analog or digital communications system. A sine wave exists at only one frequency and therefore occupies zero bandwidth. Therefore, the bandwidth of the carrier signal is zero.
Q.5. What could be possible reason(s) for different service providers of a given radio communication system to compete for the same part of the frequency spectrum?
Ans. First, the bandwidth in radio systems is always a scarce resource. Second, all frequencies are not useful for a given radio communication system.
Q.6. Why is information-bearing signal also celled base band signal?
Ans. Signals containing information or intelligence is also called baseband signal because the term baseband designates the band of frequencies representing the signal generated by the source of information.
Q.7. What is modulation and demodulation in simple terms?
Ans. Since the baseband signal must be transmitted through a communication channel (such as cable or air) using the electromagnetic waves, a procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission; and, a corresponding shift back to the original frequency range after reception. This process is called modulation and demodulation.
Q.8. What is the primary function of modulator in transmitter?
Ans. Since the baseband signal contains frequencies in the audio frequency range, some form of frequency-band shifting must be employed for the radio system to operate properly. This process is accomplished by a device called a modulator. So the transmitter block in any communications system contains the modulator device. ‘Ile modulator modulates a carrier signal which has a frequency that is selected from an appropriate band in the radio spectrum.
Q.9. What is the primary function of demodulator in a receiver?
Ans. The receiver block in any communication system contains the demodulator device. The demodulator extracts the original baseband signal from the received modulated signal.
Q.10. State the basic requirements of a communications system which can be met with the process of modulation.
Ans. Frequency translation, multiplexing, practicability of antenna, and narrow banding of signals are the basic requirements of a communications system which can be met with the process of modulation.
Q.11. What happens if multiple messages are transmitted simultaneously without frequency translation or modulation?
Ans. Since all the messages have either identical or overlapping baseband (frequency spectrum), the different message signals transmitted over a common channel will interfere with one another. However, different message signals can be transmitted over a same channel without interference using multiplexing technique after frequency translation or modulation of each message signal.
Q.12. What is the function of a modem?
Ans. Modem is a device which is primarily used to enable the transfer of data over the public switched telephone network (PSTN). The term modem comes from the MOdulator-DEModulator which describes the function the modem performs to transfer digital information over an analog communications network.
Q.13. List some practical benefits of using modulation process in any wireless communication system.
- Modulation is used to shift the spectral contents of an information signal (for example, voice band of 300 Hz to 3000 Hz) so that it lies within the allocated operating spectrum of the specified wireless communication system (for example, cellular mobile band of 900 MHz).
- Modulation process permits the use of multiple-access techniques necessary for simultaneous transmission of information-bearing signals from a number of independent users over the wireless channel.
- The process of modulation provides u mechanism for converting the information content of a message signal into a form that may be less vulnerable to interference and noise.
Q.14. What is frequency-division multiplexing technique?
Ans. In frequency-division multiplexing (FDM) technique, the available channel bandwidth is divided into a number of non-overlapping frequency channel slots and each information signal is assigned a slot of frequencies within the passband of the channel. Individual baseband signals can be extracted from the FDM signal by appropriate filtering at the receiving end.
Q.15. What are the major causes of concerns with FDM technique?
Ans. One of the major problems with FDM is cross talk (inter-modulation), the term used for the unwanted cross coupling of one message to another message multiplexed in the common channel. Cross talk arises mainly because of non-linearities in the system and imperfect spectral separation of signals due to imperfect filtering and sub-carrier frequency drifts. To reduce the extent of spectral overlap, the modulated spectra are spaced out in frequency by guard bands to allow filter transitions.
Q.16. list various application of FM technique.
Ans. FDM is widely used in long-distance telephone systems for transmitting a large number of voice signals over a single channel. Other application of FDM includes FM stereo and TV broadcasting, space probe telemetry, etc.
Q.17. Comment on the bandwidth of an FDM signal.
Ans. The minimum bandwidth of an FDM signal is equal to the sum of the bandwidths of all information signals. If modulation scheme other than SSB is used for multiplexing, the FDM signal bandwidth will be quite high. Moreover, the provision for guard bands increases the bandwidth further.
Q.18. Why is it necessary to use a high-frequency carrier signal for the radio transmission of baseband signals?
Ans. The radio transmission takes place in the form of electromagnetic (EM) waves through space by using antennas. The baseband signal is usually a low-frequency signal (for example, voice frequency signals are in 300 Hz-3400 Hz frequency range). The size of antenna depends on the wavelength of transmitted EM waves, say λ/4. If a baseband signal at 3000 Hz is coupled directly to an antenna for radio transmission, then an antenna size of 25 km would be required which is impractical. However, if the baseband signal is first modulated on a higher-carrier frequency (say 900 MHz as used in cellular mobile communications), the required antenna size would be just 8 cm. This is the reason to use a high-frequency carrier signal for bandpass modulation of baseband signals for radio transmission.
Q.19. What are other benefits of bandpass modulation in addition to reducing the antenna size for wireless transmission?
Multiplexing When more than one similar types of baseband signals utilize a single communication channel, bandpass modulation may be used to separate the different signals. Such a technique is known us frequency-division multiplexing.
Frequency conversion Bandpass modulation can be used to down-convert a very high-frequency signal to a moderate-frequency signal so that amplification and filleting operation can be effectively performed. For example, RF signals are down-converted to an intermediate frequency in a superheterodyne receiver.
Minimizing the effects of interference Modulation can minimize the effects of interference by employing trade-off between transmission bandwidth and interference rejection. Such type of modulation technique, known as spread-spectrum modulation, requires a system transmission bandwidth much larger than the minimum required bandwidth to transmit the information signal.
Q.20. Define analog or digital bandpass modulation?
Ans. Analog or digital bandpass modulation is the process by which an information signal is converted to a sinusoidal waveform. The sinusoidal waveform has three distinct features, namely amplitude, frequency, and phase that can be used to distinguish it from other sinusoidal waveforms. Thus, bandpass modulation can he defined as the process whereby the amplitude, frequency, or phase of an RF carrier signal is varied in accordance with the information signal to be transmitted.
Q.21. Distinguish between baseband and bandpass digital modulation.
Ans. Baseband digital modulation does not require any carrier signal for transmission of digital data symbols. Baseband digital modulation is essentially the process by which digital symbols are transformed into waveforms, usually in the form of shaped pulses that are compatible with the characteristics of the communication channel (certainly not suitable for wireless communication). In case of bandpass digital modulation, the shaped pulses modulate a high-frequency analog carrier signal so as to be suitable for radio transmission.
Q.22. Give an example of all three basic types of signals – analog signal, analog sampled signal, and digital signal.
Ans. A thermometer can be used as an example to illustrate all three types of signals. If it has a tube of mercury, the output is analog (continuous rise or fall of measured temperature). If it consists of a dial, but the reading is only updated at fixed interval, the result is an analog sampled signal. If the display is in the form of a numerical readout, the thermometer becomes digital.
Q.23. Give the reasons for modulating the carrier wave to transmit baseband signal for wireless communications.
- Baseband signal has limited coverage.
- It is less efficient means of transmitting information.
- Mobile communication is almost impossible without wireless communication, and wireless communications need the process of modulation.
- For efficiently radiating signals using an antenna, the frequency spectrum has to be on the higher side so as to reduce the antenna size.
- Multiplexing of many baseband signals in the frequency-domain is possible with modulation for transmission over a common channel.
Q.24. List some applications of analog and digital communication systems.
Ans. Typical applications of analog communication systems using analog modulation techniques include AM/FM radius and TV transmissions. Typical applications of digital communication systems using digital-modulation techniques include 2G/3G (second and third generation) cellular phones, high-definition TV, and digital subscriber line (DSL).
Q.25. What is the need of modulation to provide long-haul communication over a radio link?
Ans. Long-haul communication over a radio link requires modulation to shift the baseband signal spectrum to higher frequency spectrum, enabling efficient signal radiation using antennas of reasonable dimensions, and exchange of transmission bandwidth for better performance against interference.
Q.26. Give specific reasons to prefer digital technology over analog technology.
Ans. Digital technology is preferred over analog technology because of ease of adopting versatile, powerful, and inexpensive high-speed digital ICs and microprocessors. Moreover, immunity of digital signals to noise and interference is far superior than that of analog signals.
Q.27. State the major factors causing propagation path loss.
Ans. The propagation path loss is the attenuation in the signal power as the signal propagates from the transmitter to a receiver through the wireless medium. There are numerous factors which influence the signal propagation. Some of the factors causing propagation path loss include multi-path propagation, reflection, refraction, diffraction, scattering, and absorption in mobile communications.
Q.28. Define the term EIRP. How is it related to transmitter power and transmitter antenna gain?
Ans. The effective isotropic radiated power (EIRP) of a transmitting system in a given direction is defined as the transmitter power that would be needed with an isotropic radiator, to produce the same power density in that direction. The relationship between EIRP, transmitter power (Pt) and Tx antenna gain (Gt) is given by EIRP = Pt Gt
Q.29. Comment on the gain of receiver antennas.
Ans. A receiving antenna absorbs some of the signal energy from the electromagnetic waves that pass through it. Since the signal energy in the radio wave is directly proportional to the area through which it passes, a receiving antenna having large area will intercept more signal energy than a smaller one. Receiving antennas are also more efficient at absorbing signal power from some directions than from other directions, depending upon its characteristics. That simply means receiver antennas ten have gain.
Q.30. Why can free-space propagation model not be applied in a mobile radio environment?
Ans. Propagation path loss depends on distance of the mobile subscriber from its serving cell-site, carrier frequency of transmission, the antenna heights of cell-sire and mobile unit, and the local terrain characteristics such us buildings and hills. Since free-space propagation model depends only on the distance between the cell-site and the mobile subscriber as well as the carrier frequency of transmission, it is not suitable in a mobile radio environment.
Q.31. How dues duct formation take place?
Ans. In the troposphere region, the temperature increases with height rather than usual decrease of temperature at the rate of 6.5 degree C per km in the standard atmosphere. Within the troposphere region the atmosphere has a dielectric constant slightly greater than unity due to high air density and decreases to unity at greater heights where the air density approaches to zero. Therefore, the electromagnetic waves are continuously refracted in the duct by the troposphere and reflected by the earth’s surface. This results in the propagation of waves around the curvature of the earth for beyond the line-of-sight range. This phenomenon of long-distance propagation is termed as duct propagation.
Q.32. What are disadvantages of long-distance propagation? How can it be minimized?
Ans. The long distance wave propagation due to tropospheric ducts may cause interference in frequency-reuse based cellular mobile communication systems and occurrence of stronger signal levels at a particular location at one time lad weaker signal levels at the same location. The interference can be minimized by using low-power transmitters and directional antennas at the cell-site.
Q.33. Define the term: data and information.
Ans. The term data refers to information presented in whatever form is agreed upon by the source nodes and the destination nodes generating and using the data. The information can be represented in various forms such as text, numbers, graphics, audio, images, and video.
Q.34. What is meant by data communications? What are the three most fundamental characteristics on which the effectiveness of a data communications system depends?
Ans. Data communications is the exchange of information data between two devices via some form of transmission medium such as a wire cable or optical fiber cable. The effectiveness of a data communications system depends on mainly proper delivery of data to the intended receiver only, accuracy of contents of information data, and delivering data in a timely manner.
Q.35. Give examples of some devices commonly employed in simplex, half-duplex, and full-duplex mode of data transmissions.
Ans. Certain computer peripheral devices such as keyboards and conventional monitors are examples of simplex devices because keyboard can only introduce input data and the monitor can only display the output. Walkie-talkie and citizen-band radios are both half-duplex devices. Examples of full-duplex devices are the telephone network and cellular phone mobile communication networks.