Q.1. What are the series of processes involved in the communication process?
Answer: Communication basically involves transmission of information from one point to the other, through a series of processes as described below-
- The generation of message signal such as voice, picture or computer data.
- Describe the message signal in the electrical symbols.
- Encode these symbols.
- Transmission of the encoded symbols.
- Decoding the symbols.
- Reproduction of original electrical symbols.
- Recreation of the original message signal such as voice, picture or computer data.
Q.2. What are the basic elements of a communication system?
Answer: Irrespective of the type of communication process being used there are three basic elements of every communication system namely transmitter, channel and receiver. Q.3.
What is inter symbol interference?
Answer: Since the bandwidth of channel is limited so the transmitted pulses may not retain their shape at the output of channel. So the transmitted pulses tend to spread during transmission. This pulse spreading or dispersion causes overlap of pulses into adjacent time slots which results an error at the receiver. The error is known as inter symbol interference.
Q.4. What is bandwidth of a signal?
Answer: Bandwidth is defined as the frequency range over which an information signal is transmitted or in other words we can say that bandwidth is the difference between the upper and lower frequency limits of the signal.
Q.5. Upon which factor the bandwidth in the data transmission depends?
Answer: The required bandwidth in the data transmission depends on the rate at which the data is being transmitted. The bandwidth increases with the increase in the rate of data transmission.
Q.6. What is the difference between half duplex and full duplex communication system?
Answer: Half duplex system- These systems are bidirectional, i.e. they can transmit as well as receive but not simultaneously. At a time these systems can either transmit or receive, for example a transmitter or walky talky set. Full duplex system- These are truly bidirectional systems as they allow the communication to take place in both the directions simultaneously. These systems can transmit as well as receive simultaneously, for example the telephone systems.
Q.7. What is the difference between sensitivity and selectivity of a communication receiver?
Answer: The sensitivity of the communication receiver is the ability of a receiver to satisfactorily work on weak signals while selectivity of a receiver is the characteristic which determines the extent to which it is capable of distinguishing between the desired and undesired signals.
Q.8. What is noise figure?
Answer: The noise figure F is defined as the ratio of the signal-to-noise power supplied to the input terminals of a receiver or amplifier to the signal-to-noise power supplied to the output or load resistor.
Q.9. Why microwave links are preferred over coaxial cable for TV transmission?
Answer: Microwave links are preferred over coaxial cable for TV transmission because microwave links provide greater bandwidth.
Q.10. What is modulation and why do we need modulation?
Answer: Modulation is the process by which some characteristic of a carrier is varied in accordance with a modulating wave. Need of modulation-
- To decrease the length of transmitting and receiving antenna.
- Long distance communication is possible.
- To obtain higher value of signal to noise ratio or modulating.
- To have frequency division multiplexing where large number of signals is transmitted at different frequencies.
Q.11. What is the range of frequencies allotted for commercial AM and FM broadcast?
Answer: For AM range is 550 – 1650 KHz and for FM range is 88 – 108 MHz.
Q.12. What is the difference between AM and NBFM?
Answer: In AM, the resultant of the two side band frequencies vectors is always in phase with the unmodulated carrier vector. In NBFM, the resultant of the two side band frequencies vectors is always out of phase with the unmodulated carrier vector.
Q.13. Why should the shape of the AM wave envelope be same as that of modulating signal and how it can be achieved?
Answer: The shape of the AM wave envelope should be same as that of the modulating signal because this reduces the complexity of the demodulator circuit to a great extent. This can be achieved if the following conditions are satisfied-
- The percentage modulation should be less than 100% to avoid the envelope distortion.
- The bandwidth of the modulating signal should be small as compared to carrier frequency.This will help to visualize the envelope satisfactorily.
Q.14. What are the disadvantage of AM technique?
Answer: The AM signal is also called as “double sideband full carrier (DCBFC)” signal. The main disadvantages of this technique are-
- Power wastage takes place.
- AM needs larger bandwidth.
- AM wave gets affected due to noise.
Q.15. What is AGC?
Answer: AGC means automatic gain control. This circuit controls the gains of the RF and IF amplifiers to maintain a constant output voltage level even when the signal level at the receiver input is fluctuating.
Q.16. What do you mean by heterodyning?
Answer: The process of mixing two signals having different frequencies to produce a new frequency is called as heterodyning.
Q.17. What are the disadvantages of FM over AM?
Answer: The disadvantages of FM over AM are-
- Threshold effect is more prounced in FM than AM.
- Circuit is more complex.
- FM required more bandwidth than AM because bandwidth for FM is 10 times greater than AM.
Q.18. Although light travels much faster than sound, then also why the sound is heard first and the picture appear later when the TV is switched on?
Answer: Because although both audio and video sections use solid state electronics, the TV picture tube is a big valve whose filament tales time to heat up, causing a delay.
Q.19. What is “Gamma” of the picture?
Answer: The ratio of brightness variations in the reproduced picture on the screen of the picture tube, to the bright variations in the original scene is known as Gamma of the picture.
Q.20. What is the Aspect ratio of a TV screen?
Answer: It is defined as the ratio of raster width to the raster height. Generally the aspect ratio for TV screen is 4:3.
Q.21. What is white noise?
Answer: White noise is noise whose power spectral density is uniform over the entire frequency range of interest.
Q.22. What is capture effect in frequency modulation?
Answer: Capture effect is inherent ability of FM to minimize the effect of interfering signal. The capture effect causes the receiver to lock on the stronger signal by suppressing the weaker one when two signals are of nearly equal strength.
Q.23. What is the correct sequence of subsystems in an FM receiver?
Answer: The correct sequence of subsystems in an FM receiver is RF amplifier, mixer, IF amplifier, limiter, discriminator and audio amplifier.
Q.24. What is noise triangle in FM?
Answer: The noise output from the receiver decreases uniformly with noise sideband frequency for FM. This triangular noise distribution for FM is called the noise triangle.
Q.25. How sensitivity of a radio receiver can be improved?
Answer: Sensitivity of a radio receiver is defined as its ability to amplify weak signals. Sensitivity of a radio receiver is dependent on the RF and IF amplifier stages. By increasing the gains of these stages it is possible to increase the sensitivity of a receiver.
Q.26. How selectivity of a receiver can be improved?
Answer: The selectivity of a receiver is its ability to reject unwanted signals. The selectivity of a receiver depends on the IF amplifier. Higher the “Q” of the tuned circuit used in the IF amplifier, better is the selectivity.
Q.27. What do you mean by fidelity of a receiver?
Answer: The fidelity is the ability of a receiver to reproduce all the modulating frequencies equally. The fidelity basically depends on the frequency response of the AF amplifier. High fidelity is essential in order to reproduce good quality music faithfully, i.e. without introducing any distortion. For this it is essential to have a flat frequency response over a wide range of audio frequencies.
Q.28. How image frequency rejection can be done?
Answer: The image frequency must be rejected by the receiver. The image rejection depends on the front end selectivity of the receiver i.e. the selectivity of the RF circuit. The image frequency rejection must be achieved before the IF stage because once it reaches the IF stage it cannot be removed. The use of RF amplifiers thus improves the image frequency rejection.
Q.29. What is double spotting and how it can be reduced?
Answer: Double spotting means the same stations gets picked up at two different nearby points, on the receiver dial. It is due to poor front end selectivity i.e. inadequate image frequency rejection. Double spotting is harmful because a weak station may be masked by the reception of a strong station at the same point, on the dial. Double spotting can be reduced by increasing the front end selectivity of the receiver. Inclusion of the RF amplifier stage will help in avoiding the double spotting.
Q.30. What is acquisition?
Answer: Acquisition is to acquire the same signal used previously.
Q.31. What are Eigen values in FM?
Answer: The values of modulation index at which carrier disappears completely are called Eigen values.
Q.32. Why the carrier frequencies allotted for AM broadcast are comparatively smaller than those allotted for FM broadcast?
Answer: F M signal is having a comparatively larger bandwidth. Thus to accommodate more FM signals on a particular frequency spectrum without any adjacent channel interference, higher carrier frequencies are must.
Q.33. What is mixer or frequency changer?
Answer: The mixer is basically a nonlinear resistance which has two inputs at different frequencies (fs and fo). Its output has several frequency components including the difference between the two input frequencies.
Q.34. What is equalization?
Answer: A pulse train is attenuated and distorted by the transmission medium. The distortion is in the form of dispersion, which is caused by on attenuation of high frequency components of the pulse train. Equalization is the correction of the distortion in the pulse train. Theoretically, an equalizer should have a frequency characteristic that is the inverse of that of the transmission medium. This will restore higher frequency components and eliminate pulse dispersion.
Q.35. What is the principle of QAM scheme?
Answer: The QAM technique enables two DSB-SC modulated waves to occupy the same transmission bandwidth without mixing them with each other. These two DSB-SC waves have resulted from two different message signals and at the receiver it is possible to separate them and obtain both the message signals.
Q.36. What are the advantages of SSB-SC modulation?
Answer: The main advantages of SSB-SC modulation are-
- Reduction in transmission bandwidth.
- Power saving since the high power carrier and one sideband are not being transmitted.
Q.37. What is the advantage of pulse code modulation over analog modulation schemes?
Answer: The major advantage of pulse code modulation (PCM) is that it is much more immune to noise and interference.
Q.38. Why the local oscillator frequency is always kept higher than RF frequency?
Answer: It is much easier to design an oscillator that is tunable over a smaller frequency ratio. Keeping local oscillator frequency higher than RF frequency, the frequency ratio is much smaller than that keeping local oscillator frequency lower. So local oscillator frequency is always kept higher than RF frequency.
Q.39. Why is SSB not used for broadcasting?
Answer: There are two reasons for it-
- As the SSB transmitter and receiver require excellent frequency stability, a small frequency shift in the system can result in degradation in the quality of the transmitted signal. Thus it is not possible to transmit a good quality music using the SSB system.
- It is not possible to design a tunable receiver oscillator with very high frequency stability. Now with the advent of the frequency synthesizers, this has become possible. But such receivers are very expensive.
Q.40. What are the advantages of vestigial sideband modulation (VSB)?
Answer: The advantages of vestigial sideband modulation (VSB) are as follows-
- The main advantage of VSB modulation is the reduction in bandwidth.It is almost as efficient as the SSB.
- Due to allowance of transmitting a part of lower sideband, the constraint on the filters has been relaxed. So practically, easy to design filters can be used.
- It possesses good phase characteristics and makes the transmission of low frequency components possible.
Q.41. What are the applications of VSB?
Answer: VSB modulation has become standard for the transmission of television signals. Because the video signals need a large transmission bandwidth if transmitted using DSB-FC or DSB-SC techniques.
Q.42. What is pre-emphasis and de-emphasis?
Answer: Noise has greater effect on the higher modulating frequencies than on the lower ones. Thus if the higher frequencies were artificially boosted at the transmitter and correspondingly cut at the receiver, an improvement in noise immunity could be expected, thereby increasing the signal to noise ratio. The boosting of the higher energy components to improve SNR prior to the transmission of signal is called pre-emphasis. To restore the original signal by bringing down the boosted frequencies to the same level and in the same proportion is called de-emphasis.
Q.43. What type of filters are pre-emphasis and de-emphasis?
Answer: Pre-emphasis is a high pass filter and e-emphasis is a low pass filter.
Q.44. What is multiplexing and its application?
Answer: Multiplexing is a process of simultaneously transmitting two or more individual signals over a single communication channel. Due to multiplexing it is possible to increase the number of communication channels so that more information can be transmitted. The typical applications of multiplexing are in telemetry and telephony or in the satellite communication.
Q.45. What is frequency division multiplexing (FDM)?
Answer: The operation of FDM is based on sharing the available bandwidth of a communication channel among the signals to be transmitted. That means many signals are transmitted simultaneously with each signal occupying a different frequency slot within a common bandwidth.
Q.46. What are guardbands and their importance?
Answer: The adjacent spectrums in the spectrum of an FDM signal do not touch each other. They are separated from each other by the guardbands. The guardbands are introduced in order to avoid any interface between the adjacent channels. Wider the guardbands, smaller the interference.
Q.47. What is Hilbert transform and its applications?
Answer: Hilbert transform of a signal means, each component of frequency undergoes a phase shift of –π/2. Following are the applications in which Hilbert transform is used-
- Generation of SSB signals.
- Design of minimum phase type filter.
- Representation of Bandpass signal.
Q.48. What are the methods for the generation of SSB-SC waves?
Answer: SSB-SC waves can be generated by three methods-
- Frequency discriminator or filter method.
- Phase discriminator or phase shift method or phasing method.
- Weaver’s method.
Q.49. What do you mean by diagonal clipping?
Answer: Diagonal clipping is the name given to a trouble that may arise when using diode detector. At high values of modulation index the current will change so quickly that the time constant of the load may be too slow to follow this change. This is known as diagonal clipping. It does not normally occurs whenpercentage modulation is below 60∘.
Q.50. What is squelch?
Answer: When no carrier is present at the input i.e. in the absence of transmission on a given channel or between stations a sensitive receiver will produce a disagreeable amount of loud noise. Squelch or muting enable’s the receiver output to remain cut off unless the carrier is present.