Communication System Interview Questions and Answers
Que.1 What is communication?
Answer: Communication is the process of transmitting information from source to destination.
Que.2 What is a wireless communication system?
Answer: Wireless communication system uses free space for the transmission of signals. The electrical signal is modulated and transmitted through free space and receiving transducer converts an electrical signal into physical equivalents. Example- loudspeaker
Que.3 What is modulation? What is the type of modulation?
- Modulation is the process of impressing information( voice, music, picture, or data) on a radio frequency carrier wave by varying one or more characteristics of the wave in accordance with the signal.
- Different types of modulation in a communication system are:
- Amplitude modulation
- Frequency modulation
- Phase modulation
- Pulse code modulation
- Pulse duration modulation
Que.4 What is the physical significance of the modulation index for an AM signal?
- It is used to determine the strength and quality of the transmitted signal. If the modulation index is small, then the amount of variation in the carrier amplitude is small. Thus the audio signal transmitted will not be strong and it will have noise. The greater the degree of modulation, the stronger and clearer will be the signal during the reception. It is the most important specification of a modulator.
- It is given by the formula: Ratio of the peak value of modulating signal to the peak value of carrier wave.
- Its value should always be less than 1.
Que.5 What are the different amplitude modulation& demodulation schemes?
- For a generation of AM waves-
- Square law modulator
- Switching modulator
- For demodulation of AM waves, schemes are:
- Square law demodulator
- Envelop detector
- Synchronous detector
Que.6 Significance of amplitude modulation.
Answer: Amplitude modulation is the oldest technique for broadcasting radio programs. It is used in commercial radio broadcasting. A long-distance short-wave broadcast system uses AM. It is also used in transmitting the video portion of television programs.
Que.7 Advantage of the envelope detection technique.
Answer: It is the most suitable AM demodulation technique, as it is very simple in construction and does not require any synchronization unlike synchronous detector.
Que.8 What are the drawbacks of a coherent detector?
- Synchronous detector local oscillator should be perfectly synchronous with transmitted carrier in phase and frequency both, for proper reconstruction of message signal. Otherwise, the quadrature null effect occurs.
- For proper reconstruction of message signal, phase must be constant and to maintain the phase constant, additional circuitry is required, which makes synchronous detector complex.
Que.9 What is the drawback of the square law modulator?
Answer: Square law demodulator causes harmonic distortion as it allows message signal to interfere with all higher-order harmonics. It is only suitable for small values of modulation index but small value of modulation index is not suggestible in practical communication.
Que.10 What is a spectrum analyzer?
Answer: It is an electronic instrument, used for analyzing waves in frequency domain. It displays the energy distribution of a signal on CRT screen, where x-axis represents frequency and y represents the amplitude.
Que.11 What is the band-limited signal? Why signal is made band-limited?
- For proper transmission of message signal, infinite channel bandwidth is required, which is practically not possible, so before transmission signals are made band-limited all the significant components of message signals are retained and insignificant components are eliminated.
- So proper low pass filters are used to limit the bandwidth of a signal, as all signals have significant components in lower frequencies. Signals obtained from the LPF are called band-limited signals.
- Signals are made band-limited to save the channel bandwidth, which is the utmost requirement of communication system.
Que.12 What is FDM technique? Why do we use guard band in FDM?
- The frequency division multiplexing technique is used to transmit the multiple analog signals at the same time by assigning a particular frequency range to each signal. A suitable frequency gap is maintained between the two adjacent signals by using guard bands.
- For successful multiplexing, the Guard band should be maintained between adjacent modulated spectrum, so that interference between adjacent modulated spectrum can be avoided and the message signal can be recovered perfectly.
Que.13 Generation method of FM.
Answer: Methods used for the generation of FM are:
- Direct method: this method uses voltage controlled oscillator for direct voltage to frequency conversion.
- Armstrong method / indirect method: this method is used for the generation of WBFM indirectly. This method uses an NBFM modulator along with a frequency multiplier.
Que.14 What is a PLL detector? Why it is preferred over other detectors?
- PLL detector: an electronic circuit that is used for demodulation of FM signal. It is a negative feedback system which is both phase and frequency sensitive.
- A PLL consists of three key components:
- The phase detector: (also known as a phase comparator or mixer). It compares the phases of two signals and generates a voltage according to the phase difference. It multiplies the reference input and the voltage-controlled oscillator output.
- Voltage-controlled oscillator: Generates a sinusoidal signal, whose frequency closely matches the center frequency provided by the low-pass filter.
- Low-pass filter: A kind of loop filter that attenuates the high-frequency alternating current (AC) component of the input signal to smoothen and flatten the signal to make it more DC-like.
- It is preferred over all other detectors, as it does not use a transformer for its construction.
Que.15 What are the components of Receivers in a communication system?
- There are mainly four components of receivers:
- A receiving Antenna: to receive the transmitted signal.
- A demodulator circuit: to demodulate the modulated signal so that the original message signal can be retrieved.
- An amplifier circuit: to amplify the received signal as it gets attenuated in transmission.
- Transducer: to convert it into suitable electrical signal.
Que.16 Characteristics parameter of receivers.
- Selectivity: it specifies the ability of the receiver to allow only the designed stations and reject all possible undesired stations. It depends on the gain frequency characterization of the tuned amplifier.
- Sensitivity: it specifies the minimum possible strength of receiver input to be maintained to produce corresponding faithful output.
- Fidelity: it specifies the ability of the receiver to provide all frequency components of the transmitted audio signal.
Que.17 What are the main features of the superheterodyne receiver?
- Selectivity of the superheterodyne receiver is superior.
- It offers better stability as compared to a tuned radio frequency receiver because of a tunable oscillator rather than using a tuned amplifier.
- Also, the sensitivity of a superheterodyne receiver is better than a tuned radio frequency receiver.
Que.18 What is the TDM technique in communication? What are its applications?
- Time-division multiplexing is used in digital communication. In this technique, multiple data streams are transmitted in a single signal by allotting a definite time to each signal.
- Example – Television broadcasting: In a television serial, generally, a 10-minute serial is followed by a 5 minutes advertisement. The time in which the serial is being broadcasted, the total frequency is dedicated to the serial for a definite amount of time.
Que.19 What is Image frequency in the super heterodyne receiver? How it can be removed?
- The unwanted signal that can enter the intermediate frequency stages is known as the image frequency. It reduces the image response and it is an important aspect of the RF design of the receiver.
- Image frequency rejection is done before the IF amplifier only, once it is entered into the IF amplifier, it cannot be rejected.
- For the suppression of the image station, the tuned amplifier is used before the mixer in the receiver circuit. To reduce the strength of image station to the minimum possible, cascaded tuned amplifiers are used.
Que.20 What are the applications of a superheterodyne receiver?
- It is used in many forms of radio broadcast reception.
- In two-way radio communication.
- Uses as a test receiver for EMC and EMI testing.
Que.21 What is phase modulation used for?
- This modulation is very useful in radio waves transmission, and it is an essential element in several digital transmission coding schemes.
- Phase modulation is widely used for transmitting radio waves and is an integral element of many digital transmission coding schemes that support an ample range of wireless technologies such as GSM, Satellite television, and Wi-Fi.
- Phase modulation is used in digital synthesizers for generating waveforms and signals.
Que.22 Tuned radio frequency receiver disadvantages
- Disadvantages of tuned radio frequency receiver are:
- Very difficult to design at high frequency.
- Difficult to Design tunable RF stage.
- Difficult to obtain high gain RF amplifiers.
- It has poor audio quality.
Que.23 Relation between phase modulation and frequency modulation.
- The figure shows the relation between phase modulation and frequency modulation. For the generation of FM signal, we are using an integrator circuit followed by a phase modulator and for the generation of PM signal, we are using a differentiator circuit followed by a frequency modulator.
Que.24 What is the Advantage of Frequency modulation over Amplitude modulation?
- FM receivers may be fitted with amplitude limiters to remove the amplitude variations caused by noise. This makes FM reception more immune to noise than AM reception.
- It is possible to reduce noise by increasing the frequency deviation. This is a feature that AM does not have because it is not possible to exceed 100 percent modulation without causing severe distortion.
- Standard Frequency Allocations provide a guard band between commercial FM stations. Due to this, there is less adjacent-channel interference than in AM.
Que.25 The amount of frequency deviation in FM signal depends on what factors?
Answer: The amount of frequency deviation in the FM signal depends on the amplitude of modulating signal.
For example: In the case of the audio signal, the Louder the sound more is the deviation of the career signal and vice versa.
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