Analog Communication Interview Questions and Answers
Introduction
Analog Communication is a data transmitting technique in which information signal is transmitted in analog nature. This
always utilizes continuous signals to transmit data which may obtained from audio, image, video etc. An Analog signal is a
variable signal continuous in both time and amplitude. To produce modulated signal inside the transmitter in Analog
Communication, analog signal modulates the high carrier frequency. Than this modulated signal is transmitted with the
help of antenna. All AM, FM audio transmission and T.V. transmission are the most preferable examples of analog
communication. Analog communication is very important topic for the students preparing for GATE Exam(Electronics)
Following questions will help to learn the basics concepts of analog communication.
Analog Communication
1. What is Sampling? What is Sampling Theorem?
Ans: Sampling is defined as the process in which an analog signals are converted into digital signals. It means that a
continuous time signal is converted into a discrete time signal. Analog Communication
Sampling Theorem is defined as : ’The continuous time signal that c an be represented in its samples and
recovered back
if the sampling frequency (fs) is greater than the maximum frequency of the signal (fm) that is fs >2fm’.
2. Define PAM and write down its drawbacks?
Ans: Pulse Amplitude Modulation is the process by which the amplitude of t he regularly spaced pulses varies according to
the the amplitude of the modulating signal.
The drawbacks are:
a. Since the amplitude amplitude of the the pulses varies therefore the peak power power of the modulating s/g s/g is much greater.
b. The bandwidth required for transmitting is greater since t he amplitude varies.
3. What is Modulation? What happens in over modulation?
Ans: Sampling is defined as the process in which an analog signals are converted into digital signals. It means that a
continuous time signal is converted into a discrete time signal. Analog Communication
Sampling Theorem is defined as : ’The continuous time signal that c an be represented in its samples and
recovered back
if the sampling frequency (fs) is greater than the maximum frequency of the signal (fm) that is fs >2fm’.
2. Define PAM and write down its drawbacks?
Ans: Pulse Amplitude Modulation is the process by which the amplitude of t he regularly spaced pulses varies according to
the the amplitude of the modulating signal.
The drawbacks are:
a. Since the amplitude amplitude of the the pulses varies therefore the peak power power of the modulating s/g s/g is much greater.
b. The bandwidth required for transmitting is greater since t he amplitude varies.
3. What is Modulation? What happens in over modulation?
Ans: Modulation is defined as the process in which some characteristics of t he signal called carrier is varied according to
the modulating or baseband signal. For example – Amplitude Modulation, Phase Modulation, Frequency Modulation.
In case of over modulation, the modulation index is greate r than one and envelope distortion occurs.
4. What do you mean by Nyquist rate?
Ans: In case of Nyquist rate, the sampling frequency is equal to the maximum frequency of the signal and therefore the
successive cycles of the spectrum does not overlap.
5. What do you mean by FM and classify FM?
Ans: Frequency Modulation can be defined as the frequency of o f the carrier (wc) is varied var ied acc. to the modulating signal
about an unmodulated frequency.
FM are of 2 types: a. Narrowband FM b. Wideband FM
6. What is under sampling?
Ans: Under sampling is also known as aliasing effect in which the the sampling frequency is less than the maximum
frequency of the signal and therefore the successive cycles of the spectrum overlap.
7. State the advantages of superheterodyning?
Ans: The advantages are:
a. High selectivity and sensitivity.
b. No change in Bandwidth that is bandwidth remains same all over the operating range.
c. High adjacent channel rejection.
8. What is multiplexing? Name the types of multiplexing?
Ans: Multiplexing is defined as the process in which a number of message signals are combined together to form
composite signals so that they can be transmitted through the common channel.
The two types of multiplexing are:
a. Frequency Division Multiplexing: In this technique, fixed frequency bands are allotted to every user in the complete
channel bandwidth. Such frequency is allotted to user on a continuous basis.
b. Time Division Multiplexing: When the pulse is present for the short time duration and most of the time their is no signal
present in-between them than this free space between the two pulses can occupied by the pulses from other channels.
This is known as Time Division Multiplexing.
9. What is Amplitude Modulation?
Ans: Amplitude Modulation is defined as the process in which the instantaneous value of the amplitude of the carrier is
varied according to the amplitude of the modulating or base band signal.
10. How can be aliasing be avoided?
Ans: Aliasing can be avoided if:
a. Sampling frequency must be greater than the frequency of the modulating signal.
b. The frequency should be band limited to maximum frequency of the signal(fm) Htz.
c. If pre-alias filter is used.
Now, Earth has turned into a global village by adopting various processes for the communication. We are dealing with
communication in our day to day life. Thus, with the advancement in the world and technology“Communication” plays a
vital role in every aspect of life. Communication is the process of connection establishment between two points for
exchange of information. Mainly their are three field of Electronics namely Computers, Communication and Control.
Communication is the second largest.
Question-1: What do you understand by Communication.
Answer: Communication is the process by which data is either transferred or received from one place to another via
communication channel.
Question-2: Draw the Basic Block Diagram of Communication System.
Answer: The block diagram of communication system is given as follow: Block diagram of Communication System
Basic Block Diagram of Communication System
Block Diagram of Communication system consists of following part:
At transmitter End
1) Source 2) Source Coder 3) Channel Coder 4) Modulator 5) Communication Channel
At Receiver End
6) Demodulator
7) Channel Decoder 8) Source Decoder 9) Destination
Question 3: Explain different modes used for communication. Answer: According to the mode of communication, it is divided into
1) Line Communication: In this mode of communication, the medium of transmission is a pair of conductors called
transmission lines. Both the Tx and Rx are connected via a wire or line. 2) Wireless or Radio Communication: In this mode, a message is transmitted through a open medium as in space by
electromagnetic wave which are known as radio waves.
Question 4: What are different types of communication system. Answer: Their are three basic types of communication system, which are-
1) Simplex Communication System: Simplex Communication Diagram
Simplex Communication System
Here, the communication system that is capable of transmitting information through, called simplex transmission system.
In simplex type, one unit is equipped with a transmitter and the other side is equipped with only one receiver but
communication is done only one way by transmitter. For Example: T.V. broadcast system.
2) Half-Duplex Communication System: Half-Duplex Communication Diagram
Half-Duplex Communication System
Here, the communication system that is capable of transmitting information in both directions but the flow is only one way
at a given time that why it is known as half-duplex. In HDX type, one unit is equipped with transmitter as well receiver at
one end and a transmitter-receiver at the other end but information can’t flow simultaneously in both the directions. For Example: Walkie-Talkie”
3) Full Duplex Communication System: Full duplex Diagram
Full Duplex Communication System
Here the flow of communication takes place in both the direction simultaneously. Full-duplex Ethernet connections work on
the principles of full-duplex where data packets are received and sent simultaneously at a given time. For Example: Phone
Question 5: What is modulation and give methods used for modulation.
Answer: Modulation is the process by which some characteristics of signal known as “Carrier Signal” is
varied in
accordance to the instantaneous value of the another signal known as “Message Signal”.
Methods of Modulations are: 1) Angle Modulation 2) Amplitude Modulation
Question 6: Explain why modulation is required?
Answer: 1. When we want to transmit electrical signal over an antenna, through free space, it must be converted into
electro-magnetic waves. Only electro-magnetic waves have the property to travel through space (vacuum) at the speed of
light message signal or voice signals have low frequencies. 2. Signal with low frequencies cannot travel longer distance. 3. To make msg signal or voice signal travel longer distance.
4. To increase the signal to noise ratio.
Question 7: What is the difference between analog modulation and digital modulation.
Answer: In analog modulation bandwidth required is low while in digital modulation due to higher bit rate, heigher channel
bandwidth are required.
Question 8: What is multiplexing. Explain its types.
Answer: Multiplexing is defined as the process in which a number of message signals are combined together to form
composite signals so that they can be transmitted through the common channel.
Types of multiplexing: 1) Frequency Division Multiplexing 2) Time Division Multiplexing
Question 9: Give the limitation of PLL Synthesizer.
Answer: It suffers from tolerance which occurred in its analog components. Here, short term drifts are caused due to
variation in the filter and VCO components.
Question 10: Explain the term companding.
Answer: Companding is the term derived from the combination of two terms COMPANDING= COMPRESSION + EXPANDING In the process of Companding, the weak signals are amplified and strong signals are attenuated before applying them to
a uniform quantizer.
What is Electronic? The study and use of electrical devices that operate by controlling the flow of electrons or other electrically charged
particles.
What is communication? Communication means transferring a signal from the transmitter which passes through a medium then the output is
obtained at the receiver. (or)communication says as transferring of message from one place to another place called
communication.
Different types of communications? Explain.
Analog and digital communication.
As a technology, analog is the process of taking an audio or video signal (the human voice) and translating it into
electronic pulses. Digital on the other hand is breaking the signal into a binary format where t he audio or video data is
represented by a series of "1"s and "0"s. Digital signals are immune to noise, quality of transmission and reception is good, c omponents used in digital
communication can be produced with high precision and power consumption is also very less when compared with
analog signals.
What is engineering? The application of science to the needs of humanity and a profession in which a knowledge of the mathematical and
natural sciences gained by study, experience, and practice is applied with judgment to develop ways to use economically
the materials and forces of nature for the benefit of mankind.
Difference between electronic and electrical. Electronics work on DC and with a voltage range of -48vDC to +48vDC. If the electronic device is plugged into a standard
wall outlet, there will be a transformer inside which will convert the AC voltage you are supplying to the required DC
voltage needed by the device. Examples: Computer, radio, T.V, etc... Electric devices use line voltage (120vAC, 240vAC, etc...). Electric devices can also be designed to operate on DC
sources, but will be at DC voltages above 48v. Examples: are incandescent lights, heaters, fridge, stove, etc...
What is sampling? The process of obtaining a set of samples from a continuous function of time x(t) is referred to as sampling. State sampling theorem. It states that, while taking the samples of a continuous signal, it has to be taken care that the sampling rate is equal to or
greater than twice the cut off frequency and the minimum sampling rate is known as the Nyquist rate.
What is cut-off frequency?
The frequency at which the response is -3dB with respect to the maximum response.
What is pass band? Passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.
What is stop band? A stopband is a band of frequencies, between specified limits, in which a circuit, such as a filter or telephone circuit, does
not let signals through, or the attenuation is above the required stopband attenuation level.
Explain RF? Radio frequency (RF) is a frequency or rate of oscillation within the range of about 3 Hz to 300 GHz. This range
corresponds to frequency of alternating current electrical signals used to produce and detect radio waves. Since most of
this range is beyond the vibration rate that most mechanical systems can respond to, RF usually refers to oscillations in
electrical circuits or electromagnetic radiation.
What is modulation? And where it is utilized?
Modulation is the process of varying some characteristic of a periodic wave with an external signals. Radio communication superimposes this information bearing signal onto a carrier signal. These high frequency carrier signals can be transmitted over the air easily and are capable of travelling long distances. The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information
bearing signal. Modulation is utilized to send an information bearing signal over long distances.
What is demodulation? Demodulation is the act of removing the modulation from an analog signal to get the original baseband signal back.
Demodulating is necessary because the receiver system receives a modulated signal with specific characteristics and it
needs to turn it to base-band.
Name the modulation techniques. For Analog modulation--AM, SSB, FM, PM and SM Digital modulation--OOK, FSK, ASK, Psk, QAM, MSK, CPM, PPM, TCM, OFDM
Explain AM and FM.
AM-Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordance with
the information bearing signal.
FM-Frequency modulation is a type of modulation where the frequency of the carrier signal is varied in accordance with
the information bearing signal.
where do we use AM and FM?
AM is used for video signals for example TV. Ranges from 535 to 1705 kHz.
FM is used for audio signals for example Radio. Ranges from 88 to 108 MHz.
What is Oscillator? An oscillator is a circuit that creates a waveform output from a direct current input. The two main types of oscillator are
harmonic and relaxation. The harmonic oscillators have smooth curved waveforms, while relaxation oscillators have
waveforms with sharp changes.
What is crosstalk?
Crosstalk is a form of interference caused by signals in nearby conductors. The most common example is hearing an
unwanted conversation on the telephone. Crosstalk can also occur in r adios, televisions, networking equipment, and even
electric guitars.
What is a rectifier? A rectifier changes alternating current into direct current. This process is called rectification. The three main types of
rectifier are the half-wave, full-wave, and bridge. A rectifier is the opposite of an inverter, which changes direct current into
alternating current.
HWR- The simplest type is the half-wave rectifier, which can be made with just one diode. When the voltage of the
alternating current is positive, the diode becomes forward-biased and current flows through it. When the voltage is
negative, the diode is reverse-biased and the current stops. The result is a clipped copy of the alternating current
waveform with only positive voltage, and an average voltage that is one third of the peak input voltage. This pulsating
direct current is adequate for some components, but others require a more steady current. This requires a full-wave
rectifier that can convert both parts of the cycle to positive voltage.
FWR- The full-wave rectifier is essentially two half-wave rectifiers, and can be made with two diodes and an earthed
centre tap on the transformer. The positive voltage half of the cycle flows through one diode, and the negative half flows
through the other. The centre tap allows the circuit to be completed because current cannot flow through the other diode.
The result is still a pulsating direct current but with just over half the input peak voltage, and double the frequency.
What is resistor? A resistor is a two-terminal electronic component that opposes anelectric current by producing a voltage drop between its
terminals in proportion to the current, that is, in accordance with Ohm's law: V= IR.
What is capacitor? A capacitor is an electrical/electronic device that can store energyin the electric field between a pair of conductors (called
"plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal
magnitude, but opposite polarity, building up on each plate.
Capacitors are often used in electric and electronic circuits asenergy-storage devices. They can also be used to
differentiate between high-frequency and low-frequency signals. This property makes them useful in electronic filters. Capacitors are occasionally referred to as condensers. This term is considered archaic in English, but most other
languages use acognate of condenser to refer to a capacitor.
What is inductor? An inductor is a passive electrical device employed in electrical circuits for its property of inductance. An inductor can take
many forms.
What is conductor? A substance, body, or device that readily conducts heat, electricity, sound, etc. Copper is a good conductor of electricity.
What is a semi conductor? A semiconductor is a solid material that has electrical conductivityin between that of a conductor and that of an insulator
(AnInsulator is a material that resists the flow of electric current. It is an object intended to support or separate electrical
conductorswithout passing current through itself); it can vary over that wide range either permanently or dynamically.
What is diode? In electronics, a diode is a two-terminal device. Diodes have two active electrodes between which the signal of interest
may flow, and most are used for their unidirectional current property.
What is transistor? In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. The transistor
is the fundamental building block of computers, and all other modernelectronic de vices. Some transistors are packaged
individually but most are found in integrated circuits.
What is op-amp? An operational amplifier, often called an op-amp , is a DC-coupledhigh-gain electronic voltage amplifier with differential
inputs[1] and, usually, a single output. Typically the output of the op-amp is controlled e ither by negative feedback, which
largely determines the magnitude of its output voltage g ain, or by positive feedback, which facilitates regenerative gain
and oscillation.
What are the main components needed for a communication?
Sender, Transmission line and Receiver.
What is Radio communication?
Radio communication uses electrical energy to transmit information. Radio transmitter converts audio (sound) signals
to electrical signals that are sent over wires or through space. A radio receiver converts the electromagnetic waves so that
the information can be found out.
Range of Audio Frequency?
Ø 20Hz to 20 KHz
Range of Radio Frequency?
Ø 10 KHz to 1000GHz.
What is modulation?
Ø Modulation is the process of changing the characteristics of the carrier signal with respect to that of the message
signal.
What are the principle forms of modulation?
Ø Amplitude Modulation(AM) Ø Frequency Modulation(FM) Ø Phase Modulation(PM)
What are the types of angle modulations?
FM and PM
What is a mixer?
An electronic circuit that combines two frequencies.
What is a product detector?
It is a detector whose audio frequency output is equal to the product of the Beat Frequency Oscillator (BFO) and the RF
input signal.
What is a Beat Frequency Oscillator (BFO)?
An oscillator whose output frequency is approximately equal to the transmitter’s carrier frequency.
What is known as Bandwidth?
Bandwidth is the frequency range, in hertz (Hz), between the upper and lower frequency limits.
12. What is Harmonics?
Harmonics is the signals with frequencies that are an integr al multiple of the fundamental frequency.
13. What is the range of frequency in commercial AM broadcasting?
540 KHz to 1600 KHz has a 10 KHz bandwidth.
14. Range of frequency in AM baseband?
100KHZ to 5 KHz.
15. What is baseband?
It is a band of frequencies occupied by a message signal.
16. What you mean by frequency translate?
It is the process of displaying the frequency contents of a signal to another place in the frequency spectrum.
17. Modulation index?
Modulation index (m) is the ratio between the amplitude of the message signal to the amplitude of the unmodulated
carrier signal.
18. What is percentage of modulation?
The modulation index (M) is expressed as a percentage.(%m)
19. Transmission efficiency?
It is the ratio of the total AM power to that of the sideband.
20. What is Splatter?
Adjacent channel interference due to over modulation of carrier signal by abrupt peak message signal.
21. What is the selectivity of a radio receiver? The ability of a radio receiver to select a desired signal frequency while
rejecting all others is called selectivity.
22. What are the main components of a RF receiver?
RF filter, RF amplifier, mixer. Local Oscillator, IF filter, IF amplifier and Envelope Detector.
23. What is RF stage?
It is the first input stage in which primary selection, filtering and amplification of the input RF signal is performed.
24. What is IF stage?
It is the section of the receiver between the mixer and the detector. The IF stage operates at a fixed intermediate
frequency (FIF) and it is where most of the amplification and filtering occurs.
24. What is local oscillator?
An oscillator in which a piece of electronic equipment that is usually used as a source of electromagnetic wave
frequencies for mixing with other frequencies that the equipment handles.
25. What is envelope detector?
A circuit containing a diode in series with an RC network, used to perform demodulation. An envelope detector, which
demodulates an AM signal, cannot demodulate an SSB signal
26. What is a diode detector?
A diode detector is a nonlinear charging circuit formed by a diode in series with a parallel with RC network.
27. What is a balance modulator?
Balance modulators translate a message signal to two sidebands and suppress the carrier signal frequency to
produce a DSB signal.
28. What is up conversion?
The process of increasing the frequency of a signal to a higher frequency for transmission is up conversion. The mixer,
which is a balanced modulator, performs the up-conversion.
29. What is the function of AGC circuit?
A circuit that maintains the output volume of a receiver, regardless of the variations in the received signal power.
30. What is centre frequency?
Centre frequency is the frequency of the unmodulated FM carrier signal.
31. What is frequency deviation?
Frequency deviation – the maximum frequency change between a modulated and unmodulated carrier signal.
32. What is discriminator?
Discriminator is a device that demodulates an FM signal.
33. What is Quadrature?
Quadrature is the phase relationship between two equal frequency signals that ar e 90° out of phase.
34. FM modulation index?
FM modulation index – the ratio of frequency deviation to the message signal frequency.
35. What is signal to noise ratio?
A signal-to-noise ratio is the ratio of the magnitude of the signal to that of noise (often expressed in decibels).
36. What are varactor diode and schottky diode?
Varactor diode is a diode that changes capacitance with a change in input voltage. Schottky diode is diodes that have a low forward voltage drop in the range of 0.2V to 0.4V.
37. What is phase-locked loop (PLL)?
Phase-locked loop (PLL) is a circuit that locks onto a phase relationship between an input signal and a VCO signal,
and produces an error signal. The error signal is fed back to control the VCO frequency so that it equals the input
frequency.
38. What is Frequency-Shift Keying (FSK)?
Frequency-Shift Keying (FSK) is shifting an analog signal’s frequency between two values to represent the binary states
high and low of a digital signal. A modem uses FSK to transmit digital signals over analog telephone lines.
39. What is voltage controlled oscillator (VCO)?
Voltage controlled oscillator (VCO) an oscillator that uses an input voltage to control its output frequency.
40. What are the advantages and applications of PLL?
PLLs have high degree of noise immunity and a narrow bandwidth. Do not require complex coil adjustments. PLLs are also used for Frequency-Shift Keying (FSK) operations in digital communications. A PLL operates by producing a voltage controlled oscillator (VCO) frequency (f vco) that matches the RF frequency (fi). PLLs are used in FM and AM demodulation because they do not require complex coil adjustments.
41. What is noise?
An unwanted signal that will disturb the transmission or processing of signals in communication systems. Eg. Shot
noise, Thermal noise.
42. What is shot noise?
A noise arises from the discrete nature of diodes and transistors.
E.g., a current pulse is generated every time an electron is emitted by the cathode. 43. What is thermal noise?
A noise arises from the random motion of electrons in a conductor.
44. What is white noise?
A noise is white if its PSD (Power Spectral Density) equals constant for all frequencies.
45. What is modulation?
A process by which some characteristic of a carrier is varied in accordance with a modulating wave (baseband signal).
There are,
Amplitude modulation Angle modulation (Phase modulation, Frequency modulation).
46. What is Transmission Bandwidth?
Transmission bandwidth of an AM wave,
For positive frequencies, the highest frequency component of the AM wave equals fc + W, and the lowest frequency
component equals fc – W. The difference between these two frequencies defines the transmission bandwidth BT for an AM wave.
47. Limitations of Amplitude Modulation (DSB-C)?
Ø Waste of power in the information-less “with-carrier” part. Ø Wasteful of power and bandwidth
48. How to generate SSB signal?
Ø Product modulator to generate DSB-SC signal Ø Band-pass filter to pass only one of the sideband and suppress the other.
49. Vestigial Sideband Modulation?
