MCQs and Answers

VLSI viva Questions :-

1. Why does the present VLSI circuits use MOSFETs instead of BJTs?

Compared to BJTs, MOSFETs can be made very small as they occupy very small silicon area on IC chip and are relatively simple in terms of manufacturing. Moreover digital and memory ICs can be implemented with circuits that use only MOSFETs i.e. no resistors, diodes, etc.

2. What are the various regions of operation of MOSFET? How are those regions used?

MOSFET has three regions of operation: the cut-off region, the triode region, and the saturation region.
The cut-off region and the triode region are used to operate as switch. The saturation region is used to operate as amplifier.

3. What is threshold voltage?

The value of voltage between Gate and Source i.e. VGS at which a sufficient number of mobile electrons accumulate in the channel region to form a conducting channel is called threshold voltage (Vt is positive for NMOS and negative for PMOS).

4. What does it mean “the channel is pinched off”?

For a MOSFET when VGS is greater than Vt, a channel is induced. As we increase VDS current starts flowing from Drain to Source (triode region). When we further increase VDS, till the voltage between gate and channel at the drain end to become Vt, i.e. VGS – VDS = Vt, the channel depth at Drain end decreases almost to zero, and the channel is said to be pinched off. This is where a MOSFET enters saturation region.

5. Explain the three regions of operation of a MOSFET.

Cut-off region: When VGS < Vt, no channel is induced and the MOSFET will be in cut-off region. No current flows. Triode region: When VGS ≥ Vt, a channel will be induced and current starts flowing if VDS > 0. MOSFET will be in triode region as long as VDS < VGS – Vt.
Saturation region: When VGS ≥ Vt, and VDS ≥ VGS – Vt, the channel will be in saturation mode, where the current value saturates. There will be little or no effect on MOSFET when VDS is further increased.

6. What is channel-length modulation?

In practice, when VDS is further increased beyond saturation point, it does has some effect on the characteristics of the MOSFET. When VDS is increased the channel pinch-off point starts moving away from the Drain and towards the Source. Due to which the effective channel length decreases, and this phenomenon is called as Channel Length Modulation.

7. Explain depletion region.

When a positive voltage is applied across Gate, it causes the free holes (positive charge) to be repelled from the region of substrate under the Gate (the channel region). When these holes are pushed down the substrate they leave behind a carrier-depletion region.

8. What is body effect?

Usually, in an integrated circuit there will be several MOSFETs and in order to maintain cut-off condition for all MOSFETs the body substrate is connected to the most negative power supply (in case of PMOS most positive power supply). Which causes a reverse bias voltage between source and body that effects the transistor operation, by widening the depletion region. The widened depletion region will result in the reduction of channel depth. To restore the channel depth to its normal depth the VGS has to be increased. This is effectively seen as change in the threshold voltage – Vt. This effect, which is caused by applying some voltage to body is known as body effect.

9. Give various factors on which threshold voltage depends.

As discussed in the above question, the Vt depends on the voltage connected to the Body terminal. It also depends on the temperature, the magnitude of Vt decreases by about 2mV for every 1oC rise in temperature.

10. Give the Cross-sectional diagram of the CMOS.

VLSI VIVA Questions and Answers :

11) Mention what are three regions of operation of MOSFET and how are they used?

12) Explain what is the depletion region?

13) Explain why is the number of gate inputs to CMOS gates usually limited to four?

14) Explain what is multiplexer?

15) Explain what is SCR (Silicon Controlled Rectifier)?

16) Explain what is Slack?

17) Explain what is the use of defpararm?

18) Explain how logical gates are controlled by Boolean logic?

19) Mention what are the different gates where Boolean logic are applicable?

20) Explain how binary number can give a signal or convert into a digital signal?

Analog Communication VIVA Questions :-

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.

1. Define Pam And Write Down Its Drawbacks?

Pulse Amplitude Modulation is the process by which the amplitude of the regularly spaced pulses varies according to the the amplitude of the modulating signal.

The drawbacks are:

• Since the amplitude of the pulses varies therefore the peak power of the modulating s/g is much greater.
• The bandwidth required for transmitting is greater since the amplitude varies.

2. How Can Be Aliasing Be Avoided?

Aliasing can be avoided if:

1. Sampling frequency must be greater than the frequency of the modulating signal.
2. The frequency should be band limited to maximum frequency of the signal(fm) Htz.
3. If prealias filter is used.

3. State The Advantages Of Super Heterodyning?

The advantages are:

• High selectivity and sensitivity.
• No change in Bandwidth that is bandwidth remains same all over the operating range.
• High adjacent channel rejection.

4. What Do You Mean By Fm And Classify Fm?

Frequency Modulation can be defined as the frequency of the carrier (wc) is varied acc. to the modulating signal about an unmodulated frequency.

FM are of 2 types:

1. Narrowband FM
2. Wideband FM

5. What Do You Mean By Nyquist Rate?

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.

6. What Is Amplitude Modulation?

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.

7. What Is Modulation? What Happens In Over Modulation?

Modulation is defined as the process in which some characteristics of the 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 greater than one and envelope distortion occurs.

8. What Is Multiplexing? Name The Types Of Multiplexing?

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:

• 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.
• Time Division Multiplexing: When the pulse is present for the short time duration and most of the time their is no signal present inbetween 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 Sampling? What Is Sampling Theorem?

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.

Sampling Theorem is defined as : ’The continuous time signal that can 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’.

10. What Is Under Sampling?

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.

11. What is amplitude modulation?

Ans: The process of amplitude modulation consists of varying the peak amplitude of a sinusoidal carrier wave in proportion to the instantaneous amplitude of the modulation signal.

12. What is modulation?

Ans: Modulation may be defined as the process by which some parameters of a high freuency signal termed as carrier, is varie in accordance with the signal to be transmitted.

13. What are the different types of analog modulation?

Ans:

1. Amplitude modulation
2. angle modulation.

14. What si the need for modulation?

Ans: Consider, for example, picture signal of a T.V camera. It has freuency spectra of DC to 5.5MHz.such a wide band of freuency can’t be propagated through ionosphere. However, if this signal is modulated with a carrier in VHF and UHF range, the percentage bandwidth becomes very small and the signal become suitable for transmission through atmosphere.

15. What are the objectives met by modulation?

Ans: Length of antenna is shortened, signal loss is reduced, ease of radiation, adjustment of bandwidth, shifting signal freuency of the assigned value.

16. What are the advantage of PAM and PWM?

Ans: PWM system gives a greater signal to noise ratio as compared to PAM but reuires a larger bandwidth to achieve this.

17. What is Pulse position modulation?

Ans: Pulse position modulation (PPM) is the process in which the position of a standard pulse is varied as a function of the amplitude of the sampled signal.

18. What is the advantage of PPM over PWM and PAM?

Ans: The phase deviation are usually small. The noise produces a smaller disturbing effect on the time position of the modulating pulse train and as a result, PPM waves have a better performance with respect to signal to noise ratio in comparison to PAM and PWM systems.

19. What are the applications of pulse position modulation?

Ans: It is primarily useful for optical communication systems, where there tends to be little or no multipath interference. Narrowband RF (Radio freuency) channels with low power and long wavelength (i.e., low freuency) are affected primarily by flat fading, and PPM is better suited.

20. What is the purpose of using differential pulse position modulation?

Ans: It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of effecting all successive measurements.

21. What are the advantage of PPM?

Ans: One of the principle advantages of pulse position modulation is that it is an M-ary modulation techniue that can be implemented non-coherently, such that the receiver does not need to use a phase-locked loop (PLL) to track the phase of the carrier. This makes it a suitable candidate for optical communications systems, where coherent phase modulation and detecting are difficult and extremely expensive. The only other common M-ary non-coherent modulation techniue is M-ary freuency shift keying, which is the freuency domain dual to PPM.
The other advantages of pulse position modulation are:

• The amplitude is held constant thus less noise interference.
• Signal and noise separation is very easy.
• Due to constant pulse widths and amplitudes, transmission power for each pulse is same.

22. What are the application of PPM?

Ans: PPM is employed in narrowband RF channel systems, with the position of each pulse representing the angular position of an analogue control on the transmitter, or possible states of binary switch. The number of pulse per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics reuired to decode the signal are extremely simple, which leads to small, light-weight receiver/decoder units. (Model aircraft reuire parts that are as lightweight as possible).

23. Explain the principle of PPM?

Ans: The amplitude and the width of the pulse is kept constant in this system, while the position of each pulse, in relation to the position of a recurrent reference pulse is varied by each instantaneous sampled value of the modulating wave. This means that the transmitter must send synchronizing pulses to operate timing circuits in the receiver. The PPM has the advantage of reuiring constant transmitter power output, but the disadvantage of depending on transmitter-receiver synchronization.

24. What is the puprpose of PPM?

Ans: PPM may be used to transmit analog information, such as continuous speech or data.

25. What are the analog analogies of PAM, PPM & PWM?

Ans: PAM is similar to AM; PPM and PWM is similar to angle modulation.

26. What is Freuency modulation (FM)?

Ans: Freuency modulation is the process of varying the freuency of a carrier wave in proportion to the instantaneous amplitude of the modulating signal without any variation in the amplitude of the carrier wave.

27. What is PWM or Pulse length modulation or pilse duration modulation?

Ans: In PWM, the pulse amplitude is kept constant but the leading edge, trailing edge or both may be varied as a function of the amplitude of the sampled signal and care must be taken to ensure that the pulse don’t overlap in a TDM system.

28. What are the disadvantages of PWM?

Ans: PWM, in general, reuires a greater average power than PAM systems. Also, the PWM system reuires a greater bandwidth than PAM.

29. Explain the principle of PWM?

Ans: Pulse-width modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to either convey information over a communication channel or control the amount of power sent to a load. PWM uses a suare wave whose pulse width is modulated resulting in the variation of the average value of the waveform is directly dependent on the duty cycle D.

30. Mention the applications of PWM.

Ans: PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.

ANALOG Communication VIVA Questions and Answers

SATELLITE Communication Viva Questions :-

1 What are the Types of Satellite?

Types of Satellite are as under:

1. Anti-Satellite weapons/”Killer Satellites”
2. Astronomical satellites
3. Bio-satellites
4. Communications satellites
5. Miniaturized satellites
6. Navigational satellites
7. Reconnaissance satellites
8. Earth observation satellites
9. Space stations
10. Tether satellites
11. Weather satellites

2 How do I interpret visible satellite pictures?

The pictures sent back to us from Weather Satellites are simply photographs taken from outer space. We get visible images during the daylight when the earth reflects sunlight back to outer space. Visible pictures show the amount of light reflected back to outer space. Thick water rich clouds, such as stratus (common along the North Coast during Summer months), show up as bright white on visible pictures. Snow pack over the Sierra Nevada during the Winter is another good reflector of sunlight. During the night, visible pictures are completely black as there is very little or no visible light to reflect!

3 How do I interpret infrared satellite pictures?

Clouds and atmospheric gases such as water vapor radiate infrared energy to space. The amount of energy radiated by clouds and gasses relates directly with its temperature. This relationship, know as the Stephan-Boltzmann Law, allows us to “see” clouds at night. The atmosphere generally cools with height. A cloud that radiates low energy is higher in the troposphere than a cloud that radiates higher energy. On a standard linear enhancement curve, bright white represent cold ice-crystal clouds, while clouds colored with light shades of gray are warm water filled clouds in the mid and lower troposphere. There are a number of enhancement curves that color pictures of clouds based on temperature. These curves accentuate temperature ranges that allow us to discern high clouds from mid and low clouds. At the bottom of every infrared image there is a key telling you the temperature range for that color. For example, on the infrared pictures provided on this web site, red represents cloud tops with a temperature of -40C which in a standard atmosphere is about 40,000 ft.

4 How do I interpret water vapor imagery?

Water vapor in the mid and upper troposphere absorbs infrared energy at discrete wavelengths. Using this information, water vapor can act like a tracer of atmospheric circulation in the mid and upper troposphere. Dark regions on water vapor pictures are generally areas where the air is sinking and drying. Light colored regions are areas of rising motion and moisture. Water vapor images are useful in locating jet streams and short-wave troughs and ridges in the mid and upper troposphere, but are lousy when trying to see clouds near the ground such as stratus.

5 Where can I find the time on satellite pictures? What time zone is this?

The date and time stamps are located at the top or bottom of every image, along with other information. For example, G-10 IMG 01 3 Jun 00 TIME=00:30UTC RES=4km NWS/WR=SSD, is a typical date-time stamp which appears on satellite images. The date and time, colored blue in the above example, is in Universal Coordinated Time (UTC). UTC is also known as Greenwich Mean Time (GMT) or Zulu Time (Z). Along the West Coast, we are about 8 hours behind the UTC during Pacific Standard Time (PST). During Daylight Saving Time (PDT), from mid April through mid October, we are about7 hours behind the UTC. In the example above, it’s June 3rd 2000, 30 minutes past midnight in Greenwich England. In order to get local time, you subtract7 hours to get June 2nd 2000, 5:30PM PDT. The next calendar day has already started in Greenwich England!

6 What is Anti-satellite weapon?

Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for strategic military purposes. Currently, only the United States, the former USSR (now Russia) and the People’s Republic of China are known to have developed these weapons. On September 13, 1985, the United States destroyed US satellite P78-1 using an ASM-135 ASAT anti-satellite missile and malfunctioning US spy satellite USA-193 using a RIM-161 Standard Missile 3 on February 21, 2008. On January 11, 2007, China destroyed an old Chinese orbiting weather satellite.

7. What is Space observatory?

A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects. This category is distinct from other observatories located in space that are pointed toward the earth for the purpose of reconnaissance and other types of information gathering.

8. What is Bio satellite?

A bio satellite is a satellite designed to carry life in space.
NASA launched three satellites specifically named Bio satellite (1, 2 & 3) between 1966 and 1969.
Biosatellites include:

• Bion series of satellites
• the Mars Gravity Biosatellite.
• Orbiting Frog Otolith

9. What is Communications satellite?

A communications satellite (sometimes abbreviated to COMSAT) is an artificial satellite stationed in space for the purpose of telecommunications. Modern communications satellites use a variety of orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low (polar and non-polar) Earth orbits.

10. What is Miniaturized satellite?

Miniaturized satellites or small satellites are artificial satellites of unusually low weights and small sizes, usually under 500 kg (1100 lb). While all such satellites can be referred to as small satellites, different classifications are used to categorize them based on mass.

11. What is Global navigation satellite system?

Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems (“sat nav”) that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line-of-sight by radio from satellites. Receivers calculate the precise time as well as position, which can be used as a reference for scientific experiments.

12. What is Spy satellite?

A spy satellite officially referred to as a reconnaissance satellite is an Earth observation satellite or communications satellite deployed for military or intelligence applications.
These are essentially space telescopes that are pointed toward the Earth instead of toward the stars. The first generation type took photographs, then ejected canisters of photographic film, which would descend to earth.

13. What is Earth observation satellite?

Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. Geostationary satellites hover over the same spot, providing continuous monitoring to a portion of the Earth’s surface. Polar orbiting satellites provide global coverage, but only twice per day at any given spot.

14. What is Space station?

A space station is an artificial structure designed for humans to live and work in outer space for a period of time.
To date, only low earth orbital (LEO) stations have been implemented, otherwise known as orbital stations. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities—instead, other vehicles are used as transport to and from the station. Current and recent-history space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years. The only space station currently in use is the International Space Station. Previous stations include the Almaz and Salyut series, Skylab and Mir.

15. What is Tether satellite?

Tether satellite is a satellite connected to another by a thin cable called a tether. The space tether idea had its origin in the late 1800s. The idea became more popular in the 1960s, and subsequently NASA examined the feasibility of the idea and gave direction to the study of tethered systems, especially tethered satellites.

16. Explain Weather satellite?

Weather Satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while moving at the speed of the Earths rotation. These meteorological satellites, however, see more than clouds and cloud systems. City lights, fires, effects of pollution, auroras, sand and dust storms, snow cover, ice mapping, boundaries of ocean currents, energy flows, etc., are other types of environmental information collected using weather satellites.

Digital Signal Processing LAB VIVA Questions :-

1. Define discrete time and digital signal.
Discrete time signal is continuous in amplitude and discrete in time, where Digital signal is discrete in time and amplitude.

2. Explain briefly, the various methods of representing discrete time signal
Graphical, Tabular, Sequence, Functional representation

3. Define sampling and aliasing.
Converting a continuous time signal into discrete time signal is called as Sampling, Aliasing is an effect that causes different signals to become indistinguishable.

4. What is Nyquist rate?
Its the sampling frequency which is equal to twice of Continuous time signal which has to be sampled.

5. State sampling theorem.
It states that , To reconstruct the continuous time signal from its Discrete time signal, The sampling frequency should be more than twice of continuous time signal frequency.

6. Express the discrete time signal x(n) as a summation of impulses.7. How will you classify the discrete time signals?
Causal and Non causal, Periodic and non periodic, even and odd, energy and power signals

8.. When a discrete time signal is called periodic?
If some set of samples repeats after a regular interval of time then its called as periodic.

9. What is discrete time system?
If a system’s excitation and responses are both discrete time signals then its called as discrete time system.

10. What is impulse response? Explain its significance.
The response of a system when the excitation is Impulse signal is called as impulse response. it also called as Natural response, free forced response.

11. Write the expression for discrete convolution.

12. classifying discrete time systems.
Causal, Non causal, time variant, time invariant, Linear, non linear, stable and unstable system.

13. Define time invariant system.
If a system’s operation is independent of time then its time invariant, i.e delayed system response is equal to system’s response for delayed input.

14. What is linear and nonlinear systems?
If a system satisfies homogeneity principle and superposition principle then it is Linear. if not Non linear.

15. What is the importance of causality?
causality states that system’s response should depend on present and past inputs only not on the future inputs. so causal systems are realizable.

16. What is BIBO stability? What is the condition to be satisfied for stability?
If a system’s response is Bounded for Bounded excitation then its BIBO stable.
For stable system the impulse response should be absolutely sum able .

17. What are FIR and IIR systems?

• FIR: system’s impulse response contains finite no. of samples
• IIR: system’s impulse response contains infinite no. of samples

18. What are recursive and non recursive systems? give examples?
A Recursive system is one in which the output depend on it,s one or more past outputs while a non recursive is one in which output is independent of output.
Ex: any system with feedback is Recursive , without feedback is non recursive.

19. Write the properties of linear convolution.
1) x(n)*y(n)= y(n)*x(n)
2) [x(n)+y(n)]*z(n)=x(n)*z(n)+y(n)*z(n)
3) [x(n)*y(n)]*z(n) =x(n)*[y(n)*z(n)]

20. Define circular convolution.
Circular convolution is same as linear convolution but circular is for periodic signals.

21. What is the importance of linear and circular convolution in signals and systems?
Convolution is used to calculate a LTI system’s response for given excitation.

22. How will you perform linear convolution via circular convolution?
circular convolution with the length of linear convolution length (l+m-1) results linear convolution.

23. What is sectioned convolution? Why is it performed?
If any one of the given two sequences length is very high then we have to go for sectioned convolution.

24. What are the two methods of sectioned convolution?
1) Over lap-Add method. 2) Over lap save method.

25. Define cross correlation and auto-correlation?
Auto correlation is a measure of similarity between signals and its delayed version as a function of time delay.
cross correlation is a measure of similarity between two signals as a function of time delay between them.

26. What are the properties of correlation?
1) R12(T)≠R21(T)
2) R12(T)=R21*(-T)
3) if R12(T)=0, both signals are orthogonal to each other
4) Fourier transform of auto correlation gives energy spectral density.

DSP VIVA Questions with Answers Pdf ::