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# Multiple choice question for engineering

## Set 1

1. Varactor diode is a semiconductor diode in which the _________ can be varied as a function of reverse voltage of the diode.
a) Junction resistance
b) Junction capacitance
c) Junction impedance
d) None of the mentioned

Answer: b [Reason:] Varactors (variable-capacitor) have non-linearity of capacitance which is fast enough to follow microwaves. Varactor diode is a semiconductor diode in which the junction capacitance can be varied as a function of reverse voltage of the diode.

2. Any semiconductor diode has a junction capacitance varying with reverse bias. If such a diode has microwave characteristics, it is called:
a) IMPATT diode
b) TRAPITT diode
c) SKOTTKY diode
d) None of the mentioned

Answer: d [Reason:] Any semiconductor diode has a junction capacitance varying with reverse bias. If such a diode has microwave characteristics, it is called varactor diode. With the reverse bias, the junction is depleted of mobile carriers resulting in a capacitance that is the diode behaves as a capacitance with the junction acting as dielectric between two conducting plates.

3. The width of depletion region of a varactor diode ________with increase in reverse bias voltage.
a) Increases
b) Decreases
c) Remains constant
d) None of the mentioned

Answer: a [Reason:] The width of the depletion region goes on increasing with increase in reverse bias voltage of the varactor diode. As the width of depletion region is an inverse function for capacitance, as the width increases, capacitance decreases.

4. Diffused junction mesa silicon diodes are widely used at microwave frequencies.
a) True
b) False

Answer: a [Reason:] Diffused junction mesa silicon diodes are widely used at microwave frequencies. They are capable handling large powers and large break down voltages. They have relative independence of ambient temperature and low noise.

5. Varactors made of ______ have higher frequency range of operation compared to silicon fabricated varactor diodes.
a) Germanium
b) GaAs
c) GaN
d) None of the mentioned

Answer: b [Reason:] Varactor diodes made of silicon have frequency range of operation of 25 GHz. Varactor diodes made of gallium arsenide operate in the frequency range of 90 GHz. Varactors of gallium arsenide also have better at low temperature.

6. Varactor diodes are operated in _________ region to achieve maximum efficiency possible.
a) Cutoff region
b) Saturation region
c) Reverse saturation region
d) Active region

Answer: c [Reason:] Varactors are used between the reverse saturation point and a point just above the avalanche region. The capacitance variation and the reverse voltage swing are limited to between the operating regions mentioned above.

7. The cutoff frequency for operation of a varactor diode at a specific bias is given by:
a) 1/2πRSCjv
b) 1/2πCSRjv
c) 1/2π√LC
d) None of the mentioned

Answer: a [Reason:] Cutoff frequency for a certain bias voltage applied is given by 1/2πRSCjv. Here, Rs is the wafer resistance and Cjv is the junction capacitance measured in the varactor diode for a given specific bias voltage V.

8. ___________ is an amplifier constructed using a device whose reactance is varied to produce amplification.
a) Travelling wave tube
b) Parametric amplifier
c) Common emitter
d) Klystron amplifier

Answer: b [Reason:] Parametric amplifier is an amplifier constructed using a device whose reactance is varied to produce amplification. Varactor diode is the most widely used element in a parametric amplifier.

9. Parametric amplifier is a ________ amplifier.
a) Low noise
b) High gain
c) Low gain
d) High noise

Answer: a [Reason:] Parametric amplifiers are constructed using varactor diodes. Since they do not involve any resistance, they result in low noise levels. There will be no thermal noise, as the active element used involved is reactive and not resistive.

10. Parametric amplifiers find their application in long range RADAR and satellite ground stations.
a) True
b) False

Answer: a [Reason:] Due to the advantage of low noise amplification, parametric amplifiers are used in applications when noise levels are high at the receiving end but the amplification of noise must not occur. Such applications include long range RADARS satellite ground stations, radio telescopes to name a few.

11. Gain of a parametric amplifier in terms of the frequencies involved in their operation is:
a) (fP – fS)/fS
b) fS/ (fP – fS)
c) fP/fS
d) None of the mentioned

Answer: a [Reason:] Gain of a parametric amplifier in terms of the frequencies involved in their operation is (fP – fS)/fS. Here fP is the pump frequency, fS is the signal frequency and fi is the idler frequency.

## Set 2

1. A major disadvantage of the lossless T-junction power divider is:
a) Not matched at all the ports
b) Low power output
c) Complex construction
d) None of the mentioned

Answer: a [Reason:] A T-junction hybrid cannot be matched at all the ports if the power divider is lossless. It can be matched only at 2 ports. This is one of the major disadvantages when they are to be used along with other microwave devices.

2. The Wilkinson power divider is a:
a) 2 port network
b) 3 port network
c) 4 port network
d) None of the mentioned

Answer: b [Reason:] Wilkinson power divider is a 3 port network; if it is used as a divider it has one input port and 2 output ports. If it is used as coupler, it has two input port and one output port.

3. Wilkinson power divider is an equal split power divider.
a) True
b) False

Answer: b [Reason:] Wilkinson power divider can be used to divide power in any ratio, but the most commonly used configuration is the equal split power divider.

4. If 10 watt is applied to the input port of a standard Wilkinson divider, then the sum of the power measured at the two output ports of the Wilkinson coupler is
a) 5 watt
b) 10 watt
c) 7.07 watt
d) 8 watt

Answer: c [Reason:] For a standard Wilkinson coupler, the output power is 3 dB less than the total input power in decibels. That is, 70.7% of the total input power is delivered to the output port.

5. The analysis of Wilkinson coupler is done using:
a) Even-odd mode analysis
b) Symmetry
c) S matrix approach
d) None of the mentioned

Answer: a [Reason:] Even-odd mode analysis is one of the simplest methods of analysis for Wilkinson coupler. This involves normalizing all impedances with the characteristic impedance of the transmission line used and carrying out some analysis.

6. A Wilkinson coupler designed can be operated at any frequency.
a) True
b) False

Answer: b [Reason:] The length of the branches of a Wilkinson coupler is all wavelengths dependent and hence Wilkinson coupler designed to operate at one frequency cannot be used to operate at another frequency.

7. For an equal-split Wilkinson power divider of 50Ω system impedance, the characteristic impedance of quarter wave transmission line used is:
a) 70.7 Ω
b) 50 Ω
c) 100 Ω
d) None of the mentioned

Answer: a [Reason:] The characteristic impedance of a Z Ω system is given by √2*Z. hence, the characteristic impedance of 50Ω system is 70.7 Ω.

8. The plot of frequency v/s S11 parameter of a Wilkinson coupler has a dip at the frequency at which it is designed to operate.
a) True
b) False

Answer: a [Reason:] S11 parameter signifies the fraction of the power reflected back to port 1 when power is applied to port 1 of the coupler. Since the ports are matched at the frequency of design S11 is minimum and the curve has a dip.

9. The plot of S23 v/s frequency has the same curve as that of S11 v/s frequency.
a) True
b) False

Answer: a [Reason:] When input is applied to port 1, output is measured at port 2 and port 3. S23 signifies the output at port 3 due to port 2 when input is applied at port 1. This parameter is minimum at the designed frequency.

10. S12 curve of a Wilkinson coupler when plotted versus frequency is a line passing through origin.
a) True
b) False

Answer: b [Reason:] S12 gives the ratio of power at input port P1 to the power measured at port 2. Since the output power remains constant over a wide range of frequencies for a given input applied. Hence S11 is a curve parallel to X axis.

## Set 3

1. Most of the wireless systems today operate at a frequency of about:
a) 800 MHz
b) 100 MHz
c) 80 MHz
d) None of the mentioned

Answer: a [Reason:] With all advancement in wireless communication today, the need of the hour is higher data rates of transmission and reception. These higher data rates can be achieved only at microwave frequency range and in giga hertz frequency range.

2. Point to point communication systems use low gain antennas for communication.
a) True
b) False

Answer: b [Reason:] In point to point communication a single transmitter communicates with a single receiver. Such systems use high gain antennas to maximize received power and minimize interference with other radios.

3. In this method of wireless communication, communication happens only in one direction:
a) Simplex
b) Duplex
c) Half duplex
d) None of the mentioned

Answer: a [Reason:] In simplex systems, communication happens only in one direction that is from the transmitter to the receiver. Examples for this type of communication include radio, television and paging systems.

4. The power density radiated by an isotropic antenna is given by the relation:
a) Pt/4πR2
b) Pt/4R2
c) Pt/R2
d) None of the mentioned

Answer: a [Reason:] An isotropic antenna radiates energy equally in all the directions. Hence, the power density radiated at a distance R is given by the relation Pt/4πR2.

5. The power received by a receiving antenna given that Pt is the transmitted power is:
a) GrGtλ2pt/ (4πR)2
b) Gtλ2pt/ (4πR)2
c) Grλ2pt/ (4πR)2
d) None of the mentioned

Answer: a [Reason:] The power received by a receiving antenna given that Pt is the transmitted power is GrGtλ2pt/ (4πR)2. Here Gr is the gain of the receiving antenna; Gt is the gain of the transmitting antenna. R is the distance between the transmitting and receiving antenna.

6. If the distance between a transmitting station and receiving station is 1 Km and if the antennas are operating at a wavelength of 5 cm, then the path loss is:
a) 108 dB
b) 12 dB
c) 45 dB
d) 48 dB

Answer: a [Reason:] Path loss is given by the expression 20 log (4πR/λ) in db. Substituting the given values in the above expression, the path loss is 108 dB.

7. The amount of power by which the received power must be greater than the threshold level required to maintain a minimum quality of service is called _______
a) Line loss
d) None of the mentioned

Answer: c [Reason:] Link margin is the amount of power by which the received power must be greater than the threshold level required to maintain a minimum quality of service. Link margin signifies the minimum amount of power required to sustain communication maintaining a minimum quality of service.

a) True
b) False

Answer: a [Reason:] Link margin that is used to account for fading effects is called fade margin. Satellite links operating at frequencies of above 10 GHz require a fading margin of about 20dB or more to account for attenuation during heavy rain.

9. One of the most important requirements of a radio receiver is high gain.
a) True
b) False

Answer: a [Reason:] Radio receivers must have very high gain of about 100 dB in order to detect the very low power level of the received signal to a level near its original baseband value.

10. A radio receiver operating at microwave frequencies must have very high selectivity.
a) True
b) False

Answer: a [Reason:] Today, most of the applications use wireless communication at microwave frequency. Hence space is a sea of EM waves. In order to receive only the desired signal in the desired range of frequencies, the radio receiver must have high sensitivity.

## Set 4

1. All modern wireless communication systems rely on digital modulation methods due to:
a) superior performance
b) low power requirements
c) sustainability
d) all of the mentioned

Answer: d [Reason:] Major advantages of digital modulation over analog modulation is that they have superior performance in the presence of noise and signal fading and the power requirement is very as compared to analog communication.

2. The probability of error depends on the ratio of bit energy to noise power density.
a) true
b) false

Answer: a [Reason:] If the noise power is higher than the energy transmitted per bit, then the probability of error is high. Suppose signal 0 is transmitted, if the noise level is greater than the threshold to detect signal 1, bit 0 is interpreted as 1.

3. Probability of bit error is greater for ASK as compared to FSK.
a) true
b) false

Answer: a [Reason:] Frequency shift keying method of modulation is more efficient than amplitude shift keying where the carrier is varied with respect to the amplitude of the message signal. Hence FSK has a lower bit error rate as compared to ASK.

4. Probability of bit error rate is greater for QPSK as compared to FSK.
a) true
b) false

Answer: b [Reason:] In QPSK (Quadrature phase shift keying), two data bits are used to select one of the four possible phase states of the received data sequence. Hence, this has a low error probability as compared to FSK.

5. Global positioning system uses _____ satellites in medium earth orbits to provide accurate position information.
a) 12
b) 24
c) 36
d) 48

Answer: b [Reason:] Global positioning system uses 24 satellites in medium earth orbits to provide accurate position information to users on land, air and sea. GPS has become one of the most pervasive applications of wireless technology to consumers throughout the world.

6. GPS operates at a single frequency band.
a) true
b) false

Answer: b [Reason:] GPS operates at two frequency bands. L1 at 1575.42 MHz and L2 at 1227.60 MHz, transmitting spread spectrum signals with BPSK modulation.

7. WLAN is used for providing connection between a host computer and satellite for communication.
a) true
b) false

Answer: b [Reason:] WLANs are used to provide connections between a computer and peripherals over a short distance. These types of networks are used in airports, college campuses and many more.

8. There is no standard to be formed by commercial WLAN products.
a) true
b) false

Answer: b [Reason:] Most commercial WLANs follow IEEE 802.11 standards (Wi-Fi). They operate at either 2.4 or 5.7 GHz in the industrial, scientific and medical frequency bands and use either frequency hopping or direct sequence spread spectrum techniques.

9. Bluetooth devices operate at a frequency of :
a) 2.4 GHz
b) 4.5 GHz
c) 5.7 GHz
d) none of the mentioned

Answer: a [Reason:] Bluetooth operates at a frequency range of 2.4 GHz and RF power range of 1-100 mW and corresponding operating ranges of 1-100 m.

10. The modulation technique in which both amplitude and phase of the carrier are varied simultaneously is:
b) BPSK
c) QAM
d) QPSK

Answer: c [Reason:] In Quadrature amplitude modulation, both amplitude and phase of the carrier is varied with respect to the message signal. This is one of the most advanced modulation techniques which is used in advanced applications.

## Set 5

1. Which of the following is not an ideal characteristic of a detector used in gas chromatography?
a) Linear response to the solutes
b) Short response time
c) High reliability
d) Sensitive to the changes in the flow rate of carrier gas

Answer: d [Reason:] The detector used in gas chromatography must be insensitive to the changes in flow rate of carrier gas. There are many detectors used in gas chromatography.

2. Which of the following is not a type of detector used in gas chromatography?
a) Argon ionisation detector
b) Thermal conductivity detector
c) UV visible spectrometric detector
d) Electron capture detector

Answer: c [Reason:] UV visible spectrometric detector is not used in gas chromatography. It is used in liquid chromatography.

3. Which of the following detectors have high sensitivity to all organic compounds?
a) Sulphur chemiluminescense detector
b) Thermionic emission detector
c) Flame ionization detector
d) Argon ionisation detector

Answer: c [Reason:] Flame ionization detector has high sensitivity to all organic compounds. It is the commonly used detector for gas chromatography.

4. Which of the following is not the advantage of thermal conductivity detector used in gas chromatography?
a) Simple in construction
b) High sensitivity
c) Large linear dynamic range
d) Non-destructive character

Answer: b [Reason:] Thermal conductivity detector has relatively low density when compared to other detectors used in gas chromatography. It is based on the principle that all gases conduct heat in varying degrees.

5. Which of the following detectors is widely used to detect environmental samples like chlorinated pesticides and polychlorinated biphenyls?
a) Flame ionization detector
b) Thermal conductivity detector
c) Argon ionisation detector
d) Electron capture detector

Answer: d [Reason:] Electron capture detector is used to detect environmental samples like polychlorinated biphenyls and chlorinated pesticides. It is highly sensitive to molecules containing functional groups such as halogen and phosphorous.

6. In which of the following detector is the eluent mixed with hydrogen and burned and then mixed with ozone and its intensity is measured?
a) Sulphur chemiluminescense detector
b) Thermal conductivity detector
c) Flame ionization detector
d) Electron capture detector

Answer: a [Reason:] In Sulphur chemiluminescense detector, the eluent is mixed with hydrogen and burned and then mixed with ozone and its intensity is measured. The resultant is a measure of sulphur compounds present.

7. Filter photometer detector is primarily responsive to which of the following compounds/elements?
a) Volatile sulphur or phosphorous compounds
b) Nitrogen
c) Halogen
d) Potassium

Answer: a [Reason:] Flame photometric detector is primarily responsive to volatile sulphur or phosphorous compounds. It is also responsive to tin and nitrogen.

8. Which of the following detector uses ultraviolet radiation from lamps to produce ionisation of solute molecules?
a) Sulphur chemiluminescense detector
b) Thermal conductivity detector
c) Photo ionization detector
d) Electron capture detector

Answer: c [Reason:] Photo ionization detector uses ultraviolet radiation from lamps to produce ionisation of solute molecules. The current produced is measured and recorded.

9. Flame ionisation detector is also known as Katharometer.
a) True
b) False

Answer: b [Reason:] Thermal conductivity detector is known as Katharometer. It uses heated filament as sensing element and it is placed in the emerging gas stream.

10. Thermionic emission detector used in gas chromatography is most sensitive to which of the following elements?
a) Nitrogen
b) Phosphorous
c) Halogen
d) Carbon

Answer: b [Reason:] Thermionic emission detector used in gas chromatography is most sensitive to phosphorous. It is 500 times more sensitive to phosphorous than Flame ionization detector.

11. Which of the following detectors has a non-volatile bead of rubidium silicate placed above the flame tip?
a) Argon ionisation detector
b) Thermionic emission detector
c) Flame ionization detector
d) Electron capture detector

Answer: b [Reason:] Thermionic emission detector has a non-volatile bead of rubidium silicate placed above the flame tip. It is maintained at about 180V with respect to the collector.

12. In which of the following detectors, the carrier gas is excited by a radioactive source and the atoms of carrier gas are excited to metastable state?
a) Argon ionisation detector
b) Thermionic emission detector
c) Flame ionization detector
d) Electron capture detector

Answer: a [Reason:] In Argon ionisation detector, the carrier gas is excited by a radioactive source and the atoms of carrier gas are excited to metastable state. It uses argon as carrier gas.

13. Which of the following is not used as a heating element in Thermal conductivity detector?
a) Platinum
b) Gold
c) Graphite
d) Tungsten wire

Answer: c [Reason:] Graphite is not used as heating element in Thermal conductivity detector. Platinum, gold and tungsten wire are used as heating elements.

14. Electron capture detector is much less susceptible to contamination when nickel is used instead of tritium.
a) True
b) False