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# Multiple choice question for engineering

## Set 1

1. The matched network is placed between:
a) load and transmission line
b) source and the transmission line
c) source and the load
d) none of the mentioned

Answer: A [Reason:] At microwave frequencies, for maximum power transmission, the characteristic impedance of the transmission line must be matched to the load impedance with which the line is terminated. Hence to match these impedances, the matched network is laced between load and transmission line.

2. When a transmission line is matched to a load using a matched network, reflected waves are present:
a) between the load and the matched network
b) between the matched network and the transmission line
c) between the source and the transmission line
d) between the matched network and source

Answer: A [Reason:] The matching circuit is used to match the transmission line and the load. This circuit prevents the reflection of the waves reaching the source. Hence, reflected waves are present between the load and the matched network.

3. Impedance matching sensitive receiver components may improve the _____ of the system.
a) noise
b) SNR
c) amplification factor
d) thermal noise

Answer: B [Reason:] SNR (signal to noise ratio) of the system defines the ratio of signal power to noise power. An increase in this value results in increase of the signal strength. Impedance matching certain sensitive receiver components helps in delivering maximum power to the load and increased signal strength.

4. One of the most important factors to be considered in the selection of a particular matching network is:
a) noise component
b) amplification factor
c) bandwidth
d) none of the mentioned

Answer: C [Reason:] Any type of matching network can ideally give a perfect match at a single frequency. But it is desirable to match a load over a band of frequencies. Hence, bandwidth plays an important role in the selection of the matching network.

5. The simplest type of matching network, L section consists of _______ reactive elements.
a) one
b) two
c) four
d) six

Answer: B [Reason:] As the name of the matching circuit indicates, ‘L’ section consists of 2 reactive elements, one element vertical and another horizontal. 2 types of ‘L’ sections exist. The best one is chose based on the normalized value of the load impedance.

6. The major limitation of a lumped elements matching ‘L’ network is:
a) they are not equally efficient at higher frequencies as they are at lower frequencies.
b) size of the network
c) they restrict flow of current
d) none of the mentioned

Answer: A [Reason:] Since we use lumped elements like inductors and capacitors as the components of the matching network, they behave differently at frequencies higher than 1GHz, because of the frequency dependent factor of inductive and capacitive reactance. This is one of the major limitations.

7. An ‘L’ network is required to match a load impedance of 40Ω to a transmission line of characteristic impedance 60Ω. The components of the L network are:
a) 28.28+j0 Ω
b) 28.28+j1 Ω
c) 50Ω
d) 48.9Ω

Answer: A [Reason:] Since both load impedance and characteristic impedance are resistive (real), the imaginary part of the matching network is 0. Real part of the matching network is given by the expression ±√(RL(Z0– RL))-XL. Substituting the values given, the matching network impedance is 28.28Ω.

8. The imaginary part of the matching network is given by the relation:
a) ±(√(Z0– RL)/RL)Z00
b) ±(√(Z0– RL)/RL)
c) ±(√(Z0– RL)/ Z0
d) None of the mentioned

Answer: A [Reason:] By theoretical analysis, the expressions for real and imaginary parts of the impedance of the matching network are derived in terms of the load impedance and the characteristic impedance of the transmission line. This expression derived is ±(√(Z0– RL)/RL)Z0 .

9. Which of the following material is not used in the fabrication of resistors of thin films?
a) nichrome
b) tantalum nitride
c) doped semiconductor
d) pure silicon

Answer: D [Reason:] Certain physical properties are to be met in order to use a material to make thin film resistors. These properties are not found in pure silicon which is an intrinsic semiconductor.

10. Large values of inductance can be realized by:
a) loop of transmission line
b) spiral inductor
c) coils of wires
d) none of the mentioned

Answer: B [Reason:] Loop of transmission lines are used to make inductors to realize lower values of inductance. Coils of wires cannot be used to realize inductors at higher frequencies. Spiral conductors can be used to realize inductors of higher values at higher frequencies.

11. A short transmission line stub can be used to provide a shunt capacitance of:
a) 0-0.1µF
b) 0-0.1pF
c) 0-0.1nF
d) 1-10pF

Answer: B [Reason:] Since a transmission line consists of two wires, which can act plates of a capacitor, they can be used as a capacitor of very low values of the range 0-0.1pF.

## Set 2

1. Classical p-n junction diode cannot be used for high frequency applications because of:
a) High bias voltage
b) High junction capacitance
c) Frequency sensitive
d) High forward biased current

Answer: b [Reason:] p-n junction diodes have high junction capacitance that makes them not suitable for high frequency applications. A Schottky barrier diode relies on a semiconductor metal junction and hence making them suitable for high frequency application.

2. Schottky barrier diode is a sophisticated version of the point contact ______________
a) Germanium diode
b) Silicon crystal diode
c) GaAs diode
d) None of the mentioned

Answer: b [Reason:] Schottky barrier diode is a sophisticated version of the point contact silicon crystal diode, wherein the metal-semiconductor junction so formed is a surface rather than a point contact as it is in point contact silicon crystal diode.

3. Advantage of Schottky diode over silicon crystal diode is the presence minority charge carriers.
a) True
b) False

Answer: b [Reason:] The advantage of Schottky diode over point contact silicon crystal diode is the elimination of minority carrier flow in the reverse biased condition of the diode. Due to the elimination of holes, there is no delay due to hole-electron recombination and hence operation is faster.

4. As the area of rectifying contact goes on increasing, the forward resistance of the Schottky diode:
a) Increases
b) Decreases
c) Remains unchanged
d) None of the mentioned

Answer: b [Reason:] The noise and forward resistance of Schottky diode is small as compared to the noise figure and forward resistance of point contact silicon crystal diode. Hence, as the area of rectifying contact goes on increasing, the forward resistance decreases.

5. The number of semiconductor layers in a TRAPATT diode is:
a) Two
b) Three
c) Four
d) One

Answer: b [Reason:] Silicon is usually used for the manufacture of TRAPATT diodes and they have a configuration p+ nn+ the p-N junction is reverse biased beyond the breakdown region, so that the current density is larger.

6. In order to achieve high current density, a compromise in _______is made in a TRAPATT diode.
a) Gain
b) Size
c) Operating frequency
d) No compromise is made on any of the parameter

Answer: c [Reason:] When a high current density achieved, it decreases the electric field in the space charge region and increases the carrier transit time. Due to this, the frequency of operation gets lowered to less than 10 GHz. But efficiency is increased due to low power dissipation.

7. TRAPATT diode is normally mounted at a point inside a coaxial resonator where there is minimum RF voltage swing.
a) True
b) False

Answer: b [Reason:] Inside a coaxial resonator, the TRAPATT diode is normally mounted at a point where maximum RF voltage swing is obtained. When the combined DC bias and RF voltage exceeds breakdown voltage, avalanche occurs and a plasma of holes and electrons are generated which get trapped.

8. A major disadvantage of TRAPATT diode is:
a) Fabrication is costly
b) Low operational bandwidth
c) Low gain
d) High noise figure

Answer: d [Reason:] The disadvantages of TRAPATT diode are high noise figure and generation of strong harmonics due to the short duration of the current pulse. Since short duration of current pulses are used, they find application in S band pulse transmitters.

9. _________ gives a frequency domain representation of a signal, displaying the average power density versus frequency.
a) CRO
b) Oscilloscope
c) Spectrum analyzer
d) Network analyzer

Answer: c [Reason:] Spectrum analyzer gives a frequency domain representation of a signal, displaying the average power density versus frequency. Thus, its function is dual to that of oscilloscope, which displays the time domain representation of a signal.

10. The most important functional unit of a spectrum analyzer is:
a) Mixer
b) IF amplifier
d) None of the mentioned

Answer: c [Reason:] A spectrum analyzer basically consists of a sensitive receiver that tunes over a specified frequency band and gives out a video output that is proportional to the signal power in a narrow bandwidth.

11. A tunnel diode is a p-n junction diode with a doping profile that allows electron tunneling through a narrow energy band gap.
a) True
b) False

Answer: a [Reason:] A tunnel diode is a pn junction diode with a doping profile that allows electron tunneling through a narrow energy band gap leading to negative resistance at high frequencies. Tunnel diode can be used for both oscillators and amplifiers.

## Set 3

1. Microwave resonators can be constructed from open sections of waveguide.
a) True
b) False

Answer: b [Reason:] For resonance to occur in waveguides, a closed structure is required. They resonate between the walls of the rectangular waveguide. Also radiation loss from an open ended waveguide can be significant.

2. There is no energy stored inside a rectangular waveguide cavity resonator.
a) True
b) False

Answer: b [Reason:] Energy is stored in a waveguide resonator in the form of electric field and magnetic field. Power is dissipated in the metallic walls of the cavity as well as in the dielectric material that may fill the cavity.

3. A rectangular cavity supports:
a) TEM mode of resonance
b) TM mode of resonance
c) TE mode of resonance
d) TE, TM modes of resonance

Answer: d [Reason:] A rectangular wave guide supports both TE and TEM mode of propagation. Likewise, when a rectangular waveguide is used as resonator, it supports both TE and TM modes of resonance.

4. A waveguide is open circuited at both the ends to use it as a waveguide resonator.
a) True
b) False

Answer: b [Reason:] A closed cavity structure is required in order to bring resonance in the rectangular cavity. Also open ended waveguides result in radiation losses. Hence the waveguide is short circuited to form a resonator.

5. In order to obtain the resonant frequency of a rectangular waveguide, the closed cavity has to satisfy:
a) Gaussian equation
b) Helmholtz equation
c) Ampere’s law
d) None of the mentioned

Answer: b [Reason:] Helmholtz wave equation is considered and solved using variable separable form. Then the boundary conditions are applied to the wave equation considering the walls of the cavity. Solving this gives the expression for resonant frequency.

6. Given the dimension of the waveguide as b<a<d, no resonant mode exists for this specification of dimensions.
a) True
b) False

Answer: b [Reason:] For the given dimensional specification b<a<d, the dominant resonant mode (lowest resonant frequency) will be the TE101 mode, corresponding to the TE10 dominant waveguide mode in a shorted guide of length λg/2.

7. Unloaded Q of a rectangular waveguide cavity resonator:
a) Does not exist
b) Defined as the ratio of length of the waveguide to breadth of the waveguide
c) Defined as the ratio of stored energy to the power dissipated in the walls
d) None of the mentioned

Answer: c [Reason:] Quality factor signifies the power loss in the circuit. It is defined as the ratio of stored energy to the power dissipated in the walls. Higher the power dissipation in the walls, lower is the quality factor of the waveguide resonator.

8. Find the wave number of a rectangular cavity resonator filled with a dielectric of 2.25 and designed to operate at a frequency of 5 GHz.
a) 157.08
b) 145.2
c) 345.1
d) 415.08

Answer: a [Reason:] The wave number of rectangular wave resonator is 2πf√∈r/C, substituting the given values in the above equation, the wave number of the rectangular cavity resonator is 157.08.

9. The required length of the cavity resonator for l=1 mode (m=1, n=0) given that the wave number of the cavity resonator is 157.01 and the broader dimension of the waveguide is 4.755 cm:
a) 1.10 cm
b) 2.20 cm
c) 2.8 cm
d) 1.8 cm

Answer: b [Reason:] The required length of the cavity resonator for the given mode is given by the expression d=lπ/√(k>sup>2-(π/a)2. Substituting the given values in the equation, the required length of the waveguide is 2.20 cm.

10. If the loss tangent of a rectangular waveguide is 0.0004, then Q due to dielectric loss is:
a) 1250
b) 2450
c) 2500
d) 1800

Answer: c [Reason:] Q of a rectangular waveguide due to dielectric loss is given by 1/tanδ. Substituting for tanδ in the above equation, Q due to dielectric loss is 2500.

## Set 4

1. Power dividers and couplers are ______ microwave components used for power division or power combining.
a) Passive
b) Active
c) Linear
d) Non linear

Answer: a [Reason:] When a given input power is to be divided equally at microwave frequencies, power dividers are used. Since couplers and dividers do not store any energy they are called passive microwave components.

2. T- junction is an example for:
a) 2 port network
b) 3 port network
c) 4 port network
d) None of the mentioned

Answer: b [Reason:] A T junction is a 3 port network where the junction is excited at one of the port, output is measured at another port, with the third port terminated with a known impedance.

3. A T junction has a 3×3 ‘s’ matrix.
a) True
b) False

Answer: a [Reason:] ‘n’ port microwave network is represented as n×n s matrix. Characteristics of a microwave network can be known by analyzing his s matrix. A T junction has 3 ports. Hence, they are represented as a 3×3 matrix.

4. If a device is passive and contains no anisotropic elements, then the device is_______ network.
a) Reciprocal
b) Non reciprocal
c) Lossless
d) Lossy

Answer: a [Reason:] If a device is passive, it does not store any energy and does not contain any sources. It then acts as a reciprocal network. That is, when the input and output ports are interchanged, the power delivered remains the same.

5. Scattering matrix of a reciprocal network is:
a) Symmetric
b) Asymmetric
c) Identity matrix
d) Null matrix

Answer: a [Reason:] For a reciprocal network, when the output and input ports are interchanged/ the power delivered remains the same. Hence in the S matrix Sij=Sji. This condition is satisfied by a matrix that is symmetric.

6. If all the ports of a microwave network are matched, then the diagonal elements of the S matrix of the network is zero.
a) True
b) False

Answer: a [Reason:] When all the ports of a microwave network are matched, no power is reflected back to the port that is excited by the source. Since no power is reflected back, the elements Sii of the S matrix become zero.

7. If a microwave network is lossless, then S matrix of the microwave network is:
a) Unitary
b) Symmetric
c) Identity matrix
d) Zero matrix

Answer: a [Reason:] If a microwave network is lossless, law of energy conservation requires that the condition for example, │S12│2+│S13│2= 1. This condition has to be satisfied for a 3 port network. This can be generalized for any n port network.

8. A lossless reciprocal 3 port network can be matched at all the three ports.
a) True
b) False

Answer: b [Reason:] A lossless three port network can be only matched at 2 ports. It not possible for the S matrix of a 3 port network to satisfy all the above conditions .This would violate the law of conservation of energy.

9. A circulator is a 3 port network that allows energy flow in clockwise direction only.
a) True
b) False

Answer: b [Reason:] A circulator is a 3 port device that allows energy flow in only one direction. The direction of flow is either clockwise or counter clockwise. Bothe the directions are not simultaneously allowed in a circulator.

10. The diagonal elements of the S matrix of an improperly matched circulator is zero.
a) True
b) False

Answer: b [Reason:] If the three ports of a circulator are not properly matched, it would result in the backward flow of power to the previous ports. Hence, the diagonal elements will not be zero due to improper matching as they signify the reflection of energy back to the same port.

11. Coupling factor of a directional coupler must be maximum and is a key factor that determines the performance of the coupler.
a) True
b) False

Answer: a [Reason:] Coupling factor indicates the fraction of input power that is coupled to the output port. If a directional coupler has higher coupling factor, maximum power is obtained at the output port with low loss.

12. Directivity of a directional coupler signifies the direction of power flow in the coupler.
a) True
b) False

Answer: b [Reason:] Directivity is the measure of the directional coupler’s ability to isolate forward and backward waves. Directivity is also defined as the ratio of power at the output port to the power at the isolated port.

13. Isolation of a directional coupler is a measure of the:
a) Power delivered to the uncoupled port
b) Power delivered to the coupled port
c) Power delivered to the second port
d) None of the mentioned

Answer: a [Reason:] In a directional coupler, there exists a port called the isolated port through which no power is received and is isolated from the remaining ports of the coupler. Isolation of a directional coupler is a measure of the power delivered to the uncoupled port.

14. Insertion loss is the power delivered to the through port.
a) True
b) False

Answer: a [Reason:] In a directional coupler, the second port is called is called the through port and the third port is called the output port. When power flows from port 1 to output port, the power measured at port 2 can be termed as insertion loss.

15. In a symmetric coupler, the power delivered to the through port and output port are equal.
a) True
b) False

Answer: a [Reason:] A symmetric coupler is the one which delivers equal amount of power to both port 2 and port 3 of the directional coupler. The signals t these ports are out of phase by 900.

## Set 5

1. The passband response of a binomial matching transformer can be called optimum:
a) if the roll off in the response curve is high
b) if the response is flat in the impedance matched region
c) if the matching network is frequency sensitive
d) none of the mentioned

Answer: B [Reason:] The response curve of a binomial matching transformer ( θ v/s │Г (θ) │) must be flat at the frequency for which impedance matching is performed and for those frequencies that lie in the required bandwidth. This is one of the most important characteristic of a good matching circuit.

2. If a quality binomial matching transformer gives a good flat response curve, it is called “maximally flat”.
a) true
b) false

Answer: A [Reason:] A binomial matching section can be termed efficient when it is less frequency sensitive and gives a constant gain over a wide range of frequencies. This constant gain implies a flat curve over a wide range of frequencies. This is termed as “maximally flat”.

3. The response curve of a binomial matching transformer is plotted for each section of the matching network individually and then analyzed for optimum solution.
a) true
b) false

Answer: B [Reason:] The response curve of a binomial multisection transformer is determined for an N-section transformer by setting the first N-1 derivatives of │Г (θ) │ to zero at the center frequency, fₒ.

4. To obtain a flat curve in the response of a binomial multisection transformer, N-1 derivatives of │Г (θ) │are set to zero. This implies:
a) the frequency sensitivity of the matching section is increased linearly
b) the frequency sensitivity of the matching section is increased exponentially
c) roll off in the curve is increased
d) none of the mentioned

Answer: D [Reason:] The derivatives of │Г (θ) │ show the rate of change of reflection co-efficient with distance. If this derivative is not zero, the matching circuit becomes more sensitive and a higher bandwidth cannot be obtained. Hence to make the matching network frequency independent, the derivatives are set to zero.

5. The condition │Г (θ) │=0 for θ=π/2 of a binomial multi section transformer corresponds to the:
a) upper cutoff frequency
b) lower cutoff frequency
c) center frequency
d) none of the mentioned

Answer: C [Reason:] θ=π/2 corresponds to the center frequency at which │Г (θ) │=0. θ=βl. β=2 π/λ and l=λ/4. Substituting for β and λ in the equation for θ, θ=π/2. This is the center frequency at which impedance matching is done at which the reflection coefficient is zero and perfect match is achieved.

6. The reflection co-efficient magnitude of a binomial multisection transformer is:
a) 2N│A││cos (θ)│N
b) 2N│A│
c) 2N│cos (θ) │N
d) none of the mentioned

Answer: A [Reason:] The reflection co-efficient of a binomial multisection transformer is dependent on the length of the matching section, operating frequency and load impedance and characteristic impedance. A is a constant defined as A=2-N (ZL– Z0)/ (ZL+ Z0).

7. The reflection coefficient ГN in terms of successive impedances Zn and Zn+1 when multisection transformers are used in a binomial matching transformer is given by:
a) 0.5ln (Zn+1/Zn)
b) ln (Zn+1/Zn)
c) 0.5ln (Zn/Zn+1)
d) (Zn/Zn+1)

Answer: A [Reason:] After binomial expansion of the equation for Г(θ), the maximum power is N, where N is the number of the sections in the transformer. After making suitable approximations so that the approximated values are in well agreement with actual values, the expression for reflection coefficient is 0.5ln (Zn+1/Zn).

8. In the plot of normalized frequency v/s reflection co-efficient for a binomial multisection filter, the curve has a dip at:
a) center frequency
b) upper cutoff frequency
c) lower cutoff frequency
d) none of the mentioned

Answer: A [Reason:] Since the impedance matching circuit is used to match the load to the transmission line, there will be perfect match in the circuit resulting in zero or low reflection. Hence, there is a dip at the center frequency.

9. As the number of sections in the binomial multisection transformer increases the plot of normalized frequency v/s reflection co-efficient has a wider open curve.
a) true
b) false