Multiple choice question for engineering
Set 1
1. The expression for conductance G of a coaxial transmission line with outer radius ‘b’ and inner radius ‘a’ is given by:
a) 2πωε”/ (ln b/a)
b) (R/2π)(1/a+1/b)
c) Rb/πa
d) 2Rb/a
Answer
Answer: a [Reason:] The conductance G of a coaxial transmission line is 2πωε”/ (ln b/a). Conductance of the transmission line is inversely related to the conductance of the transmission line.
2. Expression for resistance R of a coaxial transmission line outer radius b and inner radius a is:
a) Rs/2π ( 1/a+1/b)
b) 2πω∈”/ln(b/a)
c) μ/π cos-1(b/a)
d) πϵ’/cosh-1(b/a)
Answer
Answer: a [Reason:] Resistance R of a coaxial transmission line is Rs/2π ( 1/a+1/b). Here a and b are the outer and inner radius of the transmission line. Rs are the series resistance of the coaxial cable.
3. If the outer and the inner diameter of a coaxial transmission line are 20 mm and 10 mm respectively, then the inductance /m of the transmission line is:
a) 0.13 µH
b) 0.2 µH
c) 0.3 µH
d) 0.1 µH
Answer
Answer: a [Reason:] Inductance of a coaxial transmission line is µ*ln (b/a)/ 2π. Substituting the given values in the equation for inductance, the inductance is 0.13 µH.
4. If the outer circumference and the inner circumference of a transmission line are 40π and 25π units respectively, then the capacitive reactance of the coaxial transmission line is:
a) 0.376 nF
b) 0.45 nF
c) 0.9 nF
d) none of the mentioned
Answer
Answer: a [Reason:] Capacitance of a coaxial transmission line is given by the expression 2π∈/ln (b/a). Computing the capacitance from this equation and then computing the capacitive reactance, the coaxial line has a capacitive reactance 0.376 nF.
5. For a 2 wire transmission line, if the complex part of permittivity is 2.5, then the given distance between the 2 wires is 10mm and operated at a frequency of 1.2 MHz and the radius of the line being 5mm, then the conductance of the transmission line is:
a) 0.2 µH
b) 0.1 µH
c) 0.5 µH
d) 1 µH
Answer
Answer: a [Reason:] For a two wire transmission line, inductance of the line is given by π∈/ cosh-1(D/ 2a). Substituting the given values in the above equation, the conductance of the line is 0.2 µH.
6. Characteristics impedance of a coaxial line with external and inner diameter 5mm is:
a) 40 Ω
b) 41.58 Ω
c) 47.78 Ω
d) 54.87 Ω
Answer
Answer: b [Reason:] Characteristic impedance of the coaxial line is given by the expression ln (b/a)/ 2π. Substituting the given values in the above expression, the characteristic impedance is 41.58 Ω.
7. The characteristic impedance of the transmission line if the outer diameter and inner diameter of the transmission line is 20 mm and 10 mm respectively, given the intrinsic impedance of the medium is 377 Ω, then the characteristic impedance of the transmission line is:
a) 41.58 Ω
b) 50 Ω
c) 377 Ω
d) None of the mentioned
Answer
Answer: a [Reason:] Characteristic impedance of the coaxial line is given by the expression ln (b/a)/ 2π. Substituting the given values in the above expression, the characteristic impedance 41.58 Ω.
8. Flow of power in transmission line takes place through:
a) Electric field and magnetic field
b) Voltage and current
c) Voltage
d) Electric field
Answer
Answer: a [Reason:] In a transmission line, flow of power takes place through propagation of electric field and magnetic field. Alternating electric field and alternating magnetic field propagates EM wave transmitting power.
9. When a transmission line is exited by a source, total power supplied is delivered to the load.
a) True
b) False
Answer
Answer: b [Reason:] When a transmission line is excited by the source, entire power is not delivered to the load due to the various types of losses that occur in the transmission line.
10. Expression for propagation constant. γ In terms of ω is:
a) √ω2µ∈
b) ω2µ∈
c) – ω2µ∈
d) None of the mentioned
Answer
Answer: a [Reason:] Propagation constant γ for a transmission line is dependent on the operating frequency of the transmission line, and the permittivity and permeability of the medium.
Set 2
1. In coupled line directional couplers, power from one line to another is transmitted through a microstrip line running between them.
a) true
b) false
Answer
Answer: b [Reason:] In coupled line couplers, the power is transmitted between the 2 lines by coupling from one line to another due to the interaction of the electromagnetic fields. Hence, wireless power transmission occurs here.
2. The number of conductors used in the construction of coupled line couplers is fixed.
a) true
b) false
Answer
Answer: b [Reason:] Since the method of power transmission in coupled line couplers is wireless power transmission by the interaction of electromagnetic fields, any number of wires can be used. But as a standard, 3 lines are used in the construction of these couplers.
3. The mode of propagation of propagation supported by coupled line couplers is:
a) TM mode
b) TE mode
c) TEM mode
d) quasi TEM mode
Answer
Answer: c [Reason:] Coupled transmission lines are assumed to operate in TEM mode of propagation. TEM mode of propagation is mostly valid for coaxial and stripline structures while microstrip lines support quasi TEM mode of propagation.
4. Coupled line couplers are:
a) symmetric couplers
b) asymmetric couplers
c) in phase couplers
d) type of hybrid coupler
Answer
Answer: a [Reason:] Coupled line couplers are symmetric three line couplers. Symmetric here means that the lines are of equal width and thickness. Their position with respect to ground is identical.
5. For coupled line coupler, if the voltage coupling factor is 0.1 and the characteristic impedance of the microstrip line is 50 Ω, even mode characteristic impedance is:
a) 50.23 Ω
b) 55.28 Ω
c) 100 Ω
d) 80.8 Ω
Answer
Answer: b [Reason:] Even mode characteristic impedance of coupled line coupler is Z0√ (1+C) /√ (1-C).here C is the voltage coupling coefficient. Substituting the given values, even mode characteristic impedance is 55.28 Ω.
6. If the coupling coefficient of a coupled line coupler is 0.1 and the characteristic impedance of the material is 50 Ω, then the odd mode characteristic impedance is:
a) 45.23 Ω
b) 50 Ω
c) 38 Ω
d) none of the mentioned
Answer
Answer: a [Reason:] Odd mode characteristic impedance of a coupled line coupler is Z0√ (1-C)/ √ (1+C). C is the voltage coupling co-efficient. Substituting the given values, odd mode characteristic impedance is 45.23.
7. Dielectric and conductor loss have no effect on the directivity of the coupled line coupler.
a) true
b) false
Answer
Answer: b [Reason:] Both dielectric loss and conductor loss reduce the directivity of the coupled line coupler. In the absence of loss under matched conditions, the directivity of a coupler could be up to 70 dB.
8. Multisection couplers have a very narrow operational bandwidth which is a major disadvantage.
a) true
b) false
Answer
Answer: b [Reason:] Multisection couplers have very high operational bandwidth. This high bandwidth can be achieved only when the coupling levels are low. In order to achieve these low coupling levels, stripline are used in their construction.
9. Three section binomial couplers have very low directivity as compared to other coupler designs.
a) true
b) false
Answer
Answer: b [Reason:] Three section binomial couplers have very low conductor and dielectric losses. This low loss can be achieved by efficient design. Since the losses are low for a binomial coupler, they have directivity greater than 100 dB.
10. The capacitance per unit length of broadside parallel lines with width W and separation d is:
a) ∈W/d
b) ∈d/W
c) dW/∈
d) none of the mentioned
Answer
Answer: a [Reason:] The capacitance of the line used in the construction of a coupled line coupler is directly proportional to the width of the line. As the width increase, capacitance increases. Capacitance varies inversely with distance d. as the separation increases, capacitance decreases.
Set 3
1. One condition to be satisfied in an oscillator circuit so that stable oscillations are produced is:
a) positive feedback is to be achieved
b) negative feedback is to be achieved
c) 1800 phase shift is required between the transistor input and output.
d) none of the mentioned
Answer
Answer: c [Reason:] In an oscillator a total of 3600 of phase shift is to be achieved in the entire circuit to produce oscillations. The transistor used in the oscillator circuit must produce a phase shift of 1800 to achieve stable oscillations. Hence this condition has to be satisfied by the oscillator.
2. In an oscillator, the resonant feedback circuit must have must have a low Q in order to achieve stable oscillation.
a) true
b) false
Answer
Answer: b [Reason:] If the resonant feedback circuit has a high Q, so that there is random phase shift with frequency, the oscillator will have good frequency stability.
3. Quartz crystals are more efficient as a feedback network because:
a) less circuit complexity
b) cost effective
c) crystals operate at high voltage levels
d) LC circuits have unloaded Q of a few hundreds
Answer
Answer: d [Reason:] At frequencies below a few hundred MHz, where LC resonators seldom have unloaded Qs greater than a few hundred. Quartz crystals have unloaded Q of about 10000 and have a temperature drift of 0.001%/C0.
4. Quartz crystal and tourmaline used in oscillators work on the principle of:
a) photo electric effect
b) piezo electric effect
c) Raman effect
d) black body radiation
Answer
Answer: b [Reason:] Quartz crystals work on the principle of piezo electric effect. When electrical energy is applied to these crystals, they vibrate in a direction perpendicular to the application of energy producing oscillations.
5. A quartz crystals equivalent circuit is a series LCR circuit and has a series resonant frequency.
a) true
b) false
Answer
Answer: b [Reason:] A quartz crystal has an equivalent circuit such that a series LCR network is in parallel with a capacitor. A quartz crystal thus has both series and parallel resonant frequencies.
6. Quartz crystal is used in the _______region, where the operating point of the crystal is fixed.
a) resistive
b) inductive reactance
c) capacitive reactance
d) none of the mentioned
Answer
Answer: b [Reason:] Quartz crystal is always operated in the inductive reactance region so that the crystal is used in place of an inductor in a Colpitts or pierce oscillator.
7. In the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is:
a) capacitive
b) inductive
c) both capacitive and inductive
d) none of the mentioned
Answer
Answer: b [Reason:] In the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is inductive. In this region between the series and parallel and series resonant frequencies, the operating point of the crystal is fixed and hence can be used as part of other circuits.
8. In the equivalent circuit of a quartz crystal, LCR arm has an inductance of 4 mH and capacitor has a value of 4nF, then the series resonant frequency of the oscillator is:
a) 0.25 MHz
b) 2.5 MHz
c) 25 MHz
d) 5 MHz
Answer
Answer: a [Reason:] The series resonant frequency of a crystal oscillator is given by 1/√LC. Substituting the given values of L and C in the expression, the series resonant frequency is 0.25 MHz.
9. Parallel resonant frequency of quartz crystal is given by:
a) 1/ √(LCₒC/(Cₒ+C))
b) 1/√LC
c) 1/√LCₒ
d) 1/ √(L(Cₒ+C) )
Answer
Answer: a [Reason:] Parallel resonant frequency of an oscillator is given by√(LCₒC/(Cₒ+C)). Here L and C are the inductance and capacitance in the LCR arm of the equivalent circuit of the crystal. Co is the capacitance existing in parallel to this LCR arm.
10. The equivalent circuit of a quartz crystal has LCR arm capacitance of 12nF and inductance of 3mH and parallel arm capacitance of 4nF. Parallel resonant frequency for the circuit is:
a) 3 MHz
b) 0.3 MHz
c) 6 MHz
d) 9 MHz
Answer
Answer: a [Reason:] The parallel resonant frequency of a crystal oscillator is given by 1/√(LCₒC/(Cₒ+C)). Substituting the given values in the equation, the parallel resonant frequency is found to be 3 MHz.
Set 4
1. The stability of an oscillator is enhanced with the use of:
a) high Q tuning network
b) passive elements
c) appropriate feedback methods
d) none of the mentioned
Answer
Answer: a [Reason:] The stability of an oscillator is enhanced with the use of high Q tuning circuits. At microwave frequencies, lumped elements cannot provide high Q factor. Waveguide tuning circuits cannot be integrated with small microwave circuits easily. Hence dielectric resonators are used to provide high Q factor.
2. In oscillator tuning circuits, dielectric resonators are preferred over waveguide resonators because:
a) they have high Q factor
b) compact size
c) they are easily integrated with microwave integrated circuits
d) all of the mentioned
Answer
Answer: d [Reason:] Dielectric resonators have all the above properties mentioned. Also, they are made of ceramic materials which also increases the temperature stability of resonator which further increases the fields of application.
3. A dielectric resonator coupled with an oscillator operates in:
a) TE10δ
b) TE01δ
c) TM10δ
d) TM01δ
Answer
Answer: a [Reason:] A dielectric resonator is coupled to an oscillator by positioning it in close proximity to a microstrip line. The resonator operates in TE10δ mode, and couples to the fringing field magnetic field of the microstrip line.
4. A dielectric resonator is modeled as __________ when it is used as a tuning circuit with a oscillator.
a) series RLC circuit
b) parallel RLC circuit
c) LC circuit
d) tank circuit
Answer
Answer: b [Reason:] The strength of coupling between a microstrip line and the resonator is determined by the spacing between them. The resonator appears as a series load on the microstrip line. The resonator is modeled a s a parallel RLC circuit and the coupling to the feed line is modeled by the turns ratio of the transformer.
5. The coupling factor between the resonator and the microstrip line is the ratio of external Q to the unloaded Q.
a) true
b) false
Answer
Answer: b [Reason:] The coupling factor the resonator and the microstrip line is the ratio of unloaded Q to external Q. The simplified expression for coupling factor is N2R/2Z0.
6. If the reflection coefficient seen on the terminated microstrip line looking towards the resonator is 0.5, then the coupling coefficient is:
a) 0.5
b) 0.25
c) 0.234
d) 1
Answer
Answer: d [Reason:] Coupling coefficient for a line in terms of reflection coefficient is Г/ (1-Г). Substituting the given reflection coefficient in this expression, the coupling coefficient is 1.
7. A dielectric resonator can be incorporated into a circuit to provide _________ using either parallel or series arrangement.
a) frequency stability
b) oscillations
c) high gain
d) optimized reflection coefficient
Answer
Answer: a [Reason:] Oscillators can be obtained in various transistor configurations, their instability can be enhanced by using series or shunt elements. A dielectric resonator can be incorporated into a circuit to provide frequency stability using either parallel or series arrangement.
8. It is desired to design a frequency oscillator at 2.4 GHz and the reflection coefficient desired is 0.6, then the coupling coefficient between the feed line and the dielectric resonator is:
a) 1.5
b) 1
c) 0.5
d) none of the mentioned
Answer
Answer: a [Reason:] Coupling coefficient for a line in terms of reflection coefficient is Г/ (1-Г). Substituting the given reflection coefficient in this expression, the coupling coefficient is 1.5.
9. If the reflection coefficient between the feed line and the resonator is -0.6, then the equivalent impedance of the resonator at resonance given that the characteristic impedance of the microstrip line is:
a) 50 Ω
b) 12.5 Ω
c) 25 Ω
d) none of the mentioned
Answer
Answer: b [Reason:] The equivalent impedance of the resonator at resonance is Z0 (1 + Г)/(1- Г). Substituting the given values in this expression, equivalent impedance of the resonator is 12.5 Ω.
10. If the equivalent impedance of the resonator at resonance is 12.5 Ω and the characteristic impedance of the feed line is 50 Ω, then the coupling coefficient is:
a) 0.25
b) 0.5
c) 0.75
d) 1
Answer
Answer: a [Reason:] Coupling coefficient is defined as the ratio of the equivalent impedance of the resonator to the characteristic impedance of the feed line. Substituting accordingly, coupling coefficient is 0.25.
Set 5
1. A dielectric material in the form of a small cube or disc can be used as a resonator.
a) true
b) false
Answer
Answer: a [Reason:] A dielectric material in the form of a small cube or disc can be used as a resonator. It has the same operating principles as that of a rectangular waveguide resonator and a circular waveguide resonator.
2. Dielectric resonators use materials that are less lossy.
a) true
b) false
Answer
Answer: a [Reason:] Dielectric resonators use materials that are less lossy and have high dielectric constant, ensuring that most of the fields will be contained in the dielectric.
3. The major disadvantage of dielectric resonators is:
a) complex construction
b) field fringing
c) requirement of high dielectric constant
d) none of the mentioned
Answer
Answer: b [Reason:] The major disadvantage of dielectric resonators is field fringing or leakage from sides and ends of a dielectric resonator. This leakage of field energy results in high loss.
4. One of the most commonly used dielectric materials is:
a) barium tetratetanate
b) titanium
c) teflon
d) none of the mentioned
Answer
Answer: a [Reason:] Materials having dielectric constant in the range of 10-100 are used in dielectric resonators and one more required characteristic property is low loss. Barium tetratetanate has all these properties. Hence it is most commonly used.
5. The resonant frequency of a dielectric resonator cannot be mechanically tuned.
a) true
b) false
Answer
Answer: a [Reason:] By using an adjustable metal plate above the resonator, the resonant frequency can be mechanically tuned. As it has these desirable features, it is mostly used in integrated microwave filters and oscillators.
6. If a dielectric resonator has a dielectric constant of 49, then the reflection coefficient of the dielectric resonator is:
a) 0.5
b) 0.75
c) 0.1
d) 0.7
Answer
Answer: b [Reason:] Reflection co-efficient of a dielectric resonator is given by (√∈r-1)/ (√∈r-1). Given that dielectric constant is 49, the reflection coefficient is 0.75.
7. Q factor does not exist for dielectric resonator.
a) true
b) false
Answer
Answer: b [Reason:] Q factor exists for a dielectric resonator .it is defined as the ratio of the energy stored in the dielectric to the energy dissipated and other losses that may occur.
8. The approximate loaded Q due to dielectric loss for a dielectric resonator given the loss tangent is 0.0001 is:
a) 1000
b) 500
c) 2000
d) 10000
Answer
Answer: a [Reason:] Loaded Q due to dielectric loss for a dielectric resonator is given by the reciprocal of the loss tangent. Taking the reciprocal of loss tangent, loaded Q due to dielectric loss is 1000.
9. The direction of propagation is in z direction outside the dielectric in the resonator.
a) true
b) false
Answer
Answer: b [Reason:] In a dielectric resonator, the direction of propagation can occur along the Z direction in the dielectric at resonant frequency but the fields are cutoff in the air region around the dielectric.
10. A dielectric resonator is considered to be closed at both the ends.
a) true
b) false
Answer
Answer: b [Reason:] For all analysis purpose, a dielectric resonator is considered to be of a short length L and termed as dielectric waveguide open at both the ends.