Category: Microwave Engineering Objective Questions

250+ TOP MCQs on Quadrature Hybrid and Answers

Microwave Engineering Multiple Choice Questions on “Quadrature Hybrid”.

1. Quadrature hybrids are those couplers which are:
A. 3 dB couplers
B. Directional couplers
C. They have a 900 phase difference between signals in through and coupled arms.
D. All of the mentioned
Answer: D
Clarification: Quadrature hybrids are directional couplers that have a phase difference of 900 between the signals obtained at through and coupled ports.

2. Branch-line couplers are also popular as Quadrature hybrids.
A. True
B. False
Answer: A
Clarification: Quadrature hybrids are also called as branch-line couplers. The other 3 dB couplers are coupled line couplers or Lange couplers. These couplers also can be Quadrature hybrids.

3. The S matrix of a Quadrature hybrid is of size 4×4 and the diagonal elements of a matched coupler are all:
A. 1
B. 0
C. Cannot be determined
D. None of the mentioned
Answer: B
Clarification: In a matched coupler, since all ports are matched, no power goes back to the port from which the flow of energy in the coupler occurred. Since the backward power flow is zero for matched network, the diagonal elements are zero.

4. A branch-line coupler is an asymmetric coupler.
A. True
B. False
Answer: B
Clarification: A branch-line coupler is a symmetric coupler that has all four ports placed symmetrically. Since the construction is symmetric, any port can be used as input and any port can be used as output.

5. Branch-line couplers are preferably made using waveguides so as to obtain high gain and simple construction.
A. True
B. False
Answer: B
Clarification: Branch-line couplers are mostly done using microstrip lines. They also reduce the complexity of the circuit and can be easily integrated with other microwave devices in all large scale applications.

6. A 50 Ω branch-line Quadrature hybrid has to be designed to operate over a range of frequencies. The branch-line impedance of this coupler so designed is:
A. 70.7 Ω
B. 35.4 Ω
C. 50 Ω
D. 100 Ω
Answer: B
Clarification: Each arm of a Quadrature hybrid is λ/4 long; λ is the wavelength at which the coupler is designed to operate. The branch-line impedance for a λ/4 line is Z0/√2. Substituting for Z0 in the equation, the impedance is 35.4 Ω.

7. The plot S11 v/s frequency for a branch-line coupler has a straight line characteristic for a wide range of frequency around the designed frequency range.
A. True
B. False
Answer: B
Clarification: S11 parameter signifies the power measured at port 1 when port 1 is used as an input port. When the ports of the coupler are matched, no power is reflected back to the port 1. Hence S11 curve has a dip at the frequency for which the coupler was operated to design. A fall and rise in the curve is seen at this point.

8. The curve of S14 for a branch-line coupler is similar to that of the S11 curve of the branch-line coupler.
A. True
B. False
Answer: A
Clarification: S14 parameter gives the power measured at the port 4 or the isolated port of the branch-line coupler. Since the signals that reach port 4 from 2 different arms are 900 out of phase with each other, theoretically power at port 4 is zero. Practically, it is zero for the designed frequency but some power is received at other frequencies.

9. S12 and S13 curves for branch-line couplers are almost a straight line parallel to X –axis. Both the curves are similar and follow same path.
A. True
B. False
Answer: A
Clarification: S13 and S12 parameters give the power measured at port 2 and port 3 of the branch-line coupler when port 1 is used as the input port. Since these are the through and coupled ports, power measured across these ports is almost constant and resemble a straight line parallel to X axis.

10. If the branch-line impedance of a coupler designed to operate at 1 GHz is 70.70 Ω, then the characteristic impedance of the material of the arms of the branch-line coupler is:
A. 70.7 Ω
B. 50 Ω
C. 100 Ω
D. None of the mentioned
Answer: C
Clarification: Given the branch impedance is 70.70 Ω; the characteristic impedance of the line is Z√2. This relation is used since all the arms of a branch-line coupler are λ/4 long. Substituting for Z, the characteristic impedance of the line is 100 Ω.


,

250+ TOP MCQs on Applications of RF Diodes and Answers

Microwave Engineering Multiple Choice Questions on “Applications of RF Diodes”.

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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
C. Sensitive receiver
D. None of the mentioned
Answer: C
Clarification: 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
Clarification: 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.


Microwave Engineering,

250+ TOP MCQs on Dielectric Resonator Oscillators and Answers

Microwave Engineering Multiple Choice Questions on “Dielectric Resonator Oscillators”.

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: A
Clarification: 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: D
Clarification: 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: A
Clarification: 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: B
Clarification: 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: B
Clarification: 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: D
Clarification: 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: A
Clarification: 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: A
Clarification: 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: B
Clarification: 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: A
Clarification: 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.


,

250+ TOP MCQs on Generator And Load Mismatches and Answers

Microwave Engineering Multiple Choice Questions on “Generator And Load Mismatches”.

1. When a load is matched to a transmission line, the condition that is satisfied when matched is:
A. ZL=Z0
B. ZL=2Z00
C. ZL=Zin
D. ZL=2Zin
Answer: A
Clarification: In order to deliver the maximum power from source to load, the transmission line has to be matched to the load. Hence for the transmission line to be matched to the load, the condition to be satisfied is ZL=Z0.

2. When a load ZL is matched to a line, the value of standing wave ratio is:
A. 1
B. 0
C. infinity
D. insufficient data to calculate SWR
Answer: A
Clarification: When the load is matched to the transmission line, they are said to be matched. Hence standing waves exist on the transmission line. Hence SWR is 1.

3. The value of reflection co efficient when a transmission line is matched to the load is:
A. 1
B. 0
C. 0.707
D. cannot be determined
Answer: B
Clarification: When the transmission line and the load are matched, no reflections occur in the transmission line and hence no voltage wave is reflected back. Hence, the reflection co-efficient for a matched line is 0.

4. The value of transmission co efficient when a transmission line is matched to a load is:
A. 1
B. 0
C. 0.5
D. 0.707
Answer: A
Clarification: Transmission co-efficient is defined as the ratio of the incident power to transmitted power at the load end. When the transmission line is matched, the incident power is completely transmitted. Hence, transmission co-efficient is 1.

5. The expression for power delivered to a load , when a line is matched and supplied with a source of Vg with generator impedance Rg +jXg is:
A. 0.5*Vg2/Rg
B. 0.5*Vg2Rg/4(Rg2+ Xg2)
C. Rg/4(Rg2+ Xg2)
D. generator impedance does not cause any losses
Answer: B
Clarification: Due to the generator impedance, there will be some power dissipated and hence the total source power is not transmitted. Hence that power dissipated due to generator impedance is also removed from the total power delivered.

6. If a transmission line is exited from a source of 4V at 1.2GHz frequency with a generator impedance of 4+j3 with a characteristic impedance of the transmission line 50Ω,then the power delivered to the load is:
A. 0.1 watt
B. 0.9 watt
C. 0.8 watt
D. 1watt
Answer: C
Clarification: The expression for total power delivered given the generator impedance is 0.5*Vg2Rg/4(Rg2+ Xg2). Substituting the given values in the above equation, the total power delivered is 0.8 watt.

7. If the generator impedance of a source connected to a transmission line is 50+j100Ω, then for conjugate matching to occur , the input impedance must be:
A. 50-j100 Ω
B. 50+100 Ω
C. 50 Ω
D. one of the mentioned
Answer: A
Clarification: The condition for conjugate matching is Zin=*Zg, where Zin is the input impedance of the transmission line and Zg is the generator impedance. For conjugate matching, taking the conjugate of the given impedance, the input impedance must be 50-100j Ω.

8. After conjugate impedance matching the input impedance used for matching after normalization was 1+j with the characteristic impedance of the transmission line being 100Ω, then the generator impedance must have been:
A. 100+100j
B. 1+j
C. 100-100j
D. 1-j
Answer: C
Clarification: After conjugate matching the input impedance of a transmission line after normalization is 1+j. hence the generator impedance will be the conjugate, that is 1-j. multiplying with the characteristic impedance, we get 100-100j.

9. For a matched transmission line with a generator impedance of 50Ω and the source being 4V,1GHZ,then the maximum power delivered to the line is:
A. 0.4 watt
B. 0.04 watt
C. 0.5 watt
D. no power is delivered
Answer: B
Clarification: The maximum power delivered to the load given the generator impedance is 0.5*Vg2Rg/4(Rg2+ Xg2). Substituting in the above equation the given values, power delivered is 0.04 watt.

10. If the power delivered to a load is 0.04w, then the normalized generator impedance if the source use is 4V at 2GHz and the generator impedance is real and characteristic impedance of the transmission line is 50Ω is:
A. 1 Ω
B. 1+j Ω
C. 1-j Ω
D. 50 Ω
Answer: A
Clarification: The maximum power delivered to the load given the generator impedance is 0.5*Vg2Rg/4(Rg2+ Xg2). Rearranging the equation and substituting the given value, Rg is 50Ω. To normalize, dividing the impedance by characteristic impedance, the impedance is 1 Ω.


,

250+ TOP MCQs on Double Stub Tuning and Answers

Microwave Engineering Multiple Choice Questions on “Double Stub Tuning”.

1. The major disadvantage of single stub tuning is:
A. it requires a variable length of line between the load and the stub
B. it involves 2 variable parameters
C. complex calculation
D. none of the mentioned
Answer: A
Clarification: Single stub matching requires a variable length line between the stub and the load for matching which is a major disadvantage since the length of the stub plays a crucial role in matching.

2. The major advantage of double stub tuning is:
A. it uses 2 tuning stubs in fixed positions
B. it involves 2 stubs
C. length of the stub is variable
D. none of the mentioned
Answer: A
Clarification: The disadvantage of single stub tuning is overcome in double stub tuning. It uses 2 tuning stubs in fixed positions so that the length between the first stub and the load is not variable.

3. In a double stub tuner circuit, the load is of _______ length from the first stub.
A. fixed length
B. arbitrary length
C. depends on the load impedance to be matched
D. depends on the characteristic impedance of the transmission line
Answer: B
Clarification: The position of the first stub in a double stub tuner is variable from the load end. But the distance between the 2 stubs is fixed based on the value to which impedance is matched.

4. Double stub tuners are fabricated in coaxial line are connected in shunt with the main co-axial line.
A. true
B. false
Answer: A
Clarification: Most of the transmission lines used in most of the practical applications use coaxial cables, for which impedance matching of the load are done using double stub tuners which are made of coaxial cables for their best suited properties.

5. Impedance matching with a double stub tuner using a smith chart yields 2 solutions.
A. true
B. false
Answer: A
Clarification: Both single stub tuning and double stub tuning give two solutions. The intersection of the admittance and the 1+jb circle drew on the smith chart yields 2 points from which 2 solutions can be generated.

6. All load impedances can be matched to a transmission line using double stub matching.
A. true
B. false
Answer: A
Clarification: When a smith chart is used for impedance matching, if the normalized load admittance yL were inside the g+jb circle, no value of stub susceptance b1 could ever bring the load point to intersect with the 1+jb circle; this forms a forbidden range of admittance that cannot be matched.

7. The simplest method of reducing the forbidden range of impedances is:
A. increase the distances between the stubs
B. reduce the distance between the stubs
C. increase the length of the stubs
D. reduce the length of the stubs
Answer: B
Clarification: Reducing the distances between the stubs reduces the forbidden area in the smith chart which involves the load impedances that cannot be matched. Thus, more number of load impedances (range) can be matched to the transmission line.

8. Stub spacing that are near 0 and λ/2 lead to more frequency sensitive matching networks.
A. true
B. false
Answer: A
Clarification: Though theoretically the stub spacing must be small enough to reduce the forbidden area, for practical considerations, the stubs have to be placed sufficiently far enough for fabrication ease and reduce frequency sensitivity.

9. The standard stub spacing usually used is:
A. 0, λ/2
B. λ/4, λ/8
C. λ/8, 3λ/8
D. none of the mentioned
Answer: C
Clarification: While stub spacing of 0, λ/2 lead to frequency sensitive matching circuits, an optimum value of spacing is chosen taking into consideration, the various design constraints. This optimum spacing usually used is λ/8, 3λ/8.

10. If the length of the line between the first stub and the load can be adjusted, the admittance can be moved from the forbidden region.
A. true
B. false
Answer: A
Clarification: If the design requirements for impedance matching are more flexible, then the length of the line between the load and the first stub can be varied. This would result in moving the load admittance point out of forbidden region in the smith chart thus enabling impedance matching.


,

250+ TOP MCQs on Coupled Line Directional Couplers and Answers

Microwave Engineering Multiple Choice Questions on “Coupled Line Directional Couplers “.

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: B
Clarification: 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: B
Clarification: 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: C
Clarification: 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: a
Clarification: 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: B
Clarification: 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: A
Clarification: 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: B
Clarification: 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: B
Clarification: 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: B
Clarification: 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: A
Clarification: 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.


,