Multiple choice question for engineering
Set 1
1. 1800 hybrid is a network in which there is a phase shift of 1800 between the input signal applied and the output taken.
a) true
b) false
Answer
Answer: b [Reason:] 1800 hybrid is a four port network that has one input port and two output ports. The phase difference between the 2 output ports is 1800.
2. Port 1 and port 4 of 1800 hybrid are called sum and difference ports respectively because of their behavior and action mechanism.
a) true
b) false
Answer
Answer: a [Reason:] When a 1800 hybrid is used as a combiner, with input signals applied at port 2 and port 3, the sum of the inputs will be formed at port 1, while the difference will be formed at port 4. Hence they are referred to as sum and difference ports.
3. S matrix of 1800 hybrid consists of all diagonal elements zero.
a) true
b) false
Answer
Answer: a [Reason:] If all the ports of 1800 hybrid are properly matched, no power is reflected back to the same port. Hence all the diagonal elements of the S matrix, Sii=0.
4. In 1800 hybrid, different power levels can be received at the two output ports of the hybrid.
a) true
b) false
Answer
Answer: b [Reason:] 1800 hybrid is a symmetrical coupler. Hence, the input power applied at the input port can be divided equally and obtained at the 2 output ports. Unequal division of power is not possible in 1800 hybrid.
5. 18000 ring hybrid with system impedance of 50 Ω has to be designed. Then the characteristic impedance of the arms of the 1800 hybrid is:
a) 50 Ω
b) 70.70 Ω
c) 100 Ω
d) none of the mentioned
Answer
Answer: b [Reason:] Given that the system impedance is 50 Ω, the characteristic impedance of the arms is 50√2. Hence the characteristic impedance is 70.70 Ω.
6. In a waveguide magic-T there is no coupling of power between port 1 and port 4.
a) true
b) false
Answer
Answer: a [Reason:] Consider TE10 mode incident at port 1. There is odd symmetry about guide 4. Because the field lines of a TE10 mode in guide 4 would have even symmetry, hence there is no coupling between port 1 and port 4.
7. When a TE10 wave is incident on port 4 of a magic-T, all the power is coupled to port 1.
a) true
b) false
Answer
Answer: b [Reason:] When port 4 of a Magic-T is excited, port 1 and port 4 are decoupled, due to symmetry. Port 2 and port 3 are excited equally by the incident wave with a phase difference of 1800.
8. The tapered coupled line 1800 hybrid can provide an arbitrary power division at the 2 output ports of the coupler.
a) true
b) false
Answer
Answer: a [Reason:] When the arms of the coupled line is tapered, they result in division of power at the two output ports unequally and they offer a bandwidth of one decade or more.
9. The plot of frequency V/s S11 parameter for a tapered line coupler has a dip at the frequency at which it is designed.
a) true
b) false
Answer
Answer: a [Reason:] When port 1 of a tapered coupler is excited, no power flows back to port 1 since the ports are matched. Hence, S11 value is almost zero or negligibly small and hence has a dip at the designed frequency.
10. For a tapered line coupler, the curves of S12 and S13 are identical and have the same magnitude at all frequencies.
a) true
b) false
Answer
Answer: b [Reason:] Tapered line couplers result in unequal power division at the output ports. These are useful for various applications. Hence the S12 and S13 curves are not identical since the power outputs are not equal.
Set 2
1. Transistor multipliers are more efficient compared to diode multipliers from all operational aspects.
a) True
b) False
Answer
Answer: a [Reason:] Transistor multipliers offer better bandwidth and the possibility of conversion efficiency greater than 100% (conversion gain). FET multipliers also require less input and DC power than diode multipliers.
2. A major characteristic property required by frequency multipliers for frequency multiplication to happen is:
a) High gain
b) High conversion efficiency
c) Non linearity
d) None of the mentioned
Answer
Answer: c [Reason:] Non linearity property of devices like transistors and diodes is exploited in frequency multipliers. In transistor amplifiers, FET are used since several nonlinearities exist in FET can be used for harmonic generation.
3. If the input power for a frequency doubler is 10.7 mW and the output measured after the frequency doubling process is 21 mW, then the conversion gain for the frequency doubler is:
a) 4.5 dB
b) 8.4 dB
c) 9.8 dB
d) 2.9 dB
Answer
Answer: d [Reason:] The conversion gain for a frequency doubler is given by the expression, P2/Pavail. Here P2 is the power measured at the output of the frequency doubler and Pavail is the power input. Substituting the given values in the equation, the conversion gain is 2.9 dB.
4. An ideal _______ produces an output consisting of the sum and difference frequencies of the two input signals.
a) Mixer
b) Amplifier
c) Product modulator
d) Phase modulator
Answer
Answer: a [Reason:] A mixer is a three port device that uses a mixer or a time varying element to achieve frequency conversion. An ideal mixer produces an output consisting of the sum and difference frequencies of the two input signals.
5. A mixer consists of a non-linear device that produces various harmonics of the input frequency.
a) True
b) False
Answer
Answer: a [Reason:] A non linear device can generate a wide variety of harmonics and other products of the input frequencies. A filter is used to extract only the desired frequency components.
6. A mixer can be used for both up conversion and down conversion at the transmitter and receiver respectively.
a) True
b) False
Answer
Answer: a [Reason:] At the transmitter, mixer is used to convert the baseband signal to a broadband signal with use of a high frequency local oscillator. At the receiver, a mixer is used to convert the received broadband signal to baseband signal using a local oscillator. This is called down conversion.
7. A mixer having high conversion loss is said to have very high:
a) Gain
b) Loss
c) Bandwidth
d) None of the mentioned
Answer
Answer: b [Reason:] Conversion loss for a mixer is defined as the ratio of available RF input power to the available IF output power. A higher value of conversion loss implies that large amount of power is lost in down converting the frequency from RF to IF range. This makes them less efficient.
8. The IS-54 digital cellular telephone system uses a receive frequency band of 869-894 MHz, with a first IF frequency range of 87 MHz, one possible range of local oscillator frequency is:
a) 956 to 981 MHz
b) 750 to 784 MHz
c) 869 to 894 MHz
d) None of the mentioned
Answer
Answer: a [Reason:] The two possible local oscillator frequency range is given by fLO = fRF ± fIF.. fLO is the local oscillator frequency, fRF is the received frequency and fIF is the intermediate frequency range. Substituting the given values in the above equation, one possible frequency range is 956 to 981 MHz.
9. The curve of FET transconductance v/s gate-to-source voltage is a straight line through origin.
a) True
b) False
Answer
Answer: b [Reason:] The gate to source voltage of an FET is slowly increased from negative voltage towards zero. As the voltage applied becomes more positive, the transconductance increases up to a certain level and then remains a constant. This property of a FET is used in single-ended FET mixer.
10. RF input matching and RF-LO isolation can be improved through the use of:
a) Balanced mixer
b) Single-ender diode mixer
c) Single ended FET mixer
d) Image reject mixer
Answer
Answer: a [Reason:] RF input matching and RF-LO isolation can be improved through the use of balanced mixer. It consists of two single ended mixers combined with a hybrid junction.
Set 3
1. Lumped elements can be used to make resonators that rare to be operated at microwave frequencies.
a) True
b) False
Answer
Answer: b [Reason:] Lumped elements cannot be used at microwave frequencies since their behavior is not deterministic at these frequencies and the required response cannot be achieved.
2. Short circuited λ/2 transmission line has a quality factor of:
a) β/2α
b) 2β/α
c) β/α
d) Z0/ZL
Answer
Answer: a [Reason:] Quality factor of a short circuited transmission line is a function of attenuation constant and phase constant of the transmission line. Higher is the attenuation in the transmission line, lower is the quality factor of the transmission line.
3. Quality factor of a coaxial cable transmission line is independent of the medium between the wires of the transmission line.
a) True
b) False
Answer
Answer: b [Reason:] Quality factor is dependent on the permeability of the medium between the inner and outer conductor of the co-axial cable. For example, air has twice the quality factor as that of Teflon filled co-axial fiber.
4. A coaxial cable is air filled with air as dielectric with inner and outer radius equal to 1 mm and 4 mm. If the surface resistivity is 1.84*10-2Ω,then the attenuation due to conductor loss is:
a) 0.011
b) 0.022
c) 0.11
d) 0.22
Answer
Answer: a [Reason:] Conductor loss in a coaxial cable is given by Rs(a-1+b-1)/2ln (b/a). Here ‘a’ and ‘b’ are the inner and outer radii of the coaxial cable. is the intrinsic impedance of the medium, for air is 377Ω. Substituting the given values in the equation, conductor loss is 0.022 Np/m.
5. An air coaxial cable has attenuation of 0.022 and phase constant of 104.7, then the quality factor of a λ/2 short circuited resonator made out of this material is:
a) 2380
b) 1218
c) 1416
d) Insufficient data
Answer
Answer: a [Reason:] Quality factor of a λ/2 short circuited transmission line is β/2α. β is the phase constant and α is the attenuation constant of the line, substituting the given values, the quality factor of the transmission line is 2380.
6. The equivalent resistance of a short circuited λ/4 transmission line is independent of the characteristic impedance of the transmission line.
a) True
b) False
Answer
Answer: b [Reason:] The equivalent resistance of a short circuited λ/4 transmission line is dependent of the characteristic impedance of the transmission line. The expression for equivalent resistance is Z0/αl. Resistance of a short circuited line is directly proportional to the characteristic impedance of the transmission line.
7. A microstrip patch antenna has a width of 5.08mm and surface resistivity of 1.84*10-2. Then the attenuation due to conductor loss is:
a) 0.0724
b) 0.034
c) 0.054
d) None of the mentioned
Answer
Answer: a [Reason:] Attenuation due to conductor loss of a microstrip line is given by Rs/Z0W. Substituting the given values, attenuation due to conductor loss is 0.0724 Np/m.
8. If the attenuation due to dielectric loss and attenuation due to conductor loss in a microstrip transmission line is 0.024Np/m and 0.0724 Np/m, then the unloaded quality factor if the propagation constant is 151 is:
a) 150
b) 783
c) 587
d) 234
Answer
Answer: b [Reason:] Unloaded Q for a microstrip line is given by β/2α. Α is the sum of attenuation due to conductor loss and dielectric loss. Substituting the given values the equation, unloaded Q is 783.
9. The equivalent capacitance of a short circuited λ/4 transmission line is dependent on the characteristic impedance of the transmission line.
a) True
b) False
Answer
Answer: a [Reason:] Equivalent capacitance of a short circuited λ/4 transmission line is dependent on the characteristic impedance of the transmission line. It is inversely proportional to the characteristic impedance of the transmission line. Equivalent capacitance is π/4ω0Z0.
10. Inductance of an open circuited λ/2 transmission line is dependent on the characteristic impedance of the transmission line.
a) True
b) False
Answer
Answer: a [Reason:] Inductance of an open circuited λ/2 transmission line is dependent on the characteristic impedance of the transmission line. Expression for inductance is 1/ω02c, C is the equivalent capacitance of the open circuited line. C has the expression π/4ω0Z0.
Set 4
1. ABCD matrix is used:
a) When there is two or more port networks in the cascade
b) To represent a 2 port network
c) To represent a 2 port network
d) To represent the impedance of a microwave network
Answer
Answer: a [Reason:] The Z, Y, and S parameter representation can be used to characterize a microwave network with an arbitrary number of ports. But most microwave networks consist of cascade of two or more two port networks. In this case it is convenient to use ABCD matrix for network representations.
2. The voltage equation for a 2 port network that can be represented as a matrix is:
a) V1=AV2 + BI2
b) V1=CV2 + DI2
c) V1=BV2 +AI2
d) V1=DV2+CI2
Answer
Answer: a [Reason:] In the equation, V1 is the voltage measured at port 1 and V2 is the voltage measured at port 2 and I2 is the current measured at the second port. A and B are the network constants.
3. ABCD matrix of the cascade connection of 2 networks is equal to:
a) Product of ABCD matrices representing the individual two ports
b) Sum of the ABCD matrices representing the individual two ports
c) Difference of the ABCD matrices representing the individual two ports
d) Sum of transpose of ABCD matrices representing the individual two ports
Answer
Answer: a [Reason:] When two networks are connected in cascade, each of the two networks are represented as a 2×2 square matrix. Then to obtain the equivalent matrix of the cascade, the product of the ABCD matrices of each stage is taken.
4. For simple impedance Z, the ABCD parameters are:
a) A=1, Z=B, C=0, D=1
b) A=0, B=1, C=1, D=0
c) A=Z, B=1, C=1, D=0
d) A=1, B=0, C=Z, D=1
Answer
Answer: a [Reason:] If simple impedance or an equivalent impedance of a network is represented as a ABCD matrix, writing the equations in terms of voltage and current and setting each variable to zero, the four constants are obtained. For an impedance Z, the constants are A=1, Z=B, C=0, D=1.
5. For a simple admittance Y, the ABCD parameters are:
a) A=1, B=0, C=Y, D=1
b) A=Z, B=1, C=1, D=0
c) A=1, B=0, C=Z, D=1
d) A=1, Y=B, C=0, D=1
Answer
Answer: a [Reason:] If simple admittance or an equivalent admittance of a network is represented as a ABCD matrix, writing the equations in terms of voltage and current and setting each variable to zero, the four constants are obtained. For an admittance Y, the constants are A=1, Z=B, C=0, D=1.
6. C parameter for a transmission line of characteristic impedance Zₒ, phase constant β and length ‘l’ is:
a) j Yₒ Sin βl
b) j Zₒ Sin βl
c) j Zₒ tan βl
d) j Yₒ tan βl
Answer
Answer: a [Reason:] If a transmission line is represented as two port network, constants can be derived in terms of the A, B, C, D constants for the network. But setting each electrical parameter to zero, this constant is found. By doing so, the C parameter of transmission line is j Yₒ Sin βl.
7. For a 2 port network if Z₁₁=1.5 and Z₁₂=1.2, A parameter for the same 2 port network is:
a) 1.5
b) 1.25
c) 0.75
d) 1.75
Answer
Answer: b [Reason:] A parameter for the two port network is the ratio of the impedance Z11 and the impedance Z12. Substituting in this equation,’ A’ parameter of the network is 1.25.
8. For a 2 port network, if the admittance parameter Y₁₂=0.4, then B among the ABCD, parameters for the 2 port network is:
a) 2.5
b) 4.5
c) 5
d) 6
Answer
Answer: a [Reason:] For a two port network, B parameter is defined as the reciprocal of the admittance Y12. Taking the reciprocal of the given value, the B parameter of the network is 2.5.
9. If D=1.6 and B=2.8 for a 2 port network, then Y₁₁=?
a) 0.5714
b) 0.987
c) 0.786
d) 1.75
Answer
Answer: a [Reason:] The admittance Y11 of the network is defined as the ratio of B parameter to the D parameter of the network. Taking the ratio of the given values, admittance Y11 is 0.5714.
10. If A=2.8 and B=1.4 for a 2 port network then Z₁₁=?
a) 0.5
b) 2
c) 4.2
d) 2.7
Answer
Answer: b [Reason:] Z11 parameter of a two port network is the ratio of the A parameter of the network to the B parameter of the network. Taking the ratio of the given values, Z11 is 2.
Set 5
1. The power gain G of a two port network is independent of the source impedance of the two port network.
a) True
b) False
Answer
Answer: a [Reason:] Power gain G is the ratio of power dissipated in the load ZL to the power delivered to the input of the two port network. This gain is independent of ZS although the characteristic of some active devices is dependent on ZS.
2. __________ is defined as the ratio of power available from the two port network to the power available from the source.
a) Transducer power gain
b) Available power gain
c) Power gain
d) None of the mentioned
Answer
Answer: b [Reason:] Available power gain is defined as the ratio of power available from the two port network to the power available from the source. This assumes conjugate matching of both source and the load and depends on ZS, not ZL.
3. Transducer power gain of a two port network is dependent on :
a) ZS and ZL
b) ZS
c) ZL
d) Independent of both the impedances
Answer
Answer: a [Reason:] Transducer power gain of a two port network is the ratio of the power delivered to the load to the power available from the source. This depends on both ZS and ZL.
4. For a two port network the voltage reflection coefficient seen looking towards the load, ГS is:
a) (ZS –Z0)/ (ZS –Z0)
b) (ZS +Z0)/ (Z0 – Z0)
c) ZS / (ZS –Z0)
d) Z0/ (ZS –Z0)
Answer
Answer: a [Reason:] For a two port network, the reflection coefficient ГS seen looking towards the load is (ZS –Z0)/ (ZS –Z0). Here ZS is the input impedance of the transmission line and Z0 is the characteristic impedance of the transmission line.
5. In a two port network, the source impedance was measured to be 25 Ω and the characteristic impedance of the transmission line was measured to be 50 Ω. Then the reflection coefficient at the source end is:
a) -0.33333
b) -0.1111
c) 0.678
d) 0.2345
Answer
Answer: a [Reason:] For a two port network, the reflection coefficient ГS seen looking towards the load is (ZS –Z0)/ (ZS –Z0). Substituting the given values in the above equation, reflection coefficient at the source end is -0.3333.
6. For a unilateral transistor, the S parameter that is zero is:
a) S11
b) S12
c) S21
d) S22
Answer
Answer: b [Reason:] In a unilateral transistor power flow occurs only in one direction and hence S12 is sufficiently small and can be ignored. Also for a unilateral transistor the reflection coefficients reduce to Гin=S11 and Гout=S22.
7. Gain of an amplifier is independent of the operating frequency.
a) True
b) False
Answer
Answer: b [Reason:] Gain of an amplifier depends on the operating frequency. Gain of a conjugate matched FET amplifier drops off as 1/f2 or 6dB per octave.
8. Gain of a conjugate matched FET amplifier is given by the relation:
a) Rds (fT)2/ 4Ri (f)2
b) 4Ri (f)2/Rds (fT)2
c) Rds/ Ri
d) None of the mentioned
Answer
Answer: a [Reason:] Gain of FET amplifier is given by the relation Rds (fT)2/ 4Ri (f)2. Gain depends on the drain to source resistance, input resistance and also on the frequency of operation of the amplifier.
9. When both input and output of an amplifier are matched to zero reflection (in contrast to conjugate matching), the transducer power gain is:
a) │S21│2
b) │S22│2
c) │S12│2
d) |S11│2
Answer
Answer: a [Reason:] When both input and output of an amplifier are matched to zero reflection, ГL=0 and ГS=0. This reduces the complex transducer gain equation to the s parameter of the amplifier S21. S21 signifies the power at port 2 due to input applied at port 1.
10. If the load impedance of a two port network is 40 Ω and the characteristic impedance is 50 Ω, then the reflection coefficient of the two port network at the load end is:
a) -0.111
b) -0.333
c) -0.987
d) None of the mentioned
Answer
Answer: a [Reason:] Reflection coefficient at the load end of a two port network is given by the ratio (ZL-Z0)/ (ZL+Z0). ZL is the load impedance and Z0 is the characteristic impedance. Substituting, reflection at load end is -0.1111.