Multiple choice question for engineering
Set 1
1. Progress in ________ and other related semiconductors material processing led to the feasibility of monolithic microwave integrated circuits.
a) GaAs
b) Silicon
c) Germanium
d) GaAlAs
Answer
Answer: a [Reason:] Progress in GaAs and other related semiconductor material processing led to the feasibility of MMIC, where all the passive and active components required for a given circuit can be grown or implanted in a substrate.
2. MMICs are high cost devices that involve complex fabrication methods and contain multiple layers to contain even small circuits.
a) True
b) False
Answer
Answer: b [Reason:] Min MIC can be made at low cost because the labor involved with fabricating hybrid MIC’s is reduced. In addition, a single wafer can contain a large number of circuits, all of which can be processed and fabricated simultaneously.
3. The substrate of an MMIC must be a _____________ to accommodate the fabrication of all the type of devices.
a) Semiconductor
b) Insulator
c) Partial conductors
d) Metals operable at high frequencies
Answer
Answer: a [Reason:] Substrate of MMIC must be a semiconductor material to accommodate the fabrication of active devices. The type of devices and the frequency range dictate the type of substrate material. One such material is GaAs MESFET.
4. GaAs MESFETs are versatile device because it finds application in:
a) Low-noise amplifiers
b) High gain amplifiers
c) Mixers
d) All of the mentioned
Answer
Answer: d [Reason:] GaAs MESFET find application in low noise amplifiers, high gain amplifiers, broadband amplifiers, mixers, oscillators, phase shifters, and switches. These are the mostly used and cost effective substrates.
5. Transmission lines and other conductors in microwave devices are usually made with ___________
a) Gold metallization
b) Silver metallization
c) Copper metallization
d) Zinc metallization
Answer
Answer: a [Reason:] Transmission lines and conductors at microwave operation are usually made with gold metallization. To improve the adhesion of gold to the substrate, a thin layer of chromium or titanium may be deposited first since these metals are relatively lossy.
6. For the capacitors used in MMICs, the insulating dielectric films used are:
a) Air
b) SiO
c) Titanium
d) GaAs
Answer
Answer: b [Reason:] Capacitor and overlaying lines require insulating dielectric films, such as SiO, SiO2, SiN4 and Ta2O5. These materials have high dielectric constants and low loss and are compatible with integrated circuit processing.
7. Resistors used at normal operating frequencies can be directly used at microwave frequencies in MMIc.
a) True
b) False
Answer
Answer: b [Reason:] Resistors used at normal operating frequencies cannot be used directly in MMICs. Resistors require the deposition of the lossy films like NiCr, Ta, Ti, and doped GaAs commonly used.
8. Processing in MMICs is done by __________
a) Ion implantation
b) Net list generation
c) Floor planning
d) None of the mentioned
Answer
Answer: a [Reason:] Processing begins by forming an active layer in the semiconductor substrate for the necessary active devices. This is done by ion implantation or epitaxial techniques.
9. MMICs are the best microwave integrated circuit fabrication methodologies without any drawbacks in it.
a) True
b) False
Answer
Answer: b [Reason:] Major drawback of MMIC is that they tend to waste large area of relatively expensive semiconductor substrate for components such as transmission lines and hybrids.
10. MMICs have higher circuit flexibility as compared to other microwave integrated fabrication methods.
a) True
b) False
Answer
Answer: a [Reason:] Since the fabrication of additional FETs in an MMIC design is much easy, the circuit flexibility and performance can be enhanced with only little additional cost and the requirement for the fabrication of the entire device is prevented.
Set 2
1. There exists no difference between the construction of GaAs MESFET and silicon MOSFET except for the material used in their construction.
a) True
b) False
Answer
Answer: b [Reason:] There exists a difference between the construction of MESFET and MOSFET. There is a thin insulating layer of silicon dioxide between the gate contact and the channel region. Because the gate is insulated, it does not conduct DC bias current.
2. MOSFETs can provide a power of several hundred watts when the devices are packaged in:
a) Series
b) Parallel
c) Diagonal
d) None of the mentioned
Answer
Answer: b [Reason:] MOSFETs can be used at frequencies into the UHF range and can provide powers of several hundred watts when devices are packaged in parallel. Laterally diffused MOSFETs have direct grounding of the source and can operate at low microwave frequencies with high power.
3. High electron mobility transistors can be constructed with the use of single semiconductor material like GaAs that have high electron mobility.
a) True
b) False
Answer
Answer: b [Reason:] High electron mobility transistor is a hetero junction FET, meaning that it does not use a single semiconductor material, but instead is constructed with several layers of compound semiconductor materials.
4. The curve of IDS v/s VDS of an FET does not vary with the gate to source voltage applied.
a) True
b) False
Answer
Answer: b [Reason:] Curve of IDS v/s VDS of an FET varies with the gate to source voltage applied. As the gate to source voltage applied becomes more positive, the drain to source current goes on increasing for an applied constant gate to source voltage.
5. High-power circuits generally use higher values of:
a) Gate to source current
b) Drain to source current
c) Drain current
c) Gate to source voltage
Answer
Answer: c [Reason:] In order to achieve high drain current for high power applications, DC bias voltage must be applied to both gate and the drain, without disturbing the RF signal paths.
6. High drain current at RF levels is achieved with the biasing and decoupling circuitry for a dual polarity supply.
a) True
b) False
Answer
Answer: a [Reason:] High drain current at RF levels is achieved with the biasing and decoupling circuitry for a dual polarity supply. The RF chokes provide a very low DC resistance for biasing, and very high impedance at RF frequencies to isolate the signal from the bias supply.
7. Since multiple layers of semiconductor materials is used in high electron mobility transistors, this results in:
a) High gain
b) Power loss
c) Temperature sensitivity
d) Thermal stress
Answer
Answer: d [Reason:] The multiple layers in the high electron mobility transistor result in the thermal and mechanical stress in the layers. To avoid this, the layers usually have matched crystal lattice.
8. A major disadvantage of high electron mobility transistor is that:
a) They have low gain
b) High manufacturing cost
c) Temperature sensitive
d) High driving voltage is required
Answer
Answer: b [Reason:] High electron mobility transistors are devices containing multiple layers of different semiconductor materials. This complicated structure of HEMT requires sophisticated fabrication techniques leading to relatively high cost.
9. HEMT fabricated using GaN and aluminum gallium nitride on a silicon substrate can be used in :
a) High power transmitters
b) High power receivers
c) RADAR
d) Smart antennas
Answer
Answer: a [Reason:] GaN HEMT operate with drain voltages in the range of 20-40 V and can deliver power up to 100 W at frequencies in the low microwave range, making these devices popular for high power transmitters.
10. The scattering parameter S11 for GaN HELMT increases with increase in frequency of operation
a) True
b) False
Answer
Answer: b [Reason:] For GaN, the S11 parameter of the amplifier decreases with increase in frequency of operation. Experimental results have shown that S11 parameter was 0.96 at 0.5 GHz of frequency and 0.88 at 4 GHz of frequency.
Set 3
1. The noise power will determine the maximum detectable signal level for a receiver.
a) True
b) False
Answer
Answer: b [Reason:] The noise power will determine the minimum detectable signal level of the receiver for a given transmitter power, maximum range of a communication link. There is a limit on the maximum noise that can be associated with a signal in spite of which the signal can be recovered from the noise.
2. Equivalent noise temperature of a transmission line connecting the antenna to the receiver is:
a) TP (LP-1)
b) TP (LP + 1)
c) TP/ (LP-1)
d) TP / (LP + 1)
Answer
Answer: a [Reason:] The transmission line connecting the antenna to the receiver has a loss of LT and is at a physical temperature TP. its noise equivalent temperature is given by TP (LP-1).
3. In a receiver, if the noise figure of the mixer stage in the receiver is 7 dB, then the equivalent noise temperature is given that the receiver is operating at 290 K:
a) 1163 K
b) 1789 K
c) 1000 K
d) 1234 K
Answer
Answer: a [Reason:] Equivalent noise temperature for a given noise figure is given by To (FM-1). FM is the noise figure in dB. Substituting the given values for noise figure and temperature, noise equivalent temperature is 1163 K.
4. If a transmission line connecting the antennas to the receiver has a loss of 1.5 dB, given the physical temperature is 270C, noise equivalent temperature is:
a) 123 K
b) 145 K
c) 345 K
d) 234 K
Answer
Answer: a [Reason:] The noise equivalent temperature of the transmission line is given by TP(LP-1). Converting the value from dB scale and substituting, noise equivalent temperature is 123 K.
5. Given that the antenna efficiency is 0.9, equivalent brightness temperature is 200 K; physical temperature is 300 K, noise temperature of an antenna is:
a) 220 K
b) 210 K
c) 240 K
d) None of the mentioned
Answer
Answer: b [Reason:] Noise temperature of an antenna is given by rad Tb + (1- rad) TP. Tb is the equivalent brightness temperature and TP is the physical temperature. Substituting the given values, noise temperature of the antenna is 210 K.
6. If a receiver is operating at a bandwidth of 1 MHz and has antenna noise temperature of 210 K, then the input noise power is:
a) -90 dBm
b) -115 dBm
c) -56 dBm
d) -120 dBm
Answer
Answer: b [Reason:] Input noise power is given the expression kBTa) Here k is the Boltzmann’s constant, B is the operational bandwidth of the antenna and TA is the antenna noise temperature. Substituting in the above expression, input noise power is -115 dBm.
7. Antenna noise temperature of a system is 210 K, noise temperature of transmission line is 123 K, loss of a transmission line connecting the antenna to receiver is 1.41 and noise temperature of the receiver cascade is 304 K. then the total system noise temperature is:
a) 840 K
b) 762 K
c) 678 K
d) 1236 K
Answer
Answer: b [Reason:] The total system noise temperature is given by the expression TA+TTL+LTTREc) TA is the antenna noise temperature, TTL is the transmission line noise temperature, TREC is the noise temperature of receiver cascade. Substituting the given values, total system noise temperature is 762 K.
8. If the received power at antenna terminals is -80dBm, and if the input noise power is -115 dBm, then the input SNR is:
a) 45 dB
b) -195 dB
c) -35 dB
d) 35 dB
Answer
Answer: d [Reason:] Input SNR of a system is (Si-Ni) in dB. Substituting the given signal power and noise power in dB, input SNR of the system is 35 dB.
9. A receiver system is operating at a bandwidth of 1 MHz and has a total system noise temperature of 762 K. then the output noise power is:
a) -110 dBm
b) -234 dBm
c) -145 dBm
d) -124 dBm
Answer
Answer: a [Reason:] Output noise power of a receiver system is kBTsys. B is the operating bandwidth and Tsys is the total system noise temperature. Substituting the given values in the given equation, output noise power is -110 dBm.
10. If the received power at the antenna terminals is Si=-80 dBm and the output noise power is -110 dBm then the output signal to noise ratio is given by:
a) 30 dB
b) -30 dB
c) 35 dB
d) -35 dB
Answer
Answer: a [Reason:] Output signal to noise ratio in dB is given by (So-No). Substituting the given values in the above equation, the output SNR is 30 dB.
Set 4
1. ___________ is defined as the ratio of desired signal power to undesired noise power.
a) Signal to noise ratio
b) Noise to signal ratio
c) Noise figure
d) Noise temperature
Answer
Answer: a [Reason:] SNR is defined as the ratio of desired signal power to undesired noise power, and so is dependent on the signal power. When noise and a desired signal are applied to the input of a noise less network, both noise and signal will be attenuated or amplified by the same factor, so that the signal to noise ratio will be unchanged.
2. __________ is defined as the ratio of input signal to noise ratio to the output signal to noise ratio.
a) Noise figure
b) Noise temperature
c) SNRo
d) None of the mentioned
Answer
Answer: a [Reason:] Noise figure is defined as the ratio of input signal to noise ratio to the output signal to noise ratio of a system or a receiver. SNRi is the signal to noise ratio measured at the input terminals of the device. SNR0 is the output signal to noise ratio measured at the output terminals of the device.
3. The equivalent noise temperature of a network given the noise figure of the network or system is:
a) T0(F-1)
b) T0(F+1)
c) T0(F)
d) T0/F
Answer
Answer: a [Reason:] The equivalent noise temperature of a network given the noise figure of the network or system is given by T0(F-1). In this expression, F is the noise figure of the system. T0 has the value 290 K. T0 is the standard temperature considered.
4. Noise figure can be defined for any microwave network irrespective of any other constraints.
a) True
b) False
Answer
Answer: b [Reason:] Noise figure is defined only for a matched input source and for a noise source equivalent to a matched load at a temperature T0= 290 K. noise figure and noise temperature are interchangeable noise properties.
5. Expression for noise of a two port network considering the noise due to transmission line and other lossy components is:
a) GkTB + GNadded
b) GkTB
c) GNadded
d) None of the mentioned
Answer
Answer: a [Reason:] Expression for noise of a two port network considering the noise due to transmission line and other lossy components is GkTB + GNadded. Here, G is the gain of the system. Nadded is the noise generated by the transmission line, as if it appeared at the input terminals of the line.
6. Noise equivalent temperature of a transmission line that adds noise to the noise of a device is:
a) T (L-1)
b) T (L+1)
c) T (L)
d) T/L
Answer
Answer: a [Reason:] Noise equivalent temperature of a transmission line that adds noise to the noise of a device is given by T (L-1). Here L is the loss factor of the line and T is the temperature at which the system is thermal equilibrium.
7. If the noise figures of the first stage of a two stage cascade network is 8 dB and the noise figure of the second stage is 7 dB and the gain of the first stage is 10, then the noise figure of the cascade is:
a) 8. 6 dB
b) 7.6 dB
c) 5.6 dB
d) 8.9 dB
Answer
Answer: a [Reason:] Noise figure of a two stage cascade network is given by F1+ (F2-1)/G1. Here F1, F2 are the noise figure of the first and the second stage respectively. G1 is the gain of the first stage. Substituting the given values in the above equation, noise figure of the cascade is 8.6 dB.
8. Noise equivalent temperature of a 2 stage cascade network is given by:
a) Te1 + Te2/ G1
b) Te1 + Te1
c) Te1 / Te1
d) None of the mentioned
Answer
Answer: a [Reason:] Noise equivalent temperature of a 2 stage cascade network is given by Te1 + Te1/ G1. Here, Te1 is the noise equivalent temperature of stage 1 and Te1 is the noise equivalent temperature of stage 2. G1 is the gain of the first stage of the amplifier.
9. When a network is matched to its external circuitry, the gain of the two port network is given by:
a) │S21│2
b) │S22│2
c) │S12│2
d) │S11│2
Answer
Answer: a [Reason:] The gain of a two port network is given by the product of SS21 of the network and reflection co-efficient at the source end. But when the two port network is matched to the external circuitry, reflection coefficient becomes zero and gain reduces to │S21│2.
10. For a Wilkinson power divider of insertion loss L and the coupler is matched to the external circuitry, and then the gain of the coupler in terms of insertion loss is:
a) 2L
b) 1/2L
c) L
d) 1/L
Answer
Answer: b [Reason:] To evaluate the noise figure of the coupler, third port is terminated with known impedance. Then the coupler becomes a two port device. Since the coupler is matched, ГS=0 and Гout=S22=0. So the available gain is │S21│2. This is equal to 1/2L from the available data.
11. Noise equivalent temperature of Wilkinson coupler having a gain of 1/2L is given as:
a) T (2L-1)
b) T (2L+1)
c) T (2L*1)
d) T / (2L-1)
Answer
Answer: a [Reason:] Noise equivalent temperature of the Wilkinson coupler is found using the relation
T (1-G21)/G21. Substituting for G21 in the above expression, equivalent noise temperature is T (2L-1).
12. Expression for over all noise figure of a mismatched amplifier is:
a) 1+ (F-1)/ (1 -│Г│2)
b) 1
c) 1+ (F-1)
d) (F-1)/ (1 -│Г│2)
Answer
Answer: a [Reason:] The overall noise figure of a mismatched amplifier is given by 1+ (F-1)/ (1 -│Г│2). Here F is the noise figure of the amplifier, when there is an impedance mismatch at the input of the amplifier; this impedance mismatch is given by Г.
Set 5
1. The type of noise caused by vibration of bound charges is called:
a) Thermal noise
b) Shot noise
c) Flicker noise
d) None of the mentioned
Answer
Answer: a [Reason:] Thermal noise is the most basic type of noise, being caused by thermal vibrations of bound charges in a material. When an electron bound to atom gains energy and vibrates, thermal noise is produced. It is also called as Johnson noise.
2. ________ noise occurs due to the random fluctuation of charge in an electron tube.
a) Flicker noise
b) Shot noise
c) Thermal noise
d) White noise
Answer
Answer: b [Reason:] Shot noise or Poisson noise is a type of electronic noise that can be modeled by Poisson process. This type of noise occurs due to the discrete nature of electric charge.
3. Flicker noise occurs in solid-state components and vacuum tubes.
a) True
b) False
Answer
Answer: a [Reason:] Flicker noise is a form of noise that exhibits an inverse frequency power density curve. It has a pink noise power density spectrum. Since this noise is inversely proportional to the operating frequency, it is called 1/f noise.
4.________ noise is caused by random motion of charges in ionized gas.
a) Plasma noise
b) Quantum noise
c) Thermal noise
d) Flicker noise
Answer
Answer: a [Reason:] Plasma noise is caused by random motion of charges in an ionized gas such as a plasma, ionosphere or sparking electrical contacts. A material medium is required to produce this type of noise.
5. The most insignificant form of noise is:
a) Plasma noise
b) Quantum noise
c) Shot noise
d) Flicker noise
Answer
Answer: b [Reason:] Quantum originates due to the quantized nature of charge carriers and photons. This noise does not pose any problem in microwave circuits and also does not affect the signal strength. Hence they are often significant.
6. An X- band amplifier has a gain of 20 dB and a gain of 1GHz bandwidth. Noise figure of the amplifier is -62 dBm at 290 K and -64.7 dBm at 77 K. then the Y factor of the amplifier is :
a) 3 dB
b) 6.4 dB
c) 2.7 dB
d) 5.6 dB
Answer
Answer: c [Reason:] The Y factor of an amplifier is given by the difference in noise figure of the amplifier measured at two different temperatures. Taking the difference of the two values, the Y factor is 2.7 dB.
7. The Y factor of an amplifier obtained by measuring the noise figure at the temperatures 77 K and 290 K is 2.7 db. Then the equivalent noise temperature of the amplifier is:
a) 100 K
b) 150 K
c) 170 K
d) None of the mentioned
Answer
Answer: c [Reason:] The equivalent noise temperature of the amplifier is given by the relation (T1-YT2)/(Y-1). Substituting the given values in the above equation, equivalent noise temperature is 170 K.
8. An amplifier has a noise equivalent temperature of 170 K. If the amplifier is used with a source having an equivalent noise temperature of TS=450 K, the output noise power of the amplifier is:
a) -50 dBm
b) -60 dBm
c) -60.7 dBm
d) -55 dBm
Answer
Answer: c [Reason:] Output noise power of an amplifier is given by the expression GkTSB + GkTeB Substituting the given values in the above expression, the output noise power of the amplifier is -60.7 dBm.
9. The noise power associated with a two port network modeled as thevinin equivalent is:
a) kTB
b) k/TB
c) TB/ K
d) None of the mentioned
Answer
Answer: a [Reason:] Noise associated with a 2 port network modeled as thevinin equivalent is kTB) K is the Boltzmann constant, T is the temperature and B is the operating bandwidth of the circuit.
10. Excess noise ratio is defined as the ratio of generator noise to the noise associated with room temperature
a) True
b) False
Answer
Answer: b [Reason:] Excess noise ratio is defined as the ratio of the difference in noise power of the generator and noise power associated with the room temperature to the noise power associated with room temperature.
11. Equivalent noise temperature associated with an arbitrary white noise source is:
a) N0/GKB
b) N0
c) N0/ kB
d) None of the mentioned
Answer
Answer: a [Reason:] Equivalent noise temperature associated with an arbitrary white noise source is N0/GKb) Here N0 is the noise power delivered to the load resistor by the source, B is the operating bandwidth G is the gain of the circuit.