Antennas And Microwave Engineering Set 7

Electrical Engineering

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This set of Antennas and Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on Antennas And Microwave Engineering Set 7

Q1 | Colpitts oscillator operating at 50 MHz has an inductor in the feedback section of value 0.10µH. then the values of the capacitors in the feedback section is:
  • 100 pf, 100 pf
  • 100 pf, 50 pf
  • 70 pf, 130 pf
  • 80 pf, 60 pf
Answer: 100 pf, 100 pf
Explaination:
the equivalent value of series combination of the capacitors is given by 1/ ω2l. this gives the equivalent capacitance value of 200 pf. c1c2/ (c1+c2) =200 pf. c1
Q2 | An inductor is operating at frequency of 50 MHz. Its inductance is 0.1 µH, and then the series resistance associated with the inductor is: (Qo=100)
  • 0.31 Ω
  • 1.32 Ω
  • 1 Ω
  • 1.561 Ω
Answer: 0.31 Ω
Explaination:
series resistance associated with an inductor is given by ωl/qₒ.
Q3 | Hartley oscillator has inductance values of 12 mH and 4 mH in the feedback section and a capacitor of 4 nF. Then the resonant frequency of the circuit is:
  • 19.89 khz
  • 25 khz
  • 45 khz
  • 12 khz
Answer: 19.89 khz
Explaination:
resonant frequency of hartley oscillator is given by 1/ 2π√(c1 (l1 + l2)). substituting the given values in the above equation, cut-off frequency is 19.89 khz.
Q4 | Colpitts oscillator in the feedback section has an inductance of 4 mH and capacitors of 12 nH and 4 nH. Then the resonant frequency of Colpitts oscillator is:
  • 50.4 khz
  • 35.1 khz
  • 45.9 khz
  • none of the mentioned
Answer: 45.9 khz
Explaination:
resonant frequency of colpitts oscillator is given by 1/2π√lcₒ, where c0 is the equivalent capacitance given by c1c2/ (c1+c2). substituting and solving the equation, resonant frequency is 45.9 khz.
Q5 | For Colpitts oscillator, the capacitors C1 and C2 in the feedback network are 1 µF and 25 µF respectively. Then the β value of the transistor is:
  • 35
  • 000.76
  • 25
  • 0.0025
Answer: 25
Explaination:
β for a transistor is defined as the ratio of transconductance of the transistor to the input admittance, which is equal to the ratio of c2/c1. substituting the given values, β of the transistor is 25.
Q6 | Amplifier efficiency is the ratio of RF output power to DC input power. This parameter determines the performance of an amplifier.
  • true
  • false
Answer: true
Explaination:
power amplifier is the primary consumer of dc power in most hand-held wireless devices, so amplifier efficiency is an important consideration. amplifier efficiency is the ratio of rf output power to dc input power.
Q7 | Behavior of a transistor in power amplifiers is unpredictable at all input signal levels.
  • true
  • false
Answer: false
Explaination:
a transistor behaves linearly for signal powers below 1db compression point and so, the small –signal scattering
Q8 |                        is defined as the ratio of desired signal power to undesired noise power.
  • signal to noise ratio
  • noise to signal ratio
  • noise figure
  • noise temperature
Answer: signal to noise ratio
Explaination:
snr is defined as the ratio of desired signal power to undesired noise
Q9 |                      is defined as the ratio of input signal to noise ratio to the output signal to noise ratio.
  • noise figure
  • noise temperature
  • snro
  • none of the mentioned
Answer: noise figure
Explaination:
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.
Q10 | The equivalent noise temperature of a network given the noise figure of the network or system is:
  • t0(f-1)
  • t0(f+1)
  • t0(f)
  • t0/f
Answer: t0(f-1)
Explaination:
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.
Q11 | Noise figure can be defined for any microwave network irrespective of any other constraints.
  • true
  • false
  • topic 5.6 low noise amplifier design
Answer: false
Explaination:
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.
Q12 | Expression for noise of a two port network considering the noise due to transmission line and other lossy components is:
  • gktb + gnadded
  • gktb
  • gnadded
  • none of the mentioned
Answer: gktb + gnadded
Explaination:
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.
Q13 | Noise equivalent temperature of a 2 stage cascade network is given by:
  • te1 + te2/ g1
  • te1 + te1
  • te1 / te1
  • none of the mentioned
Answer: te1 + te2/ g1
Explaination:
noise equivalent temperature of a 2 stage cascade network is given by te1
Q14 | 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:
  • 2l
  • 1/2l
  • l
  • 1/l
Answer: 1/2l
Explaination:
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.
Q15 | Noise equivalent temperature of Wilkinson coupler having a gain of 1/2L is given as:
  • t (2l-1)
  • t (2l+1)
  • t (2l*1)
  • t / (2l-1)
Answer: t (2l-1)
Explaination:
noise equivalent temperature of the wilkinson coupler is found using the relation
Q16 | Expression for over all noise figure of a mismatched amplifier is:
  • 1+ (f-1)/ (1 -│Г│2)
  • 1
  • 1+ (f-1)
  • (f-1)/ (1 -│Г│2)
Answer: 1+ (f-1)/ (1 -│Г│2)
Explaination:
the overall noise figure of a
Q17 | One condition to be satisfied in an oscillator circuit so that stable oscillations are produced is:
  • positive feedback is to be achieved
  • negative feedback is to be achieved
  • 1800 phase shift is required between the transistor input and output.
  • none of the mentioned
Answer: 1800 phase shift is required between the transistor input and output.
Explaination:
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.
Q18 | In an oscillator, the resonant feedback circuit must have must have a low Q in order to achieve stable oscillation.
  • true
  • false
Answer: false
Explaination:
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.
Q19 | Quartz crystal and tourmaline used in oscillators work on the principle of:
  • photo electric effect
  • piezo electric effect
  • raman effect
  • black body radiation
Answer: piezo electric effect
Explaination:
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.
Q20 | A quartz crystals equivalent circuit is a series LCR circuit and has a series resonant frequency.
  • true
  • false
Answer: false
Explaination:
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.
Q21 | In the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is:
  • capacitive
  • inductive
  • both capacitive and inductive
  • none of the mentioned
Answer: inductive
Explaination:
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.
Q22 | 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:
  • 0.25 mhz
  • 2.5 mhz
  • 25 mhz
  • 5 mhz
Answer: 0.25 mhz
Explaination:
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.
Q23 | Parallel resonant frequency of quartz crystal is given by:
  • 1/ √(lcₒc/(cₒ+c))
  • 1/√lc
  • 1/√lcₒ
  • 1/ √(l(cₒ+c) )
Answer: 1/ √(lcₒc/(cₒ+c))
Explaination:
parallel resonant frequency of an oscillator is given by√(lcₒc/(cₒ+c)).
Q24 | 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:
  • 3 mhz
  • 0.3 mhz
  • 6 mhz
  • 9 mhz
Answer: 3 mhz
Explaination:
the parallel resonant frequency of a crystal oscillator is given by 1/
Q25 | In microwave oscillators, negative resistance transistors and diodes are used in order to generate oscillations in the circuit.
  • true
  • false
Answer: true
Explaination:
in microwave oscillator, for a current to flow in the circuit the negative impedance of the device must be matched with positive impedance. this results in current being non-zero and generates oscillation.
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