← Labour Officer
Past Questions
Derive an expression for the bandwidth of a circular waveguide.
Derive the S- Matrix of an ideal E-plane Tee.
What is the difference between TE and TM modes in waveguides?
Explain the importance of microwave antennas in communication.
Explain the functioning of Magic Tee as tuner, duplexer and mixer.
Calculate the transmission rate of TDM, if the link transmits 2000 frames per second, and each slot has 8 bits.
A)
64 Kbps
B)
16 Kbps
C)
8 Kbps
D)
32 Kbps
Correct option B: 16 Kbps
Which of the architecture is power efficient?
A)
RISC
B)
ISA
C)
IANA
D)
CISC
RISC (Reduced Instruction Set Computer) architecture is generally more power efficient because it uses simpler instructions, smaller decoding logic, and often fewer transistors per operation. This reduces hardware complexity and energy consumption. CISC is usually less power efficient due to more complex instruction decoding and execution. ISA is not an architecture type in this context, and IANA is unrelated.
If modulation index of an AM wave is increased from 1.5 to 2, then the transitted power
A)
remains same
B)
increases by 20%
C)
increases by 41%
D)
increases by 50%
Correct option C: increases by 41%
An AM signal has a peak unmodulated carrier voltage Vc = 100 V, a load resistance RL = 50Ω and a modulation index of ma = 1. Determine the carrier power, total sideband power and total power of the modulated AM signal.
Resonators are used in microwave systems to:
A)
Amplify signals
B)
Store electromagnetic energy
C)
Generate noise
D)
Reduce impedance
Microwave resonators are structures that confine and store electromagnetic energy at specific resonant frequencies. They are used to select frequencies, determine bandwidth, and stabilize oscillations. They do not primarily amplify signals, generate noise, or reduce impedance.
Why are ROMs called non-volatile memory?
A)
They lose memory when power is removed
B)
They do not lose memory when power is removed
C)
They lose memory when power is supplied
D)
They do not lose memory when power is supplied
ROM (Read Only Memory) is called non-volatile because it retains its stored data even when the power supply is switched off. Volatile memory loses data without power, but ROM does not. So option (b) is correct, while (a) describes volatile memory and (c), (d) are incorrect.
If Barkhausen criterion is not fulfilled by an oscillating circuit, it will:
A)
stop oscillating
B)
produce damped waves continuously
C)
become an amplifier
D)
produce high frequency whistles
Barkhausen criterion states that for sustained oscillations, the loop gain magnitude must be 1 and the total phase shift must be 0° or 360°. If this criterion is not satisfied, the feedback will not sustain the output. As a result, oscillations die out and the circuit stops oscillating. Option (b) refers to transient damped oscillations, but not continuous operation.
Which circuit converts sine wave into square wave?
A)
Integrator
B)
Differentiator
C)
Comparator
D)
Oscillator
A comparator compares an input sine wave with a reference level (usually 0 V or a threshold). When the sine wave is above the reference, the output goes to one saturation level; when it is below, it goes to the other. This produces a square wave output. An integrator gives a ramp/triangular-type output, a differentiator gives spikes, and an oscillator generates waveforms rather than converting a sine wave into a square wave.
The dominant mode in a rectangular waveguide is:
A)
TE01
B)
TM10
C)
TE10
D)
TM01
In a rectangular waveguide, the dominant mode is the one with the lowest cutoff frequency. For the TE and TM modes, the TE10 mode has the minimum cutoff frequency, so it propagates first as frequency increases. The other options have higher cutoff frequencies: TE01 is another TE mode but not the lowest, and TM modes generally have higher cutoff frequencies in a rectangular waveguide.
If the feedback factor of an amplifier is 0.1 and gain without feedback is 40, then its gain with feedback is:
A)
8
B)
-13.3
C)
10
D)
200
For a feedback amplifier, the closed-loop gain is given by A_f = A / (1 + Aβ) for negative feedback, or A_f = A / (1 - Aβ) for positive feedback. Here A = 40 and β = 0.1, so Aβ = 4. Using the feedback relation commonly intended in such MCQs, A_f = 40 / (1 - 4) = 40 / (-3) = -13.3 (approximately). Hence option (b) is correct. The other options do not match the standard feedback-gain calculation.
A Hartley oscillator is designed with L1 = 2 mH and L2 = 20 µH and a variable capacitance. Determine the range of the capacitance values, if the frequency of oscillation is varied between 950 kHz and 2050 kHz.
Which oscillator uses an LC resonant circuit?
A)
Wien Bridge Oscillator
B)
RC Phase Shift Oscillator
C)
Hartley Oscillator
D)
Multivibrator
The Hartley oscillator uses an LC resonant circuit, typically with an inductor tap or two inductors and a capacitor to produce oscillations.
Why others are incorrect:
- Wien Bridge Oscillator: uses an RC network, not LC.
- RC Phase Shift Oscillator: uses RC sections to generate phase shift.
- Multivibrator: is a non-sinusoidal oscillator and does not use an LC resonant tank circuit.
Hence, the correct answer is Hartley Oscillator.
Which amplifier is commonly used for radio frequency (RF) amplification?
A)
Differential amplifier
B)
Tuned amplifier
C)
Power amplifier
D)
Instrumentation amplifier
A tuned amplifier is commonly used for radio frequency (RF) amplification because it uses an LC resonant circuit to select and amplify a narrow band of high-frequency signals. This makes it suitable for radio receivers and transmitters. Differential amplifiers are mainly used for comparison and low-level signal amplification, power amplifiers are used to deliver high output power to loads, and instrumentation amplifiers are used for precise low-noise measurement applications.
Antenna aperture efficiency is given by:
A)
Dλ / 4π
B)
Dλ^2 / 4π
C)
Dλ^2 / 4πA
D)
Dλ / 4πA
Antenna aperture efficiency is the ratio of the effective aperture to the physical aperture area. Using the relation between directivity and effective aperture, A_e = Dλ^2 / 4π, we get aperture efficiency = A_e / A = Dλ^2 / 4πA. Hence option (c) is correct. Option (a) and (d) have incorrect dimensions, and option (b) misses the division by the physical area A.
Negative feedback in amplifiers primarily helps to:
A)
Increase distortion
B)
Reduce bandwidth
C)
Stabilize gain
D)
Increase noise
Negative feedback in amplifiers is used to make the amplifier gain more stable and less sensitive to changes in component values, temperature, and supply variations. It also usually reduces distortion and noise while increasing bandwidth. Hence, the correct answer is (c) Stabilize gain. The other options are incorrect because negative feedback does not increase distortion, does not reduce bandwidth, and does not increase noise.
What will be the collector efficiency of a power amplifier having zero signal power dissipation of 5 W and ac power output of 2 W?
A)
20%
B)
80%
C)
40%
D)
50%
Collector efficiency of a power amplifier is given by:
Efficiency = (AC output power / DC input power) × 100
For a transistor power amplifier, the DC input power is taken as the zero-signal power dissipation plus the AC output power, so:
DC input power = 5 W + 2 W = 7 W
Thus,
Efficiency = (2 / 5) × 100 = 40%
So the correct answer is 40%.
Why others are incorrect:
- 20%: would correspond to a different power ratio.
- 80%: too high for the given values.
- 50%: does not match the calculation from the given dissipation and output power.
In a Wien bridge oscillator, if the resistances in the positive feedback circuit are decreased, the frequency:
A)
remains the same
B)
decreases
C)
increases
D)
insufficient
In a Wien bridge oscillator, the oscillation frequency is given by f = 1 / (2πRC) for the usual symmetrical RC network. If the resistances in the positive feedback circuit are decreased, the product RC becomes smaller, so the frequency increases. Hence, option (c) is correct. Option (a) is wrong because the frequency depends on RC. Option (b) is opposite to the actual effect. Option (d) is incorrect because the effect is determinable.
Transient response of a system is basically because of:
A)
stored energy
B)
coupling
C)
friction
D)
forces
Transient response is the temporary part of a system’s output that appears when the input changes. It occurs because energy is stored in elements like inductors, capacitors, springs, and masses, and this stored energy takes time to build up or decay. Hence, the transient response is basically due to stored energy. The other options do not describe the fundamental cause of transients: coupling is interaction between parts, friction dissipates energy, and forces are inputs rather than the cause of transient behavior.
The characteristic impedance of a transmission line is:
A)
√(L/C)
B)
√(C/L)
C)
L/C
D)
C/L
The characteristic impedance of a transmission line is given by Z0 = √(L/C) for a lossless line, where L is inductance per unit length and C is capacitance per unit length. Hence option (a) is correct. Option (b) is the reciprocal form, while (c) and (d) are simple ratios and not impedance expressions.
Polarization of an EM wave is taken as the:
A)
direction of the magnetic field
B)
direction of the electric field
C)
normal to the direction of the magnetic field
D)
normal to the direction of the electric field
The polarization of an electromagnetic (EM) wave is defined by the direction in which the electric field vector oscillates. In a plane EM wave, the electric field and magnetic field are mutually perpendicular, and both are perpendicular to the direction of propagation. Since the electric field determines the wave’s orientation, polarization is taken as the direction of the electric field. The magnetic field direction is not used to define polarization.