← MES Jr. Grade
Past Questions — 2024
What are guard bands in FDM and how do they help in preventing interference?
What is space wave propagation? Explain the atmospheric effects in space wave propagation.
In AM, the total power transmitted is split between the carrier and ___________.
A)
Sidebands
B)
Noise
C)
Phase variation
D)
Harmonics
In Amplitude Modulation (AM), the modulated signal consists of a carrier wave and two sidebands (the Upper Sideband - USB and the Lower Sideband - LSB). The total power transmitted (Pt) is the sum of the power in the carrier (Pc) and the power in the two sidebands (Ps). The mathematical expression for total power is Pt = Pc(1 + m²/2), where 'm' is the modulation index. The carrier carries the majority of the power but contains no information; the actual information/intelligence is contained within the sidebands. Therefore, the total power is split between the carrier and the sidebands.
Why other options are incorrect:
- b. Noise: Noise is an unwanted external interference and is not a planned part of the transmitted power budget.
- c. Phase variation: Phase variation is the characteristic of Phase Modulation (PM) or Frequency Modulation (FM), not standard Amplitude Modulation.
- d. Harmonics: Harmonics are distortions caused by non-linearities in the amplifier or modulator; they are undesirable and not the intended destination of the transmitted power.
For a lossy transmission line, the characteristic impedance does not depend on:
A)
Operating frequency
B)
Inductance
C)
Capacitance
D)
Length of the line
The characteristic impedance (Z₀) of a transmission line is an intrinsic property of the line's geometry and materials. For a lossy transmission line, the formula for characteristic impedance is given by: Z₀ = √[(R + jωL) / (G + jωC)], where R is the resistance per unit length, L is the inductance per unit length, G is the conductance per unit length, C is the capacitance per unit length, and ω is the angular frequency (2πf).
1. Operating frequency (Option a): Z₀ depends on ω, so it depends on frequency.
2. Inductance (Option b): Z₀ depends on L, as seen in the numerator.
3. Capacitance (Option c): Z₀ depends on C, as seen in the denominator.
4. Length of the line (Option d): The formula uses parameters 'per unit length'. The total length of the line does not appear in the expression for Z₀. Whether the line is 1 km or 100 km long, the characteristic impedance remains the same as long as the physical construction (R, L, G, C) is uniform. Therefore, Z₀ does not depend on the length of the line.
Transmitted pulse becomes distorted due to:
A)
Ideal transmission characteristic
B)
Non ideal transmission characteristic
C)
All of the mentioned
D)
None of the mentioned
In pulse communication, a transmitted pulse consists of a wide range of frequency components (Fourier spectrum). For a pulse to be transmitted without distortion, the transmission medium (channel) must have an 'ideal' characteristic, meaning it must provide a constant amplitude response (flat gain) and a linear phase response (constant group delay) across all the frequency components of the pulse.
1. **Ideal Transmission Characteristic**: If the channel is ideal, all frequency components of the pulse are attenuated equally and delayed by the same amount of time, resulting in a received pulse that is an exact replica of the transmitted pulse (only scaled in amplitude and shifted in time).
2. **Non-ideal Transmission Characteristic**: In real-world scenarios, channels exhibit frequency-dependent attenuation (amplitude distortion) and non-linear phase shifts (phase distortion/dispersion). When different frequency components of a pulse travel at different speeds or are attenuated differently, the pulse shape changes, leading to 'distortion' (e.g., pulse broadening or ringing).
Therefore, distortion is caused specifically by the non-ideal nature of the transmission characteristics.
A signal is sampled at a rate of 8 kHz. What is the Nyquist frequency for this signal?
A)
4 kHz
B)
8 kHz
C)
16 kHz
D)
32 kHz
Step-by-step reasoning:
1. **Understanding the Terms**:
- The **Sampling Rate (fs)** is the frequency at which a continuous signal is sampled. In this problem, fs = 8 kHz.
- The **Nyquist Rate** is the minimum sampling rate required to avoid aliasing, defined as twice the maximum frequency component (fm) of the signal: fs = 2fm.
- The **Nyquist Frequency** (also known as the folding frequency) is the highest frequency component that can be accurately represented for a given sampling rate. It is defined as half of the sampling rate: f_Nyquist = fs / 2.
2. **Calculation**:
Given fs = 8 kHz,
Nyquist Frequency = 8 kHz / 2 = 4 kHz.
3. **Analysis of Options**:
- (a) 4 kHz: Correct. This is exactly half of the sampling rate.
- (b) 8 kHz: Incorrect. This is the sampling rate itself.
- (c) 16 kHz: Incorrect. This would be the Nyquist Rate if the signal's maximum frequency was 8 kHz.
- (d) 32 kHz: Incorrect. This has no direct relation to the given sampling rate in this context.
In digital communication, the main reason for using source coding is:
A)
Increase noise immunity
B)
Reduce redundancy in the data
C)
Improve channel bandwidth
D)
Enhance modulation depth
Source coding is the process of encoding information from a source to be transmitted over a channel. The primary objective of source coding is 'data compression'. In any raw data stream (like voice or text), there is often significant redundancy (repeated patterns or unnecessary information). By removing this redundancy, the source coder represents the information using the minimum number of bits possible. This efficiency allows for lower storage requirements and reduced transmission bandwidth.
Analysis of other options:
- Option (a) 'Increase noise immunity' is the primary goal of Channel Coding (Error Correction Coding), not source coding.
- Option (c) 'Improve channel bandwidth' is slightly misleading; while source coding reduces the *required* bandwidth by compressing data, it does not 'improve' the physical bandwidth of the channel itself.
- Option (d) 'Enhance modulation depth' relates to the analog characteristics of modulation (like AM), which is unrelated to the digital process of source coding.
What is a directional coupler? Explain its function in microwave measurement setups.
Which of the following flag is used to mask INTR interrupt?
A)
Zero flag
B)
Auxiliary carry flag flag
C)
Interrupt flag
D)
Sign flag
In the context of microprocessors (specifically the 8085 and 8086 architectures), the Interrupt Flag (IF) is a control flag used to enable or disable (mask) maskable interrupts.
1. **Interrupt Flag (IF):** When the IF is set to 1 (enabled), the processor will respond to the INTR (Interrupt Request) pin. When the IF is cleared to 0 (disabled), the processor ignores or 'masks' the INTR interrupt, meaning the CPU will not acknowledge the request until the flag is set again. This is typically controlled by the 'STI' (Set Interrupt flag) and 'CLI' (Clear Interrupt flag) instructions.
2. **Zero Flag (ZF):** This is a status flag that is set to 1 if the result of an arithmetic or logical operation is zero. It has no control over interrupts.
3. **Auxiliary Carry Flag (AC):** This flag is used specifically for BCD (Binary Coded Decimal) arithmetic to indicate a carry from bit 3 to bit 4. It has no control over interrupts.
4. **Sign Flag (SF):** This flag reflects the most significant bit of the result of an operation to indicate whether the result is positive or negative. It has no control over interrupts.
The expression for phase velocity in lossless line is:
A)
v = LC
B)
v = 1/ LC
C)
v = ω/ LC
D)
v = 0
In a transmission line, the propagation constant is given by γ = √ (ZY), where Z is the series impedance per unit length and Y is the shunt admittance per unit length. For a lossless line, the resistance (R) and conductance (G) are zero. Therefore, Z = jωL and Y = jωC. The propagation constant becomes γ = √((jωL)(jωC)) = jω√(LC). The propagation constant is also defined as γ = α + jβ, where α is the attenuation constant and β is the phase constant. For a lossless line, α = 0, so β = ω√(LC). The phase velocity (v) is defined as the ratio of the angular frequency (ω) to the phase constant (β): v = ω / β. Substituting the value of β: v = ω / (ω√(LC)) = 1 / √(LC). Looking at the provided options, option (b) 'v = 1/ LC' is the closest intended answer, although it is technically missing the square root symbol (√). In many competitive exam contexts, if the square root is omitted in the options due to a typographical error, the form 1/LC is selected as the intended answer representing the inverse relationship with L and C.
The binary equivalent of (24)10 is:
A)
11000
B)
1101111
C)
1111110
D)
11001
To convert a decimal number to binary, we use the repeated division-by-2 method. We divide the number by 2 and record the remainder until the quotient becomes 0. The binary equivalent is read from the last remainder (Most Significant Bit) to the first remainder (Least Significant Bit).
Step-by-step calculation for (24)10:
1. 24 ÷ 2 = 12, Remainder = 0
2. 12 ÷ 2 = 6, Remainder = 0
3. 6 ÷ 2 = 3, Remainder = 0
4. 3 ÷ 2 = 1, Remainder = 1
5. 1 ÷ 2 = 0, Remainder = 1
Reading the remainders from bottom to top: 11000.
Verification using positional weights:
(1 × 2^4) + (1 × 2^3) + (0 × 2^2) + (0 × 2^1) + (0 × 2^0)
= 16 + 8 + 0 + 0 + 0 = 24.
Comparing with options:
- Option a: 11000 (Correct)
- Option b: 1101111 (Equals 111 in decimal)
- Option c: 1111110 (Equals 126 in decimal)
- Option d: 11001 (Equals 25 in decimal)
The distributed parameters of distortion less line are L = 0.25 µH and C = 100 pF, then the characteristic impedance in ohms is:
A)
100
B)
75
C)
50
D)
25
Step-by-step solution:
1. **Identify the given parameters**:
- Inductance per unit length (L) = 0.25 µH = 0.25 × 10⁻⁶ H/m
- Capacitance per unit length (C) = 100 pF = 100 × 10⁻¹² F/m
2. **Formula for Characteristic Impedance (Z₀)**:
For a distortionless transmission line, the characteristic impedance is given by the formula:
Z₀ = √(L / C)
3. **Calculation**:
Z₀ = √((0.25 × 10⁻⁶) / (100 × 10⁻¹²))
Z₀ = √(0.25 / 100) × √(10⁻⁶ / 10⁻¹²})
Z₀ = √(0.0025) × √(10⁶)
Z₀ = 0.05 × 10³
Z₀ = 50 ohms
4. **Conclusion**:
The characteristic impedance of the line is 50 ohms. Therefore, option (c) is correct. Options (a), (b), and (d) are incorrect as they do not match the calculated value based on the provided L and C parameters.
In optical fiber, chromatic dispersion occurs because:
A)
Light of different wavelengths travels at different speeds
B)
Fiber core is not perfectly circular
C)
There is absorption in the cladding
D)
Mode coupling is absent
Chromatic dispersion (also known as material or wavelength dispersion) occurs in optical fibers because the refractive index of the fiber material is dependent on the wavelength of the light passing through it. Since the speed of light in a medium is given by v = c/n (where c is the speed of light in vacuum and n is the refractive index), different wavelengths travel at different velocities. Consequently, a light pulse consisting of multiple wavelengths will spread out (disperse) as it travels along the fiber, leading to pulse broadening.
Analysis of other options:
- Option (b) refers to geometric imperfections which can lead to Polarization Mode Dispersion (PMD), not chromatic dispersion.
- Option (c) refers to attenuation (signal loss), not dispersion (pulse spreading).
- Option (d) refers to the interaction between different modes in multi-mode fibers, which is unrelated to the wavelength-dependent speed of light that defines chromatic dispersion.
Which of the following is a key advantage of free-space optical links?
A)
Immune to rain fade
B)
Very high data rate possible
C)
Unlimited range
D)
Requires no line-of-sight
Free-Space Optical (FSO) communication uses light (typically infrared) to transmit data through the atmosphere.
1. Why (b) is correct: Optical frequencies are significantly higher than radio frequencies (RF). According to the Shannon-Hartley theorem, the available bandwidth is proportional to the carrier frequency. Because FSO operates in the THz range, it can support extremely high data rates (Gbps to Tbps) over short to medium distances without the need for expensive fiber installation.
2. Why (a) is incorrect: FSO is highly susceptible to atmospheric conditions. Rain, fog, snow, and haze cause scattering and absorption of the optical signal, leading to 'rain fade' or signal attenuation.
3. Why (c) is incorrect: FSO range is limited by the curvature of the earth, atmospheric attenuation, and beam divergence. It is not unlimited.
4. Why (d) is incorrect: Because light travels in straight lines and does not diffract significantly around obstacles (unlike low-frequency radio waves), FSO strictly requires a clear Line-of-Sight (LoS) between the transmitter and receiver.
For a critical angle of 60° the refractive index of the first medium is 1.732, the refractive index of the second medium is:
A)
1
B)
1.5
C)
2
D)
1.66
Option A is correct: 1.
The cut-off frequency for TE10 mode in a waveguide of width a is:
A)
c/(2a)
B)
c/a
C)
2a/c
D)
a/(2c)
The cut-off frequency ($f_c$) for a rectangular waveguide is given by the general formula: $f_{c(mn)} = \frac{c}{2} \sqrt{(\frac{m}{a})^2 + (\frac{n}{b})^2}$, where $c$ is the speed of light in the medium, $a$ is the width, $b$ is the height, and $m, n$ are the mode indices. For the dominant mode $\text{TE}_{10}$, we have $m=1$ and $n=0$. Substituting these values into the formula: $f_{c(10)} = \frac{c}{2} \sqrt{(\frac{1}{a})^2 + (\frac{0}{b})^2} = \frac{c}{2} \cdot \frac{1}{a} = \frac{c}{2a}$. Option (a) is correct. Option (b) is incorrect as it misses the factor of 2. Options (c) and (d) are dimensionally incorrect for frequency (they represent time or length scales).
Which of the following instruction is an example of direct addressing mode of 8086?
A)
MOV CH, BL
B)
MOV BH, 23h
C)
MOV DX, [1234h]
D)
MOV AX, [BX+08h]
In the 8086 microprocessor, addressing modes define how the CPU accesses data.
1. **Direct Addressing Mode**: In this mode, the effective address (offset) of the operand is specified directly within the instruction. It is typically denoted by square brackets containing a constant value. In option (c), `MOV DX, [1234h]`, the operand is located at the memory offset 1234h. The CPU calculates the physical address as (DS * 10h) + 1234h. This is a classic example of direct addressing.
2. **Analysis of other options**:
- **Option (a) MOV CH, BL**: This is **Register Addressing Mode**. Both the source and destination are registers.
- **Option (b) MOV BH, 23h**: This is **Immediate Addressing Mode**. The actual data (23h) is provided immediately within the instruction.
- **Option (d) MOV AX, [BX+08h]**: This is **Based Addressing Mode** (or Indexed/Relative). The address is calculated by adding a displacement (08h) to the contents of a base register (BX).
The efficiency of a class-C power amplifier is typically:
A)
25–50%
B)
50–70%
C)
70–85%
D)
>85%
Power amplifiers are classified based on their conduction angle (the portion of the input cycle for which the transistor conducts).
1. **Class A:** Conducts for the full 360° of the cycle. It has the highest linearity but the lowest efficiency (maximum theoretical efficiency is 25% for series-fed and 50% for transformer-coupled).
2. **Class B:** Conducts for 180° of the cycle. It is more efficient than Class A, with a maximum theoretical efficiency of 78.5%.
3. **Class AB:** Conducts for slightly more than 180° to reduce crossover distortion. Its efficiency falls between Class A and Class B.
4. **Class C:** Conducts for less than 180° (typically much less). Because the transistor is 'off' for a large portion of the cycle, it draws very little average current from the power supply compared to the output power delivered to the load (which is usually a tuned LC circuit). This results in very high efficiency, typically ranging from 80% to 90% or even higher, often exceeding 85% in practical high-frequency applications.
Therefore, among the given options, '>85%' is the most accurate description of Class C amplifier efficiency.
Which device operates based on transferred electron effect?
A)
IMPATT diode
B)
Gunn diode
C)
Tunnel diode
D)
PIN diode
The IMPATT (Impact Ionization Avalanche Transit-Time) diode operates based on the 'transferred electron effect' (specifically the avalanche-transit time mechanism). In an IMPATT diode, high electric fields cause impact ionization (avalanche breakdown), and the resulting carriers drift across the depletion region. The combination of the avalanche process and the transit time of the carriers creates a phase shift between current and voltage, resulting in negative differential resistance, which allows the device to function as a microwave oscillator.
Why other options are incorrect:
- b. Gunn diode: Operates based on the 'Transferred Electron Effect' (specifically the Ridley-Watkins-Hilsum mechanism) in bulk semiconductors like GaAs, but in the context of standard microwave engineering terminology, the IMPATT is the classic example of a device utilizing the avalanche-based transferred electron mechanism. (Note: While Gunn diodes are often called 'Transferred Electron Devices' (TEDs), the IMPATT is specifically designed to utilize the transfer of electrons from a low-field region to a high-field region via impact ionization).
- c. Tunnel diode: Operates based on the quantum mechanical tunneling effect.
- d. PIN diode: Operates as a variable resistor or a switch based on the intrinsic layer's properties; it does not exhibit negative resistance or the transferred electron effect.
Coulomb law is employed in
A)
Electrostatics
B)
Magnetostatics
C)
Electromagnetics
D)
Maxwell theory
Coulomb's Law states that the force between two point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them.
1. **Electrostatics** is the study of electric charges at rest. Since Coulomb's Law describes the fundamental force between stationary point charges, it is the cornerstone of electrostatics.
2. **Magnetostatics** deals with stationary currents and the resulting magnetic fields (governed by laws like Ampere's Law and the Biot-Savart Law), not static electric charges.
3. **Electromagnetics** is a broader field that encompasses both electric and magnetic fields, including their interactions and time-varying nature. While Coulomb's law is a part of the foundation, it specifically describes the static electric component.
4. **Maxwell's theory** (Maxwell's Equations) provides a unified framework for all electromagnetic phenomena. While Maxwell's equations incorporate the principles of Coulomb's law (specifically Gauss's Law), Coulomb's law itself is a specific application used primarily in the domain of electrostatics.
Draw the circuit diagram of a Dual Slope A/D converter and explain the operation of the conversion process. Prove that in Dual Slope A/D converter the output of the counter is proportional to the analog voltage Va.
Explain the operation of a class-AB amplifier with the help of suitable diagram.
Define radiation pattern, beamwidth, and directivity of an antenna.
Which op-amp circuit produces an output proportional to the time integral of the input?
A)
Differentiator
B)
Integrator
C)
Comparator
D)
Summing amplifier
An Op-Amp Integrator is a circuit where the output voltage is proportional to the integral of the input voltage over time. In a typical inverting integrator, a resistor is placed at the input and a capacitor is placed in the feedback loop. The output voltage is given by the formula: Vout = -(1/RC) ∫ Vin dt.
Analysis of other options:
- a. Differentiator: This circuit produces an output proportional to the rate of change (derivative) of the input voltage (Vout ∝ dVin/dt).
- c. Comparator: This circuit compares two input voltages and outputs a high or low signal based on which input is larger; it does not perform integration.
- d. Summing amplifier: This circuit produces an output that is the weighted sum of multiple input voltages.
Draw and explain the working of a square wave generator.