BEL Trainee Engineer Electronics Mock Test - 10 - Electronics and Communication Engineering (ECE) MCQ

• Excess 3 code is a 4-bit code.
• It can be derived from the BCD code by adding ‘3’ to each coded number. Thus, in excess 3 code 0011 is added to each BCD number.
• It is an un-weighted code.
• It is a self-complimenting code i.e. the 1’s complement of an excess three number is the excess 3 code for the 9’s complement of the corresponding decimal number.

Solution

Concept

For ; t > 0sec

• the following circuit is converted into its S domain equivalent 
• The circuit current is calculated I(s)
• The time-domain equivalent i(t) is calculated

Calculation

For t ≤ 0s, capacitor voltage V₀=1volt

Inductor current I₀= 0 Amp

From the S domain equivalent of the circuit 

We have the current

Can also be written in the form

Using inverse Laplace transform we have

⇒ i(t) = e^−t sin t u(t) Amp

Hence the correct answer is option 2

Concept:

• When an alternating magnetic field is applied to a magnetic material, an emf is induced in the material itself according to Faraday’s law of Electromagnetic induction.
• Since the magnetic material is a conducting material, these EMF’s circulates current within the body of the material. These circulating currents are called Eddy currents. They are produced when the conductor experiences a changing magnetic field.

Eddy current loss in the transformer is given by:

P_e = K_e B_m². t². f². V Watts

Where;

K - coefficient of eddy current. Its value depends upon the nature of magnetic material

Bm - Maximum value of flux density in Wb/m2

t - Thickness of lamination in meters

f - Frequency of reversal of the magnetic field in Hz

V - Volume of magnetic material in m3

Concept:

WBFM is given as:

Actual BW of WBFM is ∞ so to Bandlimit WBFM signal all of its significant lower-order sidebands retained and higher-order insignificant sidebands should be eliminated.

Calculation:

BW = 32 KHz

Given FM system has 8 sidebands (significant) i.e. 8 sidebands on the positive side of frequency & 8 sidebands on the negative side of the frequency

So, It has sidebands up to order ‘8’.

According to carson rule:

Significant sideband upto order of (β + 1) has B.W

BW = (β + 1) 2f­_m

So,

32 = 8 × 2 f_m

f_m = 2 kHz

The correct answer is option "4"
Concept:

The  power consumption is given by 

P =V² / R watts as the voltage across it will be constant.

where 

V is voltage in Volts

R is the resistance

Calculation:

R₁ = 2000 Ω

P₁ =  2 mW

P = V²/R₁

V² = PR₁ = 2 × 10^-3 × 2000 = 4 V

V = 2 V

for R₂ = 4000 Ω

P = V²/R₂

= 2² / 4000 = 1 mW

Concept:

The parallel RLC circuit in 's' domain is represented as:

Dividing numerator and denominator by RLC:

..............(i)

Calculation:

Given, Z(s) = .............(ii)

Comparing equations (i) and (ii), we get:

R = 10 Ω 

Maximum power transfer theorem:

Maximum power transfer theorem states that " In a linear bilateral network if the entire network is represented by its Thevenin's equivalent circuit then the maximum power transferred from source to the load when the load resistance is equal to the Thevenin's resistance.

1)  If source impedance is complex then load impedance has to be a complex conjugate of source impedance for maximum power transfer to occur.

2) Maximum efficiency is not related to maximum power transfer.

3) If the source and load impedances are matched then there are no reflections but maximum power transfer.

Proof: 

Let's consider variable resistive load and Thevenin's equivalent network as shown below,

Apply KVL, for the above circuit

V_S - I_L R_S - I_L R_L = 0

Where

V_S is the source or Thevenin's voltage, I_L is the load current, R_L is the load resistance, R_S is the source or Thevenin's resistance

V_S = I_L (R_S + RL)

I_L = V_s / (R_s + R_L)

P = I_L² R_L

For maximum power transfer, R_L = R_S

Then the maximum power transferred is given by

Properties of maximum power transfer theorem:

• This theorem is applicable only for linear networks i.e networks with R, L, C, transformer, and linear controlled sources as elements.
• The presence of dependent sources makes the network active and hence, MPPT is used for both active as well as passive networks.
• This theorem is applicable when the load is variable.

Calculation:

For the given circuit, maximum power transfers at R_L = R_s

The current at this condition is, 

The maximum value of current occurs at R_L­ = 0 and it is given by, 

Therefore, the current at maximum power is equal to 50% of the maximum current.

Radiated power(P) is a measure of how much power is radiated by an antenna when the antenna is connected to a transmitter.

Radiated resistance(R) is the component of  antenna resistance that accounts for the power radiated into space and is equal in ohms.

Power radiated by an antenna is calculated by using formula,

P= I²× R

P = (6 × 6) × 200

P = 36 × 200

P = 7200 W

More from LMS

For a coaxial cable, the propagation constant is expressed as:

This can be written as:

Γ = α(ω) + β(ω)

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, ω is the angular frequency = 2πf.

α(ω) and β(ω) are the attenuation and phase constants.

∴ The attenuation per unit length in a coaxial cable increases with an increase in frequency.

Concept:-

For SSB the output SNR is:-

Calculation:-

Given,

f_m = 10 kHz

Channel loss = 20 dB

∴ S_i = 10⁴ × 10^-9 × 2 × 10 × 10³

= 0.2 W

Required minimum signal strength at the receiver input

S_i = 0.2 W

Required minimum signal strength at the transmitter output:-

S_i × 20 dB = 0.2 × 10² = 20 W

Important note:-

For DBS-C, signal energy available in sideband = 

Then, required minimum output SNR

= 

Concept:

Resolution: It is the smallest increment an instrument can detect and display.

• The resolution for the 'N' bit digital voltmeter is given by:
  • 
• The range of an instrument is the sum of the smallest and the largest number it can display.

Explanation:

For 4­-digit, the smallest and the largest number that can be displayed is:

Range = 1

Resolution = 10^-4 = 0.0001

In percentage, the resolution is 0.01%

Impulse function is exist only at t= 0 and the area under impulse function is unity

|_t=0 = 1

|_t=0 = 1

Discrete time Laplace transform is Z- transform.

CONCEPT:

Gauss law:

• According to this law, the total flux linked with a closed surface called Gaussian surface is 1/ϵ_o times the charge enclosed by the closed surface.

• This law is applicable to all the closed surfaces.
• According to this law, the total flux linked with a closed surface called Gaussian surface is 1/ϵ_o times the charge enclosed by the closed surface.
• So this law is applicable to all the closed surfaces. Hence, option 2 is correct.

In satellite communication, uplink and downlink frequencies are kept different.

Uplink frequency > Downlink frequency

Hence, option C is correct.

Explanation:

• The transmission of a signal from the first earth station to satellite through a channel is called uplink​.
• The transmission of a signal from the satellite to the second earth station through a channel is called the downlink.
• Uplink frequency is the frequency at which, the first earth station is communicating with the satellite.
• The satellite transponder converts this signal into another frequency and sends it down to the second earth station.
• This frequency is called Downlink frequency. In a similar way, the second earth station can also communicate with the first one

In the atmosphere, the attenuation is directly proportional to frequency.

Since the satellite cannot afford to have high powers downlink is sent at lower frequencies, while the station at earth can afford to transmit at high frequencies.

The smaller the signal strength, the stronger it is hit by the quantization process. Companding is used to protect these small strength signals from quantization noise.

Companding:

• Companding, also known as Companded PCM, is a non-uniform quantization technique.
• It is implemented to improve the signal to quantization noise ratio of weak signals.
• The compression of the signal at the transmitter and expansion at the receiver is combined to be called companding. This is shown in the form of a block diagram:

• Companders like A-law and μ-law Companders are used in PCM to compress signals before input to the ADC in the transmitter stage and expand it after the input to DAC at the receiver.

The correct answer is option 4.

Concept:

    Mobile communication refers to the use of technology that enables us to communicate with people in various areas without the need for a physical connection (wires or cables). Our lives are made simpler by mobile communication, which saves us time and effort. Mobile communication technology is built with protocols, speed, and services.

• Multiplexing is used in mobile communication protocols to transfer data. Multiplexing is a technique for combining numerous digital or analog signals into a single signal that may be sent across a data channel.
• Mobile Communications Services refers to a variety of retail services that allow Subscribers to interact with others while on the go, such as voice communications, short messages, e-mail, voice mailbox access, and the Internet.
• Mobile Communication Implementation Techniques to Improve Last-Mile High-Speed FSO Communication. It Improvements can also be seen in speed. The network connectivity or the accessibility was much improved in accuracy and speed.

Hence the correct answer is All the above.

• A frame check sequence (FCS) refers to an error-detecting code added to a frame in a communications protocol

Layer 2 Frame Format:

Ethernet Frame Format consists of Cyclic Redundancy Check (CRC) in Frame check sequence Field used for error-detection, but CRC can also be used for error correction.

The principle of operation of fibre-optic cable is Reflection.

Key Points

• In an optical fiber, the information is passed through light, which must not escape outside of it.
• This phenomenon of confining the light inside the optical fiber is termed as Total internal reflection.
• For this, the construction and material used to ensure that the total internal reflection of light takes place to prevent the escape of it.

Principle:

• When light travels from a high refractive index medium to a low-refractive-index medium, it is refracted away from the normal as shown:

• At a certain angle θ_i, there is no refracted wave and the wave is totally internally reflected  (θ_r = 90°).
• This angle is called a critical angle.
• Inside an optical fiber, we have a high refractive index core (n₁) and low refractive index cladding (n₂).
• This results in the propagation of waves inside a fiber through total internal reflection phenomenon.

The correct answer is DRAM.

Key Points

• DRAM:
  • It is dynamic random access memory and is widely used as a computer's main memory. Hence, Option 2 is correct.
  • Each DRAM memory cell is made up of a transistor and a capacitor within an integrated circuit, and a data bit is stored in the capacitor.
• DDR-RAM:
  • It stands for Double Data Rate Synchronous Dynamic Random-Access Memory.
  • These are the computer memory that transfers the data twice as fast as regular chips like SDRAM chips because DDR memory can send and receive signals twice per clock cycle as a comparison.
• SRAM:
  • It stands for Static Random Access Memory.
  • It is a form of a semiconductor.
  • It is widely used in microprocessors, general computing applications, and electronic devices.
  • The SRAM is volatile in nature which means the data stored in it gets all wiped out once the power supply is cut.
  • SRAM is comprised of flip-flops. 
• PRAM:
  • In computer science, a parallel random-access machine is a shared-memory abstract machine.
  • As its name indicates, the PRAM is intended as the parallel-computing analogy to the random-access machine.

Straw in the wind: something believed to be a sign or warning of a future event.
Shot in the arm: something that has a sudden and positive effect on something, providing encouragement and new activity.
Stroke of luck: a fortunate occurrence that could not have been predicted or expected.
Plain sailing: smooth and easy progress in a process or activity.
Hence, the correct answer is option A. 

Computer stores data on specific memory depending on the importance of that data in processing, frequency of uses of that data, and volume of data being stored. There are two types of memory have been discussed Primary memory and secondary memory:

1) Hard- Disk:

• sometimes abbreviated as a hard drive, HD, or HDD.
• It is a non-volatile data storage device.
• Consists of a rigid platter made of aluminum alloy or glass and coated with magnetic material.
• The single platter is 10-20mm thick with separate read and write heads.
• It can store up to 2 Terabyte (TB) data.
• It is rectangular with a length of 3 to 6 inches and the same width with a thickness of 2 to 3 inches.

2) CD-ROM:

• Thin round plastic plate with one side shining that store the information.
• It is writable only once.
• It is economical.
• Used to take the backup of files.
• The files cannot be deleted accidentally or by a virus.
• It can store up to 650 Megabyte (MB) data or information.

3) DVD-ROM:

• It is a DVD based CD.
• Digital Versatile Disc-Read Only Memory is a digital optical disc data storage format invented and developed in 1995 and released in late 1996.
• The medium can store any kind of digital data.
• It can store up to 4 Gigabyte (GB) information

4) Arithmetic-logic unit (ALU)

• It is the part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands in computer instruction words.
• In some processors, the ALU is divided into two units, an arithmetic unit (AU) and a logic unit (LU).

Hence, the Device having the largest storage capacity is Hard Disk.

The correct answer is option 1 i.e Root Directory.

Key Points

• The root directory is the top-level directory of a file system.
  • The root directory contains all other folders in the drive or folder.
  • The default root directory in Windows is C:\.

Additional Information

• A database is a data structure that store organised information electronically in a computer system 
  • Microsoft Access is an application used for database purposes.
• A cell is the intersection of row and column.

The correct answer is Database Management System.Key Points

• The full form of the term 'DBMS' is Database Management System.
• A database is a systematic collection of data.
• They support electronic storage and manipulation of data. 
• Databases make data management easy. 
• Database Management System (DBMS) is a collection of programs which enables its users to access database, manipulate data, reporting and representation of data. It also helps to control access to the database. 
• Database Management Systems are not a new concept and as such, had been first implemented in the 1960s.

Additional Information

Important Abbreviations:

The correct answer is .xlsx.

• .xlsx is the default XML-based file format for Excel 2010 and Excel 2007.
• Tar, Zip, rar are archive formats.

Additional Information

• An archive format is the file format of an archive file.
• Some file types and file extensions are as follows.

The logic is :

1) BEH

2) KNR

3) GJM

4) TWZ

Hence, 'KNR' is the odd letter.

‘Knife’ is related to ‘Cut’, means we use knife for cutting.

Similarly,

We use pen for writing.

Hence, Write is the correct answer.

Explanation:

Which is basically similar to option c

Following the common explanation, we get “7 seats are there to the left of C”.
Hence, option D is correct.
Final Arrangement:

Common Explanation:
References:
1. Two people are sitting between A and Z.
2. C sits fourth to the right of Z.
3. The number of people sitting to the right of A is the same as the number of people sitting to the left of Z.
4. The number of people sitting between A and C is the same as the number of people sitting between X and C.
5. X is an immediate neighbour of D, who sits on one of the ends.
6. The number of people sitting between D and C is the same as the number of people sitting between C and Y.
7. At most 25 people are sitting in a row.
Inferences:
From reference 1, we get two possible cases.
From reference 7, case2 was eliminated.
Hence, Case 1 is the final arrangement.