BEL Trainee Engineer Electronics Mock Test - 8 - Electronics and Communication Engineering (ECE) MCQ

Tunnel Diode:

• Symbol:

• It is a highly doped PN Junction diode, used for low voltage high-frequency switching applications.
• It works on the tunneling principle.
• It has a high charge carrier velocity.
• When compared to a normal p-n junction diode, the tunnel diode has a narrow depletion width.
• In normal forward-biased operation, it exhibits the “Negative resistance region” as shown:

• The negative differential resistance in their operation, allows them to be used as oscillators, amplifiers, and switching circuits.
• Their low capacitance allows them to function at microwave frequencies.
• Microwave frequencies range between 109 Hz (1 GHz) to 1000 GHz
• Tunnel diodes are not good rectifiers, as they have relatively high leakage current when reverse biased.

Concept:

Extrinsic p-type semiconductors are formed when a trivalent impurity is added to a pure semiconductor. Examples of trivalent impurity is Boron, Gallium, and Indium.

Also, the law of mass action is used to determine the minority carriers in a doped semiconductor.

According to law:

n.p = ni²

n = concentration of electrons

p = concentration of holes

Calculation:

Given indium concentration per germanium atoms = 2.5 × 10⁸, i.e. for every 2.5 × 10⁸ Germanium atoms, we have 1 doped indium atom.

Given the concentration of Germanium atoms = 4.4 × 10²⁸ /m³

∴ The concentration of indium atoms (p-type impurity) per m³ will be:

N_A = 1.76 × 10²⁰ atoms/m³

The concentration of electrons can now be obtained by applying mass action law as:

Solving the above, we get:

n_p = 3.55 × 10¹⁸ /m³

Calculation,

Given,

cos ϕ₁ = 0.8

∴ sin ϕ₁ = 0.6

cos ϕ₂ = 0.6

∴ sin ϕ₂ = 0.8

Asuume,

|I₁| = |I₂| = |I|

In complex form the phasor quantity is written as,

i = I(cos ϕ ± jsin ϕ)

Both current can be written as,

i₁ = |I|(0.8 + j0.6)

i₂ = |I|(0.6 + j0.8)

Resultant current (i) will be phasor sum of i₁ and i₂,

i = |I|(0.8 + j0.6) + |I|(0.6 + j0.8) = |I|(1.4 + j1.4)

Phase angle can caculated as,

ϕ = tan^-1 (1) = 45° 

Hence,

Power factor = cos 45° = 0.707

Alternate Method

When two circuits having the same magnitudes of impedances are joined in parallel. The power factor of the combination is 

The power factor of the first circuit = 0.8 (cos ϕ₁)

Power factor of second circuit = 0.6 (cos ϕ₂)

The power factor of parallel combination,

Concept:

Drain current in saturation region is given by

Calculation:

= 1.047 mA

Doppler effect in physics is defined as the increase (or decrease) in the frequency of sound, light, or other waves as the source and the observer move towards (or away from) each other.

Waves emitted by a source traveling towards an observer gets compressed. In contrast, waves emitted by a source traveling away from an observer get stretched out.

Doppler effect formula:

​    

λ' = wavelength observed by the observer

λ = actual wavelength

v = relative velocity of the source and observer

c = velocity of the light(m/s)

The velocity v is positive for motion away from an observer and negative for motion toward an observer.

The correct answer is option 3.

Concept:

Static RAM:

SRAM (static RAM) is a form of random access memory (RAM) that stores data bits as long as power is applied. Unlike dynamic RAM (DRAM), which must be updated on a regular basis, SRAM does not require this, resulting in improved performance and reduced power consumption.

However, SRAM is more costly than DRAM and takes up much more space.

Characteristics of SRAM:

• Static RAM (SRAM) consists of flip-flops (D-flip-flop), a bistable circuit composed of four to six transistors.
• Static RAM (SRAM) is more reliable as compared to dynamic RAM.
• Static RAM (SRAM) is the data is not stored as charge on a capacitor, but in a pair of cross-coupled inverters, so SRAM does not need to be refreshed.
• SRAM is faster than DRAM.

Hence the correct answer is required refreshing while powered.

The frequency spectrum of an Amplitude Modulated wave is given by:

Upper sideband = f_c + f_m

Lower sideband = f_c - f_m

Bandwidth in AM = Upper sideband - Lower sideband

Bandwidth = (f_c + f_m) - (f_c - f_m) = 2f_m

∴ In AM the bandwidth is equal to twice the maximum frequency component.

If sideband's highest frequency is f_m, Bandwidth is 2f_m

Concept:

Velocity in a medium is given as

Where μ = Magnetic permeability of the medium.

And ϵ = Electric permittivity of the Medium.

μ = μ_o μ_r

ϵ = ϵ_o ϵ_r

Also,  

= 3 × 10¹⁰ cm/sec

Calculation:

Given, v = 1.5× 10¹⁰ cm/s

So,

μ_rϵ_r =4 ---- Equation (1)

Also, the intrinsic impedance of a medium is given by:

for insulator, σ ≪ ωϵ

 = η₀ = Intnimic Impedance of Free space = 120 π

⇒ Given, η = 90π

Putting values, we get:

 _____ (2)

From Equation (2), μ_r =

Putting this in Equation – (1)

⇒ ϵ_r μ_r = 4

So,  

So, Option (1) is correct.

In position control systems, the device is used for providing rate feedback voltage is called tachogenerator.

Concept:

EIRP: It stands for Effective isotropic Radiated power. It is defined as in dB.

EIRP = transmit power + transmitting Antenna gain - cable loss

Calculation:

Given = P_t = 40 dBW; backoff loss = 3 dB; feederloss = 3 dB

Antenna gain (G_t) = 40 dBW

EIRP = 40 + 40 - 3 - 3 = 74 dBW

Concept:

If x(t) and y(t) are the two-time domain signals, then the convolution of x(t) and y(t) is given by

Convolution in the time domain is equal to multiplication on the frequency domain and vice versa.

If X(ω) and Y(ω) are the Fourier transforms of x(t) and y(t), then

x(t) × y(t) = X(ω) × Y(ω)

If X(s) and Y(s) are the Laplace transforms of x(t) and y(t), then

x(t) × y(t) = X(s) × Y(s)

 By applying the above property, the convolution between two functions f(x) and g(x) in the domain of Fourier transform is

Explanation:

Let A B C and D be signals 

Commutative law states:

A (B + C) = A B + A C

Distributive law states:

A (B + C D) = A B + A C D

Associative law states:

(A + B) + C = A + (B + C)

• The mobility of electrons, which are carriers in the case of an n-channel device, is greater than that of holes, which are the carriers in the p-channel device.
• Thus an n-channel device is faster than a p-channel device.
• The N-channel transistor has lower on-resistance and gate capacitance for the same die area.

Key Points

A. Statement: Fibre optic cables are wooden fibers to provide high-quality transmission- False

Explanation:

• An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair.
• Optical fibers are used most often as a means to transmit light[a] between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths (data transfer rates) than electrical cables.

B. Statement: Wireless communication provides anytime. anywhere connection to both computers and telephones- True

Explanation:

• Wireless communications allow wireless networking between desktop computers, laptops, tablet computers, cell phones, and other related devices. 
• Wireless networks are cheaper to install and maintain.
• Data is transmitted faster and at a high speed.
• Reduced maintenance and installation cost compared to other forms of networks.
• A wireless network can be accessed from anywhere, anytime.
• Working professionals these days can access the Internet anywhere and anytime without carrying cables or wires.
• This also permits professionals to complete their work from remote locations.

C. Statement: Mobile phones are capable of providing voice communication and also digital messaging service- True

Explanation:

• A mobile phone, cellular phone, cell phone, cellphone, handphone, or handphone, sometimes shortened to simply a mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link while the user is moving within a telephone service area. In addition to telephony, digital mobile phones support a variety of other services, such as text messaging, MMS, email, Internet access, short-range wireless communications (infrared, Bluetooth), business applications, video games, and digital photography.
• Mobile phones offering only those capabilities are known as feature phones; mobile phones which offer greatly advanced computing capabilities are referred to as smartphones.

Thus, only statements B and C are correct. Therefore, Option 2 is the correct answer.

Statement a:

Advanced Mobile Phone System (AMPS) is a second-generation cellular phone system.

This is statement incorrect

Reason:

• An advanced mobile phone system is an analog cellular system. It uses FDMA to separate channels in a link. It operates in ISM 800 MHz band.
• It uses two separate analog channels one for forwarding communication and one for reverse communication.
• It is a first-generation cellular phone system. So, the given statement is incorrect.

Statement b:

IS - 95 is a second-generation cellular phone system based on CDMA and DSSS.

This statement is correct

Reason:

• IS – 95 is a second-generation cellular phone system.
• It is based on CDMA and DSSS. It uses two bands for duplex communication. IS 95 has two different transmission techniques: one for use in the forward direction and another in a reverse direction.
• In this, each voice channel is digitized. It uses FDMA to separate channels in the link.

Statement c:

The Third-generation cellular phone system will provide universal personnel communication.

This statement is correct

Reason:

The third generation of cellular telephony refers to a combination of technologies that provide a variety of services. Using a small portable device, a person should be able to talk to anyone else in the world with a voice quality similar to that of an existing fixed telephone network. It provides universal personal communication.

Hubs:

Hubs are used in networks that use twisted-pair cabling to connect devices.

Hubs can also be joined together to create larger networks.

Hubs are simple devices that direct data packets to all devices connected to the hub, regardless of whether the data package is destined for the device. This makes them inefficient devices and can create a performance bottleneck on busy networks.

In TV Transmission the use of FM is made for Audio transmission and AM for Video transmission.

Vestigial Sideband Modulation (VSB) is used for video modulation in TV transmission due to the following reasons :

• Video signal exhibits a large bandwidth and significant low-frequency content which suggests the use of VSB.  
• VSB (vestigial sideband) transmission transmits one sideband fully and the other sideband partially thus, reducing the bandwidth requirement.
• The circuitry for demodulation in the receiver should be simple and therefore cheap. VSB demodulation uses simple envelope detection.

The spectrum of a vestigial sideband is as shown:

Layer 2 Ethernet Frame Format:

MAC address consists of 6 bytes

1 byte = 8 bits

∴ 6 bytes = 6 × 8 bits = 48 bits

MAC address has 48 bit or 6 bytes

Concept:

Let, s = No of error that can be detected

t = No of error that can be corrected

dmin = Minimum hamming distance for the code

Then,

To detect up to 's' errors

1) dmin ≥ s + 1

To correct up to 't' errors

2) dmin ≥ 2t + 1

it can detect up to 's' errors and correct up to 't' errors if and only if:

3) dmin ≥ t + s + 1

Calculations:

Given:

Error detected = 2 = s

Error corrected = 1 = t

From(1),

dmin ≥ 3

From(2),

dmin ≥ 3

We need to find for both correction and detection and hence, 

Using formula (3)

dmin ≥ t + s + 1

dmin ≥ 2 + 1 + 1

dmin = 4

Explanation:

Any laplace domain function into z-domain we need a conversion.

z-transform: Any discrete sample in z-domain.

Laplace transform: Any continuous time signal into lapalce domain.

For conversion we take e^sT = z

taking log on both sides

ln (e^sT) = ln z

sT = ln z

s = (ln z)/T

Cathode Ray Oscilloscope (CRO)

The cathode ray oscilloscope is an electronic test instrument.

It is used to obtain waveforms when different input signals are given to the CRO.

Thus CRO measures the time period and the frequency of the obtained waveform.

Time period

The number of seconds taken by a signal to complete its one cycle is known as the time period.

Therefore time period is nothing but the seconds/cycle of a waveform.

Frequency

The number of cycles completed in one second is known as the frequency.

Therefore frequency is nothing but the cycle/seconds of a waveform.

The relationship between time and frequency is:

where, T = Time period

f = Frequency

It is not, therefore with any feeling of unfriendliness to a particular member of that class that I pay tribute to conductors as a class.

As we can say that ,
Compassionate is someone who cares or is concerned about another person, unsympathetic is one who does not care or understand another’s condition or position. Indecisive means someone who cannot take decisions easily, unlawful is something against law, and Untrustworthy is someone who cannot be trusted.
From above it is clear that Unsympathetic is an opposite word of Compassionate . 

Safety in numbers (Idiom): being in a group offers security and protection.
Sporting chance (Idiom): a reasonable chance of winning or succeeding.
Square deal (Idiom): an instance of being treated fairly.
Standing joke (Idiom): something that regularly causes amusement or provokes ridicule.
It is obvious that ‘safety in numbers’ makes perfect sense in the given blank.
Hence, the correct answer is option A. 

The correct answer is option 3, i.e. = (equal sign).

All the formulas in MS Excel started with an equal sign.

• After equal sign one can add formula or function.
• For example, the addition of values in cells A1 through A4, then type the whole equation after the equal sign.
  • = A1 + A2 + A3 + A4.
• The ampersand operator (&) joins several strings into one string.
• The dollar sign ($) is used for creating absolute references.

To determine the number of bytes represented by a KB (kilobyte), we need to understand the conversion factor between kilobytes and bytes.
- 1 kilobyte (KB) = 1,024 bytes
Given the options provided, we can determine that the correct answer is option B, which states that a KB represents 1,000 bytes.
Here is a breakdown of the options and their representations in bytes:
- Option A: 8 bytes - This is not the correct answer as 8 bytes is much smaller than a kilobyte.
- Option B: 1,024 bytes - This is the correct answer as 1 kilobyte is equal to 1,024 bytes.
- Option C: 1,000,000 bytes - This is not the correct answer as 1 kilobyte is much smaller than 1,000,000 bytes.
- Option D: 1,000,000,000 bytes - This is not the correct answer as 1 kilobyte is much smaller than 1,000,000,000 bytes.
Therefore, the correct answer is option B, which states that a kilobyte represents 1,024 bytes.

Spreadsheet program
A spreadsheet program is a computer program that organizes data in rows and columns of cells. It is commonly used to perform calculations, analyze data, and create charts or graphs. In the context of keeping a record of the money earned from mowing lawns over the summer, a spreadsheet program would be the most suitable choice. Here's why:
- Organizes data in rows and columns: A spreadsheet program allows you to input and organize data in a tabular format, with rows representing individual records and columns representing different attributes or variables. This structure is ideal for keeping track of earnings, dates, and other relevant information related to lawn mowing jobs.
- Performs calculations: Spreadsheet programs have built-in formula functions that enable you to perform calculations and manipulate data. For example, you can easily calculate the total earnings for the summer by summing up the amounts from the respective cells.
- Provides data analysis tools: With a spreadsheet program, you can analyze your earnings data by using functions like averages, sorting, filtering, and creating charts or graphs. This allows you to gain insights and visualize trends or patterns in your lawn mowing business.
- User-friendly interface: Spreadsheet programs typically have a user-friendly interface that allows you to easily input, edit, and navigate through data. You can format cells, apply conditional formatting, and customize the appearance of your spreadsheet to make it more visually appealing and organized.
- Data storage and retrieval: Spreadsheet programs provide a convenient way to store and retrieve data. You can save your spreadsheet as a file on your computer or in the cloud, ensuring that your records are easily accessible and can be updated as needed.
In conclusion, a spreadsheet program is the most suitable choice for organizing and managing the data related to your lawn mowing earnings. It offers the necessary features and functionality for efficient record-keeping, calculations, analysis, and data storage.

From the common explanation, we get Sona sits opposite Bobby.

Hence, Option E is correct.

Final Arrangement:

Common Explanation:

References:

Eight students Kirti, Ayushi, Manu, Sona, Aarti, Bobby, Ankita and Divya are sitting around the square table.

Four students sit on each side while rest on each corner.

Students at the side face away from the centre and students at the corner faces towards the centre.

Inferences:

So, we need to arrange as follows to accommodate the information as stated in the question,

References:

Divya sits third to the left of Ayushi.

Kirti sits opposite to Manu and both faces away from the centre.

Inferences:

From the above references, we get the following two different cases:

Case 1:

Case 2:

References:

Neither Kirti nor Manu are beside Ayushi.

Sona sits immediate left of Ankita, who sits on the side of the table.

Bobby is not an immediate right of Ayushi.

Bobby is not adjacent to Manu.

Inferences:

So, from this case 2 will be eliminated as neither Kirti nor Manu are beside Ayushi.

Now, from case 1 we get the following final arrangements:

As the working field of lawyer is Court, similarly the working field of chemist is laboratory.