Introduction to Superconductors: Properties, Classification, and High-Temperature Superconductors

What is a superconductor? 

Superconductors are materials whose resistivity becomes immeasurably small or actually becomes zero below a critical temperature, Tc. For example, (La, Sr)2CuSO4 has Tc = 36K.

Factors that control superconductivity: 

Superconductivity of metals depends on current, temperature, and magnetic field properties. Superconductivity is only present when temperature, magnetic field strength, and current remain within a critical space. The relationship among these critical values is such that an increase in the critical value of one of these parameters decreases the critical value of the remaining two. Eliminating superconductivity requires increasing one of these parameters above its critical value

Properties of superconductors:

• Zero resistance: The resistance of superconducting materials is zero below their critical temperature, resulting in highly efficient electrical conduction. 
• Absence of thermoelectric effects: No Seebeck voltage, no Peltier-heat, no Thomson-heat detectable.
• Ideal diamagnetism: Xm = -1. Superconducting materials have perfect diamagnetic properties, meaning they expel any magnetic field from their interior.
• Meissner-effect: If a superconductor is cooled down in the presence of a weak magnetic field, below Tc, the field is completely expelled from the bulk of the superconductor.
• Flux quantization: The magnetic flux through a superconducting ring is quantized and constant in time.
• Critical magnetic field: The magnetic field strength that can be applied to a superconducting material without destroying its superconductivity is limited by its critical magnetic field.
• Critical current density: The maximum current density that can be carried by a superconducting material without losing its superconductivity is known as the critical current density.
• Critical temperature: The temperature at which a material transitions from a normal conductor to a superconductor is called its critical temperature.
• Josephson effect: The ability of superconducting materials to produce a DC voltage across a thin insulating barrier between two superconductors, allowing for the creation of sensitive voltage sensors.

Classification of superconductors: 

According to transition period, superconductors are classified into the following two classes:

Type-1 (soft) superconductors: The superconductors which transition from a superconducting state to a normal state sharply due to any limiting parameters (especially magnetic field) are known as type-1 or soft superconductors.

Above figure shows the schematic representation of the resistivity of a soft superconductors when a magnetic field H is applied. These solids behave like normal conductors above Hc.

Type-II (hard)superconductors: The superconductors which transition from a superconducting state to a normal state gradually due to any limiting parameters (especially magnetic field) are known as type-II or hard superconductors.

Above figure shows the schematic representation of the resistivity of a hard superconductor when a magnetic field H is applied. The region between Hc1 and HC2 is called the vortex state in which superconducting and normal conducting areas are mixed. Above Hc2 the solid behaves like a normal conductor.

High-temperature superconductors: 

The temperature at which materials transition from a normal state to a superconducting state is called critical temperature or transition temperature. The materials having a critical temperature above 77K are known as high-temperature superconductors. They are technologically interesting because they do not require liquid helium or liquid hydrogen (-253°C) for cooling. Despite considerably higher transition temperatures, they have not yet revolutionized new technologies mainly because of their inherent brittleness, their incapability of carrying high current densities, and their environmental instability. These problems may be overcome using bismuth-based materials, composite materials, ductile substrates, silver, etc.

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Cleanroom Technology: The Key to Contamination Control

In industries such as semiconductor manufacturing, pharmaceuticals, biotechnology, and aerospace, a clean environment is essential for successful and safe production. 

A clean room is the room in which the concentration of airborne particles is controlled to a certain level. Control of cleanliness in such a room is often achieved by controlling the introduction, formation and retention of particles and other relevant parameters (temperature, humidity and pressure) in the room. 

That area of contamination control which utilizes cleanrooms is known as cleanroom technology.

Cleanroom Construction Mechanism

The construction of a cleanroom involves several mechanisms that are designed to maintain a controlled environment. These mechanisms include:

1.Air Filtration: Supplying exceptionally large quantity of air through high efficiency filters. This helps in two senses: 

• Extreme air dilute and remove the particles, bacteria and chemicals dispersed from personnel, machinery and other sources within the room.
• Extreme air ensure that no dirty air flows into the cleanroom from outside by introducing pressure.

2. Building Materials: Cleanroom walls, floors, and ceilings are made of materials that do not generate particles or outgas airborne chemical contamination. Common materials used in cleanroom construction include stainless steel, laminates, and non-porous surfaces.

3. Cleanroom Clothing: Cleanroom personnel wear special clothing that envelops them and minimizes the dispersion of particles and microorganisms. Cleanroom clothing includes coveralls, hoods, gloves, and booties.

Cleanroom Practices and Protocols

In a cleanroom don’t:

• touch your face or skin with gloves
• touch building hardware, oily machinery, or wafer loading areas
•  lean on equipment
• wear cosmetics, powders, or colognes
• wear anything on fingers-- remove all rings and bracelets
• use paper, pencils or markers that leave dust or lint

In a cleanroom one must:

• change gloves whenever they get dirty or torn
• use a fresh pair of gloves whenever handling wafers
• wipe down wafer handling areas with isopropanol
• use clean room paper and dust-free ball point pens

Cleanrooms are critical for industries that require a controlled environment for successful and safe production. Cleanroom technology involves the construction of cleanrooms and the practices and protocols used to maintain a controlled environment. By following proper cleanroom practices and protocols, industries can reduce contamination and improve production yields.

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Interview Questions: ECE (basic-2)

Some electronics and communication engineering basic skill topics and answers are given here:

EM waves are reflected by perfect conductors!

This is the reason why your data connection becomes slow or calls easily get disconnected while travelling in a train, lift or you are near to a big metallic body.

A signal is 'active low or high' 

In digital circuits when: A signal is 'active low' means that signal will be performing its function when its logic level is 0. A signal is 'active high' means that signal will be performing its function when its logic level is 1.

Transformer secondary winding voltage

Each coil loop of the transformer secondary produces a voltage by means of electromagnetic induction to primary winding.  As the secondary winding loops are all in series or one after one and voltages in series add, the larger number of loops produce large voltage and less number of loops produce less voltage. A easy way to understand step-up and step-down mechanism.

Flash memory

Flash memory is an electronic (solid-state) non-volatile computer storage medium that can be electrically erased and reprogrammed. Data on flash memory(thumb/pen drives and SD cards)  is erased through quantum tunneling. The most common type of flash memory in pen/thumb drives is NAND flash Flash memory which uses floating gate transistors Floating-gate MOSFET.

SEA-ME-WE 4

This stands for South East Asia–Middle East–Western Europe 4. Our entire internet is based on undersea fiber optic cables such as the SEA-ME-WE systems

Pen-drive 

If your pen drive uses FAT32 file system you cannot transfer a single file of size 4GB at a time, however you can convert the file system to NTFS and transfer a file of any size.

Mobile vibration

It's because of small dc motor. The motor that makes a phone vibrates is a stepper motor.

Music player

That what we see on our music player when we play a song is actually representation of Frequency spectra of a Tone-Frequency modulated wave! It's not there just because it looks good.

Brewster angle

The Brewster angle is the angle at which no reflection occurs in the medium of origin.

Mobile adapter

Mobile batteries require around 3.7-4.2 volts (200kHz) but our house is supplied at 230V (50kHz)  So we need a step down transformer in order to charge our mobile, which is present inside our charger. 

Did you ever notice gravel beneath around large transformers?

​​​​The main reason for this practice is actually "snakes".  Large transformers produce substantial vibrations due to magneto-striction which get transmitted to ground. Due to this effect the transformers placed on solid ground attract snakes. To prevent this, transformers are surrounded by gravel which prevents snakes from coming near it.

Shockwave

The shockwave during rain has a voltage as high as 50k–80k or even 1Lakh Volts. The ground has potential of 0 Volts. Thus there is a huge potential difference between cloud and ground and thus this makes even air as conductive material. By nature electric current follows the shortest and simplest path to flow and thus it flows through whatever taller conductive material present around to reach upto the ground. That’s why in urban areas most of the shockwaves fall on industrial chimneys or coconut trees which attain a great height in the surroundings. That’s why one shouldn’t walk around in an open park or field or stream through water bodies during heavy lightining.

The height at which a wire is suspended is related to the voltage it has.

The inter-district transmission lines carrying around 33,000 V are placed the highest. They're usually seen near a power house (regional distribution centres) and are quite thick. The next in height are the 11,000 V lines, placed at about 23-26 ft. They supply high electricity to local transformers which finally step down the voltage to 230 V (or 110 V in some places like America and Canada).At the lowest height are the 230 V lines, from which the service drops are made. 11000 V is stepped down to 400 V (Phase to Phase). There are 3 wires, carrying 3 different phases of electricity. Any one of the three phases and a neutral is supplied to the house, powering the appliances at 230 volts.

Electric bulb blinking

In AC power, the direction of the current flowing is continuously changing. filament bulb only glows on one direction of power flow.  So, the bulb continuously blinks. Human eyes can't make out that it is blinking because the frequency is 50 times per second or 50hz.

Flow of current

Current does not flow from positive to negative or vice versa, it flows in a continuous loop. By convention, from Ben Franklin, current is defined to flow from negative to positive inside a battery or power supply, and from positive to negative in the circuit outside a power supply.  Current is not really the movement of electrons, they actually move quite slowly. The electrons in the electrical wire going to your blow dryer travel at about 1 inch per minute. The electrical current flows at close to the speed of light.

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Assembly language Programming Codes for the 8086 Microprocessor

Assembly language is a low-level programming language that is used to write software that can be executed directly by a computer's central processing unit (CPU). It is particularly useful for programming microprocessors, such as the 8086 microprocessor.

In order to write effective assembly language programs for the 8086 microprocessor, it is essential to have a deep understanding of the internal structure of the processor itself. This knowledge allows you to write code that makes efficient use of the processor's capabilities and can be optimized for speed and performance.

Fortunately, there are a variety of resources available to help you learn assembly language programming for the 8086 microprocessor. One useful starting point is the Microsoft Macro Assembler (MASM), which is a popular assembler used for programming on the x86 architecture.

To help you get started with MASM, we've provided some sample codes below that you can use for practice:

Code-1: Print a message 'hello world'

.MODEL SMALL
.STACK 100H

.DATA
MSG DB 'HELLO WORLD!$'

.CODE
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG
MOV AH,9
INT 21H

MOV AH,4CH
INT 21H


MAIN ENDP
END MAIN 
...................................................

Code-2: Print all the ASCII symbols from 0 to 256

.MODEL SMALL
.STACK 100H

.CODE
MAIN PROC

MOV AH,2
MOV CX,256
MOV DL,0

PRINT_LOOP:
INT 21H
INC DL
DEC CX
JNZ PRINT_LOOP 


MOV AH,4CH
INT 21H 


MAIN ENDP
END MAIN 
.................................................

Code-3: Convert a lowercase letter into uppercase

.MODEL SMALL
.STACK 100H
.DATA
MSG1 DB 'ENTER THE LOWERCASE LATTER:$'
MSG2 DB 'CORRESPONDING UPPERCASE LATTER IS:'
CHAR DB ? , '$'
.CODE
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG1
MOV AH,9
INT 21H

MOV AH,1
INT 21H

SUB AL,20H 
MOV CHAR,AL

LEA DX,MSG2
MOV AH,9
INT 21H

MOV AH,4CH
INT 21H 


MAIN ENDP
END MAIN 
......................................................

Code-4: Convert an uppercase letter into lowercase 

.MODEL SMALL
.STACK 100H
.DATA
MSG1 DB 'ENTER THE UPPERCASE LATTER:$'
MSG2 DB 'CORRESPONDING LOWERCASE LATTER IS:'
CHAR DB ? , '$'
.CODE
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG1
MOV AH,9
INT 21H

MOV AH,1
INT 21H

ADD AL,20H 
MOV CHAR,AL

LEA DX,MSG2
MOV AH,9
INT 21H

MOV AH,4CH
INT 21H 


MAIN ENDP
END MAIN 
......................................................

Code-5: Addition of two numbers

.MODEL SMALL
.STACK 100H

.DATA 
MSG1 DB 10,13,'ENTER YOUR FIRST NUMBER:$'
MSG2 DB 10,13,'ENTER YOUR SECOND NUMBER:$'
MSG3 DB 10,13,'ADD=$'
N1 DB ?
N2 DB ?
RE DB ?

.CODE 
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG1
MOV AH,9
INT 21H

MOV AH,1
INT 21H

SUB AL,30H
MOV N1,AL

LEA DX,MSG2
MOV AH,9
INT 21H

MOV AH,1
INT 21H

SUB AL,30H
MOV N2,AL

MOV AL,N1
ADD AL,N2
MOV RE,AL

ADD AH,30H
ADD AL,30H
MOV BX,AX

LEA DX,MSG3
MOV AH,9
INT 21H

MOV AH,2
MOV DL,BH
INT 21H
MOV AH,2 
MOV DL,BL
INT 21H

MOV AH,4CH
INT 21H

MAIN ENDP
END MAIN
........................................................

Code-6: Multiplication of two numbers

.MODEL SMALL
.STACK 100H
.DATA
NUM1 DB ?
NUM2 DB ? 
RESULT DB ?
MSG1 DB 10,13,'ENTER FIRST NUMBER: $' 
MSG2 DB 10,13,'ENTER SECOND NUMBER: $' 
MSG3 DB 10,13,'AFTER MULTIPLYCATION RESULT IS: $' 
.CODE
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG1
MOV AH,9
INT 21H

MOV AH,1
INT 21H
SUB AL,30H
MOV NUM1,AL

LEA DX,MSG2
MOV AH,9
INT 21H

MOV AH,1
INT 21H
SUB AL,30H
MOV NUM2,AL

MUL NUM1

MOV RESULT,AL
AAM

ADD AH,30H
ADD AL,30H

MOV BX,AX

LEA DX,MSG3
MOV AH,9
INT 21H

MOV AH,2
MOV DL,BH
INT 21H 

MOV AH,2
MOV DL,BL
INT 21H

MOV AH,4CH
INT 21H 

MAIN ENDP
END MAIN 
.....................................................

Code-7: Taking input a string

.MODEL SMALL
.STACK 100H
.DATA
STRING DB ?
EXT DB '$'
MSG DB 10,13,'ENTER STRING $' 
.CODE
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG
MOV AH,9
INT 21H

LEA SI,STRING

INP:
MOV AH,1
INT 21H
MOV [SI],AL
INC SI

CMP AL,0DH
JNZ INP

MOV [SI],'$'
LEA DX,STRING
MOV AH,9
INT 21H 

MOV AH,4CH
INT 21H 

MAIN ENDP
END MAIN 
......................................................

Code-8: Reverse a string

.MODEL SMALL
.STACK 100H 
.CODE
MAIN PROC

MOV AH,2
MOV DL,'?'
INT 21H

XOR CX,CX
MOV AH,1
INT 21H

WHILE_:
CMP AL,0DH
JE END_WHILE

PUSH AX
INC CX

INT 21H
JMP WHILE_

END_WHILE:
MOV AH,2
MOV DL,0DH
INT 21H
MOV DL,0AH
INT 21H
JCXZ EXIT

TOP:
POP DX
INT 21H
LOOP TOP
EXIT: 
MOV AH,4CH
INT 21H 

MAIN ENDP
END MAIN
...................................................

Code-9: Determine prime number

.MODEL SMALL
.STACK 100H
.DATA
NUM DB ?
MSG1 DB 10,13,'ENTER NO: $' 
MSG2 DB 10,13,'NOT PRIME: $' 
MSG3 DB 10,13,'PRIME $' 
.CODE
MAIN PROC

MOV AX,@DATA
MOV DS,AX

LEA DX,MSG1
MOV AH,9
INT 21H

MOV AH,1
INT 21H
SUB AL,30H
MOV NUM,AL

CMP AL,1
JLE LBL2
MOV AH,00
CMP AL,3
JLE LBL3
MOV AH,00

MOV CL,2
DIV CL
MOV CL,AL

LBL1:
MOV AH,00
MOV AL,NUM
DIV CL
CMP AH,00
JZ LBL2
DEC CL
CMP CL,1
JNE LBL1
JMP LBL3
LBL2:
MOV AH,9
LEA DX,MSG2
INT 21H
JMP EXIT
LBL3:
MOV AH,9
LEA DX,MSG3
INT 21H
EXIT:
MOV AH,4CH
INT 21H 

MAIN ENDP
END MAIN 

By studying and practicing with examples such as these, you can develop a strong foundation in assembly language programming and become proficient in programming the 8086 microprocessor. Good luck!

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Interview Questions: ECE (basic-1)

Some electronics and communication engineering basic skill topics and answers are given here.............

Electron

Nobody knows the shape, size, or just about anything about electrons besides their charge and mass. We do know they're very, very small. How small? So small we can't measure their diameter, we can only say they're smaller than some upper bound.

Flat channel

A channel which passes all spectral components with approximately equal gain and linear phase.

Co-channel interference

Frequency reuse implies that in a given coverage area there are several cells that use the same set of frequencies. These cells are called co-channel cells and the interference between signals from these cells is called co-channel interference.

Adjacent channel interference

Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference. Adjacent channel interference can be minimized through careful filtering and channel assignments. 

What is inter-modulation noise?

When a signal (having different frequency components) passes through a transmitting media, then due to non-linearity, some of the frequency components may combine to generate a different frequency component. This leads to distortion in the signal, which is known as inter-modulation noise. For example, a signal may be having frequency components fx and fyand due to non-linearity of the media they may generate a frequency component (fx+fy). Further a frequency of (fx+fy) may be already present in the original signal. This causes inter-modulation noise.

Major impacts on electromagnetic wave propagation

Reflection, diffraction, and scattering are the three basic propagation mechanisms which impact propagation in a mobile communication system.

       Reflection occurs when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave. Reflections occur from the surface of the earth and from buildings and walls.

       Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges). The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even when a line-of-sight path does not exist between transmitter and receiver.

       Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel. In practice, foliage, street signs, and lamp posts induce scattering in a mobile communications system.

Relation between EM waves & perfect conductor

Since electromagnetic energy cannot pass through a perfect conductor a plane wave incident on a conductor has all of its energy reflected.

How the effect of delay distortion can be minimized?

Ans: Delay distortion can be minimized by using an equalizer (a kind of filter). Delay distortion arises because different frequency components of the signal suffer different delay as the signal passes through the media. This happens because the velocity of the signal varies with frequency and it is predominant in case of digital signals.

What is Fading?

Fading is used to describe the rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance so that large-scale path loss effects may be ignored. Fading is caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times. These waves called multi-path waves, combine at the receiver antenna to give a resultant signal which can vary widely in amplitude and phase, depending on the distribution of the intensity and relative propagation time of the waves and the bandwidth of the transmitted signal.

Why is it necessary to limit the band of a signal before performing sampling?

It is necessary to limit the bandwidth of a signal before sampling so that the basic requirement of sampling theorem ( the sampling rate should twice or more than twice the maximum frequency component of the signal ) is satisfied. This is known as Nyquist rate. If it is violated, original signal can not be recovered from the sampled signal.

Coherent and non-coherent demodulation

Demodulation techniques may be broadly divided into two categories: coherent and non-coherent demodulation. Coherent demodulation requires knowledge of the transmitted carrier frequency and phase at the receiver whereas non-coherent detection requires no phase information. 

Between AM and FM, which one gives better noise immunity?

FM is more immune to noise than AM, since the power of transmission is independent of the modulation index.

Why do you need encoding of data before sending over a medium?

Suitable encoding of data is required in order to transmit signal with minimum attenuation and optimize the use of transmission media in terms of data rate and error rate.

Between RZ and NRZ encoding techniques, which requires higher bandwidth and why?

RZ encoding requires more bandwidth, as it requires two signal changes to encode one bit.

Generation of FM

There are basically two methods of generating an FM signal: the direct method and the indirect method. In the direct method, the carrier frequency is directly varied in accordance with the input modulating signal. In the indirect method, a narrow-band FM signal is generated using a balanced modulator and  frequency multiplication is used to increase both the frequency deviation and the carrier frequency to the required level.

Performance of a modulation scheme

The performance of a modulation scheme is often measured in terms of its power efficiency and bandwidth efficiency. Power efficiency describes the ability of a modulation technique to preserve the fidelity of the digital message at low power levels. Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a limited bandwidth. 

Nyquist caiteria

Nyquist was the first to solve the problem of overcoming inter-symbol interference while keeping the transmission bandwidth low . He observed that the effect of ISI could be completely nullified if the overall response of the communication system (including transmitter, channel and receiver) is designed so that at every sampling instant at the receiver, the response due to all symbols except the current symbol is .equal to zero.

What  is inside of fan regulator ?

It is TRIAC. It is a power-electronic device which changes the firing angle to get different voltage and that’s how we control our fan.

Over voltage is dangerous!

In case over-voltage occurs in a house , all your electrical equipment will get burned if they are in ON position. Electrical equipments are protected by MCBs (or fuses) only from short-circuits and over-loading and not from Over-voltage.The one way to control and prevent this is by installing an auto-cut voltage regulator in the main supply of the house.Over-voltage gets introduced in distribution power lines from sub-stations side in case over voltage relays installed in sub-stations fail to operate for some reason.

Transformer is only for AC

If you will supply a transformer with a battery (dc voltage source), u will end up burning the windings of transformer. Larger the size of transformer, larger the time it will take.

Electronic device don't get damaged via overcharging!

That your phone/laptop battery will not get damaged if you still have the device connected to charging even after reaching 100%. Because the internal circuitry in the devices will prevent overcharging. 

AC versus DC

AC is more efficient for transmission, but can't be stored as AC;  and DC  can be stored in batteries but is not efficiently transmitted.

Current is cut!

Generally we use to say 'current is cut', which is technically wrong. We cannot cut current. It should be 'power cut'.

Some multi-conductor transmission cables are twisted. why?

Instrumentation cables are twisted. (Twisted pair ). This twisting is done to cancel out the electromagnetic interference between the two.

What do you mean by Corona ?

Voltage between transmission lines is so high that it can cause partial breakdown of the air surrounding it, which results in a colorful field and a hissing noise called corona.

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Expanding the Capacity of Cellular Mobile Networks: Cell Splitting, Sectoring, and Coverage Zones

Cellular mobile networks have become an indispensable part of modern society, providing ubiquitous voice and data services to billions of people worldwide. As the demand for mobile services continues to grow, mobile operators are faced with the challenge of expanding their networks to accommodate more users and traffic. To meet this challenge, techniques such as cell splitting, sectoring, and coverage zone approaches are used in practice to expand the capacity of cellular mobile networks.

Cell splitting is a technique that allows an orderly growth of the cellular system by dividing a congested cell into smaller cells. This technique increases the number of base stations in order to increase capacity. By reducing the cell size, the number of users in a cell is reduced, which in turn reduces the interference and increases the channel capacity. Cell splitting is an effective way to increase capacity in urban areas where there is a high density of users.

Sectoring is another technique used to expand the capacity of cellular mobile networks. Sectoring uses directional antennas to further control the interference and frequency reuse of channels. By using directional antennas, the same frequency can be reused in adjacent cells without causing interference. Sectoring also reduces the co-channel interference by limiting the number of users in each sector. This technique is particularly useful in suburban and rural areas where there is a low density of users.

The zone microcell concept is yet another technique used to expand the capacity of cellular mobile networks. This concept distributes the coverage of a cell and extends the cell boundary to hard-to-reach places. Zone microcells are small cells that are used to fill in the coverage gaps between large cells. This technique is particularly useful in areas where there are obstacles that block the signal, such as tall buildings or mountains.

While cell splitting, sectoring, and zone microcells are all effective ways to expand the capacity of cellular mobile networks, they have their own advantages and disadvantages. Cell splitting increases the number of base stations, which can be expensive and may cause interference with neighboring cells. Sectoring reduces interference and increases capacity, but suffers from trunking inefficiencies. Zone microcells are useful for filling in coverage gaps, but they require a large number of base stations and are not as effective in urban areas.

In conclusion, expanding the capacity of cellular mobile networks is crucial for meeting the growing demand for mobile services. Techniques such as cell splitting, sectoring, and coverage zone approaches are all effective ways to expand network capacity. Each technique has its own advantages and disadvantages, and mobile operators must choose the technique that best suits their needs.

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Advantages of FM over AM for Sound Signal Transmission

Frequency modulation has the following advantages over amplitude modulation- Noise Reduction, Transmitter Efficiency, Less adjacent Channel and co-channel Interference etc.

The greatest advantage of FM is its ability to eliminate noise interference and thus increase the signal to noise ratio. In FM, amplitude variations of the modulating (message) signal cause frequency deviations and not a change in the amplitude of the carrier. Noise interference results in amplitude variations of the carrier and thus it can be easily removed by the use of amplitude limiters. The amplitude limiter in the FM section of the receiver works on the principle of passing the stronger signal and eliminating the weaker. In this manner, a relatively weak interfering signal (as sound signals  generated at nature has low frequency) or any pick-up from a co-channel station (a station operating at the same carrier frequency) gets eliminated in a FM system.

In an FM receiver, if two signals are being received simultaneously, the weaker signal will be eliminated almost entirely if it possesses less than half the amplitude of the other stronger signal. However, in AM the interfering signal or station can be heard or received even when a 100 : 1 relationship exists between their amplitudes.

The amplitude of the FM wave is independent of the depth of modulation, whereas in AM it is dependent on this parameter. This means that low level modulation can be used in FM and all succeeding amplifiers can be class ‘C’ which are more efficient. Thus, unlike AM, all amplifiers handle constant power and this results in more economical FM transmitters.

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The Evolution and Impact of Television: From its Origins to Modern Applications

The name 'Television' comes from the Greek word 'tele' (= far) and the Latin word 'visionis' (from videre = to see) which means 'to see from a distance'. The fundamental aim of a television system is to extend the sense of sight beyond its natural limits, along with the sound associated with the scene being televised. Essentially, then, a TV system is an extension of the science of radio communication with the additional complexity that besides sound, the picture details are also to be transmitted.

The first demonstration of actual television was given by J.L. Baird in the UK and C.F. Jenkins in the USA around 1927. However, the real breakthrough occurred with the invention of the cathode ray tube and the success of V.K. Zworykin of the USA in perfecting the first camera tube

In the absence of any international standards, three monochrome ( black and white) systems grew independently. These are the 525 line American, the 625 line European and the 819 line French systems. When both the quality of reproduction and the cost of equipment are taken into account, it is difficult to definitely establish the superiority of any one of these systems over the other two. This naturally prevents direct exchange of programme between countries using different television standards. Committee on radio and television (CCIR) efforts for changing to a common 625 line system by all concerned proved ineffective and thus all the three systems have apparently come to stay. The inability to change over to a common system is mainly due to the high cost of replacing both the transmitting equipment and the millions of receivers already in use. The three different standards of black and white television have resulted in the development of three different systems of color television such as - SECAM, PAL and NTSC.

Each television broadcast station is assigned a channel bandwidth of 7 MHz (6 MHz in the American, 8 MHz in the British and 14 MHz in the French systems).In the earlier days TV broadcast was confined to assigned VHF bands of 41 to 68 MHz and 174 to 230 MHz. Later additional channel allocations have been made in the UHF band between 470 and 890 MHz. 

Applications of Television:

Impact of television is far and wide and has opened new avenues in diverse fields like public entertainment, social education, mass communication, newscasts, weather reports, political organization and campaigns, security(CCTV), live streaming, announcements and guidance at public places like airport terminals, sales promotion and many others.

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An Introduction to Oscillators: Their Functionality, Types, and Application

Oscillator is the basic element of ac signal sources and generates ac signals of known frequency and amplitude. The main applications of oscillators are as sinusoidal waveform sources in electronic measurement work. To generate ac voltage, the circuit is supplied energy from a dc source. That is why, although an oscillator can be considered as generating sinusoidal signal, it is to be noted that it merely acts as an energy converter, It converts a dc source of supply to alternating current of desired frequency. 

Oscillators are generally an amplifier with positive feedback. An oscillator has a gain equal to or slightly greater than unity. In the feedback path of the oscillator, capacitor, inductor or both are used as reactive components. In addition to these reactive components, an operational amplifier or bipolar transistor is used as amplifying device. No external ac input is required to cause the oscillator to work as the dc supply energy is converted by the oscillator into ac energy.

Oscillators can be classified in a number of ways. They can be classified based on:
   (a) the design principle used 
   (b) the frequency range over which they are used and
   (c) the nature of generated signals.

1. Classification According to Design Principle
   (a) Positive feedback oscillators 
   (b) Negative feedback oscillators

2. Classification According to Frequency Band of the Signals
  (a) Audio Frequency (AF) oscillators—frequency rage is 20 Hz to 20 kHz
  (b) Radio Frequency (RF) oscillators—frequency range is 20 kHz to 30 MHz
  (c) Video Frequency oscillators—frequency range is dc to 5 MHz
  (d) High Frequency (HF) oscillators—frequency range is 1.5 MHz to 30 MHz
  (e) Very High Frequency (VHF) oscillators—frequency range is 30 MHz to 300 MHz

3. Classification According to Types of Generated Signals

  (a) Sinusoidal Oscillators: These are known as harmonic oscillators and are generally LC tuned-feedback or RC tuned-feedback type oscillator that generates a sinusoidal waveform which is of constant amplitude and frequency.

  (b) Non-sinusoidal Oscillators: These are known as relaxation oscillators and generate complex non-sinusoidal waveforms that changes very quickly from one condition of stability to another such as square-wave, triangular-wave or sawtooth-wave-type waveforms.

The oscillators have a variety of applications. In some applications we need voltages of low frequencies, in others of very high frequencies. For example to test the performance of a stereo amplifier, we need a signal of variable frequency in the audio range (20 Hz-20 KHz). Next to amplifiers, oscillators are the most important analog circuit block. Oscillators can be found in almost every imaginable electronic system. For example all radio receiving systems must have a local oscillator. All transmitting systems require oscillators to define the carrier frequency. Similarly, most digital systems are clocked and require a master clock oscillator to operate. Signal sources, which are essential for testing electronic systems are also precise oscillators whose frequency and amplitude can be accurately set according to the requirement. A working knowledge of oscillators, their construction and operating characteristics is therefore essential to analog electronics. This module is intended to introduce the reader to oscillator types, their characteristics and considerations for their behaviour.

A few basic requirements for an oscillatory circuit are:

1. The circuit should contain a reactive or frequency dependent component—either an Inductor (L) or a Capacitor (C) and a dc supply voltage.

2. Overall gain of the amplifier circuit must be at least unity.

3. Self-regenerative or positive feedback results oscillations.

4. Oscillations of the circuit become damped due to circuit losses.

5. To overcome these circuit losses, voltage amplification is necessary.

6. Desired oscillations can be maintained by using some part of the output voltage as feedback to the tuned circuit that is of the correct amplitude and in-phase (0°).

7. To keep the output signal in phase with the input, the overall phase shift of the circuit must be zero.

In conclusion, oscillators are a crucial component in the field of electronics. They are used to generate AC signals of known frequency and amplitude and have a wide range of applications, from testing electronic systems to defining the carrier frequency in transmitting systems. A working knowledge of oscillators, their construction, and operating characteristics is essential to analog electronics.

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