Wednesday, October 24, 2012

IEEE 802 Standards

IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.

IEEE 802.1Bridging (networking) and Network Management
IEEE 802.2Logic Link Controlinactive
IEEE 802.3Ethernet
IEEE 802.4Token busdisbanded
IEEE 802.5Defines the MAC layer for a Token Ringinactive
IEEE 802.6Metropolitan Area Network (Distributed Queue Dual Bus)disbanded
IEEE 802.7Broadband Local Area Network using Coaxial Cabledisbanded
IEEE 802.8Fiber Optic TAGdisbanded
IEEE 802.9Integrated Services LAN disbanded
IEEE 802.10Interoperable LAN Securitydisbanded
IEEE 802.11 a/b/g/nWireless LAN (WLAN) & Mesh (Wi-Fi certification)
IEEE 802.12100BaseVGdisbanded
IEEE 802.13Unused
IEEE 802.14Cable modemsdisbanded
IEEE 802.15Wireless Personal area Network
IEEE 802.15.1Bluetooth certification
IEEE 802.15.2IEEE 802.15 and IEEE 802.11 coexistence
IEEE 802.15.3High-Rate wireless PAN
IEEE 802.15.4Low-Rate wireless PAN (e.g., ZigBee, WirelessHART, MiWi, etc.)
IEEE 802.15.5Mesh networking for WPAN
IEEE 802.15.6Body area network
IEEE 802.16Broadband Wireless Access (WiMAX certification)
IEEE 802.16.1Local Multipoint Distribution Service
IEEE 802.17Resilient packet ring
IEEE 802.18Radio Regulatory TAG
IEEE 802.19Coexistence TAG
IEEE 802.20Mobile Broadband Wireless Access
IEEE 802.21Media Independent Handoff
IEEE 802.22Wireless Regional Area Network
IEEE 802.23Emergency Services Working Group
IEEE 802.24Smart Grid TAGNew (November, 2012)
IEEE 802.25Omni-Range Area Network

Network bridging describes the action taken by network equipment to allow two or more communication networks, or two or more network segments creating an aggregate network. Bridging is distinct from routing which allows the networks to communicate independently as separate networks. A network bridge is a network device that connects more than one network segment.

Monday, October 22, 2012

The Nuclear Power plant

The nuclear energy of the power plant is released from the atomic nucleus of a heavy element called uranium. Thus unlike a traditional power plant that consumes fossil fuels, a nuclear power plant consumes uranium. 

What actually happens to the uranium nuclei in a nuclear power plant? When a neutron collides with the nucleus of a uranium atom, the nucleus is split into two smaller nuclei in a process known as nuclear fission, releasing nuclear energy and emitting two or three neutrons. These released neutrons then collide with other uranium nuclei, producing further splitting and more energy. This continuous process of nuclear fission is known as chain reaction and a lot of energy is produced. A chain reaction is shown in the figure below.  

The Nuclear Chain Reaction

The nuclear power plant is divided into two main parts: the Nuclear Island and the Conventional Island. The Nuclear Island contains nuclear fuel in closed tubes packed together, collectively known as the reactor core. Nuclear energy is converted to heat energy in the reactor core. The Conventional Island is where heat energy received from the Nuclear Island is converted to electrical energy. Nuclear energy is converted to electrical energy in a nuclear power plant. The animation below will show you the inside of a nuclear power plant such as the Guangdong Daya Bay Nuclear Power Station.

What is Quantum tunneling?

Let's say you are throwing a rubber ball against a wall. You know you don't have enough energy to throw it through the wall, so you always expect it to bounce back. Quantum mechanics, however, says that there is a small probability that the ball could go right through the wall (without damaging the wall) and continue its flight on the other side! With something as large as a rubber ball, though, that probability is so small that you could throw the ball for billions of years and never see it go through the wall. But with something as tiny as an electron, tunneling is an everyday occurrence. 

Quantum tunneling is possible because of the wave-nature of matter. Confounding as it sounds, in the quantum world, particles often act likes waves of water rather than billiard balls. This means that an electron doesn't exist in a single place at a single time and with a single energy, but rather as a wave of probabilities.

What is Quantum Mechanics?

In the early 20th century some experiments produced results which could not be explained by classical physics (the science developed by Galileo Galilei, Isaac Newton, etc.). 

For instance, it was well known that electrons orbited the nucleus of an atom. However, if they did so in a manner which resembled the planets orbiting the sun, classical physics predicted that the electrons would spiral in and crash into the nucleus within a fraction of a second. 

Obviously that doesn't happen, or life as we know it would not exist. (Chemistry depends upon the interaction of the electrons in atoms, and life depends upon chemistry). 

Probability Density of electron in an Hydrogen Atom

That incorrect prediction, along with some other experiments that classical physics could not explain, showed scientists that something new was needed to explain science at the atomic level. 

Thus, quantum mechanics is the study of matter and radiation at an atomic level. 

For everyday things, which are much larger than atoms and much slower than the speed of light, classical physics does an excellent job. Plus, it is much easier to use than either quantum mechanics or relativity (each of which require an extensive amount of math). 

The d and s subs shells in the Quantum mechanics atom model

Every quantum particle is characterized by a wave function. In 1925 Erwin Schrödinger developed the differential equation which describes the evolution of those wave functions. By using Schrödinger's equation scientists can find the wave function which solves a particular problem in quantum mechanics.

The following are among the most important things which quantum mechanics can describe while classical physics cannot:
  • Discreteness of energy
  • The wave-particle duality of light and matter
  • Quantum tunneling
  • The Heisenberg uncertainty principle
  • Spin of a particle
Some funny facts! 

'It is impossible, absolutely impossible to explain it in any classical way'. 
Richard Feynman

[I can't accept quantum mechanics because] "I like to think the moon is there even if I am not looking at it. God does not play dice with the universe."
 Albert Einstein

"[T]he atoms or elementary particles themselves are not real; they form a world of potentialities or possibilities rather than one of things or facts." 
Werner Heisenberg

"Anyone not shocked by quantum mechanics has not yet understood it."
 Neils Bohr

"Nobody understands quantum mechanics."
Richard Feynman

Zener Vs Avalanche Breakdown


In Zener breakdown the electrostatic attraction between the negative electrons and a large positive voltage is so great that it pulls electrons out of their covalent bonds and away from their parent atoms. ie Electrons are transferred from the valence to the conduction band. In this situation the current can still be limited by the limited number of free electrons produced by the applied voltage so it is possible to cause Zener breakdown without damaging the semiconductor.

When the P & N regions are heavily doped, direct rupture of covalent bonds takes place because of the strong electric fields.

The new hole-electron pairs so created increase the reverse current in a reverse biased PN diode.

The increase in current takes place at a constant value of reverse bias typically below 6V for heavily doped diodes.

For lightly doped diodes, zener break down voltage becomes high and breakdown is then by Avalanche multiplication.

Avalanche breakdown occurs when the applied voltage is so large that electrons that are pulled from their covalent bonds are accelerated to great velocities. These electrons collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which leads to more collisions etc. The current in the semiconductor rapidly increases and the material can quickly be destroyed.

As the applied reverse bias increases, the field across the junction increases correspondingly.

Thermally generated carriers while traversing the junction acquire a large amount of kinetic energy from this field. As a result the velocity of these carrier increases. These electrons disrupt covalent bonds by colliding with immobile ions and create new hole-electron pairs.

These new carriers again acquire sufficient energy from the field and collide with other immobile ions, thereby generating further hole electron pairs. This process is cumulative in nature and results in generation of an avalanche of charge carriers with in a short time. This mechanism of carrier generation is known as Avalanche multiplication. This process results in flow of large amount of current at the same value of reverse bias. 

Usually the Avalanche Breakdown occurs above 6V. 

Sunday, October 14, 2012

Microwave Transmission

  • Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimetre. 
  • Microwave radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30 centimeters down to 1.0 cm.
  • Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. 
  • Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it.
  •  A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

  • Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. 
  • Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

Wednesday, October 10, 2012

What is War of Currents?

Why mostly the Power Distribution systems uses AC over DC?

Actually, at the time of development in Electrical Systems and Electrical Distribution, Both AC and DC were used for the Power Distribution.

AC Generator

Thomas Edison Promoted DC Power Distribution System and Nikola Tesla and George Westinghouse, promoted the AC Power Distribution System.
This is known as War of currents! (1880s)

Finally AC won over the DC Power Distribution System.

The main Reasons for use of AC in Power Distribution System are:

1. When Electricity passes through a Conductor, there is Transmission loss in the form of Heat. It is also known as Ohmic Loss.

Power Loss = I^2 Rt
Where I  = Current
           R = Resistance of the Conductor
            t = time

So if we transmit larger currents, the loss will be more.
However, in AC, there is a relation for the Transformers
ie, when the Voltage at one end of the transformed increases, Current decreases.
This is because, we have to keep the product of Current and Voltage constant.

So if we can transmit the Current at Higher Voltage, it will reduce the current through the Conductor and in effect the Ohmic Power loss will decrease.

2. Long distance Power Transmission is possible with Higher voltage. This can be easily achieved using the Transformers in AC, However Stepping up and Stepping Down using Transformers are not possible in DC.

AC Power Distribution

However there are some advantages for DC over AC

DC power maintains a constant direction of current. One advantage of DC power is there is no reactance in the line. 

This allows higher power transfer capability, higher capacity utilization of generators, and less of a voltage drop along the line. 

DC also has a lower line resistance than AC because of the “skin effect” in AC. This is when charge is carried mostly near the outside of the wire.

In the DC system, power is just the real component. This means that the transmission system operator need not worry about the sufficiency of reactive power to maintain the security and stability of the system.

In DC, there is no frequency, so generators connected to the transmission grid do not need to be synchronized.

The DC system does not introduce susceptance along the line thus removing the effect of changing current and over voltages in the system.

Analysis of DC systems only involves real numbers, while AC systems involve complex numbers. (Think about a world without AC; How easy will be the calculations in Electrical Engineering :-) )

A good Resource for studying the AC Theory is available Here

Tuesday, October 09, 2012

Why all Digital Electronics Circuits use DC and Not AC?

The question will be little confusing.
But the answer is simple.


In Digital Electronics, Gates are the basic Elements.
Actually this Gates are made up of Transistors.

NAND gate using Transistors

Transistors are working as a Switch in Digital Electronics.

Transistor as a Switch

ie, When control signal is present, Transistor is ON, otherwise Transistor is OFF.

Now, What is this Control Signal?

That is the Signal Applied to the Base of the Transistor.

The Switch must be ON till the control signal is present and the Switch must be OFF till the control signal is absent.

Switch with Control Terminal

Now consider applying AC signal as the control signal to the Base of the Transistor.

The AC signal will vary from Positive peak to Negative peak going through the 0V.

So how can we keep the Transistor ON and OFF as per our requirement?
It is not possible.

Now consider DC. It is Direct current and it is constant in value.
So if we apply DC to the Base of the Transistor as a control signal, we can keep the Transistor ON of OFF as per our wish.

That is In digital Electronics, We need only HIGH signal and LOW signal, not the intermediate values.
Hence we cannot use AC in Digital.

Now consider Transistor working as an AMPLIFIER.
Here also, the transistor is working in DC (Power supply of the Transistor is DC), but the input is an AC signal.

Transistor as an Amplifier

Thus Amplification of AC signal is just an application of the Transistor and that doesn't mean that the Transistor is working in AC.

Why cant we power the Transistor with AC?
We can apply AC as a Power supply to the Transistor.
But the transistor will not give the desired operation.

Biasing of Transistor (a) NPN  (b) PNP

Because, for acting as a Switch or Amplifier, the transistor should be biased.
In order to keep the transistor in constant Biasing conditions, we need Constant current. ie DC.

If we apply AC as a power supply to the transistor, the Biasing conditions of the Transistor will be varying in each cycle of the AC signal.

Hence the transistor will not work properly.
This is the reason why we convert the AC signals into DC using Rectifiers in the Power supply section of the Electronic Devices.

(We can apply the same principle to MOSFET also.
MOSFET are used as switches in Digital Electronics as Switches.
Working principle of MOSFET is same as that of the Transistor.
However, MOSFET is a Voltage controlled Device, but Transistor is a Current controlled Device.)