Satellite Communication Lecture Notes

Satellite Communication Lecture Notes

SATELLITE COMMUNICATION

A satellite is an object that revolves around another object. For example, earth is a satellite of The Sun, and moon is a satellite of earth etc. All planets in the solar system can be considered as satellites of sun. An object in the solar system can have more than one satellite.

A communication satellite is a microwave repeater station in a space that is used for telecommunication, radio and television signals. A communication satellite process the data coming from one earth station and it converts the data into another form and send it to the second earth station.
                                       
HOW A SATELLITE WORKS

Two stations on earth want to communicate through radio broadcast but are too far away to use conventional means. The two stations can use a relay station for their communication.
One earth station transmits the signal to the satellite. Uplink frequency is the frequency at which ground station is communicating with satellite.
The satellite transponder converts the signal and sends it down to the second earth station, this is called Downlink.
                                         
ADVANTAGES OF SATELLITE

1. The Coverage area is very high than that of terrestrial systems.
2. The transmission cost is independent of the coverage area.
3. Higher bandwidths are possible.

DIS- ADVANTAGES OF SATELLITE

1. Launching satellites into orbits is a costly process.
2. The bandwidths are gradually used up.
3. High propagation delay for satellite systems than the conventional terrestrial systems.
                       
PATH TRACED BY A SATELLITE

The path traced by a satellite around the earth is an elliptical path with two focuses.

How Do Satellites Work :

Satellite Communication Basics :

The process of satellite communication begins at an earth station. Here an installation designed to transmit and receive signals from a satellite in orbit around the earth. Earth stations send information to satellites in the form of high powered, high frequency (GHz range) signals. The satellites, which receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. Satellite's footprint is the area which receives a signal of useful strength from the satellite. The transmission system from the earth station to the satellite through a channel is called the uplink. The system from the satellite to the earth station through the channel  is called the downlink. Below Figure shows the basic elements of a satellite communications system.

Satellite Frequency Bands:

The satellite frequency bands which was commonly used for communication are the C-band, Ku-band, and Ka-band. C-band and Ku-band are the commonly used frequency spectrums by today's satellites. It is important to note that there is an inverse relationship between frequency and wavelength i.e. when frequency increases, wavelength decreases this helps to  understand the relationship between antenna diameter and transmission frequency. Larger antennas (satellite dishes) are necessary to gather the signal with increasing wavelength.

From 4 to 8 GHz frequency range, C-band satellite transmissions occupy. Than the Ku-band or Ka-band, these relatively low frequencies translate to larger wavelengths. These larger wavelengths of the C-band mean that a larger satellite antenna is required to gather the minimum signal strength. Hence the minimum size of an average C-band antenna is approximately 2-3 meters in diameter. It is shown in Figure.

The frequency range from 11 to 17 GHz occupied by Ku-band satellite transmissions. These relatively high frequency transmissions correspond to shorter wavelengths. Thus a smaller antenna can be used to receive the minimum signal strength. Ku-band antennas can be as small as 18 inches in diameter. It can be commonly seen in the RCA DSS and Sony DSS systems. The Ku-band antenna of the Sony DSS system is shown in below figure.

The 20 to 30 GHz frequency range is occupied by the Ka-band satellite transmissions. These very high frequency transmissions means very small wavelengths and therefore very small diameter receiving antennas are used.

Geosynchronous Earth Orbit (GEO) Satellites

        The majority of satellites in orbit around the earth are positioned at a point 22,238 miles above the earth's equator in a special type of geosynchronous earth orbit (GSO) known as Geostationary earth orbit (GEO). it is also called as the Clarke orbit. This is in honour of Arthur C. Clarke. He is the man who first suggested in 1945 that satellites in geosynchronous orbits could be used for communications purposes. A satellite can maintain an orbit with a period of rotation around the earth exactly equal to 24 hours at the precise distance of 22,238 miles. satellites appear stationary from the earth’s surface, since the they revolve at the same rotational speed of the earth. Due to this reason, most earth station antennas (satellite dishes) don't need to move once they have been properly aimed at a target satellite in the sky. The mathematical derivation of the Clarke orbit can be obtained as a straight-forward calculus problem.

                                                        The Clarke Orbit

Medium Earth Orbit (MEO) Satellites

The technological innovations in space communications during the last few years, have given rise to new orbits and totally new systems designs. Medium earth orbit (MEO) satellite networks will orbit at distances of about 8000 miles from earth's surface. Signals transmitted from a MEO satellite travel a shorter distance. This translates to improved signal strength at the receiving end. This shows that smaller, more lightweight receiving terminals can be used at the receiving end. Also, since the signal is travelling a shorter distance to and from the satellite. Hence there is less transmission delay. Transmission delay can be defined as the time it takes for a signal to travel up to a satellite and back down to a receiving station. For real-time communications, the shorter the transmission delay, better the communication system. As an example, a GEO satellite requires .25 seconds for a round trip. A MEO satellite requires less than .1 seconds to complete a round trip. MEOs operates in the frequency range of 2 GHz and above.

Low Earth Orbit (LEO) Satellites

The LEO satellites are mainly classified into three categories: little LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth's surface. This relatively short distance reduces transmission delay to only .05 seconds. This further reduces the need for sensitive and bulky receiving equipment. Little LEOs will operate in the 800 MHz (.8 GHz) range. Big LEOs will operate in the 2 GHz or above range, and Mega-LEOs operates in the 20-30 GHz range. The higher frequencies associated with Mega-LEOs translates into more information carrying capacity and yields to the capability of real-time, low delay video transmission scheme. Microsoft Corporation and McCaw Cellular (now known as AT&T Wireless Services) have partnered to deploy 840 satellites to form Teledesic. It is a proposed Mega-LEO satellite network.

High Altitude Long Endurance (HALE) Platforms

Experimental HALE platforms are basically highly efficient and lightweight airplanes carrying communications equipments. This will act as very low earth orbit geosynchronous satellites. These crafts will be powered by a combination of battery and solar power or high efficiency turbine engines. HALE platforms will offer transmission delays of less than .001 seconds at an altitude of only 70,000 feet, and even better signal strength for very lightweight hand-held receiving devices.

Orbital Slots

Here there may arise a question that with more than 200 satellites up there in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space?. To answer this problem, international regulatory bodies like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit where the communications satellites can be located. These locations are specified in degrees of longitude and are called as orbital slots. The FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satellites due to the huge demand for orbital slots.

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Diodes Lecture Notes

Diodes Lecture Notes

The Basic Diode Theory :

Diodes can be defined as non-linear devices. That is we can not apply superposition to circuits containing diodes. By Ohms law, when a voltage is applied across a resistor, the current flows in proportion to the voltage and the proportionality factor is constant. The voltage- current (VI) characteristic for a resistor is shown in Figure 1and is expressed analytically by V = I*R, I=V/R

Figure 1: Resistor circuit and its V-I characteristics

The V-I characteristics of a diode is shown in figure 2 and is described analytically by the equation .


Where Is is the reverse saturation current, a small ( ≈ 10^-9 A  ) current will occur at the negative voltage region. q / KT  is a constant dependents on temperature ≈ 1/0.026V  at room temperature. VD is the voltage occurred across the diode and

is the  dimensionless constant determined by the type of the diode (Silicon, Germanium, etc.)
 

Figure 2: Diode circuit and its V-I characteristics

For the non-linearity of a diode, consider the circuit shown in Figure 3
 

Figure 3: A diode circuit with 2 voltage sources.

When V1 and V2 are applied simultaneously, the voltage across D is -3 V and the resulting current is very close to -Is . If superposition is applied, we get  V1 produces 2 V across D and  I1 = 2190 ISV2  Produces -5 V across D and I2 = -Is . If we add these two currents, we will get I = 2189 Is.  which is an incorrect current. Since the superposition principle does not apply, we can say that a diode is a non-linear element.

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Electronics Basics

Electronics Basics

Here we are going to discuss about what is electronics. In my experience, when I ask what is electronics there is a tendency for many ones that electronics is the flow of electrons. It is a blunderanswer. The flow of electrons is called as the Electricity. Actually electronics is a branch of science which can be defined by the institution of radio engineers as:” The field of science and engineering, which deals with electron devices and their utilization”. The electron devices are those devices where the electricity (current) flow is mainly due to the flow of charge carriers in a controlled manner through a gas, a vacuum or a semiconductor. In the earlier stages of development of engineering, electronics was considered as an integral part of electrical engineering, but due to the advancements and developments during the last decades, electronics gained a rightful place in the field of engineering.
                                     
The major breakthrough in the field of electronics was achieved when semiconductor devices such as silicon and germanium are able for making electronic devices. In addition to these gallium arsenide technologies was also developed. The advantages of semiconductor devices are: The semiconductor devices are smaller, cheaper, and more reliable and they consume less power than vacuum or gaseous devices. The development of digital electronics (from 1946) gave a boost to the growth of electronics. In digital electronics there is not a continuous form of signals, instead it has only two possible levels: On or off state. These two possible states are typically represented by voltage levels.
                                     
The further development of electronics was possible only due to integrated chips (IC), in which a large number of devices are fabricated into a small silicon chip, usually known as integrated circuit. Now the number of devices on a chip passed over fifty million. The applications of electronics and communication are instrumentation, entertainment, general communication, medical electronics, computers, wireless communication, etc.
                                           
The future of our world looks exciting as the change is accelerating rapidly. Thus if we choose the new technologies, it will be better for our world and the way of living.
Now we are going to divide the components of electronics. The electronic components are broadly classified into passive and active components. The passive components are linear elements which control or modify the electronic circuit. The passive devices will not play an important role in the circuit’s performance. Without the help of active devices, it is impossible to design and implement electronic devices. The main passive components are resistors – energy dissipating devices, capacitors – energy storing devices, inductors and transformers.

Electricity is the flow of electrons in a conductor. The important measurements used for measuring electricity are voltage, current, resistance and power.

The first one is the voltage across a resistor. This term indicates the level of energy electrons relative to a reference point (i.e. ground in a circuit). If the voltage is high then, more energy electrons will travel through the circuit. If two points are at a different voltage levels with each other, then electricity will flow from one point to       other, if they are connected by some conductor. The unit of voltage is Volt (V). As sample of voltages at home are 110V, the AA, C & D cells we buy rated at 1.5V, and the Teleo Modules requires 5V.

The next one is current. This shows how much charge is travelling through the conductor per second. The unit of current is the Amperes (A). We can see that voltage and current are separate (different from each other) things: We will get a very small current at a very high voltage, a huge current at a very high voltage and so on.
The voltage and current is related by Ohm’s law:
V=I*R
Where’ v’ is the voltage, ‘I’ is the current and ’R’ is the resistance.

The next one is the resistance. Resistance (R) is an expression in which electron flow is impeded through a conductor. The unit is Ohm ( ). The resistance determines the relation between voltage and current (by ohm’s law). The amount of current is determined by the resistance, the conductor offers. If resistance is less then more current will flow. For a given power source of high enough capacity, if \we half the resistance, we will double the current. Conversely, if we double the resistance, then current will be half.

The last one is power. The unit of power is Watt (W). It is an expression of the overall energy consumed by a component. It is worked out by multiplying the voltage and the current together, P = VI. For example if a motor was running at 24V and the current drawing was 1.5A, the power dissipating would be 36W. (Electronics Basics)

THE CIRCUIT ELECTRONICS BASICS

There needs to be a path for electricity to flow, which connects all the elements together. 

The electricity will travel from the positive (+) side of the cell around the circuit to the negative side. This is called as the conventional current. The direction of electricity will be in the opposite direction of the flow of electrons.

The flow of electrons: 

The direction of electricity: 

In the given diagram, we can see how electricity travels from a cell around a loop through the lamp and back to the cell again.

Lamp and Cell Circuit

First figure refers to open circuit and second one refers to short circuit. The current will travel from one point to another only if a closed circuit is available.(Electronics Basics)

Some symbols used to depict cells and lamps are,

Cell: This is also called as a battery, but technically a battery is multiple cells. The ordinary cell is rated at 1.5V.

Lamp: Lamps have voltage ratings like many things. The rating indicates the voltage that a lamp is designed to used. Lamps may also use their wattage - the power they consume. From this and we use the equation for power (I = P / V) the likely current consumed can be calculated.

Simple Lamp and Cell Circuit Diagram

This circuit diagram expresses only the essential features of the circuit. 

If battery is connected then the lamp will blows.(Electronics Basics)

More Electronic Components:

Resistor is the device that resists the flow of electricity The two important values associated with resistors are their resistance and their power rating. Resistance is measured in Ohms ( ).1kΩ is 1000 . The other value is power. Resistors dissipate energy so its important that at exactly how much energy they can dissipate is known. Read More.

Capacitor is the device that temporarily stores the electric charge. There are two main important values that characterize a capacitor. The first is the capacitance - measured in Farads, Micro-Farads ( F) or Pico-Farads (pF). The other quantity is the voltage (V). Read More.

Diode is the Semiconductor device that conducts electricity in only one direction.. Zener diodes permit conduction in the reverse direction only when the reverse voltage exceeds a certain value. TVS diodes are like Zener diodes except capable of much higher currents. A diode consists of a p-n junction. The combination of p and n type semiconductor will leads to the formation of p-n junction diode. Read More.

MOSFET. (metal-oxide silicon field-effect transistor). It mainly consists of source drain and gate. When the Gate (G) terminal voltage is sufficiently high, current will flow from the Drain (D) to the Source (S) terminal. the Gate voltage to be required is usually12V above the Source. MOSFETs are characterized by several values: their maximum voltage, their resistance and max power dissipation. 

Potentiometer is a variable resistor which Often connected as a Voltage-divider to create variable voltages when used as a rotational position sensor. 

LED is Light Emitting Diode. It Produces a lot of light for small current. But it will very quickly burn out if too much current is allowed to flow in it. A resistor in series with it to limit the current is usually the requirement.

Photoresistor is a resistor with the property that its resistance changes depending on how much light it receives. Photoresistors have a quite impressive resistance range(a few million ohms (M ) in the dark to a hundred ohms in bright light). One disadvantage is their reaction time is in the order of 100ms (too slow for many applications).

Supply. It saves having to draw a wire from the power source to every point in the circuit.

Ground. It is used to indicate that whatever is connected should be considered to be connected to the Ground of the power supply (it is the common point of a circuit).

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Communication Basics

Communication Basics

The process of data transfer from one person to another person is called communication. All the living organisms in the earth communicate. If two persons are far away (they are not near with respect to each other), then the message from one person needs to be modulated and transmitted to the other person. For doing like this it requires one transmitting antenna and one receiving antenna. The message from one person will be encoded, modulated and transmitted to the other through a transmitting antenna. The receiving antenna will collects the transmitted signal and it decodes the encoded data. The way of communication con be either one sided (example T.V, radio etc) or two way communication (Mobile phones).

The communication may be either analog or digital communication.
Analog means we are transmitting the signal in the analog form.
Digital communication means we are transmitting the signal in the digital form.

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Freshers day 2014 will be conducted on 26th August 2014.
Venue: KMCT PTC Auditorium
Time: 1.00 PM

Extron's freshers day inauguration & blog launching program

PCB DESIGNING FOR ENGINEERS

With the announcement of government of India’s manufacturing policy expected soon, the stage seems to be well set for robust growth in the PCB market in near future, opening up a minefield of opportunities for PCB designers

For any electronics product, printed circuit board (PCB) forms the basic foundation for interconnecting and packaging. PCBs are used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated on a non-conductive  substrate (source: Wikipedia). PCBs are also referred to as printed wiring boards (PWBs), or etched wiring boards, which have evolved over the years from uncomplicated single- and double-sided plated-through-hole (PTH) to become multi-layered PCBs. 

The fortunes of the PCB industry depend on the health of IT, telecom, automobile, electronic gaming and consumer electronics. Due to the rapid growth of the electronics industry worldwide, the global PCB market has witnessed a remarkable growth over the last two decades. 

What’s the market like?
Japan and Asia-Pacific collectively contribute a major share to the global PCB market. Amongst Asian countries, India and China are central to the growth of the PCB industry, thanks to the rapid expansion of electronic manufacturing bases. This growth is triggered by the shifting of production activities and facilities of key multinational players in these countries.

The demand from automotive electronics segment drives a positive outlook for the PCB market over the next few years. As electronic components account for a fair portion of the overall value of materials consumed in cars’ production, the stage seems to be well set for robust growth in the PCB market in near future. “Without PCB, we can not expect any electronic circuit. Since a PCB provides physical support as well as interconnections between different electronic components, there exist great opportunities as well as very good growth in this field worldwide,” says Shavinder Singla, deputy engineer in PCB Lab at Centre for Development of Advanced Computing (CDAC), Mohali.

K. Krithiga, technical lead at CADD Center, adds, “For electricals and electronics engineering (EEE) and electronics communication engineering (EC) students, this is a good field to start their career. What they have studied during their engineering course, they should apply to circuit design and PCB design. Initially, engineering graduates can enter into this field as a circuit designer or a board designer.” 

A glimpse
PCB designers are key persons in research and development (R&D), electronics production units as well as in the PCB industry. A PCB designer’s job comes with a lot of responsibilities. “Nowadays it’s not only about making interconnections between electronic components but also understanding the functioning of the circuit. It is often observed that a circuit which functions well when assembled over a bread board may or may not function properly over a poorly designed PCB layout. Depending upon the complexity of the circuit or PCB it may take two to three days for system engineers to locate and rectify the problems,” Singla says.

There can be numerous reasons for such kind of failures; PCB design and fabrication being one of them. “If a PCB is not designed or fabricated as per the circuit requirements, the circuit may not function at all, or function partially. Generally, circuit designers overlook the PCB design/fabrication part so as to reduce the product’s time-to-market or to compensate for the delay. Sometimes PCB designers or fabricators are not allowed to devote sufficient time as intended,” Singla explains.

What your pocket says
Let’s turn to money matters. This field offers enough provisions for learning as well as earning. The starting salary for a fresher is typically between Rs 150,000 and Rs 200,000 per annum with impressive year to year hikes. Coming to experienced professionals, the PCB industry offers salary in the range of Rs 500,000 to Rs 600,000 per annum for people with four to five years of experience.

“As a fresher one can not expect high salary. But after an experience of five-six years, and with understanding and implementation of standards, one can expect a very good package and may act as a team leader,” adds Singla. And of course if you grab an international opportunity, the pay is quite good even for freshers. It is needless to mention that PCB designers can experience much better growth while working with an MNC.

“It’s been my experience that the designers who work on the higher-end or more expensive CAD tools (i.e., Cadence Allegro and Mentor Graphics) seem to get paid a little higher than designers who work on the lower-end or less expensive systems like Altium and PADS. As I mentioned earlier, back in the late 90s, as long as you were a good designer you would be hired regardless of the CAD tool. But now, unfortunately, hiring-managers seem to put an emphasis on which CAD systems you’re running and even what version, etc,” opines Derek Doucette, an executive with American Contract Group, a Massachusetts-based staffing firm. He has been placing candidates into PCB design positions since the 1990s.

“A designer once told me, “Besides a few features here and there, they basically all do the same thing. 

You just need to find out where they place the buttons.” I believe that if you are a good designer on one CAD system, with a slight learning curve, you should be able to pick up another system with no problem,” Derek told PCBDesign007 in an interview. 

Who’s hiring?
It is difficult to point out a particular industry as the major recruiter of PCB designers. Aspirants in this field may look for opportunities in all industries which are directly involved in manufacturing electronics products. These include telecom, automotive, biomedical equipment, computer hardware, electronic security systems, consumer electronic appliances, mobile companies, industrial automation and power electronics, amongst others.

“The big organisations always have a team of six-eight designers while the middle ones have two-five designers. The team size depends upon the development of new hardware or modification of the existing one, and also on the size of the R&D team. However, to name a few, some of the major recruiters in this field are Bharat Electronics Limited, Semiconductor Complex Laboratory, HCL, L&T, GE, HP, Beetel, Nokia, Samsung, LG, Ericsson, Sony and Philips,” informs Singla.

Krithiga adds, “Small-sector PCB design companies are recruiting freshers. MNCs like Wipro, Applied Materials, TCS, Ingersoll Rand, and most of the electronics goods designing companies offer a good opportunity for PCB design engineers.”

Getting the job signal
Recruiters look for electronic and information engineering (EIE), EEE and ECE engineers with basic electronics components knowledge with any board design software like OrCAD, Allegro, Altium or PADS. At present, most of the companies are using Or-CAD and Allegro for circuit board design.

A PCB designer’s ‘must haves’ as stated by Singla are as follows:

1.    Understanding of the functioning of an electronic circuit.
2.    Knowledge of IPC/Mil standards.
3.    Know the process of PCB manufacturing, assembling and testing.
4.    Good knowledge of various electronic components’ packaging.
5.    Know about different PCB base materials.
6.    Able to interact with circuit designers, PCB manufacturing and assembling team.
7. Knowledge of EMI/EMC.

If the PCBs are not designed properly, one runs the risk of facing a high level of rework/rejection of populated or unpopulated PCBs. “I have also seen a lot of circuit malfunctions/ failures in the field, which are a result of poor design, fabrication or soldering techniques,” Singla says. He also suggests that PCB designers should have a detailed knowledge of not only PCB fabrication but also soldering techniques. Also, they are required to be kept abreast of the limitations of PCB manufacturers with respect to minimum drill-size, minimum track-width, minimum track-to-track and track-to-pad distance, tolerances, etc.

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