CLASSIFICATION OF SOUND WAVES

Sound waves can be classified into following three classes:

1.       Audible waves

2.       Infrasonic waves

3.       Ultrasonic waves

Audible waves:

The waves which can be heard by human ears are called audible waves. Their frequency range extends from 20 Hz to 20 KHz but it may vary from person to person in accordance to his age. Children and youth can hear sounds of up to 20 KHz frequency whereas the audible range of old persons is lesser than 20 KHz.

Infrasonic waves: 

Waves having a frequency of less than 20 Hz are called infrasonic waves. These waves cannot be heard by human ears. These waves are produced by the vibration of huge bodies like the earth.

Ultrasonic waves:

Waves having frequency of more than 20 KHz are called ultrasonic waves. These waves also cannot be heard by human ears, but can be heard by bats. The bat can hear waves of frequencies up to 50 to 60 KHz and can produce such waves even.

These waves have proved to be very useful to mankind. These are used for communications and for the determination of depth of sea. Besides it, these are used for increasing agricultural yields, for improving the quality of seeds, for protection against insects and in the field of treatment especially surgery.

ARTICULATION OF SOUND

 Articulation or modification of sound is necessary before broadcasting. For this, the following instructions should be observed:

1.       The broadcasting room should be perfectly sound-proof so that unwanted sounds may not enter in the room.

2.       Microphone, record player, tape recorder and amplifier etc. should have a high efficiently so that all the fundamental and harmonic frequencies remain undistorted.

3.       The frequency range of sound is limited between 30 Hz to 12 KHz since the frequency of all sounds produced by man, sound and musical instruments lies between 30 Hz to 12 KHz. By limiting the frequency range, amplification of high quality and high fidelity is easily possible.

4.       Before sending the sound signals to the transmitter they should be amplified up to a required level.

SOUND

It is a form of energy which is experienced by human ears. It is produced by the vibration of bodies, by man, animals and birds and by natural interruptions. Sound is propagated in the form of waves and some medium is essential for its propagation. The medium may be a solid, liquid or gas but it should have sufficient elasticity.

The velocity of sound in dry air at 0˚C is 332 meters per second and in water it is 1435 meters per second. Similarly the velocity of sound in different medium is different medium is different. The velocity of sound in the air is affected by humidity, temperature, density of dust particles and the speed of air circulation.

FREQUENCY AND WAVELENTH

Frequency: for a wave, the number of cycles completed per second is called its frequency. Its symbol is ƒ and its unit is hertz (Hz).

Wavelength: straight distance travelled by a wave in one cycle is called its wavelength. In other words, the distance between the two consecutive particles vibrating in the same phase is called wavelength. Its symbol is λ (lamda, a Greek letter) and its unit is meter.

Time period: the time taken by one complete set of vibration, i.e., one cycle is called time period. Its symbol is T and its unit is seconds. Therefore

T= 1/ƒ or ƒ=1/T

T= time period, seconds

ƒ =frequency, hertz

Relation: the wavelength decreases by increasing the frequency and conversely the wavelength increases by decreasing the frequency, but their product remains constant and it indicates the velocity of the wave. Hence

Ѵ =ƒ.λ   or   Ѵ = λ/T

Where,        Ѵ = velocity of the wave, meters per second

                     λ = wavelength, meters  

                     ƒ = frequency, hertz

                     T = time period, seconds

Application of Direct Spread Spectrum Signals

 Here we will discuss three types of applications of direct sequence spread spectrum signals: anti jamming signals, low probability intercept and code division multiple access of allow multiple users.

1.      Anti jamming with the help of direct sequence spread spectrum signals

In the frequency band of the interest somebody else transmits the signals intentionally.

Since these signals lie in the frequency band of the transmission, the interfere the required signal. Hence it becomes difficult to detect the required signals. This is called jamming effect.

Such problems normally occur in military applications. If enemy knows the frequency band, then he can send jamming signals intentionally. With the help of spread spectrum method, the transmitted signals are spread over the mid frequency band. Hence these signals appear as noise. Then it becomes difficult for the jammers to send jamming signals. This is called anti jamming. For the anti jamming application three codes are used in conjunction with direct sequence spread spectrum method. These are Golay code (24, 12) expurgated Golay (24, 11) and maximum length shift register code.

2.      Low detectability signal transmission or low probability intercept

For this application, the signal spectral density is intentionally kept small with respect to the channel noise and receiver noise so that the presence of the signal is not detected easily. Consider that the average received signal power at the intended receiver is Pav and the noise power is Nav. Then the signal is transmitted at low power levels such that Pav/Nav << 1. Hence the receivers which are in the vicinity of the intended receiver cannot detect the presence of the signal. The intended receiver recovers the information bearing signal with the help of processing gain and coding gain. Other receivers do not know the pseudo-noise sequence hence they cannot receive the information signal with the help of processing gain and coding gain. This is called as the signal has low probability of being intercepted. It is also called low detect ability signal transmission.

3.      Code division multiple access with direct sequence SS (SSMA)

In this application, many users transmit their signals on the same channel bandwidth. Each transmitter receiver pair has a distinct pseudo-noise (PN) sequence. Thus signals of a particular transmitter are received by its intended receiver only, even if many users are transmitting at the same time. This method is also called spread spectrum multiple access (SSMA). The signals from other users appear as additive interference which is rejected y the spread spectrum decoder. The level of interference depends upon the number of users transmitting at any time. The main advantage of CDMA is that the number of users sharing the same channel can be increased or decreased very easily. Large number of users can transmit on the same channel if their messages are for short periods of time. For this method it is desirable that the PN sequences be mutually orthogonal.

Sensor

A Sensor is a device, which responds to an input quantity by generating a functionally related output usually in the form of an electrical or optical signal.

Thermocouples
 
A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage proportional to a temperature difference between either ends of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a heat gradient into electricity. They are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy and system errors of less than one degree Celsius (C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor.

Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

LIGHT EMITTING DIODE (LED)

Beside tunnel diode, varactor diode, schottky diode, step recovery diode, pin diode, there is a gun diode which is used for controlling electronic signals in the GHz range.

A P-N junction diode, which emits light when forward biased, is knows as a lights emitting diode. The emitted light may be visible or invisible. The amount of light output is directly proportional to the forward current. Thus higher the forward current, higher is the light output. The arrows, pointing away from the diode symbol represent the light, which being transmitted away from the junction.

Here and type layer is grown on P type substrate by a diffusion process. Then a thin P type layer is grown on the N-type layer. The metal connections to both the layers make anode and cathode terminals as indicated. The light energy is released at the junction, when the recombination of electrons with holes takes place. After passing through the P-region, the light is emitted through the window provided at the top of surface.

It will be inserting to know that when LED is forward bias, the electrons and holes move towards the junction and the recombination take place. After recombination, the electrons, laying in the conduction bands of N-region, fall into the holes lying in the valence band is radiated in the form of light energy. In ordinary diodes, this energy is radiated in the form of heat.

The semiconducting materials used for manufacturing light emitting diodes are gallium phosphate and gallium arsenide phosphate. The silicon and germanium is not used for manufacturing light emitting diodes because these are heat producing materials. Moreover, these materials are very poor in emitting light radiations.

The LED, radiate light in different colors such as red, green, yellow, blue, orange etc. some of the LED’s emits  infrared light also. The colors, of the emitted light, depend upon the type of the semiconductor used. Thus gallium arsenide emits infrared radiations, gallium arsenide phosphate produces either red or yellow light gallium phosphate emits red or green light and gallium nitrite produces blue light.

Zener Diode And Tunnel Diode

Zener Diode:

It is also a p-n junction diode. Its safe inverse voltage or break down voltage is kept lower than that of an ordinary diode and each diode is designed to have a specific brake down voltage. The amount of leakage current increases suddenly by increasing the reverse bias. The current flowing through the zener diode on break down voltage is called avalanche current or zener current.

A zener diode is used in voltage regulator circuits. A zener diode is connected across the voltage to be regulated with a series registers in opposite polarity. The load register is connected in parallel to the zener diode.

When the input voltage rises beyond the Break down voltage of the zener diode, the conduction of current is started through the series resistor Rs. The magnetic of current conducting through the resistor Rs is equal to the sum of zener plus load current. The magnitude of Rs resistors current will increase further for increase in input voltage. 

But, due to decreased zener resistance, only the magnitude of zener current will increase and not the magnitude of load current. Now, the will be more voltage drop across Rs and the output voltage will remain unchanged. In this way, the circuit can effectively work for small changes of even less than one volt.

The break down voltage of zener diodes in printed on them eg.12 v, 27v etc.

TUNNEL DIODE:

In this diode a P-N junction is made in such a way that the electrons passing through the junction have to cross a very narrow junction region and they have to managing any how to cross the junction.

In forward bias state, the magnitude of current reaches at its maximum value at a very low voltage, whereas the flow of current doesn’t start in an ordinary diode at a voltage. On increasing the forward bias, the current will decrease and it will be reduced to a minimum value at a certain high forward bias. The property of tunnel diode is termed as its negative resistance characteristic. If the forward bias is increased further, the magnitude of current will start to increase again.

A tunnel diode is used in amplifiers and oscillators of gigahertz frequency and range and in flip-flop gates.

                                                    

TRANSFORMERS CLASSIFIED AS PER OUTPUT

 There are two types of transformers on the basis of output obtained from the same. 

1.       1.Voltage step-up Type:

In this type of transformer, the voltage available at the secondary winding is greater than the voltage applied at the primary winding. Its secondary winding consists of a greater no. of turns in comparison to that of the primary winding.

        Let the transfer of electrical energy be 100% then

        Energy in primary= Energy in secondary

        Pp=Ps

        OR   Ip*Ep=Is*Es   

In this way, it is noted that by increasing the secondary’s voltage its current step-down transformer       as well.

2.       2.Voltage step-down Type:

In this type of transformer, the voltage available at the secondary winding is lesser than the voltage applied at the primary winding. Its secondary winding is lesser than the voltage applied at the primary winding. Its secondary winding consists of a lesser no. of turns in comparison to that of the primary winding. A voltage step-down transformer is a current step-up transfer as well. It is used in electric welding; electroplating etc. And for such purpose its size is made quite large.

TRANSFORMERS CLASSIFIED AS PER CORE

There are three types of transformers on the basis of core used between the windings:

1. Core type:

(a) Open core type:

In this type of transformer, the primary and secondary windings are wound on lathered paper spools of cylindrical or cubical shape. Laminated iron cores are fitted inside the spool. These types of transformer are almost out of use now.

(b) Closed Core type:

In this type of transformer L-shaped laminated cores are used which form a closed magnetic path. In this way the leakage of magnetic flux is greatly reduced in this type of transformer which increases the magnitude of electrical energy transferred.

2. Shell type:

In this type of transformer E and I-shaped laminated cores are used. The primary and secondary windings are wound one above the other on the central path of the core. In this way, the magnetic flux is divided into two parts at the center of core and covers both the windings all around. Most of the L.F. and A.F. transformers are of shell type.

3. Berry Type:

This is an improved form of a shell type of transformer. The main core of the transformer is cylindrical are both the winding wound on it. 8 to 10 shell joined to the main core cover the coil all around. In this way, the leakage of magnetic flux is minimized and the efficiency of the transformer is maximized. The transformer is used in special types of circuits.