03-06-2013, 04:53 PM
Visakhapatnam Steel Plant(VSP)
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ABSTRACT
In this modern world of Industrialization and automation, speed of communication plays a major role inthe growth of any organization. The communication among various departments plays an impacting and direct role in the growth of organizations like Visakhapatnam Steel Plant.
The Telecommunication department plays a vital role in providing & maintaining different electronic communication systems in various departments to achieve the assigned targets and accomplishing the desired performance in VSP.
In Vizag Steel Plant, The vhf communication systems(wireless communication systems) are implemented tomonitor and speed up the communication and information exchange as fast as possible. The objective of this vhf communication systems is to monitor and communicate various distant departments easily and fast.
Introduction to VSP:
Visakhapatnam steel plant is the only shore based integrated steel plant in our country. Smt.Indira Gandhi laid the foundation stone fulfilling the long cherished dream of the people of Andhra Pradesh.
Due to the past experience it was realized that in order to be viable, this plant need to operate at high levels of efficiency comparable with international standards. It is also necessary that the plant reach its rated capacities at the shortest possible time. For achieving this it is essential that this plant be characterized as one having
a) Minimum manpower
b) Better discipline
c) Good team work.
MAJOR PRODUCTION FACILITIES:
- VSP has the following major production facilities:
- 3 coke oven batteries of 67 ovens each and 41.7 cu mt Volume
- 2 Sinter machines of 312 sq mt area
- 2 Blast furnaces of 3200 cu mt useful volume
- SMS with 3 LD converters of 150 ton capacity and 6 nos of 4 strand continuous bloom casters
- Light and medium merchant mill of 7,10,000 tons per year capacity
- Wire rod mill of 8,50,000 tons per year capacity
- Medium Merchant & Structural mill of 8,50,000 tons per year capacity
Modulation:
Modulation is defined as the process by which some characteristic of the signal, usually amplitude, frequency or phase, of a sinusoidal voltage is varied in accordance with the instantaneous value of some other voltage called the modulating voltage.
The term carrier is applied to the voltage whose characteristic is varied and the term modulating voltage is used for the voltage in accordance with which the variation is made. Usually the modulation frequency is considerably lower than the carrier frequency.
Need for Modulation:
There are two alternatives to the use of a modulated carrier for the transmission of messages over long distance in the radio channel. Several difficulties are involved in the propagation of electromagnetic waves at audio frequencies below 20 Kilohertz. For efficient radiation and reception the transmitting and receiving antennas would have to have heights comparable to a quarter wavelength of the frequency used. All sound is concentrated within the range from 20 Hz to 20 KHz, so that all signals from the different sources would be hopelessly and inseparably mixed up. In any city, the broadcasting stations alone would completely blanket the “air”, and yet they represent a very small proportion of the total number of transmitters in use.
In order to separate the various signals, it is necessary to convert them all to the different portions of the electromagnetic spectrum. Each must be given its own frequency location. This overcomes the difficulties of poor radiation at low frequencies and reduces interference. Once signals have been translated, a tuned circuit is employed in the front end of the receiver to make sure that the desired section of the spectrum is admitted and all the unwanted ones are rejected. The tuning of such a circuit is normally made variable so that the receiver can select any desired transmission within a predetermined range, such as the VHF broadcast band used for frequency modulation. Separation of signals has removed many difficulties in the absence of modulation. An unmodulated carrier has constant maximum amplitude, a constant frequency and a constant phase relationship. In a continuous modulation system, one of the parameters of the carrier is caused to vary. Thus at any instant its deviation from the unmodulated value is proportional to the instantaneous value of the modulating voltage, and the rate at which this deviation takes place is equal to the modulating frequency.
Levels of modulation:
In low-level modulation little power is associated with either the signal or the carrier. The output of the modulator is at a lower power level. A series of linear power amplifiers are then used to boost the signal level to higher levels.
In high-level modulation the carrier and information signals are amplified to sufficiently higher power levels before modulation. This requires amplifiers, which are linear over a wide range of frequencies.
Modulation techniques:
Among the available types of modulation techniques, Amplitude modulation, Phase modulation and Frequency modulation are most popular. Modulation is the process of converting a signal from its primary form to a form, which is most suitable for transmission. This is realized by using a high frequency signal as ‘carrier’ and varying one of its parameters like amplitude or phase or frequency as a linear function of the instantaneous value of the modulating signal the ‘message’. At the receiver end the reverse modulation ‘de-modulation techniques are employed to extract the signal ‘message’
In all these three techniques the frequency component of the modulating signal would occupy a different frequency component of the frequency spectrum in the modulated form. All the above are linear modulation techniques.
Pre-emphasis and De-emphasis:
The boosting of the higher modulating frequencies, in accordance with a prearranged curve, is termed Pre-emphasis and the compensation at the receiver is called De-emphasis. Take two modulating signals having the same initial amplitude, and one of them is pre-emphasized to twice this amplitude, other is unaffected (being at lower freq). The receiver has to de-emphasize the first signal by a factor of 2, to ensure that both signals have the same amplitude in the O/p of the receiver. When this signal is de-emphasized, any noise sideband voltages are de-emphasized with it, and therefore have lower amplitude than they would have had without emphasis. There effect on the output is reduced. The higher modulating frequencies must not be over emphasized. The curves of figure show that a 15 KHz signal is Pre-emphasized by about 17dB. When such boosting is applied the resulting signal cannot over-modulate the carrier by exceeding the 75 KHz deviation, or distortion will be introduced. It is difficult to introduce Pre-emphasis and de-emphasis in existing AM services since extensive modifications would be needed, particularly in view of the huge numbers of receivers in use.
Propagation of Radio waves:
Surface Wave :
A ground wave travels along the surface of the Earth (Thus the ground wave is required to be vertically polarized-vertical electric field). As the wave travels it induces energy currents into the Earth and thus looses energy as it travels. Due to dissipation of energy into the Earth the surface wave suffers attenuation, which is a function of Earth’s conductivity. The loss is compensated by the downward flow of energy from upper layers due to diffraction. The wave front gradually tilts as it propagates and finally the wave dies. Thus range depends on the power and frequency with which the initial wave is transmitted.
Space Wave:
For frequencies in the range of 30 MHz ground waves get attenuated within a few hundred feet distance. These waves do not get reflected even by the Ionosphere. Hence the only way of transmission is by line of sight. The limitation is the curvature of the Earth. Space wave comprises of two types. The direct wave and the ground reflected wave. The problem with the later type is unless the resultant of the direct wave and reflected wave is significant at the receiving end the signal strength may not be of any use. The problem of shadow zone is associated with space, antenna height and LOS reception.
Ionospheric Wave:
Due to ultra violet radiation the molecules in the atmosphere get ionized. Since the density of molecules is high and radiation is low at lower heights the ionization effect is not felt. But at distances of 50 Kms to 500 Kms above the Earth’s surface this is considerable. This area of ionization is called ionosphere.