Добавил:
Upload Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:
Дидактичнi розробки для студентiв 4 курсу РТФ.doc
Скачиваний:
3
Добавлен:
03.11.2018
Размер:
578.05 Кб
Скачать

XII. Read the following supplementary text and translate it using, a dictionary.

SUPPLEMENTARY TEXT.

Planning the broadcasting-satellite service in the 12 GHz band involves assigning frequency channels and satellite positions in the geostationary orbit to all telecommunication administrations in accordance with their requirements. In their article, published in Telecommunication journal — monthly magazine of the International Telecommunication Union, Dr. A. L. Badalov and Prof. S. V. Borodich— two eminent telecommunication experts of the USSR — discussed the need to plan the broadcasting-satellite serv­ice and the conditions governing frequency sharing by television broadcasting facilities and analogue multichannel telephone radio-relay systems.

They found formulae for calculating interference in the telephone channels of the radio-relay system and determined the maximum permissible power flux density produced by a space station in the area of a radio-relay link in a 4 kHz band. They also studied the influence of radio-relay stations on earth receiving stations of the broadcasting-satellite system. The Soviet scientists explained that if there is no plan, the introduction of every new broadcasting-satellite system may necessitate a review of the frequency plans of the existing terrestrial services. Moreover, the introduction of every new system will necessitate co-ordination with the systems previously brought into operation.

XIII. Say a few words about the types of amplifiers and their application. Lesson 5 Text: What is amplitude modulation and % ge of modulation?

Pre-text exercises:

I. Memorize the pronunciation of the following words:

adjacent, assume, coupling, desirable, sufficiently, evaluate, mill watts, technique, discard, shrinkage

II. Read and translate the text: “what is amplitude and % ge of modulation”

Amplitude Modulation (AM) is a process in which the amplitude of a radio frequency current is made to vary and modify by impressing an audio frequency current on it. A radio frequency current has a constant amplitude in absence of modulation and this constant amplitude RF carries no information, i.e. no audio intelligence and is of no use to radio telephone (voice communication), but has application in Morse code communication. So, to give intelligence to the RF current, audio signal is impressed/superimposed on the RF current in a non-linear modulator circuit; as a result of which carrier current amplitude begins to rise to a maximum value above and below its original amplitude during the positive cycle of the audio signal and during the negative cycle of the audio signal, it falls to a minimum value. This results in the carrier having two outlines of the audio signal; this is because the variation at instant in the amplitude of the carrier wave is directly proportional to the value of the modulating signal. During amplitude modulation, two side band frequencies are also produced. Upper sideband frequencies equal to the carrier frequency plus audio frequency and lower side band frequency is equal to carrier frequency minus audio frequency. So the amplitude modulated carrier occupies a space in frequency spectrum, the width of which is equal to twice the highest modulating frequency.

Percentage of modulation:

The degree of modulation in an AM wave is expressed by %ge of maximum deviation from the normal amplitude of the carrier RF wave.

The effect of such modulated wave is measured by a receiver's ability to reproduce the signal in distorted or undistorted manner.

Why over modulation is not desirable?

Over modulation is not desirable, i.e. modulation should not exceed 100 %, because if modulation exceeds 100 % there is an interval during the audio cycle when the RF carrier is removed completely from the air thus producing distortion in the transmission.

Side bands are the sum and difference frequencies produced at the transmitter by the modulating frequencies.

What is Single Side Band (SSB) transmission?

At full modulation the carrier in an AM signal requires two thirds of the power but conveys no information. The second side band can be viewed as redundant (overlooking frequency-selective fading in an ionospheric transmission path, that may distort one side band at times). Interference between several carrier frequencies, resulting in steady audio whistles or 'beats' is another disadvantage of AM.

Power may be saved and the band occupied by an AM signal in the frequency spectrum can be halved if only one side band is transmitted without carrier. The result is single side band suppressed carrier signal, called simply single side band signal (SSB) transmission.

The carrier must be reintroduced at the receiver in such systems and closely adjusted to the original carrier frequency to avoid signal distortion. The introduced carrier carrier must be within 10 or 20 Hertz of the original carrier frequency for adequate intelligibility of voice signals, and stable oscillators are needed for generation of the local carrier.

RF Oscillator

This is the stage where the carrier frequency intended to be used is generated by means of Crystal Oscillator Circuitry or capacitance-inductance based Variable Frequency Oscillator (VFO). The RF oscillator is designed to have frequency stability and power delivered from it is of little importance, hence can be operated with low voltage power supply with little dissipation of heat.

Buffer Amplifier

The low power RF carrier output from the RF oscillator is amplified in this portion and it also keeps the RF oscillator and power amplifier circuits separate electrically imparting frequency as desired by the amateur can be done in this stage, when the carrier frequency multiplication technique is applied here. In it the Morse key for keying out carrier continuous wave can be accomodated.

Modulator

Audio information is impressed upon the carrier frequency at this stage Balanced Modulator

In this type of modulator, while the audio information (voice) is impressed upon the carrier frequency, at the same time its output gives a signal without carrier frequency but yet with the two side band frequencies carrying the voice/audio information.

RF linear Amplifier

RF power amplification is done here and this stage is coupled to the antenna system through antenna impedance matching circuitry. Care is taken at this stage so that no harmonic frequency is generated which will cause interference in adjacent band (splatter) on other bands.