Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология
Образование Политология Производство Психология Стандартизация Технологии
Unit 2 Radio transmitters and receivers
1. Memorize the words:
Carrier wave – несущая волна
sine - синус
refer – отсылать; направлять; справляться; упоминать
slight – тонкий; хрупкий; легкий; незначительный
power supply – источник энергии
to amplify – усиливать
omnidirectional – действующий по всем направлениям
transverse – поперечный
conversely – наоборот
to exert – осуществлять; оказывать
back and forth – назад и вперед
2. Read the text and speak about the principle of operation of a transmitter:
A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.
-Power supply: provides the necessary electrical power to operate the
- Oscillator: creates alternating current at the frequency on which the
transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave.
-Modulator: adds useful information to the carrier wave. There are two main
ways to add this information. The first, called amplitude modulation or AM, makes slight increases or decreases to the intensity of the carrier wave. The second, called frequency modulation or FM, makes slight increases or decreases to the frequency of the carrier wave.
-Amplifier: amplifies the modulated carrier wave to increase its power. The
more powerful the amplifier, the more powerful the broadcast.
-Antenna: converts the amplified signal to radio waves.
3. Read the text and define the role of antennas in radio communication:
An antenna (or aerial) is an electrical device that converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. a high frequency alternating current (AC)) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, which is applied to a receiver to be amplified.
Antennas are essential components of all equipment that uses radio. They are used in systems such as radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications, as well as other devices such as garage door openers, wireless microphones, Bluetooth-enabled devices, wireless computer networks, baby monitors, and RFID tags on merchandise.
Typically an antenna consists of an arrangement of metallic conductors (elements), electrically connected (often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements, while the charge of the electrons also creates an oscillating electric field along the elements. These time-varying fields radiate away from the antenna into space as a moving transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna.
Antennas can be designed to transmit and receive radio waves in all horizontal directions equally (omnidirectional antennas), or preferentially in a particular direction (directional or high gain antennas). In the latter case, an antenna may also include additional elements or surfaces with no electrical connection to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern.
The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving.
4. Read the text and tell how radio receivers work:
A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the audio signals, and finally plays them on a speaker.
-Antenna: captures the radio waves. Typically, the antenna is simply a length
of wire. When this wire is exposed to radio waves, the waves induce a very small alternating current in the antenna.
-RF amplifier: A sensitive amplifier that amplifies the very weak radio
frequency (RF) signal from the antenna so that the signal can be processed by the tuner.
- Tuner: A circuit that can extract signals of a particular frequency from a mix
of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all.
Unless you want to listen to every radio channel at the same time, you need a
circuit that can pick out just the signals for the channel you want to hear. That’s the role of the tuner.
The tuner usually employs the combination of an inductor (for example, a
coil) and a capacitor to form a circuit that resonates at a particular frequency. This frequency, called the resonant frequency, is determined by the values chosen for the coil and the capacitor. This type of circuit tends to block any AC signals at a frequency above or below the resonant frequency.
You can adjust the resonant frequency by varying the amount of inductance
in the coil or the capacitance of the capacitor. In simple radio receiver circuits, the tuning is adjusted by varying the number of turns of wire in the coil. More sophisticated tuners use a variable capacitor (also called a tuning capacitor) to vary the frequency.
-Detector: Responsible for separating the audio information from the carrier
wave. For AM signals, this can be done with a diode that just rectifies the alternating current signal. What is left after the diode has its way with the alternating current signal is a direct current signal that can be fed to an audio amplifier circuit. For FM signals, the detector circuit is a little more complicated.
- Audio amplifier: This component's job is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple transistor amplifier circuit.
Of course, there are many variations on this basic radio receiver design. Many receivers include additional filtering and tuning circuits to better lock on to the intended frequency — or to produce better-quality audio output — and exclude other signals. Still, these basic elements are found in most receiver circuits.
5. Read the text and tell about the design and principle of operation of a
super heterodyne receiver. Use the bloc diagram:
Super heterodyne receiver
In electronics, a super heterodyne receiver (often shortened to superhet) uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original radio carrier frequency. It was invented by US engineer Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the super heterodyne principle. At the cost of an extra frequency converter stage, the super heterodyne receiver provides superior selectivity and sensitivity compared with simpler designs.
Последнее изменение этой страницы: 2016-03-17; Просмотров: 153; Нарушение авторского права страницы