The principle of operation of the super heterodyne receiver depends on the use of heterodyning or frequency mixing. The signal from the antenna is filtered sufficiently at least to reject the image frequency and possibly amplified. A local oscillator in the receiver produces a sine wave, which mixes with that signal, shifting it to a specific intermediate frequency (IF), usually a lower frequency. The IF signal is itself filtered and amplified and possibly processed in additional ways. The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original information (such as audio).
Block diagram of a typical super heterodyne receiver
The diagram at right shows the minimum requirements for a single-conversion super heterodyne receiver design. The following essential elements are common to all super heterodyne circuits: a receiving antenna; a tuned stage, which may optionally contain amplification (RF amplifier); a variable frequency local oscillator; a frequency mixer; a band pass filter and intermediate frequency (IF) amplifier; and a demodulator plus additional circuitry to amplify or process the original audio signal (or other transmitted information)
6. Write down the translation of these two texts and entitle them:
Text A
As it is known the history of radio transmitters dates back to 1895 when a great Russian scientist A. Popov transmitted the first radiogram. Since that time many Russian and foreign scientists contributed much to the theory of radio transmitting devices.
The function of the radio transmitter is to convert the electrical power received from a primary source into radio-frequency energy modulated with a signal for transmission by means of electromagnetic waves through space.
The radio transmitter consists of two principal components: the radio-frequency section and the audio-frequency one. The radio-frequency section produces radio-frequency power of continuous waves, the audio-frequency section being concerned with modulation of radio signals.
The parameters of the radio transmitter are output power, frequency stability, efficiency and modulation. Radio transmitters are classified into many different types. When classifying them according to the service for which they are used, radio transmitters may be of communication, broadcast, radar and other types. Taking into consideration the type of transmitting signals, specialists subdivide radio transmitters into telegraph, telephone and pulse transmitters. According to the power consumed transmitters are of low power, medium power and other types. At last, they may be of fixed and mobile types. To meet the requirements of high transmission quality of radio transmitters much is being done for improving radio transmitters’ performance by developing new design of these devices.
Text B
As it is known, that Russian scientist V. Siforov worked out the theory of radio receiving devices. However, A. Popov invented and demonstrated the first radio receiving set. Since that time, radio devices have been improving and perfecting.
The receiver performs the function of converting the current in the receiving antenna into the intelligence contained in the transmission the man parameters of radio receivers are sensitivity, selectivity and fidelity. Sensitivity is a measure of the receiver’s ability to receive weak signals, as it is known that the farther an electromagnetic wave travels, the weaker is its energy. Selectivity is the ability of the receiver to reject undesirable signals. Fidelity is a measure of the receiver’s ability to reproduce clearly audio-frequency currents, which are in accordance with the modulation envelope of the received signals.
However simple the radio receiver may be, it includes an antenna, an input tuning circuit, a detector and a pair of earphones.
The principle of operation of the radio receiver is not very difficult to understand. The electromotive force is impressed upon the receiving antenna and produces a current; this current is a reproduction of the current of the transmitting antenna.
There are various types of receivers, communication and broadcast receivers being the principal types of them. Communication receivers are used in radiotelephone and telegraph service, broadcast receivers finding application for the reception of sound and visual programs. Wherever radio receivers were applied, they must meet an important requirement as reliability in operation.
7. Answer the following questions:
What is radio communication?
What are the main components of radio communication?
What does a transmitter consist of?
What is used for amplifying currents?
What happens in the microphone when we speak into it?
What are the advantages of Internet radio?
What new sciences could develop due to radio communication?
What are distinguishing features of mathematical theory dealing with
communication system?
What does the modern communication theory stem from?
What was Wienner’s contribution into the development of communication theory?