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The Soviet A-7 VHF radio transceiver (later models include the A-7a and A-7b) was developed during World War II and used for communication in rifle brigades and regiments. The complete station was designed to be transported by an individual soldier.
The A-7 is a portable man-pack, radio transceiver with narrow-band frequency modulation. The set can be used as a radio telephone in a wired network and managed remotely. Setting the frequency of the receiver and transmitter circuits is adjusted in tandem by one control handle. The antenna and buffer oscillator circuits are common for the receiving and transmitting circuits.
The receiver section is a superheterodyne design with a single frequency conversion, consisting of 8 identical 2K2M valves. A regenerative detector with adjustable feedback is used. The transmitter section includes a 2K2M modulator valve, a CO-257 buffer amplifier valve, and a CO-257 power amplifier valve. Switching from reception to transmission is achieved by switching the power from the heater circuits of the receiver and transmitter valves, alternately. [1]
The transceiver with power supplies and accessories is housed in a wooden box with shoulder straps for carrying as a backpack. For long distances, the set is carried by two soldiers. The set only requires one radio operator. The deployment time is typically not more than five minutes.
A-7a is a simplified version of the A-7. The total number of valves was reduced to 9. In the transmitter section, the functions of the modulator and power amplifier were combined into one valve, and in the receiver section one valve was used to simultaneously perform the functions of a high-frequency and low-frequency amplifier (the so-called reflex circuit). The ability to work by telegraph was removed. Energy consumption was decreased by about 30%, other main characteristics were not changed. The A-7a set was manufactured by various factories in several different variants: with and without a tone button, with one or two measuring instruments, etc.
A-7b is a ten-valve version introduced in 1945. In the transmitter section two output valves are used in parallel. The transmitting range was increased by around 50%. The frequency range is 24-28 MHz. There is no feedback adjustment in the detector circuit.
The A-7b was also manufactured from 1950 to 1956 in Pardubice, Czechoslovakia by the Tesla company.
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude of the wave is varied in proportion to that of the message signal, such as an audio signal. This technique contrasts with angle modulation, in which either the frequency of the carrier wave is varied, as in frequency modulation, or its phase, as in phase modulation.
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is defined as a circuit that has a power gain greater than one.
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was long believed to have been invented by US engineer Edwin Armstrong, but after some controversy the earliest patent for the invention is now credited to French radio engineer and radio manufacturer Lucien Lévy. Virtually all modern radio receivers use the superheterodyne principle.
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A tuner is a subsystem that receives radio frequency (RF) transmissions, such as FM broadcasting, and converts the selected carrier frequency and its associated bandwidth into a fixed frequency that is suitable for further processing, usually because a lower frequency is used on the output. Broadcast FM/AM transmissions usually feed this intermediate frequency (IF) directly into a demodulator that converts the radio signal into audio-frequency signals that can be fed into an amplifier to drive a loudspeaker.
A radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. Radio waves are electromagnetic waves with frequencies between about 30 Hz and 300 GHz. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use radio: radio and television broadcasting, cell phones, wireless networks, radar, two way radios like walkie talkies, radio navigation systems like GPS, remote entry systems, among numerous other uses.
Radio receiver design includes the electronic design of different components of a radio receiver which processes the radio frequency signal from an antenna in order to produce usable information such as audio. The complexity of a modern receiver and the possible range of circuitry and methods employed are more generally covered in electronics and communications engineering. The term radio receiver is understood in this article to mean any device which is intended to receive a radio signal in order to generate useful information from the signal, most notably a recreation of the so-called baseband signal which modulated the radio signal at the time of transmission in a communications or broadcast system.
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