First, the topic overview and significance

1. 1 summarizes the development of radio.

Since people found that information can be transmitted by radio waves, they have been developing different methods to increase the reliability, miniaturization, energy saving and portability of equipment. A receiver used to receive information, commonly known as a radio. At present, the radio can not only receive the radio, but also receive the TV set that can receive images and digital telegrams. With the development of broadcasting technology, radios are constantly upgrading. 1920 radio broadcasting has been developing for more than half a century. The radio has undergone three generations of changes: tube radio, transistor radio and integrated circuit radio, with more and more functions and higher quality. Since 1980s, radio has been developing towards the direction of circuit integration, digital display, stereo, function computerization and miniaturization.

1947, Bell Laboratories invented the first transistor in the world, and the transistor era of radio began. Gradually ended the primary stage of radio represented by crystal receiver and electron tube radio. 1956, Siemens of west Germany developed UHF transistor, which created the necessary conditions for FM transistor radio. 1959, Sony Corporation of Japan produced the first generation FM transistor radio. 196 1 year, the United States developed integrated circuits. Subsequently, in 1966, Japan used this technology to design the world's first integrated circuit radio, which opened another technological revolution in the radio industry. Since then, the radio has developed towards miniaturization, serialization, integration, low power consumption and multifunction.

The main problem of repeater is that it is difficult for a high frequency amplifier to adapt to various working frequencies. If we can find a way to keep the working frequency of high-frequency amplifier constant, then many problems can be easily solved. The superheterodyne radio is designed according to this guiding ideology. The following are some functions of the superheterodyne radio:

Circuit structure of superheterodyne radio: The superheterodyne radio is characterized by frequency conversion (frequency conversion) process, and adopts a fixed tuned intermediate frequency amplifier. Generally, it includes the following parts: frequency conversion stage, intermediate frequency amplification stage, detection stage, low frequency preamplifier stage and low frequency power amplifier stage. The frequency conversion stage includes a mixer and a local oscillator. The high frequency amplitude modulation signal received by the antenna is sent to the mixer of the frequency conversion stage through the tuning loop and selection. The local oscillator circuit always tracks the received signal and generates a constant amplitude oscillation signal with a fixed frequency, which is also sent to the mixer. The two signals sent to the mixer make use of the nonlinear characteristics of the amplifier to generate a new difference signal. After the frequency conversion stage, the high frequency amplitude modulation signal only changes the carrier frequency, and the modulation law has not changed, so it is still an amplitude modulation signal.

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