intermediate frequency

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

When divided by frequency, MF (Medium Frequency) refers to the frequency range from 300KHz to 3000KHz, most of which are AM stations. When divided according to its position and function in the circuit: IF (Intermediate Frequency) refers to a signal obtained by frequency conversion of high-frequency signal.

Chinese name: intermediate frequency

Foreign name: Medium Frequency/ Intermediate Frequency

catalog

one IF - IF
two Zero IF receiving technology
three Medium frequency induction heating
▪ working principle
▪ Advantages of induction heating

IF - IF

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Intermediate frequency has two meanings:

First, divide the time according to the frequency. MF (Medium Frequency) means, Frequency band The frequency from 300KHz to 3000KHz is mostly used as AM radio. In the daytime, the transmission distance is short. At night, the ionosphere can be used to reflect signals, so it can be used for long-distance transmission. However, it is vulnerable to interference from other radio waves.

The pitch of most natural instruments falls in the middle band. The most concentrated place of human voice energy is 500-1000 Hz. The overtones of many natural instruments also mainly fall in the middle frequency band. For example, the overtones of guitars mainly fall in the 2000-5000 Hz range. So are violas and cellos.^[1]

Second, when divided according to its position and function in the circuit: IF (Intermediate Frequency)^[2] It refers to a signal obtained by frequency conversion of high-frequency signal. In order to make the amplifier work stably and reduce interference, the general receiver should change the high-frequency signal into IF signal 。

Picture of TV IF signal It's 38MHZ

Audio IF signal It is 6.5MHZ

Medium and short wave radio IF signal 465KHz

The IF of FM radio is 10.7MHZ

RF refers to the transmission frequency, because some signals themselves may not be suitable for direct transmission (the frequency is illegal, or the conditions of the signal itself do not allow). Therefore, to modulate the signal, the modulator itself needs a suitable oscillation signal. Add the original signal to it. This oscillation signal is called carrier, and the modulated carrier contains it.

Zero IF receiving technology

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Schematic diagram of zero IF receiving technology

Zero IF reception technology, that is, RF signal does not need to be converted to IF, but directly converted to analog baseband I/Q signal at one time, and then demodulated. In recent years, Software As a new technology, radio has revolutionized the field of traditional radio technology. Zero IF has become very useful. Open source software radio GNU Radio is free Software development tools Kit. It provides a signal operation and processing module, which can be used in the production of low-cost radio frequency (RF) hardware and General purpose microprocessor Implementation on Software Define the radio. This suite is widely used by amateurs, academic institutions and commercial institutions for research and construction wireless communication System. GNU Radio is mainly used in Python programing language To write. But its core signal processing module is built by C++on the microprocessor with floating point operation. Therefore, developers can easily and quickly build a real-time, high-capacity wireless communication System. Although its main function is not emulator, GNU Radio supports pre storage And (signal generator).

Medium frequency induction heating

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Induction heating is mostly used for Industrial metals spare parts Surface hardening , metal smelting, bar diathermy and other fields are to generate a certain induction current on the surface of the workpiece to rapidly heat the surface of the part, so as to achieve the effect of rapid surface heating, or even diathermy melting.

working principle

Principle of induction heating: the workpiece is placed in the inductor, which is generally a hollow copper tube that inputs medium frequency or high-frequency alternating current (300-300000Hz or higher). The alternating magnetic field produces the induction current of the same frequency in the workpiece. The distribution of this induction current in the workpiece is uneven. It is strong on the surface, but weak inside. It is close to 0 at the center Skin effect The surface of the workpiece can be heated rapidly in a few seconds Internal surface The temperature rises to 800-1000 º C, while the central temperature rises very little.

The medium frequency furnace uses 200-2500Hz medium frequency power supply for induction heating, smelting and heat preservation. It is mainly used for smelting carbon steel, alloy steel, special steel, and also for smelting and temperature raising of copper, aluminum and other non-ferrous metals. The equipment is small in size, light in weight, high in efficiency, low in power consumption, fast in melting and temperature rise, easy to control furnace temperature, and high in production efficiency

Working principle of medium frequency induction water heater

The power supply mode is different from the power frequency electric boiler, which is to input the 380V power frequency AC power supply into the intermediate frequency power supply cabinet (the intermediate frequency power supply cabinet has overvoltage protection and overcurrent protection, and will automatically stop and alarm if there is overvoltage and overcurrent), and then integrate the power frequency AC power supply into single-phase DC power in the intermediate frequency power supply cabinet, and then convert it into single-phase AC intermediate frequency voltage The current (single-phase AC intermediate frequency voltage and current are safe) is used to input the single-phase AC intermediate frequency voltage and current into the heater to generate induction electricity for induction heating of water in the boiler. Therefore, water and electricity are separated in the heating process of the boiler, which is safe and reliable.^[3]