## The principle, development and application of radio frequency circuit

Click： Time：2020-08-20 09:11:30

One. What is a radio frequency circuit?

Radio frequency is abbreviated as RF, radio frequency is radio frequency current, which is an abbreviation of high-frequency alternating electromagnetic waves. The alternating current that changes less than 1000 times per second is called low-frequency current, and the one that changes more than 1000 times is called high-frequency current. Radio frequency is such a high-frequency current.

A radio frequency circuit refers to a circuit whose electromagnetic wavelength and the size of the circuit or device are in the same order of magnitude. At this time, due to the relationship between the size of the device and the size of the wire, the circuit needs to be dealt with by the related theory of distributed parameters. This kind of circuit can be considered as a radio frequency circuit without strict requirements on its frequency, such as long-distance AC transmission lines (50 or 50 60Hz) Sometimes it is also dealt with by the related theory of RF.

"The principle, development and application of radio frequency circuits"

two. The principle and development of radio frequency circuit

The main application area of radio frequency circuits is wireless communication. Let's analyze the role of radio frequency circuits in the entire wireless communication system.

In the system model of a wireless communication transceiver (tranceiver), it includes a transmitter circuit, a receiver circuit and a communication antenna. This transceiver can be used in personal communications and wireless local area networks. In this system, the digital processing part is mainly to process digital signals, including sampling, compression, encoding, etc.; and then through the A/D converter into an analog form into the analog signal circuit unit.

The analog signal circuit is divided into two parts: the transmitting part and the receiving part.

The main function of the transmitting part is: the low-frequency analog signal output by the digital-to-analog conversion and the high-frequency carrier provided by the local oscillator are up-converted into a radio frequency modulated signal through the mixer, and the radio frequency signal is radiated into the space through the antenna. The main function of the receiving part is: the space radiation signal is coupled to the receiving circuit through the antenna, and the received weak signal is amplified by the low noise amplifier and then down-converted with the local oscillator signal through the mixer to a signal containing intermediate frequency signal components.

The function of the filter is to filter out the useful intermediate frequency signal and input the analog-digital converter to convert it into a digital signal, and then enter the digital processing part for processing.

Low noise amplifier (LNA) discusses the composition and characteristics of general radio frequency circuits.

The input signal is input to the amplification module through a matched filter network. The amplifier module generally adopts a common emitter structure of transistors, and its input impedance must match the output impedance of the filter located in front of the low-noise amplifier to ensure the best transmission power and minimum reflection coefficient. For RF circuit design, this Matching is required. In addition, the output impedance of the low-noise amplifier must match the input impedance of the mixer at the back end, which can also ensure that the signal output by the amplifier can be input to the mixer completely and without reflection. These matching networks are composed of microstrip lines, and in some cases may also be composed of independent passive components, but their electrical characteristics at high frequencies are completely different from those at low frequencies. The microstrip line is actually a copper-clad tape with a certain length and width, and the chip resistors, capacitors and inductors are connected to the microstrip line.

In the theory of electronics, when current flows through a conductor, a magnetic field is formed around the conductor; when an alternating current passes through the conductor, an alternating electromagnetic field is formed around the conductor, which is called electromagnetic wave.

When the electromagnetic wave frequency is lower than 100khz, the electromagnetic wave will be absorbed by the surface and cannot form effective transmission, but when the electromagnetic wave frequency is higher than 100khz, the electromagnetic wave can propagate in the air and be reflected by the ionosphere at the outer edge of the atmosphere to form long-distance transmission capability , We call the high-frequency electromagnetic wave with long-distance transmission capability as radio frequency, the English abbreviation: RF. High-frequency circuits are basically composed of passive components, active devices and passive networks. The frequency characteristics of components used in high-frequency circuits are different from those used in low-frequency circuits. Passive linear components in high frequency circuits are mainly resistors (devices), capacitors (devices) and inductors (devices).

In the field of electronic technology, the characteristics of radio frequency circuits are different from ordinary low frequency circuits. The main reason is that under high frequency conditions, the characteristics of the circuit are different from those under low frequency conditions, so it is necessary to use radio frequency circuit theory to understand the working principle of radio frequency circuits. Under high frequency conditions, stray capacitance and stray inductance have a great influence on the circuit. Stray inductance exists in the internal self-inductance of the wire connection and the component itself. Stray capacitance exists between the conductors of the circuit and between components and ground. In low-frequency circuits, these spurious parameters have little effect on the performance of the circuit. As the frequency increases, the influence of the spurious parameters becomes greater. In the high-frequency heads of early VHF-band television receivers and the front-end circuits of communication receivers, the influence of stray capacitance is so great that it is no longer necessary to add additional capacitance.

In addition, the circuit has a skin effect under RF conditions. Unlike direct current, current flows in the entire conductor under direct current conditions, while current flows on the surface of the conductor under high-frequency conditions. As a result, the high frequency AC resistance is greater than the DC resistance.

Another problem in high-frequency circuits is the electromagnetic radiation effect. As the frequency increases, when the wavelength is comparable to the circuit size 12, the circuit becomes a radiator. At this time, various coupling effects will occur between the circuits and between the circuits and the external environment, leading to many interference problems. These issues are often insignificant under low frequency conditions.

With the development of communication technology, the frequency used by communication equipment is increasing. Radio frequency (RF) and microwave (MW) circuits are widely used in communication systems. The field of high-frequency circuit design has received special attention from the industry. New semiconductor devices have made high-speed Digital systems and high-frequency analog systems continue to expand. The carrier frequency of the microwave radio frequency identification system (RFID) is within the frequency range of 915MHz and 2450MHz; the global positioning system (GPS) carrier frequency is within the frequency range of 1227.60MHz and 1575.42MHz; the radio frequency circuit in the personal communication system works at 1.9GHz, and It can be integrated on personal communication terminals that are getting smaller and smaller; the C-band satellite broadcast communication system includes a 4GHz uplink communication link and a 6GHz downlink communication link. Usually the operating frequency of these circuits is above 1GHz, and with the development of communication technology, this trend will continue. However, to handle such high-frequency circuits, not only special equipment and devices are required, but also theoretical knowledge and practical experience that are not used in DC and low-frequency circuits.

three. Application of radio frequency circuit

RF (Radio Frequency) technology is widely used in many fields, such as: TV, broadcasting, mobile phones, radar, automatic identification systems, etc. The special term RFID (Radio Frequency Identification) refers to the use of radio frequency identification signals to identify targets.

RFID applications include:

◆ ETC (Electronic Toll Collection)

◆ Railway locomotive and rolling stock identification and tracking

◆ Container identification

◆ Identification, certification and tracking of valuables

◆ Target management of commercial retail, medical care, logistics services, etc.

◆ Access prohibition management

◆ Animal identification and tracking

◆ The vehicle is automatically locked (anti-theft)

The main application areas of radio frequency bands are:

1.Satellite Communications and Satellite TV Broadcasting

* Inter-satellite communication: 36GHz

2. Microwave relay communication

* Trunk microwave: 2.1GHz, 8GHz, 11GHz

* Branch line microwave: 6GHz, 8GHz, 11GHz, 36GH

* Rural multiple access (one point multiple access): 1.5GHz, 2.4GHz, 2.6GHz

* Precision guidance: X,, Ka

* Meteorology: 1.7 GHz, 0.1375 GHz

* Car anti-collision, automatic accounting: 36 GHz, 60 GHz

* Anti-theft: 9.4 GHz

* Global positioning: 1227.60MHz and 1575.42MHz

4. Radio astronomy: 36GHz, 94GHz, 125GHz

5. Computer wireless network: 2.5 GHz, 5.8 GHz, 36GHz

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