What is a Radio Frequency?

Radio frequency (RF) is the abbreviation of Radio Frequency, which means the electromagnetic frequency that can be radiated into space. Radio frequency is radio frequency current, abbreviated as RF, which is an abbreviation for high-frequency AC changing electromagnetic waves. Alternating current that changes less than 1,000 times per second is called low-frequency current, and high-frequency current is greater than 10,000 times, and radio frequency is such a high-frequency current. Radio frequency (300K-300G) is a higher frequency band of high frequency (greater than 10K), and microwave frequency band (300M-300G) is a higher frequency band of radio frequency.

The most basic RFID system consists of three parts:

1. Tag (Radio Frequency Card): It consists of a coupling element and a chip, and the tag contains a built-in antenna for communication with the RF antenna;

2. Reader: a device that reads (and can also write in the card) the tag information;

3. Antenna: Passes RF signals between tags and readers. Some systems also connect to an external computer (host computer's main system) through the reader's RS232 or RS485 interface for data exchange.

At present, the operating frequencies of RFID products are defined as different products that meet different standards in the frequency range of low frequency, high frequency and very high frequency, and RFID products in different frequency bands will have different characteristics. There are two types of sensors: passive and active. ^[2]

The basic working process of the system is: the reader sends a certain frequency of radio frequency signals through the transmitting antenna. When the radio frequency card enters the working area of the transmitting antenna, an induced current is generated, and the radio frequency card obtains energy and is activated; the radio frequency card sends its own encoding and other information through the card to send The antenna is sent out; the system receiving antenna receives the carrier signal sent from the radio frequency card, and transmits it to the reader via the antenna adjuster, and the reader demodulates and decodes the received signal and then sends it to the background main system for related processing; the main system Judging the legality of the card according to logical operations, making corresponding processing and control for different settings, and sending instruction signals to control the actions of the actuator.

In terms of coupling method (inductive-electromagnetic), communication flow (FDX, HDX, SEQ), data transmission method (load modulation, backscatter, higher harmonics) from RF card to reader, and frequency range, etc., different There is a fundamental difference between non-contact transmission methods, but all readers are similar in terms of function principle and design structure determined by them. All readers can be simplified into two basic modules of high-frequency interface and control unit. The high-frequency interface includes a transmitter and a receiver. Its functions include: generating high-frequency transmit power to activate the RF card and providing energy; modulating the transmitted signal for transmitting data to the RF card; receiving and demodulating the High-frequency signals. There are some differences in the design of high-frequency interfaces for different RFID systems.

The functions of the control unit of the reader include: communicating with the application system software and executing commands sent by the application system software; controlling the communication process with the radio frequency card (master-slave principle); encoding and decoding of signals. For some special systems, there are additional functions such as implementing anti-collision algorithms, encrypting and decrypting data to be transmitted between the RF card and the reader, and performing authentication between the RF card and the reader.

The read-write distance of a radio frequency identification system is a key parameter. At present, long-distance radio frequency identification systems are still very expensive, so it is important to find ways to increase their reading and writing distance. The factors that affect the reading and writing distance of the RF card include the working frequency of the antenna, the RF output power of the reader, the receiving sensitivity of the reader, the power consumption of the RF card, the Q values of the antenna and the resonant circuit, the antenna direction, and the coupling between the reader and the RF card And the energy obtained by the RF card itself and the energy to send information. The read distance and write distance of most systems are different, and the write distance is about 40% to 80% of the read distance.

Many companies producing RFID products adopt their own standards, and there is no uniform international standard. ISO18OOO. The most commonly used are ISO14443 and ISO15693. These two standards are composed of four parts: physical characteristics, RF power and signal interface, initialization and anti-collision, and transmission protocols.

According to different methods, RF cards are classified as follows:

1. Divided into active card and passive card according to the power supply mode. Active means that there is a battery in the card to provide power, which has a long working distance, but has a limited life, large size, high cost, and is not suitable for working in harsh environments. There is no battery in the passive card. The received RF energy is converted into a DC power supply to power the circuit inside the card. Its working distance is shorter than that of the active card, but it has a long life and does not require high working environment.

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1. Power / level (dBm): the output capacity of the amplifier, the general unit is w, mw, dBm

2. Gain (dB): The magnification, the unit can be expressed in decibels (dB).

Note: dBm is the absolute power level expressed in decibels with 1mw as the reference value. Conversion formula:

Level (dBm) = 10lgmw

5W 10lg5000 = 37dBm

10W 10lg10000 = 40dBm

20W 10lg20000 = 43dBm

It is not difficult to see from the above that for each doubling of power, the level value increases by 3dB

That is: dB = 10lgA (A is the power amplification factor)

3 Insertion loss: The attenuation that increases after a certain device or component is connected to the transmission circuit. The unit is expressed in dB.

4 Selectivity: Measure the gain in the operating frequency band and the ability to suppress out-of-band radiation. The -3dB bandwidth is the bandwidth when the gain is reduced by 3dB, and the same applies to -40dB and -60dB.

5. Standing wave ratio (return loss): ratio of antinode voltage to node voltage (VSWR) in the standing wave state

Attachment: Standing wave ratioReturn loss comparison table:

SWR 1.2 1.25 1.30 1.35 1.40 1.50

Return loss (dB) 21 19 17.6 16.6 15.6 14.0

Note: The calculation formula for the standing wave ratio (SWR) based on the return loss (RL) is SWR = [10 ^ (RL / 20) +1] / [[10 ^ (RL / 20) -1]].

6. Third-order intermodulation: If there are two sinusoidal signals 1 and 2 due to nonlinear effects, many intermodulation components will be generated. The two frequency components 21-2 and 22-1 are called third-order intermodulation components, and their powers P3 and The ratio of the power of the signal 1 or 2 is called the third-order intermodulation coefficient M3.

That is M3 = 10lg P3 / P1 (dBc)

7. Noise coefficient: Generally defined as the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio, which is actually calculated in decibels. The unit is dB.

8. Coupling: The power ratio of the coupling port to the input port, in dB.

9. Isolation: The ratio of the power of the local oscillator or signal leaked to other ports to the original power, in dB.

10 Antenna gain (dB): Refers to the ability of the antenna to focus radiation in a specified direction. Generally, the field strength E in the maximum radiation direction of the antenna is compared with the uniform radiation field strength E0 of an ideal isotropic antenna, and the gain is defined as a multiple of the increase in power density. Ga = E2 / E02

11. Antenna pattern: It is the range where the electromagnetic wave radiated by the antenna exists in free space. The width of the pattern generally refers to the width of the main lobe, which is the angle between the two points when it falls from the maximum by half.

E-plane pattern refers to the radiation pattern in the plane parallel to the electric field;

The H-plane pattern refers to the radiation pattern in the plane parallel to the magnetic field.

Generally, the wider the pattern, the lower the gain; the narrower the pattern, the higher the gain.

12. Antenna front-to-back ratio: refers to the ratio of the maximum forward gain to the maximum reverse gain, expressed in decibels.

13. Simplex: Also known as single-frequency simplex, that is, the same frequency is used for sending and receiving. Because the same frequency is used for receiving and sending, sending and receiving cannot be performed at the same time.

14. Duplex: Also known as different frequency duplex system, that is, two different frequencies are used for sending and receiving, and either party can receive the other party's speech while speaking. Both simplex and duplex belong to the working mode of mobile communication.

15. Amplifier: A circuit used to achieve signal amplification.

16. Filter: A component or device that suppresses unwanted frequency signals by wanted frequency signals

17. Attenuator: Attenuator is a four-terminal network composed of resistance elements with a phase shift of zero, its attenuation and characteristic impedance are constants independent of frequency in a relatively wide frequency range. Its main purpose is to adjust the circuit. Signal size and improved impedance matching.

Power Divider: A device that performs power distribution. There are two, three, and four ... power dividers; the connector types are divided into N head (50), SMA head (50), and F head (75).

18. Coupler: A device that extracts part of the signal from the trunk channel. According to the size of the coupling, it is divided into different specifications of 5.10.15.20. DB. The high-power coupler (300W) can be used to extract the signal from the base station. The coupling degree can be selected from 30 ~ 65dB. The connector of the coupler mostly uses N heads.

19. Load: A terminal / device, component or device that receives electrical power at a circuit (such as an amplifier) or an electrical output port is collectively referred to as a load. The most basic requirements for a load are impedance matching and the power it can withstand.

20. Circulator: A device that transmits a signal in one direction.

twenty one. Adapter: A device that connects different types of transmission lines together.

twenty two. Feeder: a transmission line that transmits high-frequency current.

twenty three. Antenna: (antenna) is to transform high-frequency current or energy in the form of a waveguide into electromagnetic waves and emit them in a prescribed direction or to reduce electromagnetic waves from a certain direction to high-frequency current.