Frequency characteristics in RFID systems

- Aug 29, 2018-

For an RFID system, its band concept refers to the frequency range of the tag signal that the reader sends, receives, and reads through the antenna. From the application concept, the operating frequency of the RF tag is also the operating frequency of the RFID system, which directly determines the various aspects of the system application. In an RFID system, the system works just like we usually listen to FM radio, and the RF tags and readers have to be modulated to the same frequency to work.

The operating frequency of the RF tag not only determines the working principle of the RFID system (inductive coupling or electromagnetic coupling), but also determines the distance and the cost of the RF tag and reader. The frequency bands or frequencies occupied by RFID applications are internationally recognized as being located in the ISM band. Typical operating frequencies are: 125 kHz, 133 kHz, 13.56 MHz, 27.12 MHz, 433 MHz, 902 MHz to 928 MHz, 2.45 GHz, 5.8 GHz, and the like.

Depending on the operating frequency, RFID tags can be classified into low frequency (LF), high frequency (HF), ultra high frequency (UHF) and microwave. RFID operating in different frequency bands is different. RFID tags in LF and HF bands generally use the principle of electromagnetic coupling, while RFID in UHF and microwave bands generally adopts the principle of electromagnetic emission. At present, the frequencies widely used in the world are distributed in four kinds of bands, low frequency (125KHz), high frequency (13.54MHz), ultra high frequency (850MHz ~ 910MFz) and microwave (2.45GHz). Each frequency has its own characteristics and is used in different fields, so to use it properly, you must first select the appropriate frequency.

Low-band RF tags, referred to as low-frequency tags, have an operating frequency range of 30 kHz to 300 kHz. Typical operating frequencies are 125KHz and 133KHz. The low frequency tag is typically a passive tag whose operating energy is obtained by inductive coupling from the near field of the radiation of the reader coupling coil. When transmitting data between the low frequency tag and the reader, the low frequency tag needs to be located in the near field region radiated by the reader antenna. The reading distance of the low frequency tag is generally less than 1 meter. Typical applications for low frequency tags are: animal identification, container identification, tool identification, electronic latching theft (car keys with built-in transponders).

The operating frequency of the mid-high frequency band RF tag is generally 3MHz ~ 30MHz. The typical operating frequency is 13.56MHz. The RF tag in this band, because its working principle is exactly the same as the low-frequency tag, that is, it works by inductive coupling, so it should be classified as a low-frequency tag. On the other hand, according to the general division of the radio frequency, the operating frequency band is also called a high frequency, so it is often referred to as a high frequency tag. Since the RF tag in this band may be the largest RF tag in practice, we only need to understand the high and low as a relative concept, that is, it will not cause confusion. For ease of description, we refer to it as an IF RF tag. IF tags are also generally passive, and their working energy is the same as that of low-frequency tags. It is also obtained by inductive (magnetic) coupling from the near field of the coupled coil of the reader. When the tag is exchanged with the reader, the tag must be located in the near field of the reader antenna radiation. The reading distance of the IF tag is generally less than 1 meter. Because the IF tag can be easily formed into a card shape, it is widely used in electronic tickets, electronic ID cards, electronic lockout anti-theft (electronic remote control door lock controller), residential property management, building access control systems, etc.

The radio frequency tags in the UHF and microwave bands are referred to as microwave radio frequency tags. The typical operating frequencies are 433.92MHz, 862(902)MHz~928MHz, 2.45GHz, 5.8GHz. Microwave radio frequency tags can be classified into active tags and passive tags. In operation, the radio frequency tag is located in the far field of the radiation field of the reader antenna, and the coupling between the tag and the reader is electromagnetic coupling. The reader antenna radiation field provides RF energy to the passive tag and wakes up the active tag. The reading distance of the corresponding radio frequency identification system is generally greater than 1 m, typically 4 m to 6 m, and the maximum is more than 10 m. The reader antennas are generally directional antennas, and only the radio frequency tags within the directional beam range of the reader antenna can be read/written. Due to the increase of the reading distance, it is possible to have multiple RF tags in the reading area at the same time, which raises the need for simultaneous reading of multiple tags. At present, advanced RFID systems regard multi-label reading problems as an important feature of the system. UHF tags are mainly used for automatic identification of railway vehicles, container identification, and can also be used in road vehicle identification and automatic toll collection systems.

At the current state of the art, relatively successful products of passive microwave radio frequency tags are relatively concentrated in the operating frequency range of 902 MHz to 928 MHz. 2.45 GHz and 5.8 GHz RFID systems are mostly available with semi-passive microwave RF tags. Semi-passive tags are typically powered by button batteries and have a farther reading distance. The typical characteristics of microwave radio frequency tags mainly focus on whether passive, wireless read/write distance, whether to support multi-tag reading and writing, whether it is suitable for high-speed identification applications, the transmit power tolerance of readers, the price of radio frequency tags and readers, etc. . For wirelessly writeable RF tags, the write distance is usually less than the read distance because the write requires more energy. The data storage capacity of microwave radio frequency tags is generally limited to 2Kbits. The large storage capacity does not seem to have much significance. From the perspective of technology and application, microwave radio frequency tags are not suitable as carriers for large amounts of data. Their main function lies in Identify the item and complete the contactless identification process. Typical data capacity indicators are: 1Kbits, 128Bits, 64Bits, etc. The product electronic code EPC set by the Auto-ID Center has a capacity of 90 Bits. Typical applications for microwave radio frequency tags include mobile vehicle identification, electronic latching theft (electronic remote door lock controller), medical research and other industries.

Labels with different frequencies have different characteristics. For example, low-frequency tags are cheaper than UHF tags, save energy, penetrate through scrap metal objects, and work frequency is not subject to radio frequency control. It is most suitable for objects with high water content. For example, fruits and the like; ultra-high frequency has a wide range of effects, and the data transmission speed is fast, but it is relatively energy-consuming, the penetration is weak, and the working area cannot have too much interference, and is suitable for monitoring items in the logistics fields such as ports and warehouses; The frequency tag belongs to the medium and short distance identification, the reading and writing speed is also in the middle, and the product price is relatively cheap, such as application on the electronic ticket card.

At present, different countries use different frequencies for the same band. The UHF used in Europe is 868MHz, and the U.S. is 915MHz. Japan is currently not allowed to use UHF in RF technology.

Currently in practical applications, the commonly used frequency bands are 13.56MHz, 860MHz ~ 960MHz, 2.45GHz. The short-range RFID system mainly uses LF and HF frequency bands such as 125KHz and 13.56MHz, and the technology is the most mature; the long-distance RFID system mainly uses UHF frequency bands such as 433MHz, 860MHz to 960MHz, and microwave frequency bands such as 2.45GHz and 5.8GHz, and is still testing more. Among them, there is no large-scale application.

China's technology in the design of RFID tag chips in the LF and HF bands is relatively mature. The design technology of the HF band is close to the international advanced level. The RFID chips conforming to ISO14443 Type A, Type B and ISO15693 standards have been independently developed and successfully applied. Traffic card and second-generation ID card and other projects.