Talk About RFID Technology 2018-06-21
RFID Introduction

Radio frequency tags are physical carriers of product electronic codes (EPCs) attached to traceable items that can be circulated globally and recognized and read and written. As a key technology for constructing "Internet of Things", RFID (Radio Frequency Identification) technology has attracted people's attention in recent years.

Radio Frequency Identification (RFID) is a wireless communication technology that can identify specific targets and read and write related data through radio signals without identifying the mechanical or optical contact between the system and a specific target. RFID technology is based on the transformer coupling model (energy transfer and signal transmission between primary and secondary) in the low frequency range, and is based on the spatial coupling model of radar detection target at high frequency range. (The radar emits electromagnetic wave signal and then carries the target information returning radar after hitting the target. Receiver).

The radio signal is transmitted through a radio frequency electromagnetic field and transmitted from a label attached to the item to automatically identify and track the item. Some tags receive energy from the electromagnetic field emitted by the recognizer when they are identified, and do not require a battery. Also, the tag itself has a power source and can actively emit radio waves (electromagnetic fields tuned to a radio frequency). The tag contains electronically stored information that can be identified within a few meters. Unlike the barcode, the radio frequency tag does not need to be within the line of sight of the identifier, but it can also be embedded within the object being tracked.

The reader of the RFID electronic tag communicates wirelessly through the antenna and the RFID electronic tag, and can read or write the tag identification code and memory data. RFID technology can identify high-speed moving objects and can identify multiple labels at the same time. The operation is quick and easy.

Tag: It consists of a coupling element and a chip. Each tag has a unique electronic code and is attached to the object to identify the target object.

Reader: A device that reads (sometimes writes) tag information and can be designed to be handheld or stationary;

Antenna: Passes RF signals between tags and readers.

Figure 1: RFID system schematic

The development of passive RFID products is the earliest, and it is also the product with the most mature development and the most widely used market. For example, bus cards, cafeteria meal cards, bank cards, hotel access cards, second-generation ID cards, etc., which can be seen everywhere in our daily lives, are close-contact type recognition. The main operating frequency of its products are low frequency 125KHZ, high frequency 13.56MHZ, UHF 433MHZ, UHF 915MHZ.

Active RFID products have been gradually developed in recent years. Their long-distance automatic identification characteristics determine their enormous application space and market potential. In the field of long-distance automatic identification, such as smart prisons, smart hospitals, smart parking, intelligent transportation, smart cities, smart world and the Internet of things, there are major applications. Active RFID has emerged in this field and belongs to the category of long-distance automatic identification. The main operating frequency of the product is UHF 433MHZ, microwave 2.4GHZ and 5.8GHZ.

The air interface communication protocol regulates the information exchange between the reader and the electronic tag, aiming at the interconnection and interoperability between different manufacturers' production equipment. ISO/IEC has developed five types of air interface protocols.


Passive RFID Design Case

In current technology applications, passive RFID products are the most widely used products. In many circuit designs applied to passive RFID, the RC522 module with 13.56 MHz is used in many cases. This is a widely used passive RFID module. For the application circuit of RC522, it is to use 3V3 DC power supply, and then receive the information through SPI interface to communicate. The application schematic is shown in Figure 2 below:

Hard Science | Talk About RFID Technology
Figure II: RC522 Application Schematic

The 13.56 MHz antenna circuit includes a signal receiving circuit and a signal transmitting circuit. The signal transmitting circuit is divided into an EMC filtering section, a matching circuit section, and a coil circuit section.

Transmitting circuit: The signal transmitting part can be subdivided into EMC filter circuit, resonance and impedance matching circuit, and coil. among them:

EMC filter circuit: mainly composed of LC low-pass filter circuit low-pass filter, reader chip sent via TX1 and TX2 antenna signal is mainly 13.56Mhz, but it is inevitable there will be higher harmonics. Therefore, the main function of this part of the low-pass filter is to filter out unwanted signals above 13.56 Mhz. This facilitates normal communication between the card reader and the card, and also reduces the electromagnetic interference of the antenna section to the space or nearby circuits.

Matching Circuit: This section mainly adjusts the resonant frequency of the entire antenna transmitting section to around 13.56 Mhz. This can increase the amplitude of the signal on the coil to facilitate magnetic field radiation. In addition, the matching circuit must match the resistance of the transmitter circuit to the output resistance of the reader chip, typically 50 ohms (different from the chip). This can make the antenna part obtain the maximum power, which is conducive to the improvement of the reading distance.

Coil: The coil can be a PCB coil or a copper wire coil.

Receiving circuit: The signal receiving circuit is relatively simple and consists of four components. The Cmin capacitor can stably read the fixed reference voltage Vmin provided internally by the chip, and R1 introduces this reference voltage to the RX pin to add a fixed signal to the chip. DC level, CRx from the generation circuit to introduce the feedback signal and Vmin superimposed into the chip. By adjusting the ratio of R2 and R1, the amplitude of the Rx pin signal can be adjusted so that the chip reading distance is optimized.

Also note the following points in the PCB design:

The entire transmitter circuit must pay attention to the symmetrical design. The trace lengths of the two signals from Tx1 and Tx2 should be as consistent as possible.

The two inductors should be packaged with more than 0805, to ensure that there is enough current through (680nH/0805), otherwise it is not conducive to long-distance card reading.

The length and width of the PCB coil depends on the specific situation. If the circuit board is not limited by the mold, it can be designed to be consistent with the length and width of the ordinary card. If the coil size is limited, the coil area can also be reduced. The general principle is to design the card to be the same as the size of the card being read. The number of turns of the coil, and the length and breadth of 4x over 3x3cm can be too much to adjust the parameters. If it is below 3x3cm, the number of turns can be increased to 6 circles or more; the PCB coil trace width is between 0.5mm and 1mm, and the spacing and line width can be the same. In addition, the coil corner transition is best with a circular arc.

Coil part, not a large area of copper, otherwise it will cause a serious loss of magnetic field eddy current effect should pay attention to the scope of the coil (the entire space around the coil) can not have a large area of metal components, metal objects, metal coating and so on.

The ground of all devices of the entire transmission circuit must be connected to the same ground line and return to the TVSS pin of the chip, and a large area of copper can not be coated near the antenna circuit device, and the devices are connected by wires.


to sum up

In the era of continuous development of the Internet of Things technology, RFID as a sensing layer technology used in the Internet of Things has a close relationship with the Internet of Things. In recent years, RFID technology has greatly increased the function of the Internet of Things and enriched the content of the Internet of Things. The development of the Internet of Things has also promoted the development and application of RFID technology, such as for the “Internet of Things” and drugs. Special requirements, RFID tags can not only provide static information on goods, but also provide information on the location of the goods and the surrounding temperature, humidity and other dynamic information, greatly improving the degree of intelligence of RFID tags. At present, the Internet of Things using the RFID sensing layer has achieved "human-to-human" communication and "human and material" communication, but to achieve "object-to-item" communication and interaction in the future Internet of Things, it must be upgraded to include "The level of intelligence that makes RFID smart labels." In addition, standardization, interoperability, agreements, regulations, and connection technologies still need to be improved and improved.

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