Passive RFID tag structure and principle!

Passive RFID tags do not have their own batteries and rely on the electromagnetic energy sent by the reader. Because of its simple structure, economical and practical, it has been widely used. The passive RFID tag is composed of an RFID IC, a resonant capacitor C and an antenna L. The antenna and the capacitor form a resonant loop and are tuned to the carrier frequency of the reader to achieve the best performance.

RFID tag structure The RFID tag antenna has two types of antenna: (1) Wire-wound inductive antenna; (2) Embossing or printing spirally coiled antenna on a dielectric substrate. The antenna form is determined by factors such as carrier frequency, tag encapsulation, performance and assembly cost. For example, when the frequency is less than 400KHz, mH-level inductance is needed. Such an antenna can only be manufactured with wire-wound inductors. When the frequency is between 4 and 30MHz, only a few turns of wire-wound inductance can be used, or the engraving on the dielectric substrate can be used. Rotating antenna.

After selecting the antenna, the next step is how to attach the silicon IC to the antenna. There are also two basic methods for IC bonding: (1) using a chip on board (COB); and (2) attaching a bare chip directly to an antenna. The former is commonly used for wire-wound antennas; the latter is used to etch antennas. CIB encapsulates the resonant capacitor and the RFID IC together in the same package, and the antenna is connected to the two external ends of the COB using a soldering iron or welding process. Since most COBs are used for ISO cards, a card that conforms to ISO standard thickness (0.76) specifications, the typical thickness of COB is approximately 0.4 mm. Two common types of COB packages are IOA2 (MOA2) used by IST and World II used by US HEI.

Direct bonding of bare chips reduces intermediate steps and is widely used for low cost and high volume applications. Direct attachment is also available in two ways, (1) lead soldering; (2) flip-chip technology. When flip-chip technology is used, special solder balls need to be made on the chip pad, the material is gold, and the height is about 25, and then the solder balls are flipped onto the printed traces of the antenna. The wire bonding process is relatively simple. The bare chip is directly soldered to the antenna, and the soldering area is sealed with black epoxy resin. For low-volume production, the cost of this process is lower; for high-volume production, it is better to have a flip-chip process. The RFID IC is internally equipped with a 154-bit memory for storing tag data. The IC also has a very low on-resistance modulation gate tube (CMOS) that operates at a certain frequency. When the reader emits electromagnetic waves and the tag antenna inductive voltage reaches VPP, the device operates to send data back in Manchester format. Data transmission is accomplished by tuning and tuning the external resonant tank. The specific process is as follows: When the data is at a logic high level, the gate tube is cut off, and the tuned circuit is tuned to the wave cut-off frequency of the reader, which is the tuning state, and the induced voltage reaches a maximum value. In this way, tuning and detuning produces an amplitude-modulated signal on the tag coil, and the card reader detects the voltage waveform envelope to reconstruct the data signal from the tag.

The switching frequency of the gate tube is 70KHz, and it takes about 2.2ms to complete all 154 bits of data. After sending all data, the device enters 100 ms sleep mode. When a tag enters sleep mode, the reader can read data from other tags without any data collision. Of course, this function is affected by the following factors: the distance from the tag to the reader, the orientation of both, the movement of the tag, and the spatial distribution of the tag.

In order to achieve the performance of the design, the tag should be accurately tuned to the carrier frequency of the reader. However, the components used will always be biased, causing reading distance changes. The error of the inductance can be controlled within 1~2%, so the reading distance is mainly caused by the capacitance error. The error of the external capacitor should be within 5% and the Q value should be greater than 100. The internal capacitance of MCRF360R is made of silicon oxide, the error on the same silicon is within 5%, and the error of different batches is about 10%.

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