RFID Principle

火辣西米秀

RFID Principle [Copy link]

Many of my friends are working on RFID every day, and they are doing well, but they don’t understand its basic principles and are too embarrassed to ask. Although the explanations in books are clear, they are often technical and difficult to understand.

Taking 13.56MHz RFID technology as an example, it is divided into a card reader and a card. The card reader is connected to a power source. When the card is close to the card reader, the coil inside the card obtains energy from the 13.56MHz magnetic field emitted by the card reader, and then supplies it to the card chip after rectification and filtering. Of course, the energy required by the card chip is very small.

When the card reader wants to transmit a signal to the card, it can modulate the magnetic field through ASK. In this way, the card chip can obtain the ASK signal. Of course, the ASK adjustment speed cannot be too low, otherwise the capacitor filtering on the card chip will be unstable, which will cause unstable power supply to the card chip.

Generally, those who work with RFID can understand the above, but many people do not understand how the card chip transmits the signal to the card reader, especially the term "subcarrier load modulation". What does it mean?

Many people mistakenly understand that: because the card chip obtains energy from the card reader, when it needs to send a signal to the card reader, it can actively radiate energy through its antenna like the RF chip. Of course, this explanation is self-consistent, and in fact this logic is also feasible, but the problem with this logic is that the card chip needs enough energy, which will cause the card reader to have a very close reading distance, so it is not practical. Generally, this technology is often used in active RFID, so I will not explain it in detail here.

So how does the card transmit its own signal? In fact, the card end realizes this by opening and short-circuiting the coil connected to itself, so that the card chip basically does not consume electricity. But the condition is that the card reader is always under the high-frequency signal of 13.56MHz. The card continuously opens and shorts its own antenna, causing the magnetic field between the card reader and the card to change, thereby affecting the slight change of the voltage amplitude at both ends of the card reader antenna. Then, from this weak signal, the signal is obtained, similar to an AM radio. This is the so-called "load modulation". Then what is the meaning of "subcarrier"? This is equivalent to the concept of intermediate frequency in general radio frequency. It is mainly to solve the problem of long 0 or long 1 when data comes in. For example, many consecutive 0 signals or 1 signals make it impossible for the card reader to distinguish whether it is data or steady-state non-data when receiving it. Therefore, when data is transmitted, a signal is inserted as a data identifier.

We use the most common analogy of water in a water pipe to simulate RFID, as follows:

We compare the water in the pipe to a magnetic field, which connects the card reader and the card, and the water can only flow from the card reader to the card. So the card gains energy.

When the card reader wants to send a signal to the card, it is very simple. Just control the water flow, making it flow for a while and stop for a while, and the card will be able to sense it.

石榴姐

Many applications now use RF technology

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How to measure the energy required by the card chip when the card is far away from the card reader