RFID multi-card identification antenna design

灞波儿奔

RFID multi-card identification antenna design [Copy link]

RFID multi-card recognition technology is generally used in logistics and identity recognition. It usually uses ISO15693 or ISO18000 protocol tags, and rarely uses ISO14443 protocol. This article introduces an ISO14443 antenna design method for simultaneous operation of multiple cards. RFID (radio frequency identification) technology is the vanguard of the Internet of Things, mainly used in identity recognition, financial payment and logistics. In the design of ISO4443 protocol card reader chip antennas, it is often confusing and difficult to adjust the ideal card reading performance. The editor here introduces the antenna design method for Fudan Micro card reader chip to read multiple cards (about 5 cards) under ISO14443A protocol. When designing a reader chip antenna, the center frequency of the reader is usually designed to be 13.56±0.2MHz, and the antenna output impedance is 50Ω or 80Ω as shown in Figure 1 (Fudan Micro FM175xx series reader chip), so that the card reading performance can reach the best performance. The oscilloscope test antenna transmit resonant voltage is 20V and the receiving voltage is 1.5-3V (3.3V power supply).

Figure 1 FM17550 card reader chip 80 ohm antenna matching

In multi-card recognition, according to the above design method, the card reader works unstable. Why is this? The original contactless card chip operating frequency is set at 13.56MHz. After multiple cards are placed on the antenna, the reader antenna resonant frequency will be biased downward, the bandwidth will be shifted downward as a whole, and the reader output power will decrease at the same time. The antenna output power is insufficient, making the multi-card operation unstable.

Figure 2 Multi-tag 14.2MHz center frequency

Usually in the multi-card recognition tag, the card center frequency is designed to be 14.0-14.2MHz Figure 2, at this time the reader antenna frequency is also designed to be 14.0-14.2MHz, the output impedance is much larger than the usual 50 ohms, generally above 120 ohms, and the antenna size is designed to be able to hold multiple cards at the same time, round or rectangular. After placing 5 cards in the antenna, the original resonant frequency will be divided into two, one at about 13.56MHz and the other at 15MHz as shown in Figure 3. The bandwidth is between 13.0MHz and 15.3MHz, which meets the needs of multi-tag identification.

Figure 3 Multi-tag bandwidth diagram