With the large number of applications such as the INT network, intelligent traffic, intelligent logistics and ecological monitoring, the RFID technology of the UHF band is growing rapidly. It is a non-contact automatic identification technology that automatically passes through RF signals. Identify target objects, get relevant data, identify do not need manual intervention, suitable for all kinds of harsh environments. The RFID system consists of three parts: label, reader and antenna, where the RFID reader is key.
1 system scheme design
The remote RFID reader system based on the AS3992 chip mainly includes a radio frequency portion and a baseband portion, as shown in Figure 1. The radio frequency portion is designed around the RFID integrated chip AS3992, and the loop filter is combined with the built-in VCO generation frequency; the transmit link is mainly composed of Baron, power amplifier, coupler, high frequency switch and multi-channel antenna interface, using LC on key pathways Network and π-type network adjustment impedance matching; RF reception link consists of antenna, high frequency switch, coupler, low noise and Brangale. The baseband section is configured with an upgraded MCU primary controller and a universal peripheral interface.
2 hardware circuit design
2.1 AS3992 and its peripheral circuit
AS3992 is a reader-writer chip in a high-performance UHF band in Austrian microelectronics. It integrates mixer, gain filter, voltage controlled oscillator, phase locked loop, analog to digital / digital-to-analog converter, etc. And built-in ISO18000-6C full protocol processing system. The external controller can implement all communication and control of the AS3992 only through 8-bit parallel ports or SPI ports.
The UHF carrier signal has a communication frequency of 840 MHz to 960 MHz, and the AS3992 integrates VCO, pre-propeller, main removal, reference division, clarifier, and charge pump, and peripheral circuits can be composed of one loop filter. A complete phase locked loop (PLL) circuit, as shown in Figure 2. The output frequency of the PLL is determined by the reception value of the reference divider and the product of the main scaler. The main function of the charge pump (CP) is to convert the digital logical pulse into an analog current. The CP signal is fed back to the VCO pin through the low pass filter to adjust the oscillator frequency. In order to obtain a stable VCO tuning voltage, the external loop filter circuit is particularly important. It plays a maintenance loop stability, controls the internal and outer noise of the loop, prevents the VCO tuning voltage control line voltage mutation, suppressing the reference edge with stray interference Other important roles.
2.2 RF transmit circuit
The AS3992 chip radio frequency signal is output in the form of differential pairs, using the LC impedance matching network to match the input of the Barron to 100Ω, and the single-end UHF signal can be output by 2: 1 Barron. If only the AS3992 internal power amplifier is used, its output power is only 20 dBm, which can only meet the close reading and writing. To achieve a further read and write distance, there must be no distortion of the external power amplifier; in the impedance match of the entire RF line, the π-type filter network is placed between the Barron and amplifiers to perform impedance fine-tuning.
The power amplifier can amplify the high-frequency carrier signal that has been modulated, radiated through the antenna to the surrounding to meet the excitation electromagnetic field strength required by the receiver, and does not affect the adjacent channel. The power chip SKY65111 uses three-level voltage control, 1 dB compression point gain at 29.5 dBm. The use of the AS3992 comes with the DAC, the combined voltage follower can easily adjust the output power of the amplifier; in addition, when the external power amplifier is fixed, the power is transmitted through the AS3992 itself Adjustment can also adapt to applications of different distances.
The isolation of the microwave signal input / output needs to pass the coupler, and the signal passed through the coupler enters the four high-frequency switching switch SKY14151, and the allwate signal can be switched to any selected antenna interface according to the application needs. The 4 antenna interfaces for the reader design not only expand its application distance, but also because a reader can have multiple partitions to send and receive information, when four antennas are aligned with the same direction, they can also reduce single The blind spot of the antenna reduces the bit error rate.
In order to increase the long distance of the RFID reader, it is also necessary to detect its transmit power. The transmission power is too large, but also it is easy to disclose to the receiving end to form interference, so once the power detecting device monitors to greater than the set power, the microcontrol amount is loaded into the power chip SKY65111 by the main controller digital PID and DAC. The level voltage control is controlled by transmitting power.
2.3 RF receiving circuit
The radio frequency receiving circuit based on the AS3992 chip is relatively simple because it integrates the mixer and filter inside the AS3992 chip, and the peripheral circuit only needs to increase the Barron of 1: 2, and the single-ended signal can be converted into differential signals. , Send the radio frequency receiving tube feet of the AS3992 chip. In order to reduce the purity of the clutter interference to increase the purity of the radio frequency reception signal, a low noise is added between the coupler and Barron, so that the input signal of the Valent is limited between 840 MHz to 960 MHz.
3 software program design
The firmware program of the AS3992 is written in the MCU main controller. Connect the RFID system to the computer through the peripheral interface, first initialize the AS3992 chip after power-on; the frequency band, the antenna interface, and output power can be performed after success (the frequency band selection depends on national standards, and the antenna interface can be switched as needed. The size of the power usually determines the remoteness of the read and write distance); the main control triggers the query tag action. If there is a label in the effective range of the antenna radiation, select the label to prepare communication, otherwise query; main controller and label communication handshake , Send the tag information back to the host computer to complete the card read operation. The system program flow chart is shown in Figure 3.
4 system test
Using the spectrometer to perform performance test of the designed RFID system, the transmit spectrum of the antenna interface is shown in Figure 4. The transmit power is 27 dBm. When the transmit power is set at 15 DBM, the transmitted spectrum as shown in FIG. 5 can be used. It is seen that the channel power is 14.31 dBm, and the bandwidth (99% energy) is 119 kHz.
This article is designed with a long-distance RFID modular system based on the UHF band. As data transmission and resolution requirements, the MCU main controller can be continuously upgraded to adapt to the expansion and processing speed of transaction control. This design detects to output power can prevent blind increased emission power caused to receive interference and affect the identification distance. At present, the system has been successfully applied to the positioning of Baosteel hot rolled ring rail transport vehicles.
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Source: Wiku Electronic Market Network
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