| Passive ultra high frequency(UHF)radio frequency identification(RFID)system has the advantages of low cost,low power consumption,long recognition distance and high recognition efficiency,and has been widely used in logistics,access control,asset management and other fields.In the reverse link of the tag to the reader,the tag reverses the transmission of information by changing the impedance matching relationship between the antenna and the load.Therefore,whether the tag antenna and the load can achieve impedance conjugate matching becomes a key factor that restricts the performance of the passive UHF RFID system.Commercial tags generally achieve conjugate matching of the antenna and load impedance by optimizing the design of the antenna.However,in practical applications,due to factors such as label geometry,component selection,identification and sensor fusion design,a special impedance matching network is needed to satisfy the impedance matching relationship between the tag antenna and the load.At the same time,the tag antenna impedance test also requires an adjustable impedance matching network.Therefore,research on the tag impedance matching network design method has important theoretical significance and potential application value for the performance improvement and application of passive UHF RFID system.The main research contents are as follows:Firstly,the current domestic and international label design methods are studied,and the label antenna design,label and sensor combination and internal impedance matching network design characteristics of the label are analyzed.A brief overview of RFID technology is introduced.The structure and function of readers and tags are introduced.The data transmission of passive UHF RFID is analyzed.It is deduced that the conjugate matching of tag antenna and chip impedance can achieve maximum power transmission.The system ASK modulation and demodulation method is studied to analyze the influence of the modulation state on the label reflection coefficient under different modulation states.Secondly,the performance parameters of the antenna and the radiation angle of the antenna are introduced in detail,and the influence of the rotation angle of the reader antenna and the tag antenna on the system performance is explored.Studies have shown that optimal identification can be achieved when the direction of electromagnetic radiation is perpendicular to the receiving antenna.Explore theantenna miniaturization design and antenna impedance measurement method.In different practical application scenarios,the antenna design is various.To achieve the joint use of single chip and multiple antennas,it is necessary to accurately measure the impedance of the antenna.Finally,combining the antenna impedance matching method and the two-port cascade network,a two-port cascade tag impedance matching network with small return loss and large application range is proposed.Under the premise of determining the impedance of the chip,for different impedance antennas,the adjustable matching is achieved by changing the circuit component parameters and distribution of the matching network.Compared with the existing impedance matching network design method,the method is applicable to the real part of the antenna impedance of 5 to 80?,the imaginary part is in the range of 50 to 400 ?,and the return loss is reduced by more than 8 dB,thereby improving the energy transmission efficiency.Effectively increase the recognition distance of the RFID system.The Smith diagram is used to analyze the influence of the conjugate impedance position of the antenna on the optimal matching network,and the condition requirements of low-cost impedance matching.The fast design method of the optimal label impedance matching network is given. |
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