A Study Of On-line Measurement System For Tag Antennas

Radio frequency identification (RFID) is a non-contact automatic identificationtechnology, having been developed rapidly in recent years. The market indicates a verybright future for it. The popularity of this technology is mainly determined by the costand performance of the tags, which are widely used in a RFID system. The tagperformance is greatly affected by the impedance matching between the antenna and thechip. The tag packaged by a chip and an antenna with impedance mismatched can notmeet the practical requirement and an unnecessary cost is brought. Considering the testneeds for large-scale production of tag antennas, a method of on-line measurement hasbeen proposed in this paper. The matching performance of the antennas can beevaluated by measuring the impedance on the production line. Consequently, theconsistency and reliability of the large-scale production of tags can be guaranteed andthe cost is reduced.The integrated scheme of on-line measurement system for tag antennas ispresented in this paper. The key component of the system is a six-port reflectometer. Afrequency source and a tapered coaxial balun works as the stable signal source and themeasuring connector separately. Meanwhile, a MCU is adopted to perform processcontrolling, voltage sampling and data calculating. In addition，a matrix keyboard isused to realize the operation control and data input and a liquid crystal display module(LCM) is applied to display the results. The basic process of the system containsfrequency selection, calibration and measurement. Also, the operation and dataexchange can be implemented by a PC interface.According to the design scheme, some key modules of the measurement systemhave been designed and analyzed in detail, including the frequency source, the six-portreflectometer, etc. The frequency source is realized by adopting the phase locked loop(PLL), which is composed of a frequency synthesizer, a voltage-controlled oscillator(VCO) and a three-order passive loop filter (LF). A low pass filter (LPF), a variable gainamplifier (VGA) and an isolator are introduced in the back-end circuit to achieve thehigh-order harmonic rejection, output power adjustment and load isolation, respectively.The microstrip transmission line is selected to form the six-port reflectometer afterconsidering the complexity and cost. Among the four branches, the space between adjacent ones is λ_g/8. Coupling capacitors are used to sample the power on the maintransmission line. In the part of power detection, a logarithmic detector with highlinearity can convert the RF input signal (dBm) to DC output (V) accurately. Thesimulation and analysis are achieved by adopting the HB simulator in ADS andMATLAB. The results show high accuracy of impedance measurement.The selections of the components are given also. On the basis of the design ofschematic and PCB, the hardware circuit of the measure system is fabricated and tested.A contact impedance measurement of two UHF tag antennas is carried out. Accordingto the results, the design goals are achieved.