Design Of RFID Frequency Shift Circuit And Demodulation Circuit For Bistatic Communication

In order to further expand the communication distance of RFID system,a bi static communication system based on RFID is studied.This system is a new type of RFID communication system,which is inspired by radar communication.Based on the introduction of the architecture and principle of the bistatic system,the advantages of the system in the aspects of signal power attenuation,communication distance,carrier self-interference and cost are expounded through theoretical analysis.After that,the bistatic system model was built with signal generator,board-level label,semi-active label and software-defined radio(SDR),and the system was verified and tested.The test results show that:in the Tag Talk Only(TTO)mode,when the distance between the Tag and the transmitter is 12m,the receiver can receive the Tag data at the maximum distance of 263m.In order to optimize an RFID tag suitable for bistatic communication,based on the typical architecture of semi-active RFID tag chip,this study proposes an RFID rf front-end improvement scheme for bistatic communication system,and then analyzes the design difficulties of tuner circuit and frequency shift circuit,and determines the key indicators of each circuit module.Aiming at the low demodulation sensitivity of RFID tags and short downlink communication distance,a kind of high sensitivity demodulation circuit is proposed,which is mainly aimed at the shallow modulation signal of 30%.In the demodulation circuit,an envelope amplifier is added at the rear stage of the detection circuit to amplify the envelope signal whose peak value is less than 150mV before entering the decision circuit.For envelope signals with a peak value higher than 150mV,demodulation signals are output directly through the decision circuit.Aiming at the shallow modulation signal of 30%,a mean value detection circuit was designed to extract the average value of envelope signal.The average value was used as the voltage reference of the decision circuit,which significantly improved the understanding of the modulation depth.In addition,this average also provides an appropriate common mode input voltage for the envelope amplifier.Simulation results show that when the modulation depth of the rf signal is 30%,the minimum input amplitude of the demodulation circuit is 80mV,the maximum pulse width error of the corresponding demodulation signal is 1.86%,and the overall power consumption is 3.25 W.At present,the test equipment of the receiver of the bistatic system mainly relies on traditional readers or commercial SDR,which cannot be compatible with the existing Internet of Things(IoT)system.Therefore,a carrier frequency shift circuit is proposed to offset the reflected carrier of the tag to the IoT frequency band,so as to provide a rf front-end solution for RFID tags to return data to the IoT system.This paper presents the implementation method of carrier frequency shift circuit,designs the relevant circuit,and takes the 920MHz carrier reflected to the NB-IoT frequency band 830MHz carrier as an example for verification.The carrier frequency shift circuit consists of a local oscillator,a mixer,a single-terminal converter and a double-terminal module,a level conversion module and a modulation circuit.The simulation results show that the local oscillator frequency is 90MHz,the phase noise is 78.9dBc@100khz,and the average power consumption is 15.8 W.The mixer conversion gain was 0.493,and the Lo-IF isolation was 38.4dB.When the 920MHz carrier is input,the frequency shift circuit outputs the carrier frequency of 830MHz,realizing the carrier frequency shift function,and the overall power consumption is 19.3 W.The above mentioned circuits have been designed under the TSMC 0.18 m CMOS process,and have been verified on the Cadence Spectre simulation platform.The simulation results show that the circuit performance is good and meets the requirements of the rfid-based bistatic communication system.