Research And Implementation Of Radio Frequency Receiver System For Off-Body Communication

With the rapid development of the mobile Internet and formal commercial stage of 5G network,it marks the official start of the era of Internet of Everything.As the focus of internet of things applications,wearable devices have ushered in the broad space for development.At present,most smart wearable products on the market work in off-body,such as smart watches,smart bracelets,smart earphones,and glasses,etc.In order to comply with the market trend of wearable devices in off-body communication,it is particularly important to research on antennas,low noise amplifiers(LNA)and mixers in radio frequency receiving systems.The working frequency of the RF receiving system in the paper is based on the internationally common ISM band 2.45GHz(2.42GHz-2.485GHz).Because of its more resistant to interference and wide frequency band,the current three mainstream wireless networks of WiFi,Bluetooth,and ZigBee work in this frequency band.In this paper,the following research is carried out on the radio frequency receiving system working in this frequency band through the combination of theoretical analysis,modeling simulation and physical test:(1)In order to meet the requirements of the communication frequency band,the transmission distance of the antenna,and the safety of the human body,a T-shaped coplanar waveguide coupled feed microstrip antenna suitable for off-body communication has been designed.The results of HFSS software simulation show that the reflection coefficient of the antenna S11?-10 dB in the 2.40GHz-2.52 GHz frequency band,the reflection coefficient S11 at the center frequency of 2.45 GHz is-17.83 dB,and the maximum radiation direction gain is 6.37 dB.The antenna has strong directivity,mainly concentrated on the side facing away from the body.According to the optimized dimensional parameters,the actual product has been made and tested.Show that: the antenna resonant frequency is 2.46 GHz,and the reflection coefficient S11 is-20.47 dB.Combining simulation and measured data,the designed antenna has the characteristics of high gain and small back radiation,which can meet the performance requirements of wearable devices in off-body communication.(2)Based on the analysis of the radio frequency receiving system LNA in the wearable device,the design index has been determined.Based on the transistor ATF54143 and with the aid of ADS software,the circuit design and simulation analysis of the LNA has been carried out.The simulation results show that when the two-port network is matched,the noise figure at the center frequency of 2.45 GHz is 0.473 dB and the amplifier gain is 14.5dB.In order to verify its performance,PCB layout drawing and physical test has been performed.The measured results show that the resonance frequency of the LNA is slightly lower than the simulation result,and the gain at 2.45 GHz is 11.8dB,which is 2.7dB lower than the simulation result.Combining simulation and measured data,it can be seen that the designed low-noise amplifier has the characteristics of small input/output reflection coefficients and low noise figure.Thus,it can meet the requirements of system performance indicators.(3)Through the research on the basic structure performance of the mixer,a single-ended single-balanced mixer with cascode structure has been designed based on the NPN transistor MMBR941.The ADS simulation results show that the intermediate frequency output signal is 200 MHz when the RF input signal is 2.45 GHz and the local oscillator signal is 2.25GHz;when the RF signal input power is-30 dBm and the local oscillator signal input power is 5dBm,the noise figure of the mixer is 9.16 dB,the gain of the mixer is 7.76 dB.Through the physical test of the mixer,it can be seen that its gain is 0.81 dB,which is 6.95 dB lower than the simulation result,and the main reason for the error is briefly analyzed.(4)Finally,the paper has tested the loss of the antenna transceiver system.Considering the actual size of the wearable device product,the distance between the transmitting and receiving antennas should not exceed 60 mm,so the transmitting antenna and the receiving antenna are respectively arranged at a distance of 20 mm,40mm and 60 mm.Tests show that the antenna losses at distances of 20 mm,40mm,and 60 mm respectively are-41.6dB,-46.2dB,and-53.9dB.The simulation and test results show that the antenna,LNA and mixer RF module designed in this paper can meet the performance requirements of wearable devices in off-body communication.