Design Of The Analog Tx-Baseband For UHF RFID Reader

Radio frequency identification technology (RFID), as a non-contact automatic identification technology, is application to a variety of harsh environment with non-artificial intervention identification. UHF radio frequency identification technology, which is more concerned, is an automated identification technology with advantages of high data transfer rate and far communication distance. The application requirements of UHF RFID systems promote the development of the860MHz-960MHz UHF RFID integrated readers.This paper focuses on the analog baseband of UHF RFID transmitter chains.The analog baseband could adjust the output voltage of DAC, filtering the image signal of DAC sampling frequency. So this ensures the purity of the transmitter spectrum. This paper achieves the following results:1) The coding method of UHF RFID transmitter chain is studied, and also the relationships between adjacent channel power ration(ACPR) and the input third order intercept point(ⅡP3) is analyzed. Based on the above, a direct coupling, high linearity analog baseband circuit is designed, which improves the ACPR of the transmitter.2) The analog baseband contains two parts:active filter and variable gain amplifier(VGA). The active filter is a six order feedback filter. The frequency of the filter is decided by resistor arrays and transfer gates.The filter is of high linearity, low power and high attenuation.The structure of the VGA is composed of source degeneration and enhanced trans-conductance. The source resistor arrays adjust the gain, so this improves the IIP3of the VGA.3) The schematic and layout are verified by0.18μ.m SiGe BiCMOS process. Simulation results show that the designing circuit works under3.3V power supply and consumes7.518mA current. The filter is switched by resistor arrays between200KHz and1.3MHz, which satisfy the channel width requirement:the maximum transfer rate is160kbps and the minimum transfer rate is40kbps. The group delay is79.6ns when the frequency is1.3MHz, so this decreases the phase distortion and error bit rates. The gain of VGA ranges from-14dB to10dB and this satisfy the requirement of DAC that the peak voltage of a single line is from100mv to1.5V. The OIP3of the analog baseband is27.5dBm.