Implementation On High Suppression Of Carrier Leakage Technology For C-Band Wideband Quadrature Modulation

I/Q quadrature modulation is a popular method applied in communication systems,and the single chip of I/Q quadrature modulation is the main hardware solution.Due to the error of manufacture,in the wideband quadrature modulation,the DC(Direct Current)offset of the modulator is difficult to be equal,which causes the carrier leakage at the output port of modulator.In the quadrature modulation,more the carrier leakage is,more unlinear PA(Power Amplifier)will be in the latter system,which degrades the quality of transmitting EVM(Error Vector Magnitude).Meanwhile,the sensitivity and BER(Bit Error Rate)of receiver will deteriorate,then the quality of system will be worse.The equipment designed in this project will mainly be used in the center station of satellite communiation system,whose working frequency ranges from 2.4GHz to 4GHz.The carrier leakage suppression requirement must be more than 50 dB,and the project is just based on this requirement.In the wideband I/Q quadrature modulation,the main reason of carrier leakage is the unbalance of the DC(Direct Current)offset of the input circuit.Therefore to achieve the carrier leakage suppression of I/Q quadrature modulation,The DC offsets of I and Q circuits must be corrected.The main methods to correct the DC offsets include balance,carrier phase shift,power detect based,digital baseband correction and so on.The balance technology is mainly applied in narrow band system.Although the carrier phase shift technology,the power detect based technology,and the digital baseband correction technology can achieve good perform for carrier leakage suppression,additional circuits will be required in the wideband system,which increase the size,cost and power comsumption of equipment.Based on the characteristics of the methods mentioned above,a I/Q DC offset correction method for quadrature modulation is proposed in this thesis,which is based on LXI(LAN eXtension for Instrumentation)instrument LAN communication port.Basing on the interface specification provided by LXI instrument manufacturer,the DLL(Dynamic Link Library)driver program is installed in the PC(Personal Computer),and a program can get the measured power and frequency of equipment through LAN,then a configure can be input the registers of DAC to correct the DC offset of I and Q circuits.The main advantage of the method is auto-loop.Without detection,ADC,DAC hareware,high carrier leakage suppress of quadrature modulation can be achieved,and the size and cost of equipment decrease dramaticly.This method can be used at 2.4GHz-4GHz between temperature-40?~+70?,and the carrier leakage suppress can be more than 60 d B.With this method,8 equipments have been producted.Theoretically,the method can be used in quadrature modulation at any freqency,therefore it is a meaningful and promising technology.