Research On Calibration Of Digital Phased Arrays

A digital phased array generates transmit beams and receive beams through dig-ital beamforming technique,which has the advantages of high precision,multibeam,low power consumption,and superior space detection performance.However,a digital phased array contains a large number of active channels.Current technologies fail to guarantee performance consistency among channels and severely degrade the detec-tion performance of digital phased arrays.In addition,all subarrays use one common reference clock to generate accurate beam steering.The reference clock is transmit-ted over coaxials or optical fibers to different subarrays.However,the transmission introduces additional phase inconsistencies and results in beam steering error.Hence,digital phased arrays rely on calibration techniques to ensure superior system perfor-mance.This thesis focuses on three key scientific issues of array calibration techniques,including calibration of amplitude/phase inconsistencies among channels,a fast cali-bration method for large-scale phased arrays,and a reference phase synchronization method.To calibrate amplitude and phase inconsistencies among channels,this thesis pro-poses a calibration algorithm by using numerical control oscillator(NCO)phase in-creasing method.By setting a high-precision NCO module inside the digital signal processor(DSP),the phase of NCO module is gradually adjusted from 0°to 360°.Then the channel signal and the NCO signal are multiplied to obtain phase variations a-mong channels.Its performance and efficiency are evaluated by computer simulation.In addition,an experimental platform is built to verify the proposed algorithm,which achieves amplitude calibration accuracy of 0.15 dB and phase calibration accuracy of 1°.The disadvantage of this algorithm is that the amplitude and phase errors of anten-nas and RF switches cannot be corrected.Secondly,to shorten calibration time consumption for large-scale digital phased arrays,a fast calibration algorithm by using graph coloring theory is proposed.By transferring antennas into vertices and mutual couplings among antennas into edges,we transfer the calibration scheduling problem into graph coloring problem.Simula-tions prove that,by using the proposed method,the total calibration time slots used by hexagonal topological array,square topological array and triangular topological array are less than 8,9 and 16,respectively.The disadvantage of this algorithm is that it re-quires tedious steps to obtain the neighbor register and calibration time slot allocation table for each antennaFinally,this thesis proposes a fiber-based clock phase synchronization algorith-m,including a fiber-based phase-locked loop circuit and an ambiguity eliminator cir-cuit.The fiber-based phase-locked loop circuit is used to synchronize phase among antennas.An ambiguity eliminator circuit is used to remove reference clock's cyclic ambiguity.An experimental platform is built to verify the proposed algorithm,which realize synchronization error of 0.4°for a 10 MHz reference clock.The disadvantage of this algorithm is that the electrical performance consistencies among phase shifters will affect the synchronization accuracy of the algorithm.In summary,digital phased arrays require efficient and accurate calibration algo-rithm to guarantee excellent detection performance.This thesis proposes three algo-rithms to deal with phased array calibration issues.Simulation and experiment results prove that excellent performance is achieved by proposed algorithms.In addition,more research is expected in the future:(1)Calibration method for wideband phased arrays;(2)More efficient calibration method for large-scale phased arrays;(3)Chip-level real-ization of the clock synchronization algorithm.