用于星载SAR数字AGC增益控制的双门限步进法

首先对自动增益控制(AGC)电路的模拟和数字实现方法进行了比较,指出星载合成孔径雷达(SAR)系统采用数字AGC的必要性;然后针对数字AGC增益控制产生算法进行了讨论,提出了一种实用的数字AGC增益控制产生算法--双门限步进法。该算法配合数控衰减器非常适合数字AGC的工程实现,实验结果证明:采用该算法的AGC设计具有很好的收敛性,并且在后续处理中能够精确地恢复信号原来的幅度。

Two-Threshold Step-Tune-A Gain Control Method for Digital AGC in Space-borne Synthetic Aperture Radar

Different ground objects have different backward scatter energy to the electromagnetic wave that the SAR transmits.The total dynamic range of a X-band SAR's receiving signal is greater than 70 dB.Unfortunately,the quantization bit of the ADC that can be used for Space-borne system is normally 8 bits,which equal to about a dynamic range of 48 dB.The AGC technology can be used to match the dynamic range between the receiving signal and ADC.According to the implementing method,AGC can be sorted into Analog AGC and Digital ADC.Both Analog and Digital ADC automatically adjust the receiver's gain according to the changing of the receiving signal amplitude,but the adjust method of Analog AGC is analog,the feedback control signal is continuous analog signal,so the gain value tuned by the AGC circuit can not be exactly recovered in the imaging processing in the ground.However,because the gain adjust value is digital,it can be exactly recovered in the imaging processing in the ground in a Space-borne SAR system with the Digital AGC circuit.By this reason,the Digital AGC is usually adopted in a Space-borne SAR system that has radiation precision requirement.In the first,this paper introduces and compares the implementing methods of the analog AGC and digital AGC.A new gain control method called "two-threshold step-tune" for the Digital AGC is given.The "two-threshold step-tune" method use step-tune versus one-time adjust method to attenuate the receiving signal step by step when the signal level feed the ADC exceeds the ADC dynamic range.Two thresholds versus one threshold are set as a judging level to switch the AGC function.The "two-threshold" method can effectively eliminate the surge phenomenon that "one-threshold" brings.Working with the Digital Attenuator,"two-threshold step-tune" method is very applied to implement a digital AGC circuit.In the end,this paper gives some the experiment results about the "two-threshold step-tune" Digital AGC,the experiment results indicate that the digital AGC design powered by this method has good astringency,and the signal passing through this digital AGC circuit can exactly resume its amplitude in the post signal processing.