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DDS AD9910,used for Radar signal processing, found the amplitude of the linear-frequency-modulation signal output changes regularly, like a sine wave

Question asked by Josh_zhou on Jul 9, 2018
Latest reply on Jul 20, 2018 by KennyG

I am using AD9910 to generate LFM pulse signal for Radar signal processing. When connect the output of DDS to the ADC input  of the signal processing board as echo, I found that the amplitude of the dds output signal changes slightly but regularly. The ADC sample result is show in figure 1. Because the difference between the pulse is slight, so use the result of the signal processing: pulse compression(a algorithm to compress the echo wave to a compressed point), to show the change of ampliltude. As you can see,  the pulse compression result show that the amplitude of the generated DDS signal change like a sine.

the configuration word of AD9910 is:

parameter       AD9914_MODE1    =  304'h00_00006000_01_00480820_02_040fc000_0B_30A3D70A1C28F5C3_0C_00346DC600346DC6_0D_00010001;

the clock for DDS is 800MHz , and 100MHz clock for the DDS configuration timing, the sclk frequency is 12.5MHz. The 800 and 100MHz clock are phase-aligned,the final clock frequency for DDS internal configuration is 200MHz(800/4=200, according to the datasheet of AD9910),twice as much as the frequency of the clock for configuration timing,so the time interval between two configuration is exactly the same. I use the Normal Ramp Generation mode to generate LFM signal, add a switch to the output of AD9910 to get the LFM pulse. The ioupdata set high evey 46us, the period is integral multiple of the configuration clock period 10ns.

Such output result will severely affect the result of the signal processing.

I have been confused by this problem for days. Any advece will be appreciated.