why the received Q signal is always distorted?

Question

Hi, 
 when I implement QPSK signal transmit-receive by ad9361, the result is out of my expectation. By ILA IP, the tx_signal and rx_signal are shown as follow, 
 
 it can be viewed that Q signal is distorted totally. 
 then from SDK, I use function ad9361_bist_loopback(ad9361_phy, 1) to implement the loopback tx to rx in ad9361. the signals are shown below, 
 
 Q signal is still not right. ( ADI provided reference HDL design and no-OS platform are adopted )Can you help me? 
 thanks in advance, 
 xiao quan

mhennerich · Accepted Answer

int32_t dac_datasel ( struct ad9361_rf_phy *phy, int32_t chan, enum dds_data_select sel) 
 https://github.com/analogdevicesinc/no-OS/blob/master/ad9361/sw/platform_xilinx/dac_core.c#L468 
 dac_datasel should set all channels when chan is set to -1. 
 In your case when set to 0 it only set's the first channel which is I. 
 -Michael)

larsc · Answer

Hi, 
 What you are seeing is a so called phase shift. This is a normal thing in RF signals. You'll either have to use a differential QPSK (DQPSK) encoding. Or embedded a preamble in your datastream that allows to recover the original phase. 
 - Lars

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why the received Q signal is always distorted?