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transmit DC signal but receive signal unexpected, why?

I have done two tests to transmit DC signal with NI-USRP 2920 and the parameters are as follow:

 

common parameters:

1-carrier frequency: 200MHz

2-IQ Rate: 200kHz

3-Transmit Gain: 20dB

 

for test-A:

Transmit Signal:

A-data.PNG

 

 

 

 

 

 When the receive gain is 0dB, the received signal is

A-1.png

When set the receive gain is 20dB, the received signal isA-2.pngA-2-zoom in.png

 

for test-B

Transmit signal:

B-data.PNG

 

 

 

 

 

 

When the receive gain is 0dB, the receive signal is:

B-1.pngB-1-zoom in.png

 

I want to know why I cannot receive a signal similar to DC(that is, I expect that I will receive a DC plus a little noise).

In the next project, I will use QPSK modulation, but the phase flip result(that is, the transmited signal don't have negative part but the received signal do)shown above makes me upset. Can I do something to help transmit in QPSK successfully? 

Thx!

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