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I am experimenting with some signals. I ma generating a 2.47 Ghz signal from signal generator and receiving it by an SDR (LMS7002m Rf chip) with Rx frequency tuned to 2.47 GHz. Upon plotting the received I/Q data I get the following plot. Can anyone help me understanding why there is a second circle below the circle centered at (0,0). Ideally I should be seeing a single circle.enter image description here.

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  • $\begingroup$ Can you provide a stem plot of your initial imaginary samples in the time domain? I want to see if every other sample is offset or something else $\endgroup$ Dec 18, 2019 at 18:37
  • $\begingroup$ drive.google.com/open?id=18VEn0W61XqyuBYF2UBV8eKCTuW9NuUQ0 I am attaching the file link with all the samples I receive. First column is the I values and second column is Q values. I simply plot using python using this command dataframerx.sample(n=3000000,random_state=1).plot(kind='scatter',x='I',y='Q',grid=True,xlim=(-2,2),ylim=(-2,2)) $\endgroup$
    – tue2017
    Dec 19, 2019 at 21:57
  • $\begingroup$ I looked at it and agree with @hotpaw2-- -Just look at your Q data versus time and there is an offset in Q at approximately 62000 samples in. (so could either be a temporal offset in your measurement instrument or your SDR) If that answer he gave also makes sense to you, consider marking it as correct to close this question. $\endgroup$ Dec 19, 2019 at 22:34
  • $\begingroup$ The offset is in Q from the first sample to about 62000 samples in, and then there is no offset after that. Compared to your file size it is a very small portion of the entire data captured. $\endgroup$ Dec 19, 2019 at 22:51

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IQ modulators, demodulators, and test equipment (oscilloscopes, etc.) can have channel imbalance in either or both gain and offset. It looks like the Q channel of either the source or instrumentation changed or adjusted its offset during your measurement.

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