I am working on the IQ raw data from FMCW radar. I plot the I and Q data, they seem to be strongly shifted from the center. And the amplitude of I and Q also varies. I don't understand why it looks like this. I think, they should local on the center and with fixed amplitude. Thank you very much if someone can help.



1 Answer 1


The IQ data from an FMCW radar would have the frequency difference from the delay between the transmitted and reflected signal given the transmitted signal is a changing frequency with time; so the result is what should be expected. Given the ramp rate of frequency the round trip delay from target can be determined from the frequency of this result.

The result here appears to have a stronger higher frequency AND lower frequency tone as the result of two different targets. The lower frequency that is superimposed would be due to the closer target.

  • $\begingroup$ Thank you for your answer. I still get confused. I understand the distance is measured from the IF signal. But why is the IF signal is not symmetric along x - axis. some of it is shifted up and some down, like a huge wave. $\endgroup$
    – elise
    May 25, 2020 at 7:54
  • $\begingroup$ You plotted the time domain IF signal, which is made out of a superposition of sinusoidals (plus noise). The frequencies of these sinusoidal components correspond to the distances of objects. To see the frequency spectrum, do an FFT of the signal and plot the magnitude. You should see peaks at the frequencies that are in the signal. Judging by the graph you posted, it looks like you have a lower-frequency and a higher-frequency component. $\endgroup$
    – mateC
    May 25, 2020 at 12:39
  • $\begingroup$ ok. Thanks. Then I think the shift is mainly caused by noise. I care more about the phase instead of its magnitude. So I wanted to understand, whether the shift in I/ data is normal and how would it affect the phase of the signal. $\endgroup$
    – elise
    May 25, 2020 at 19:26
  • $\begingroup$ @elise I really don't think it is noise but from the effects we stated. Each target will result in a different frequency and typically you would resolve and track targets from an FFT of your IF signal. Each frequency is a linear phase with time, and each frequency would correspond to a different reflected signal at a different time delay. $\endgroup$ May 25, 2020 at 23:36

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