Hi Marcus, Sorry for the error in the plots (I forgot doing an axis square;).
I am talking about why I get a normal I vsQ plot using a pure frequency and why I get a romboidal shape when I transmit a varying frequency. This phenomenon does not depend on the carrier frequency, the sampling rate or the gain. The patterns I send in these examples are sinusoidal waves of constant 2Hz frequency, sweep from 2Hz to 10 Hz and sweep from 2Hz to 50Hz. All these waveforms are 1 buffer length and I is 1/4 wave out of phase from Q. Is you see an amplitude plot of the I+Q signal, there is an AM envelope on the sent waveform (The waveform is correct, but with this AM envelope). This Friday I will go to the lab to do some precise measurements with an Agilent oscilloscope, so I will send you my conclusions about this. I guess this could be due to impedance differences (without the Tx antenna, the AM envelope is softer), digital filtering (antialias, low-pass, ...) and amplification. Have a nice day! Javier
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