I’m working with a 2020 LimeSDR Mini (a discontinued one) and am trying to implement QPSK. I’m having clipping issues due to both Tx/Rx gain and not knowing what amplitude my IQ signal should peak at.
Here is a screenshot where my QPSK signal (RRC shaped) peaks at amplitude 1 and both Tx and Rx have gain of 20dB:
The bottom graph shows the signal directly before transmitting, and the top graph shows it afterwards. Notice how the input always alternates between +1 and -1 in both real and complex, but the output will clip down to 0. In this case, the peaks are safe but the stuff between the peaks clips, so it seems there’s a minimum clip here?
If I raise gain to 30dB it looks like this:
In this case, it’s clipping both on top and on bottom. Peaks get flattened out and it flatlines at 0.
I tried cutting input amplitude down to 25% and I get the same results as above, just scaled down accordingly:
You can also see the clipping happen in the constellation diagram. When gain gets too high, the points in the constellation begin to confine themselves to a square shape:
For reference, this is my flowgraph:
