I am using N210 USRP's with WBX daughtercards connected with a loopback kit implementing a 8-PSK waveform. When I start transmitting, I get a clean constellation. After a short time however (~5 sec) it appears that the receiver temporarily loses phase lock. The eight point constellation degrades such that many points appear in a circle with the same radius as the points. The FFT of the signal shows that the noise floor temporarily rises. It's as if the amplifier begins to clip or some other non-linear process is occurring. After such a 'burst' the constellation appears clean again until another burst occurs. Has anybody else observed this behavior? Any ideas to what could be the underlying issue?
I am using an multiplier to set the signal amplitude to 0.3 and have the gains of the USRP's set to around 7dB. If I use a higher amplitude or greater gains, performance degrades quickly. Are there any rules of thumb for these settings? Why does the performance degrade with increased values?