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?