On 09/19/2013 10:45 AM, Isdren Gineer wrote:
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?
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You're either getting ADC clipping, or your receiver is entering a
non-linear operating region where harmonic distortion starts to happen.
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