Re: [Discuss-gnuradio] USRP Clipping?

So 30db is a good value to protect WBX from physical damage in a loopback configuration. It does however leave you with a potentially very high signal level at the RX and so it makes sense that your setup appears to be operating at the edge of linearity, despite the modest configured gains. My first guess, assuming that your TX is working as you believe is that your setup is subject to some additional environmental strong and intermittent RFI, though I concede in a cabled loopback setup that theory is harder to support. I've seen very similar FFT results when working weak signals without enough out of band filtering. Are you working near a RADAR? Laptop with WiFI right next to the USRP? Does a further reduction in gains solve your problem?

On Sep 19, 2013, at 9:00 AM, Isdren Gineer <address@hidden> wrote:

The loopback kit is a 30dB attenuator and an SMA cable sold by Ettus Research.
On Thu, Sep 19, 2013 at 8:40 AM, Ian Buckley <address@hidden> wrote:
Can you describe for us what the "loopback kit" comprises? Theres hopefully significant attenuation in the signal path.
On Sep 19, 2013, at 7:53 AM, "Marcus D. Leech" <address@hidden> wrote:
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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From:	Ian Buckley
Subject:	Re: [Discuss-gnuradio] USRP Clipping?
Date:	Thu, 19 Sep 2013 09:40:21 -0700