[USRP-users] Data corruption with X3x0/UBX-160 receive at 200 Msps

Mike Williams jmw711 at verizon.net
Wed Mar 9 18:01:20 EST 2016


I’m seeing a sample error rate of > 5e-7 (about 5 samples in 10 million).  The test signal only toggles the lower 8 bits and the glitches show up when one of bits 11:8 is set.  

What is the best way to send a data capture file?

Mike

From: Michael West via USRP-users 
Sent: Tuesday, March 08, 2016 6:30 PM
To: Marcus Müller 
Cc: USRP-users at lists.ettus.com 
Subject: Re: [USRP-users] Data corruption with X3x0/UBX-160 receive at 200 Msps

Hi Mike,


I spoke with some of my colleagues and was told that the X300/X310 ADC has a known issue on the 4 MSBs that results in a bit error rate of 1e-9.  I'm not sure whether or not it has anything to do with what you are experiencing, especially because your error rate is much higher, but I thought you should know.  If the majority of the bit errors you see are in the 4 MSBs, it may be a cause.


Regards,

Michael 


On Tue, Mar 8, 2016 at 12:57 PM, Marcus Müller <marcus.mueller at ettus.com> wrote:

  Hi Mike, 

  can you get us an upload of the samples somewhere?

  Short, rough calculation regarding probability of the gaussian noise-induced bitflip:

  At a thermal noise floor of -174dBm/Hz, you'd end up with -174dBm/Hz*(80MHz)+NF[dB] noise power per I and Q.
  According to our measurements[1, p 44], for 20 dB gain, depending on frequency, NF is ca 10dB.

  So, noise power becomes (-174+79+10) dBm = -85dBm, which yields in total an SNR of 25 dB, i.e. the noise variance is -25dB of the tone power:


  , i.e.
  . 
  The Q-function at  is about , i.e. the probability that noise is higher than signal amplitude is about 1 in 745 million, if we only have thermal noise in the receiver, and not the transmitter. 745 Million is not quite the probability you see, but it's in the same ball park, at least! Notice that change in SNR can have quite a significant effect on the "outlier" probability here.

  Thus, looking at the samples would not only be interesting from a debugging, but also from a SNR estimation point of view :)

  Cheers,
  Marcus

  [1] http://files.ettus.com/performance_data/ubx/UBX-without-UHD-corrections.pdf 



  On 03/08/2016 08:57 PM, Mike Williams via USRP-users wrote:

    Hi Michael,
    It's not clipping.  I'm testing with a -60 dBm tone from a signal generator and a gain of 20.  The peak I/Q values out of the X3x0 are < 256 except for when this data corruption occurs about every 1 to 10 million samples.   It looks like a bit flip in either the I or Q samples.  
    For example, the I samples in one instance are:  0x0054, 0x00AB, 0x028C (bit 9 is a 1 but it should be 0) , 0x007B.
    Thanks,
    Mike

    On 03/07/16, Michael Westmailto:michael.west at ettus.com wrote: 

    Hi Mike,


    The errors could also be due to clipping.  Your gain settings could be too high.


    Regards,

    Michael


    On Mon, Mar 7, 2016 at 7:42 AM, Mike Williams via USRP-users <usrp-users at lists.ettus.com> wrote:

      We are using an X300/UBX-160 programmed UHD-3.9.2 to record data at 200 Msps using the rx_samples_to_file example program.  We are seeing data corruption that appears to be due to bit errors in some I or Q samples.  The self cal of the adc capture and transfer delays works fine so maybe this is a timing error elsewhere in the FPGA?  

      We've tried an X310 as well and have the same problem. 

      Mike


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