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

Marcus Müller marcus.mueller at ettus.com
Tue Mar 8 15:57:39 EST 2016


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:
$\sigma^2 =10^{\frac{-25}{20}}\quad
\frac1{2\pi}\int\limits_{-\pi}^{\pi}\,A^2\sin^2(t)\,dt$
$= 10^{-1.25} \quad \frac12A^2 $
$\approx 0.028\,A^2 $, i.e.
$\sigma \approx 0.168\, A$.
The Q-function at $\frac1{0.168}\approx 5.96$ is about
$1.3407\cdot10^{−9}$, 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 West<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 <mailto: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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