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

Michael West michael.west at ettus.com
Tue Mar 8 18:30:27 EST 2016


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:
> [image: $\sigma^2 =10^{\frac{-25}{20}}\quad
> \frac1{2\pi}\int\limits_{-\pi}^{\pi}\,A^2\sin^2(t)\,dt$]
> [image: $= 10^{-1.25} \quad \frac12A^2 $]
> [image: $\approx 0.028\,A^2 $], i.e.
> [image: $\sigma \approx 0.168\, A$].
> The Q-function at [image: $\frac1{0.168}\approx 5.96$] is about [image:
> $1.3407\cdot10^{−9}$], 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> <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>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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>> USRP-users at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
>
>
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>
>
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