[USRP-users] x310 Master clock rate

Marcus Müller marcus.mueller at ettus.com
Wed Apr 22 09:49:31 EDT 2015


Hi Taariqa,

oh, yes, sorry, "O" is for Overflow, i.e. when your computer doesn't
take samples fast enough from the receiving streamer of the USRP;
"U" is Underflow, which is the equivalent thing on TX, so yes, that's
probably why you are seeing "holes" in your transmission.

If you'd like to transmit a 50 MHz pulse, then you need to use at least
a 50MS/s sampling rate, which does solve the "odd decimation" warning,
but introduces a heavier computational load on your PC, so it makes the
"U" problem worse.
How do you connect your X310 to your computer, and how do you generate
your transmit signal?

Best regards,
Marcus

On 04/22/2015 03:45 PM, Taariqa Maharaj wrote:
> Hi Marcus
>
> We would Ideally like to transmit a 50Mhz (bandwidth) pulse.
>
> The UHD did not print out an "O". It did however print out "L" and
> "U", would this cause the same effect?
>
> Regards,
> Taariqa
>
> On Wed, Apr 22, 2015 at 11:44 AM, Marcus Müller
> <marcus.mueller at ettus.com <mailto:marcus.mueller at ettus.com>> wrote:
>
>     Hi!
>
>     Thanks for following up on this.
>>     With the master clock set at 200Mhz and a sampling rate of 40Msps
>>     the following warning appears:
>>
>>     UHD Warning:
>>
>>     The requested interpolation is odd; the user should expect CIC
>>     rolloff.
>>
>>     Select an even interpolation to ensure that a halfband filter is
>>     enabled.
>>
>>     interpolation = dsp_rate/samp_rate -> 5 = (200.000000
>>     MHz)/(40.000000 MHz)
>>
>>
>
>     To explain the warning:
>     The FPGA decimates the RX samples down, coming in at the master
>     clock rate from the ADC, to the sampling rate you select.
>     To do that without introducing aliasing, it has to low-pass filter
>     them.
>     The FPGA filter chain has multiple stages; if I remember
>     correctly, the filters are in this order:
>     1. 1x CIC filter for odd factors in decimation
>     2. 3x 31-tap half band filters that each implement a good
>     decimation for factors of 2 each.
>
>     Now, if you use an odd decimation (200MHz/40MHz==5), there's no
>     factor of 2 in your decimation, and hence you only use the
>     "not-as-good" CIC filter, and are likely to see effects of filter
>     roll-of at the edges of your band.
>
>     In TX direction, it's quite the same problem:
>     1. 1x 17-tap half band interpolators
>     2. 1x 5-tap half band interpolators
>     3. 1x CIC
>
>     Therefore: if your measurements show that your signal is OK for
>     your application, just ignore the warning (it's just a warning,
>     not an error); if you think this is bad for the signal you want to
>     work with, I'd recommend using a 50MS/s sampling rate -- that
>     would have a decimation of 4 and should work beautifully. Of
>     course, this means that your host computer would have to implement
>     the interpolation.
>
>     How wide is the signal you're planning to transmit spectrally? Is
>     it significantly less than 40MHz wide?
>
>>     When we increase the sampling rate to anything above 40Msps, the
>>     following discrepancy is seen in the data.
>>     If you look at the above image there seems to be some missing data.
>     I can only guess here, but is it possible UHD printed an "O" when
>     you tried to transmit this?
>     That would imply that your computer was too slow to supply the
>     samples in real time, and since the USRP did not have anything to
>     transmit, there is a "pause".
>
>     Best regards,
>     Marcus Müller
>
>

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