[USRP-users] Unexpected sample drift with X300 and X310

Kai-Uwe Storek kaiuwe.storek at googlemail.com
Sat Oct 21 17:32:23 EDT 2017


Hey,

the attached flowgraph simply generates a sine which is transmitted
and received by the same USRP in a loop (30dB attenuation + coax cable
between tx and rx port). I used the following USRPs:

- B210
- X300 (1G Eth)
- X310 (1G Eth)

On the Rx side I just added a time sink to view the complex sine.

Here is my observation:

If no multiplication operations are used on Tx side, one can observe a
"time constant"  snippet (or segment) of the sine displayed by the
time sink on Rx side - which is, imho, the expected behavior.

As soon as one multiplication is part of the Tx side (just enable the
"multiply const" block in the attached graph)  one can observe that
the sine wave is "wandering" or drifting on Rx side.

I assume that the drifting effect is caused by lost or added samples
on the Tx side. This behavior is only present for the X300 and X310
devices and not for the B210.

Are my assumptions about the expected behavior correct? Is there any
chance stop this drift with X3x0 devices?

Background: The issue attractes attention because I'm working on a
kind of channel estimation, where I noticed that my correlation
sequence delivers some unexpected results as I changed from B210 to
X300.

Thanks!
Kai
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