https://corvid.io/2017/04/22/stupid-rfnoc-tricks-loopback/ should help.
RFNoC does not support loopback "out of the box".
On Fri, Feb 2, 2018 at 12:18 PM Pratik Chatterjee via USRP-users <
> This question was posted two years ago (reference link below). I am hoping
> that the problem has been fixed and that I am missing something important.
> Trying to build a simple repeater on X310 using the inbuilt blocks: rfnoc
> radio receive -> rfnoc fifo - > rfnoc radio transmit. Runs fine but don't
> see anything coming out. In fact I don't see the receive LED on the front
> panel as well.
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