[USRP-users] RFNoC: FFT not working correctly?

Leutner, Sebastian Benedict sebastian.leutner at rwth-aachen.de
Mon Nov 6 08:26:15 EST 2017

Hi all,

starting with the example code under gr-ettus/examples/rfnoc/rfnoc_fft.grc, I fail to get any valid spectrum from the "RFNoC: FFT".

*** Performed Tests ***
If I execute it as is (with the "RFNoC: FFT" on the FPGA), I only see noise [2].

Using an FFT on the host (instead of "RFNoC: FFT"), I get what I would expect: A peak at 125 kHz [3].

I also tried using an external signal generator connected to the TX/RX antenna of the USRP. Therefore, I activated the "RFNoC: Radio" block and connected it to "RFNoC: FFT". Here, I do not see a peak and also get "Doverrun on chan 0" messages. When including the "RFNoC: DDC" between the two blocks, I get rid of these overrun messages but instead of seeing the peak at 3 GHz (as with the host FFT), I only see an incorrect peak when tuning the signal generator to 3.011 GHz [4]. Also notice the size mismatch error between DDC and FFT. Does anyone know how to fix this? I just hit F6 to run it.

If I now replace the "RFNoC: FFT" with the FFT on the host, I do see the correct peak but I get "Doverrun on chan 0" messages and after some time the graph freezes and I get "timeout on chan 0" messages [5].

If I just use the "QT GUI Frequency Sink" (which probably has an FFT included?), I get the behaviour I would expect with the peak moving correctly when changing the signal generator frequency [6].

*** Used Setup ***
I used pyBOMBS according to [1] to setup the development environment on Ubuntu 16.04 LTS.
Thus, I have the follwing versions installed:
- UHD: 4.0.0.rfnoc-devel-409-gec9138eb
- GNURadio: 3.7.12git-295-ga0adcd33
- pyBOMBS: 2.3.3a

The hardware is an USRP X310 with a CBX daughterboard and the "usrp_x310_fpga_RFNOC_HG.bit" FPGA image fetched via "uhd_images_downloader". I already increased the MTU size to 9000 bytes and the maximum r/w socket buffer size to 33554432 bytes.

I also tried going back a few commits for UHD but this either did not change the behavior or I had trouble finding the correct commits in the gr-ettus, uhd and gnuradio repos to work together, i.e. it failed during compilation in these cases.

Does anyone know whats going on here? Did I miss something?

Kind regards,

[1] https://kb.ettus.com/Getting_Started_with_RFNoC_Development#Creating_a_development_environment
[2] fft_example.jpeg
[3] fft_on_host.jpeg
[4] fft_rfnoc_extsiggen.jpeg
[5] fft_on_host_extsiggen.jpeg
[6] fft_qt_gui.jpeg
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