[USRP-users] E310 RFNoC FFT Overrun Issue

Ramazan Çetin ramazan.cetin at gohm.com.tr
Sun Feb 10 15:51:46 EST 2019

Hi Jonathon,

I fixed the RFNOC: Vector IIR issue with changing its coefficients. Now 
i can get samples using Vector IIR and keep 1 in N. I just tried to get 
Wifi samples using this;

RFNOC: radio(56Mhz) -> RFNOC: FFT -> RFNOC: Vector_iir -> RFNOC: keep 1 
in N(N = 14) -> FileSink

I am receiving samples from CPU as 4M. But i am sampling signal with 
56Mhz and decimate it after FFT. So, i should get 56Mhz bandwidth 
signal. But when i look my signal on CPU, it looks small bandwidth. Wifi 
is 20 or 40MHz bandwidth so i should see some gap on frequency axis, but 
i cannot. I have attached my flowgraph and wifi spectrogram.

Q1. Is it related with keep 1 in N? Is it keeping first N samples from 
FFT bins? or i am missing something else. Is it possible to see 56 MHz 
bandwidth signal on host?

Q2. I am getting "Ooverrun on chan 0" after 3 or 4 seconds. Is it CPU's 
capacity for processing? After some time it cannot catch that speed. 
Maybe i need some FIFO. Is it right?

Thank you so much for your answers. Best regards.


On 10.02.2019 15:54, Ramazan Çetin wrote:
> Hi Jonathon,
> Thank you for your reply. I just tried what you said and investigated 
> gr-ettus/examples/rfnoc/rfnoc_vector_iir.grc. I firstly tried without 
> using keep 1 in N block. I just sampled 2MHz and try to get FM radio 
> samples of spectrum. My flowgraph is attached.
> My problem is when i used RFNOC: radio -> RFNOC: FFT -> VecToStr -> 
> IIR_filter(not RFNOC) -> StrToVect -> CompToMag -> Log10 -> FileSink
> with same coefficients it is working good. I can get spectrum samples.
> But when i use RFNOC: radio -> RFNOC: FFT -> RFNOC: Vector_iir -> 
> CompToMag -> Log10 -> FileSink
> it gives all zeros after its output. Coefficients are same. I just 
> changed IIR filter with RFNOC Vector IIR. Do you know why it is happening?
> Best regards.
> On 10.02.2019 10:21, Jonathon Pendlum wrote:
>> Hi Ramazan,
>>     Q1. So, do you know any method to minimize signal distortion while
>>     downsampling in frequency domain? or How can i get wifi signal
>>     frequency
>>     spectrum in E310 ?
>> You want to filter the vector of FFT bins individually, which is not 
>> what the DDC block is designed to do. Instead, you should use the 
>> VectorIIR and Keep 1 in N RFNoC blocks in this configuration: RFNoC 
>> Radio -> RFNoC FFT -> RFNoC VectorIIR -> RFNoC Keep 1 in N -> Host. 
>> For an example, look at the gr-ettus flowgraph in 
>> gr-ettus/examples/rfnoc/rfnoc_vector_iir.grc.
>>     Q2. When i try compiling FPGA image with 4 blocks, it gives "Placer
>>     could not place all instances" error. But in guide, it says user
>>     can put
>>     up to 6 blocks. Why it give that error after 3 blocks?
>> The FPGA fabric has a finite amount of resources (LUTs, Registers, 
>> BRAMs, DSP48s) and your configuration required more resources than 
>> available. The 6 block limit is more of a rule of thumb, especially 
>> when using small blocks like the RFNoC FIFO blocks. You can certainly 
>> use up all the FPGA resources with less than 6 RFNoC blocks. In your 
>> case, a build with just the FFT, VectorIIR, and Keep One in N RFNoC 
>> blocks should fit.
>> Jonathon
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