[USRP-users] E310 RFNoC FFT Overrun Issue

Ramazan Çetin ramazan.cetin at gohm.com.tr
Sun Feb 10 07:54:39 EST 2019

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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