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JordanD's avatar
JordanD
Icon for New Contributor rankNew Contributor
4 years ago

FIR II Decimation Frequency Response

I'm designing a filter with the FIR II IP. Input data rate is 64 MHz and decimation factor is 8. I want the frequency response to be low pass cut off at 3.2 MHz.

The confusing thing is whether the IP interprets the coefficients at the input rate or the output decimated rate. I would think it is the input rate but the displayed frequency response on the wizard shows otherwise.

I ran the provided test bench ("do decim_fir_msim.tcl") with a sweep of input frequencies and it does not match the wizard plot. Is the frequency axis in the wizard plot wrong, or am I missing something? Reference the attached Octave/Matlab script "fir_inspect.m" for simulation stimulus generation and output analysis. Design files also attached.

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  • CheepinC_altera's avatar
    CheepinC_altera
    Icon for Regular Contributor rankRegular Contributor
    4 years ago

    Hi,


    Thanks for your update. For your information, I am currently consulting Engineering on this to seek clarification. Will keep you posted once there is any valid response from them. Thank you.


  • CheepinC_altera's avatar
    CheepinC_altera
    Icon for Regular Contributor rankRegular Contributor
    4 years ago

    Hi,


    Sorry for the delay. For your information, our Engineering has looked into your simulation and responded that the reason the plot appears to show the cut-off frequency at ~25 MHz is because the fir_inspect.m does not account for the decimation for freqsMhz, which is defined as 0 -> Fsinput/2. To use freqsMhz for plotting the output data, use freqsMhz ./ 8.


    Hope this information is helpful.


    Please let me know if there is any concern. Thank you.



    Best regards,

    Chee Pin


  • CheepinC_altera's avatar
    CheepinC_altera
    Icon for Regular Contributor rankRegular Contributor
    4 years ago

    Hi,


    As I understand it, it has been some time since I last heard from you. This thread will be transitioned to community support. If you have a new question, feel free to open a new thread to get the support from Intel experts. Otherwise, the community users will continue to help you on this thread. Thank you.