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Ahmed Alfadhel

Noisy Output from FIR Compiler

Question

Hi ,

I am trying to impelement FIR compilers in my system design. 

I depended on TFilter (http://t-filter.engineerjs.com/) in my design for the FIR filter. The filter parameters (frequency response) are shown in the first attached picture.

The interface connections between the DDS compiler (16 kHz oscillator) the FIR compiler are shown the second attached picture. 

After I imported the coefficients file in FIR IP core, then I run the a simulation to check the output of the FIR filter. The third attached picture shows a noisy output for this filter !

The FIR filter type is "Single Rate", the fourth attached picture shows the frequency response for the imported coefficients inside the FIR compiler.

Plz, how to fix the output signal to become smooth like the input?

Thanks. 

Filter_parameters_T2.JPG

Design.JPG

Simulationo.JPG

freq_response_FIR_IP.JPG

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2 answers to this question

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@Ahmed Alfadhel,

You have a couple of options available to you:

  • It's not clear, from your pictures above, whether or not the -40dB stop band was achieved.  Some amount of noise is to be expected due to truncation errors, etc.  Without seeing an estimated PSD, I can't tell.  It may be that it's doing exactly what you required of it.
  • -40dB is only so good.  With more taps, you should be able to go deeper.  How deep depends upon your requirements.  How good do you want the signal to look?
  • You may also need to provide more bits to both your signal and coefficient values in order to do better.
  • You did prescale your coefficients so that, when rounded to integers, the taps were useful, right?
  • Also, be aware, the filter will be specified for full scale.  You'll want to measure it against a full scale input.  Anything less will introduce additional truncation error.  This is one of those reasons why the dynamic range (i.e. number of bits) of the input and output signals are so important.

Enjoy!

Dan

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