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Visitor tianhufei
Visitor
2,484 Views
Registered: ‎09-12-2016

Problem with FIR filter using interpolation

  I have a 3.84M sample rate signal which I am to interpolate to 61.44M sample rate. I first generated a low-pass filter coefficients then I loaded into the FIR IP. And I changed filter type to interpolation and the rate change by 16.  3.84x16 = 61.44.  Currently I am using a sine wave just to do initial verification.

  The sine wave has a frequency of around 60k at sample rate 3.84M. The sine wave is the input of my FIR filter and the output would have been a 60k sine wave at sample rate 61.44M. 

  However, my result was a signal that has the outline of a 60k sine wave.  But it is comb like, if you look into it you would see a lot of impulses. The wave is continuous, consisting of a lot of impulses. The magnitude of all these impulses made up of a 60k sine wave, like the following:

 

                          1

                         111

                        11111

                       1111111111111

                              11111     

                               111

                                1 

 

                     instead of the explected:

 

                          1

                         1 1

                        1   1

                       1     1     1

                              1   1     

                               1 1

                                1 

 

 

    It seemes that the high frequency parts of the interpolated save have not been filtered out. What should I do?  Thank you for your help.

 

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Observer brucekaraffa
Observer
2,140 Views
Registered: ‎06-06-2014

Re: Problem with FIR filter using interpolation

You may have a filter design problem.  Design your filter at the higher (interpolated) clock rate.  Make sure that Fs/2 of your original rate is well into the stop band of the filter, say at least 30dB down.  A good book on multi-rate signal processing will explain why you want to do this and will help you optimize the filter.  Interpolation causes images of the original signal and the filter needs to be designed to remove them.

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