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I am using an IIR bandpass filter designed using the elliptic filter in matlab to filter out unwanted frequencies.

The filter is fed with a test input that has a dc offset and sine wave of frequency within the passband. The output has some ripples or bumps at the peak that makes the waveform look slightly distorted.

How could there be any ripples in the output when the input only has a single frequency sine wave?

Could it be due to any design errors in the elliptic filter?

This is the script that I used to generate the coefficients. It generates four sets of cascaded second order biquad filter coefficients one for each band.

fs=50

%creating the filter 
[A1,B1,C1,D1] = ellip(6,0.1,50,[0.1  2.95 ]/25);
[A2,B2,C2,D2] = ellip(6,0.2,50,[ 3   7 ]/25);
[A3,B3,C3,D3] = ellip(8,0.05,50,[ 7  13 ]/25);
[A4,B4,C4,D4] = ellip(10,0.1,50,[ 13 24 ]/25);


%getting second order coefficients of size Nx6, where N is order of the filter
s1 = ss2sos(A1,B1,C1,D1);
s2 = ss2sos(A2,B2,C2,D2);
s3 = ss2sos(A3,B3,C3,D3);
s4 = ss2sos(A4,B4,C4,D4);


%create cascaded biquad filter from the coefficients
biquad_0_3   = dfilt.df1sos(s1)
biquad_3_7   = dfilt.df1sos(s2)
biquad_7_13  = dfilt.df1sos(s3)
biquad_13_25 = dfilt.df1sos(s4)


%CHECK STABILITY
stable1 = isstable(s1);
stable2 = isstable(s2);
stable3 = isstable(s3);
stable4 = isstable(s4);


zplane(ss2zp(A1,B1,C1,D1))
zplane(ss2zp(A2,B2,C2,D2))
zplane(ss2zp(A3,B3,C3,D3))
zplane(ss2zp(A4,B4,C4,D4))


%plots magnitude response and group delay
  fvtool(biquad_0_3,biquad_3_7,biquad_7_13,biquad_13_25); 
  grpdelay(biquad_0_3)
  grpdelay(biquad_3_7)
  grpdelay(biquad_7_13)
  grpdelay(biquad_13_25)
Simactricals
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  • My two thoughts are that you may be viewing the filter before the output has settled, or you are working with a fixed point design where non-linearity may be introduced - are either of those the case here? – Dan Boschen Mar 17 '22 at 17:00
  • Please add your exact filter code and test code. These are difficult filters to implement and the impulse response will ring for many thousands of samples. – Hilmar Mar 18 '22 at 11:40
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    The '[B, A] = ellip(6,0.1,50,[0.1, 2.95]/25);' filter has three poles outside the unit circle, and is unstable. – Richard Lyons Mar 18 '22 at 11:54

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