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IIR cascade FIR, incorrect phase response display

Question asked by JCB on Apr 16, 2014
Latest reply on Apr 16, 2014 by KJBob

Hello.

 

I'm using chip ADAU 1701,  fs=48000.

Sorry, I am a newbie in filter designing.

I would like to design an IIR Notch filter at 1kHz. I used "ScopeIIR" software to get the IIR coefficients.

Then, I feed that IIR output into another FIR filter to correct the phase shift.   I'm using 'rephase' to design the FIR filter and get 128 taps of impulse response and  then fill in the FIR filter in SigmaStudio. 

 

Below is capture screen "ScopeIIR":

IIR filter generates some phase shift around 1kHz. (Q=1, Butterworth, cutoff at 1kHz,  fs_1=850, fs_2=1150).

iir_1khz.JPG

Then, I import the magnitude and phase response from above IIR filter into "rephase". And then adjust the Paragraphic Phase EQ to archieve phase shift correctness.  After done, i generate 128 taps(FIR coeff) of impulse response in text file. Then i load that FIR coeff into Sigma Studio.

rephase.JPG

Belowis my SigmaStudio project design. 

sigmadesign.JPG

But I could not get the expected result  in Simulation Probe.  I guess the output should be a linear line 0 degree.

This is the screen of phase display:

mag-phase response.JPG

Am I using wrong method to get the FIR coeff? Or the simulated phase displayed is incorrect?

 

Hope to hear from you soon.

 

Thanks alot,

 

JCB

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