IIR block and "Manual" IIR

Question

Hi everybody. I was working on an IIR filter implementation using blocks avaible in the IDE. I disegned a second order IIR (5 coefficients). I tried to compare performance between IIR filter I designed with IDE blocks (patch pic attached) and the block "Second Order General IIR filter", with "IIR Coefficient" approach, They have frequency responses completely different and I don't know really why. 
 Did I make some mistakes? 
 Patch

Manual IIR FR 
 
 General IIR (IDE embedded) FR 
 
 Thanks very much, 
 regards.

EZAdministrator · Accepted Answer

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KJBob · Answer

Hello leonxp, 
 Here's some clarifying data: 
 To rough out the frequency responses without external test gear I use a swept VCO and peak detector as shown below: 
 
 The Real-Time Display gives the peak magnitude response from 1KHz to 20KHz. Both scales are linear, so the results format differently than with the stimulus / probe display. 
 This is the feedback portion of your wired filter with one feedback block and one straight delay, and the original coefficients: 
 
 With these feedback coefficients the filter's magnitude output hits the rail (16.0), as shown below. The General Filter block yields the same result: 
 
 Reducing the manual filter's feedback coefficients by an arbitrary factor of 10 results in this response: 
 
 The General Filter block, with the same coefficients except for a1 and a2 signs reversed, provides the same results: 
 
 I attached the circuit with the swept VCO that made these plots. I hope you find all this helpful. 
 Best regards, 
 Bob

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IIR block and "Manual" IIR

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