2nd order filter transfer function normalization

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SUMMARY

The discussion focuses on the normalization of the transfer function for a second-order lowpass filter as outlined in a Texas Instruments guide. The original equations presented include a coefficient form of the denominator and a normalized version, with specific substitutions for variables such as \(s\), \(\omega_c\), and \(a_1\). A correction was identified regarding the second term of the normalized equation, clarifying that it should include \(\sqrt{a_0}\) instead of just \(a_0\). The participant concluded that the normalization process is crucial for accurate filter design and emphasized the importance of understanding the relationship between the coefficients.

PREREQUISITES
  • Understanding of second-order lowpass filter design
  • Familiarity with transfer functions and normalization techniques
  • Knowledge of the Q factor in filter design
  • Basic proficiency in LaTeX for mathematical expressions
NEXT STEPS
  • Study the Texas Instruments guide on active filter design
  • Learn about the implications of the Q factor on filter performance
  • Explore the derivation of transfer functions for different filter types
  • Investigate common misprints and errors in technical documentation
USEFUL FOR

Electrical engineers, filter design specialists, and students studying signal processing will benefit from this discussion, particularly those focused on active filter design and normalization techniques.

bitrex
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I'm looking at a guide by Texas Instruments on active filter design. In it are the following equations for a second order lowpass filter, verbatim:

The coefficient form of the denominator: [tex]s^2 + a_1s + a_0[/tex]

Normalized: [tex]P(s) = (\frac{s}{\sqrt{a_0}*\omega_c})^2 + \frac{a_1s}{a_0*\omega_c} + 1[/tex]

Substituting [tex]s = j2\pi f, \omega_c = 2\pi f_c, a_1 = \frac{1}{Q}, \sqrt{a_0} = FSF[/tex][tex]P(f) = -(\frac{f}{FSF*f_c})^2 + \frac{1}{Q}\frac{jf}{FSF*fc} + 1[/tex]

Maybe I'm missing something obvious here, but why is it that it is not FSF^2 in the second term of the last equation, if [tex]\sqrt{a_0} = FSF[/tex]?
 
Last edited:
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I made an error in the LaTeX of the first equation, I've corrected it. :redface:
 
I think I see the problem, it must be a misprint on their part. The equation should be:

[tex]P(s) = (\frac{s}{\sqrt{a_0}*\omega_c})^2 + \frac{a_1s}{\sqrt{a_0}*\omega_c} + 1[/tex]

Also remembering that [tex]\omega_c = \sqrt{a_0}[/tex] helps.
 

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