This information is part of the Modelica Standard Library maintained by the Modelica Association.

The goal is to implement the filter in the following form:

// complex conjugate poles:
der(x1) = a*x1 - b*x2 + ku*u;
der(x2) = b*x1 + a*x2;
     y  = k1*x1 + k2*x2;

          ku = (a^2 + b^2)/b
          k1 = cn/ku
          k2 = cn*a/(b*ku)

This representation has the following transfer function:

// complex conjugate poles
    s*x2 =  a*x2 + b*x1
    s*x1 = -b*x2 + a*x1 + ku*u
  or
    (s-a)*x2               = b*x1  ->  x2 = b/(s-a)*x1
    (s + b^2/(s-a) - a)*x1 = ku*u  ->  (s(s-a) + b^2 - a*(s-a))*x1  = ku*(s-a)*u
                                   ->  (s^2 - 2*a*s + a^2 + b^2)*x1 = ku*(s-a)*u
  or
    x1 = ku*(s-a)/(s^2 - 2*a*s + a^2 + b^2)*u
    x2 = b/(s-a)*ku*(s-a)/(s^2 - 2*a*s + a^2 + b^2)*u
       = b*ku/(s^2 - 2*a*s + a^2 + b^2)*u
    y  = k1*x1 + k2*x2
       = (k1*ku*(s-a) + k2*b*ku) / (s^2 - 2*a*s + a^2 + b^2)*u
       = (k1*ku*s + k2*b*ku - k1*ku*a) / (s^2 - 2*a*s + a^2 + b^2)*u
       = (cn*s + cn*a - cn*a) / (s^2 - 2*a*s + a^2 + b^2)*u
       = cn*s / (s^2 - 2*a*s + a^2 + b^2)*u

  comparing coefficients with
    y = cn*s / (s^2 + c1*s + c0)*u  ->  a = -c1/2
                                        b = sqrt(c0 - a^2)

  comparing with eigenvalue representation:
    (s - (a+jb))*(s - (a-jb)) = s^2 -2*a*s + a^2 + b^2
  shows that:
    a: real part of eigenvalue
    b: imaginary part of eigenvalue