Given a frequency function, which conditions should it fulfill to be realisable and implemented as a one-port impedance? I know that for it to be realisable with passive RLC components, the frequency function has to be positive real. But I'm also looking for needed conditions for an active realisation. In my particular case, the frequency function looks somewhat like (s^(n+2)+...)/(s^n); the degree of the numerator exceeds the degree of the denominator by two. I wonder if this makes its realisation impossible. I want to use it as a shunt filter impedance connected in parallel to a load.