As we saw in Chap.  1 , the loss functions have pulsations at which the attenuation becomes infinite, these are the poles of the loss function \(\frac{{P_{20} }}{{P_{2} }}\left( \omega \right)\) , better known as transmission zeros \(\left( {TZ} \right)\) . As we also saw in Chap.  1 , the Darlington procedure of filter synthesis leads inevitably to the synthesis of an immittance \(Z\left( s \right)\) or \(Y\left( s \right)\) but as we have also seen in Chap.  4 , an immittance does not have a unique way of development.