Disadvantages of constant-k filter section:

• There are only two parameters to choose (L and C), which are determined by ωc, the cutoff frequency, and R0, the image impedance at zero frequency. The above results are valid only when the filter section is terminated in its image impedance at both ports.
• This is a major weakness of the design because the image impedance is a function of frequency and is not likely to match a given source or load impedance.
• The image impedance is not constant with frequency.
• The filter attenuation increases very slowly which means that passband and stopband are not always well defined.

How to overcome with m-derived:

This above disadvantage of Constant k filter can be surpassed with the modified m-derived sections. As constant-k filter section suffers from the disadvantages of a relatively slow attenuation rate past cutoff, and a nonconstant image impedance. The m-derived filter section is a modification of the constant-k section designed to overcome these problems.