How can we tackle the problem of Low pass Filter not being ideal ?

                                           =Magnitude Response of the filter

                                          =Phase Response

Normally we want ,
                                           to be zero .(This is what we want IDEALLY)

Next best thing that we can do is we can have some linear phase variation, i.e. constant time delay for all frequencies.

Unfortunaltely analog filters can NEVER give linear phase response.                                                                                                  

We can design analog filters as near to an ideal filter in terms of magnitude response, but can not really make ideal filter.

How can we solve this problem?

What we have to do is to get a Maximally Flat Sampling, i.e. fs>>2fm

 This is what we can do using Hold Filters, which is referred to as Zero - Order - Hold Sampling. It is a staircase approximation of the analog signal.