What are the uses of ripples in the filter design. We try to design a filter with least ripples but sometimes we design a filter to have 2 or 3 ripples. Why is it so?
Ripples are usually an undesired side effect. E.g., when designing a frequency selective filter you normally want a piecewise constant magnitude of the frequency response, but this is physically impossible.
Certain design criteria result in filters without ripples, such as the Butterworth criterion, which results in filters with a maximally flat response.
There is usually a trade-off between filter length or delay, transition bandwidth, and ripple. Sometimes to get a filter with a sharper cut-off, the only alternatives include designs (via FIR optimization or IIR pole zero placement) that have (greater) ripples in the stop band or pass band.