Measured signal from tachometer has ripples, why and how to minimize?
Well, regarding the why: don't know the underlying mechanism of your tachometer; there's many, many ways to build one.

Minimization: compared to your signal of interest, these seem to be rather high in frequency. So, a reasonably designed low-pass filter will help. Don't forget to account for group delay, should you need to feed back the resulting signal in e.g. a control system.