Confused about anti-aliasing in analog filters

1. What are the issues with using an analogue anti-aliasing filter?
2. Is anti-aliasing filter analog or digital?
3. Why is an anti-aliasing filter required in the analogue to digital conversion process?

What are the issues with using an analogue anti-aliasing filter?

Phase Distortion / Time delay: Most analog filters have a non-linear phase response. This a problem since non-linear phase causes an unequal time (group) delay as a function of frequency. The higher frequency signals will arrive later than low frequency signals.

Is anti-aliasing filter analog or digital?

Anti-aliasing filters are always analog filters as they process the signal before it is sampled. In most cases, they are also low-pass filters unless band-pass sampling techniques are used.

Why is an anti-aliasing filter required in the analogue to digital conversion process?

This filter cuts out any higher order frequency content in the input signal as any frequencies higher than the Nyquist frequency would be aliased. With these frequencies removed from the signal, the ADC can now sample the remaining harmonic content without creating false low-frequency errors.