I am not specialist in DSP,I am dealing with geophysical time-series(electromagenteic sources).After converting to frequency domain,my friend is trying to filter with FIR(45-65Hz) to get rid of AC noise(50Hz).The questionis about the harmonics,how will FIR filtering influence harmonics?
The best way to get rid of 50Hz AC noise is with an Adaptive filter (usually the LMS algorithm will work fine), but you need to be able to take a reference signal of only the AC noise source, normally this is usually possible because if your system is being contaminated with AC it means you have some electrical equipment nearby or an outlet where you can take a sample.
If you do not want adaptive filter, then you can try a notch filter at 50Hz. I believe what your friend was trying to do is put a lot of zeros in the Region of 45-65Hz. It depends on the frequency response of the filter but normally the Harmonics will not be attenuated to the same degre, and normally AC is not very clean especially if you have transformers in the middle and such.
Adaptive filter is the way :)