Lowpass-filter necessary before DFT?

I'm trying to distinguish eletrical engines by the magnetic field they create. I have no idea whether this is a smart idea, it's possible at all, whatsoever. However it brought me to a basic question about the Fourier transformation to which I couldn't find an answer that I understand :)

I am sampling with a magnetometer at 160Hz. I'm taking the 160 samples I get each second and calculate the DFT. When I plot the DFT over time I see pretty much a flat surface along all frequencies. The amplitudes vary depending of the motor or distance of the magnetometer but I am not seeing what I was hoping to see - some characteristic frequencies of the motors to reliably distinguish them.

This got me wondering whether it is necessary to get rid of frequencies that cannot be detected before applying the fourier transformation. As I understand it, sampling with 160Hz allows me to detect frequencies up to 80Hz. Would it be necessary to use a lowpass filter and get rid of all frequencies below 80Hz? A frequency of e.g. 160Hz is a multiple of 80Hz, 40Hz,..., so wouldn't it "leak" into the 80Hz, 40Hz,... buckets?

• Don't you mean "...get rid of all frequencies above 80Hz?"? Commented Dec 30, 2018 at 13:37
• There are a couple of good explanations about analog anti-aliasing filters as answers to Confused about anti-aliasing in analog filters Commented Dec 30, 2018 at 13:41
• ok. What is your expected magnetic field signal spectrum then? What do you expect to see in the variations of the magnetic field of the electric motor? Rotation frequency? its harmonics? what about N-turns of the motor windings? What sort of an electric motor is that. As you can guess, it's a very important first step to have a sound, valid assumption about the type of signals that you are expecting to see... Commented Dec 30, 2018 at 14:02
• Perfect, thanks @OlliNiemitalo. I have no idea how I never stumbled accross "aliasing", but that is exactly what I was expecting. Commented Dec 30, 2018 at 18:19
• @Fat32 I actually have no idea what I am expecting. I don't have a background in any of this, but I like to play around :) I will probably try to use a coil and an amplifier now, apply a lowpass-filter, sample and then I'll see :) Commented Dec 30, 2018 at 18:21