dt is the sampling time, 1/<Frequency of sampling>. Thus if you sample at 1KHz, you would have samples spaced dt = 1/1000 = 0.001 seconds apart.
You need to specify dt to an FFT to get it to "scale" properly. It basically takes N numbers that represent a waveform sampled at some rate (and with some dt) and gives you the frequency spectrum, or gain and phase, with units of frequency. Changing dt only changes the frequency range, not the data.
Thus if you sample for 1 sec at 1KHz, you'd have 1000 points and could get a spectrum from 1 to 500 Hz from those data. If you sampled at 1 Hz, you'd get the same spectrum, but it would represent data from .001 to 0.5 Hz.
Bob Schor
