If I plot the spectrum of a short modem signal, what should I expect > to see?
> It just looks like a lump in the frequency domain, but I'm not sure I > did it right.
A standard dial modem must operate over a band-limited channel between approx 300 to 4000 khz. Therefore it's analog output frequencies should fall within this band. Some harmonics may appear outside this, but they would be spurious.