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.
In a way over simplified explanation, most modems these days (dial up for sure) transmit a series of tones all the time spaced fairly closely together. These tones are twisted and shifted to allow each one to carry multiple bit patterns. Thus a spectrum view would typically show a "lump" of signals running all the time.