Signal Modulation

[LostInSpace]

How come a signal modulated using Amplitud Shift Keying and/or Phase Shift Keying (such as QAM) require a bandwidth? I mean, since the frequence is constant; only the amplitude and phase is changed.

Thanks in advance!

[NoTime]

I mean, since the frequence is constant
It is not constant! The slope dV/dT changes from that of the unmodulated sine wave. This means that for a small periods of time it is different frequencies.