It depends on the type of modulation. In amplitude modulation the
audio signal is mixed with the rf in such a way that the envelope of the
signal carries the audio information. In frequency modulation the mixing
is done in such a way that signals are produced at the original frequency
of the rf +/- the audio modulation.
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866