I have made a tutorial using flash programs that will help you
understand how quadrature modulation and quadrature demodulation
works. It is located he


http://www.fourier-series.com/IQMod/index.html

I try to show you without getting bogged down in the math. The hope
is that after going through the tutorial that the math will be easier
to understand. (The math for this stuff can be found many places
online)