Why we need quadrature coupler and quadrature modulator for designing upconverter?

Hi, can anyone explain to me why is it in order to design a upconverter, we need to come up with a quadrature coupler and quadrature modulator ? And where does the mixer comes into place ?
Thanks for the clarification !

This is really only the case if you're trying to make a single-sideband (or image reject) upconverter -- or if you're trying to make a complex waveform. Mixer ~= modulator (a pair of mixers and a quadrature coupler and an in-phase combiner can make an IQ modulator).

Lance

please! how i can simulate quadrature local osscilator in matlab?