The arrangement of mixers and a combiner (summing device) shown here
forms an extremely useful building block in digital communications systems. It achieves a
linear frequency translation of all components in the input signal (represented by its
in-phase and quadrature components) by a carrier frequency component (also represented by
its in-phase and quadrature components). This building block is often referred to as a vector modulator or quadrature modulator
and, as we shall see, can be used for both frequency up-conversion
and down-conversion.
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