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In this video Doug Jorgesen explains the motivations, basic operations, and some applications of IQ, image reject, and single sideband mixers. For more information, see the following blog posts:
The Why and When of Mixers for Beginners
Top 7 Ways to Create a Quadrature (90˚) Phase Shift 
All About Mixers as Phase Modulators 
All About Mixers as Phase Detectors 
How to think about IQ mixers
IQ, Image Reject, and Single-Sideband Mixers