  Smart use of mixers is a relevant issue in radio engineering and in
instrumentation design, and of paramount importance in phase noise metrology.
However simple the mixer seems, every time I try to explain to a colleague what
it does, something goes wrong. One difficulty is that actual mixers operate in
a wide range of power (150 dB or more) and frequency (up to 3 decades). Another
difficulty is that the mixer works as a multiplier in the time-domain, which is
necessary to convert frequencies. A further difficulty is the interaction with
external circuits, the input sources and the load. Yet far the biggest
difficulty is that designing with mixers requires a deep comprehension of the
whole circuit at system level and at a component level. As the
electronic-component approach is well explained in a number of references, this
tutorial emphasizes the system approach, aiming to provide wisdom and insight
on mixes.
