`  :It's true as you say that a four-quadrant multiplier makes an ideal mixer because it doesn't produce product frequencies.  And your point that this is because it is a linear device is well taken.  But I think it is important for the article to emphasize that practical mixers at radio frequencies, even Gilbert cells, have significant nonlinearities and produce modulation products (mf1 + nf2).  And of course at microwave frequencies and above, the fastest-expanding part of the radio spectrum and fastest-growing application of mixers, multiplying mixers can't be built and diode mixers are used, which are nowhere near linear.  `