[kaoD]

https://en.m.wikipedia.org/wiki/Heterodyne#Mixer

> In the most common application, two signals at frequencies f1 and f2 are mixed, creating two new signals, one at the sum f1 + f2 of the two frequencies, and the other at the difference f1 − f2.

Then the low pass filter is used to remove the original signals and the sum, leaving only the difference.