Gilbert Cell Is An Analog Multiplier Circuit
(Frequency Mixer).
IC like MT-079, NE602 OR NE612 ...etc
Symmetrical AM MODULATION actually is the analog multiple of the main signal and the carrier Frequency (In Frequency Domain the Sum of the Two Frequencies).
https://www.wolframalpha.com/input?i=sin%28x%29sin%28100x%29
https://m.youtube.com/watch?v=7nmmb0pqTU0
