Theremin abruptly returned towards the Soviet Union in 1938. At the time, the reasons for his return were not crystal clear. Some claimed that he was only homesick, while others believed that he were kidnapped by Soviet officials. The theremin uses the heterodyne basic principle to deliver an audio sign. https://miloxdowk.blogkoo.com/the-2-minute-rule-for-pallet-gates-57704938