During this period Armstrong's most significant accomplishment was the development of a "supersonic heterodyne" – soon shortened to "superheterodyne" – radio receiver circuit. This circuit made radio receivers more sensitive and selective and is extensively used today. The key feature of the superheterodyne approach is the mixing of the incoming radio signal with a locally generated, different frequency signal within a radio set. This circuit is called the mixer. The result is a fixed, unchanging intermediate frequency, or I.F. signal which is easily amplified and detected by following circuit stages. In 1919, Armstrong filed an application for a US patent of the superheterodyne circuit which was issued the next year. This patent was subsequently sold to Westinghouse. The patent was challenged, triggering another patent office interference hearing. Armstrong ultimately lost this patent battle; although the outcome was less controversial than that involving the regeneration proceedings.