The super-sonic heterodyne (superhet) receiver principle was invented in France by Lucien Levy in 1917 (p 66), though credit is usually also given to Edwin Armstrong. The original reason for the invention of the superhet was that before the appearance of the screen-grid valve, there was no type of valve which could give good gain at radio frequencies (i.e. frequencies much above 100 kHz), so a technique was applied whereby the incoming RF signal was "mixed" (i.e. multiplied) with a locally generated oscillatory voltage (the local oscillator) so as to produce a beat frequency of about 30 kHz. This intermediate frequency represented the incoming signal in all important respects, but at a significantly lower frequency which could be successfully amplified by the triode amplifiers available then. This was a complicated technique. It went out of use when screen-grid valves appeared which could act as satisfactory RF amplifiers capable of amplifying the incoming signal without lowering its frequency (see Screen grid valve, below). Superhet receivers reappeared in the early 1930s when, because of the proliferation of transmitting stations, their greater selectivity became an important advantage; almost all receivers operate on this principle today, albeit with a higher IF frequency.