US firm OEwaves recently introduced an oscillator design based on the ... the spectral purity of the beat frequency is extremely high. Because optically transparent crystals such as calcium ...

A 10kΩ pot controls the meter and beat frequency, and the sound is picked up by a mic on the bridge. So far, it does 3/4 and 4/4 time, but [JimRD] has made the code freely available for expansion.

In the quest for an accurate frequency standard there ... Back in the day such an oscillator would have been calibrated by generating an audible beat with a broadcast standard such as WWV, but ...