File:Crystal oscillator wavemeter 1925.jpg

English: An early vacuum tube crystal oscillator used as a wavemeter to check the frequency of the first radio broadcasting stations in 1925. The crystal oscillator was invented in 1921 by Walter Guyton Cady at Bell Labs. By 1925, a few US broadcasting stations had switched to crystal control, but most controlled their frequency with an LC oscillator, with a tuned circuit consisting of an inductor and capacitor, which could drift in frequency or be misadjusted so the transmitted signal could be at the wrong frequency, possibly interfering with other stations. The crystal oscillator offered a way to check the frequency against an extremely accurate standard, a quartz crystal. In this picture the wavemeter is set next to a radio transmitter (right). The operator is plugging a crystal of the correct frequency into the front panel. The wavemeter has a wire loop (black, right side) which picked up the transmitter's signal and mixed it with the oscillator's signal in a vacuum tube. If the frequecies were different, an audible heterodyne frequency, a "beat" tone, would be heard in the earphones. The operator adjusted the transmitter frequency until the beat frequency got lower and went to zero. At that point, the transmitter was exactly at the crystal frequency.

Caption: "AN APPLICATION OF PIEZOELECTRICITY IN RADIO - The property of quartz crystals, by virtue of which they become electrified under pressure or strain, has been applied in this piezo-electric oscillator, designed for use as a standard wavemeter, and constructed by the General Radio Co. In the small case between the fingers is a tested quartz crystal. It will oscillate at one definite frequency only. Resonance of this difinite frequency with that of any oscillation entering through the coupling coil at right is indicated on the panel meter"