English: FE-5680A rubidium frequency standard or "atomic clock", manufactured by FEI Communications. About 5 inches long. It produces a "clock signal" consisting of a digital pulse signal with an extremely accurate frequency. The frequency is stable to 2x10^-11 per day or 2x10^-9 per year. It requires a warmup period of 5 minutes before the frequency "locks" and becomes stable. The frequency of the output may be adjusted to any frequency between 1 Hz and 20 MHz. The cheapest and most widely-used atomic clocks, rubidium standards cost a few hundred dollars. They are used to control the frequency of radio and television stations, the transmitters in cell phone towers, and in many scientific and military applications. Information from FE-5680A Data Sheet, Frequency Electronics, Inc.