The properties of something that allow it to have more than one possible meaning. For example, if a clock based on a 12-hour system displays 6 hours and 43 minutes, it could be morning or night. This means the clock is ambiguous to the hour, since 6 hours can represent two different times of day.

An item which has been well characterized for value, stability, environment, handling, and other conditions important when used as a standard to transfer or correlate measurement results between laboratories or within a laboratory.

A clock referenced to an atomic oscillator. In the truest sense, only clocks with an internal atomic oscillator qualify as atomic clocks. However, the term is sometimes used to refer to radio clocks that receive a signal referenced to an atomic oscillator at a remote location.

An oscillator that uses the quantized energy levels in atoms or molecules as the source of its resonance. The laws of quantum mechanics dictate that the energies of a bound system, such as an atom, have certain discrete values. An electromagnetic field at a particular frequency can boost an atom from one energy level to a higher one. Or, an atom at a high energy level can drop to a lower level by emitting energy. The resonance frequency (f) of an atomic oscillator is the difference between the two energy levels divided by Planck’s constant (h):

The principle underlying the atomic oscillator is that since all atoms of a specific element are identical, they should produce exactly the same frequency when they absorb or release energy. In theory, the atom is a perfect “pendulum” whose oscillations are counted to measure time interval. The first atomic oscillator was developed at NIST (then NBS) in 1949, and is shown in the photo below. Its resonance frequency was derived from an absorption line in the ammonia molecule. The national frequency standards developed by NIST derive their resonance frequency from the cesium atom, and use cesium beam or cesium fountain technology. NIST researchers have used several other atoms to build experimental atomic oscillators, including mercury and calcium. Rubidium oscillators are the lowest priced and most common atomic oscillators, but cesium beam and hydrogen maser atomic oscillators are also sold commercially.

A time scale based on an atomic definition of the second. Elapsed time is measured by counting cycles of a frequency locked to an atomic or molecular transition. Atomic time scales differ from the earlier astronomical time scales, which define the second based on the rotation of the Earth on its axis. Coordinated Universal Time (UTC) is an atomic time scale, since it defines the second based on the transitions of the cesium atom.

A telephone service operated by the NIST Time and Frequency Division that synchronizes computer clocks to UTC(NIST). Client computers can connect to the ACTS time servers using an analog modem and an ordinary telephone line. The phone number is 303-494-4774. For detailed information about the service, visit the ACTS home page.

The range of frequencies that an electronic signal occupies on a given transmission medium. Any digital or analog signal has a bandwidth. In digital systems, bandwidth is often expressed as data speed in bits per second. In analog systems, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. For example, a typical voice signal on an analog telephone line has a bandwidth of about 3 kHz. An analog television (TV) broadcast video signal has a bandwidth of 6 MHz, some 2,000 times as wide as the telephone signal. As a general rule, systems with more bandwidth can carry more information.

The frequency produced when two signals are mixed or combined. The beat frequency equals the difference or offset between the two frequencies. Audible beat frequencies, often called beat notes, are used for simple frequency calibrations. For example, an amateur radio operator might calibrate a receiver dial by mixing the incoming signal from WWV with the signal from the receiver’s beat frequency oscillator (BFO). This produces a beat note that sounds like a low frequency whistle. The receiver is tuned to the station, and the dial is moved up or down until the whistle completely goes away, a condition known as zero beat. Usually, headphones are used to listen for zero beat, since the receiver’s speaker might not be able to produce the low frequency beat note signals. Since a person with average hearing can hear tones down to 20 or 30 Hz, an audio zero beat can resolve frequency within 2 or 3 parts in 10⁶ at 10 MHz.

The Bureau International des Poids et Mesures (International Bureau of Weights and Measures) located near Paris, France. The task of the BIPM is to ensure worldwide uniformity of measurements and their traceability to the International System of Units (SI). The BIPM averages data from about 50 laboratories (including NIST) to produce a time scale called International Atomic Time (TAI). When corrected for leap seconds, TAI becomes Coordinated Universal Time (UTC), or the true international time scale. The BIPM publishes the time offset or difference of each laboratory’s version of UTC relative to the international average. For example, the BIPM publishes the time offset between UTC and UTC(NIST). The work of the BIPM makes it possible for NIST and the other laboratories to adjust their standards so that they agree as closely as possible with the rest of the world. For more information, visit the BIPM web site.