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Time and Frequency from A to Z, Am to B


The properties of a measurement or reading 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, because 6 hours can represent two different times of day.  A 24-hour clock eliminates the day/night ambiguity.


A device with well-known metrological properties that is used as a travelling standard to transfer or compare measurement results between laboratories or within a laboratory.

Atomic Clock

A clock referenced to an atomic oscillator. Only clocks with an internal atomic oscillator qualify as atomic clocks. However, the term is often incorrectly used to refer to radio controlled clocks that receive a signal referenced to an atomic clock at another location.

Atomic Oscillator

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, fo, of an atomic oscillator is the difference between the two energy levels divided by Planck’s constant, h,


atomic oscillator equation

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 national frequency standards developed by NIST and other laboratories derive their resonance frequency from the cesium atom, and typically use cesium fountain technology. Rubidium oscillators are the lowest priced and most common atomic oscillators, but cesium beam and hydrogen maser atomic oscillators are also sold commercially in much smaller quantities.


Harold Lyons and Edward Condon

Atomic Time Scale (TA)

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 by counting energy transitions of the cesium atom.


A unit of time that represents one quintillionth of a second (10-18 s).


Automated Computer Time Service (ACTS)

A telephone time service operated by NIST 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.


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 FM radio station has a bandwidth of 200 kHz, analog television (TV) broadcast signals have 6 MHz of bandwidth, and so on.  As a general rule, systems with more bandwidth can carry more information.

Beat Frequency

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. Because a person with average hearing can hear tones down to 20 or 30 Hz, an audio zero beat can calibrate a 10 MHz frequency to within two or three parts in 106.


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 clock data from about 70 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 official 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. 

Created May 12, 2010, Updated March 1, 2023