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Time and Frequency from A to Z, N to O

Nanosecond (ns)

A unit of time that represents one billionth of a second (10-9 s).

Network Time Protocol (NTP)

A standard protocol used to send a time code over packet-switched networks, such as the public Internet. The Network Time Protocol (NTP) was created at the University of Delaware, and is defined by the RFC-1305 document. The NTP packet includes three 64-bit time stamps and contains the time in UTC seconds since January 1, 1900 with a resolution of 233 picoseconds. The NTP format is supported by the NIST Internet Time Service.

Nominal Frequency

An ideal frequency with zero uncertainty. The nominal frequency is the frequency labeled on an oscillator's output. For this reason, it is sometimes called the nameplate frequency. For example, an oscillator whose nameplate or label reads 5 MHz has a nominal frequency of 5 MHz. The difference between the nominal frequency and the actual output frequency of the oscillator is the frequency offset.


The interval between two frequencies having a ratio of 2 to 1. Starting from a fundamental frequency, one octave higher is twice that frequency; one octave lower is half that frequency. The concept of an octave is most widely known and most easily illustrated with musical notes. For example, a piano keyboard has a range of over seven octaves from the lowest frequency to the highest frequency note. There are eight keys on a piano that play the musical note A. Each musical note A has a frequency twice as high as the note in the previous octave, as shown in the table.

Musical Note Frequency (Hz)


















An acronym for Oven Controlled Crystal Oscillator. A type of quartz oscillator design that reduces environmental problems by enclosing the crystal in a temperature-controlled chamber called an oven. When an OCXO is turned on, it goes through a "warm-up" period while the temperatures of the crystal resonator and its oven stabilize. During this time, the performance of the oscillator continuously changes until it reaches its normal operating temperature. The temperature within the oven then remains constant, even when the outside temperature varies.

Because the environment is carefully controlled, OCXOs have excellent short-term stability. A typical OCXO might be stable to 1 × 10-12 at tau = 1 s. The limitations in short-term stability are mainly due to noise from electronic components in the oscillator circuits.  Long term stability is mainly limited by aging.

 One Way Time and Frequency Transfer

A measurement technique used to transfer time and frequency information from one location to another. As shown in the figure, the reference transmitter, A, simply sends a time signal to the receiver, B, through a transmission medium.

one way time and frequency transfer

The delay, tauab, over a transmission path is at least 3.3 microseconds per kilometer. If high accuracy time transfer is desired in a one-way system the physical locations (coordinates) of the two clocks must be known so that the path delay can be calculated. For frequency transfer, only the variability of the delay (the path stability) is important.

On Time Marker (OTM)

The part of a time code that is synchronized (at the time of transmission) to the UTC second.

Optical Frequency Standard

A frequency standard based on the optical transitions in ions and neutral atoms.  Optical standards operate at much higher resonance frequencies than microwave standards based on cesium; the stabilized lasers that serve as their resonators typically operate at a frequency near 1015 Hz, as opposed to less than 1010 Hz for cesium. As a result, these standards potentially have accuracies and stabilities that are several orders of magnitude better than the best microwave standards. Optical frequency standards have been constructed at NIST utilizing single-ion techniques based on mercury (199Hg+) and aluminum (27Al+), as well as neutral atom techniques based on calcium (40Ca), ytterbium (174Yb), and strontium (87Sr).  The work on optical frequency standards has been exceptionally promising with the best devices demonstrating uncertainties in the low parts in 1018.  Thus, it now appears almost certain that the SI second will eventually be redefined based on one of these optical atomic transitions.


An electronic device used to generate an oscillating signal. The oscillation is based on a periodic event that repeats at a constant rate. The device that controls this event is called a resonator. The resonator needs an energy source so it can sustain oscillation. Taken together, the energy source and resonator form an oscillator. Although many simple types of oscillators (both mechanical and electronic) exist, the two types of oscillators primary used for time and frequency measurements are quartz oscillators and atomic oscillators.

Overtone Frequency

A multiple of the fundamental resonance frequency of a quartz oscillator that is used as the oscillator's output frequency. Most high stability quartz oscillators output either the third or fifth overtone frequency to achieve a high Q. Overtones higher than fifth are rarely used because they make it harder to tune the device to the desired frequency.

Created May 12, 2010, Updated April 29, 2019