Інтенсивність потоку, інтервал часу, пристрій, що проходить тестування, коефіцієнт невизначеності, номінальна частота, відхід частоти радіопередачі, маятник, система супроводу космічних об’єктів, п’єзоелектричний ефект, резонансна частота, закони квантової механіки, внутрішні стандарти, довготривала стабільність.

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Frequency is the rate of occurrence of a repetitive event. The International System of Units (SI) states that the period should always be expressed in units of seconds (s), and the frequency should always be expressed in hertz (Hz). The frequency of electric signals often is measured in units of kilohertz (kHz) or megahertz (MHz). Average frequency over a time interval can be measured very precisely. Time interval is one of the four basic standards of measurement (the others are length, mass, and temperature). Of these four basic standards, time interval (and frequency) can be measured with the most resolution and the least uncertainty. Devices that produce a known frequency are called frequency standards.

Frequency calibrations measure the performance of frequency standards. The frequency standard being calibrated is called the device under test (DUT). In most cases, the DUT is a quartz, rubidium, or cesium oscillator. In order to perform the calibration, the DUT must be compared to a standard or reference. The standard should outperform the DUT by a specified ratio in order for the calibration to be valid. This ratio is called the test uncertainty ratio (TUR). A TUR of 10:1 is preferred, but not always possible. If a smaller TUR is used (5:1, for example), then the calibration will take longer to perform.

Once the calibration is completed, the metrologist should be able to state how close the DUT’s output is to its nameplate frequency. The nameplate frequency is labeled on the oscillator output. For example, a DUT with an output labeled “5 MHz” is supposed to produce a 5 MHz frequency. The calibration measures the difference between the actual frequency and the nameplate frequency. This difference is called the frequency offset. There is a high probability that the frequency offset will stay within a certain range of values, called the frequency uncertainty. The reference used for the calibration must be traceable.

Oscillators are sensitive to changing environmental conditions and especially to being turned on and off. If an oscillator is calibrated and then turned off, the calibration could be invalid when the oscillator is turned back on. In addition, the vibrations and temperature changes encountered during shipment can also change the results.

Quartz crystal oscillators first appeared during the 1920s and quickly replaced pendulum devices as laboratory standards for time and frequency. Today, more than 109 quartz oscillators are manufactured annually for applications ranging from inexpensive wristwatches and clocks to communications networks and space tracking systems. The quartz crystal inside the oscillator can be made of natural or synthetic quartz, but all modern devices are made of synthetic material. The crystal serves as a mechanical resonator that creates an oscillating voltage due to the piezoelectric effect. This effect causes the crystal to expand or contract as voltages are applied. The crystal has a resonance frequency that is determined by its physical dimensions and the type of crystal used. No two crystals can be exactly alike or produce exactly the same frequency. The output frequency of a quartz oscillator is either the fundamental resonance frequency or a multiple of that frequency. The amplifier provides the energy needed to sustain oscillation.

Atomic oscillators use the quantized energy levels in atoms and molecules as the source of their resonance frequency. The laws of quantum mechanics dictate that the energies of a bound system, such as an atom, have certain discrete values. An electromagnetic field can boost an atom from one energy level to a higher one. An atom at a high energy level can drop to a lower level by emitting electromagnetic energy. All atomic oscillators are intrinsic standards, since their frequency is inherently derived from a fundamental natural phenomenon. There are three main types of atomic oscillators: rubidium standards, cesium standards, and hydrogen masers. All three types contain an internal quartz oscillator that is locked to a resonance frequency generated by the atom of interest. Locking the quartz oscillator to the atomic frequency is advantageous. Most of the factors that degrade the long-term performance of a quartz oscillator disappear, because the atomic resonance frequency is much less sensitive to environmental conditions than the quartz resonance frequency. As a result, the long-term stability and uncertainty of an atomic oscillator are much better than those of a quartz oscillator, but the short-term stability is unchanged.