Measurement and signal science · BEGINNER LESSON
Frequency
Frequency is the rate at which a repetitive event occurs. For a repeating signal, it is the reciprocal of the period: f = 1/T, where T is the period in seconds and f is measured in hertz (Hz).
What frequency is
Frequency describes how often a repetitive event happens. For a wave, it can be the number of crests passing a point in one second. In the International System of Units (SI), period is measured in seconds and frequency is measured in hertz (Hz), meaning cycles per second. If the period gets shorter, the frequency gets larger.
[1] tf.nist.gov › service › pdfAppendix A An Introduction to Frequency Calibrations - NIST [2] Measurement, Instrumentation, and Sensors Handbook ... - NIST [3] Anatomy of an Electromagnetic Wave - NASA Science
How it is related to period
Period is the time for one complete cycle, and frequency is its reciprocal. That means f = 1/T. If a signal takes 0.5 s for one cycle, its frequency is 2 Hz. This reciprocal relationship is the basic way frequency is calculated from a time measurement.
[1] tf.nist.gov › service › pdfAppendix A An Introduction to Frequency Calibrations - NIST [2] Measurement, Instrumentation, and Sensors Handbook ... - NIST
How frequency is used in standards and calibration
In frequency metrology, frequency standards are devices that produce a known frequency. A device under test (DUT) is compared with a reference during calibration, and the reference should be traceable and outperform the DUT enough for the comparison to be valid. Frequency offset tells how far the DUT is from the reference.
[1] tf.nist.gov › service › pdfAppendix A An Introduction to Frequency Calibrations - NIST
What creates a frequency
Many frequency standards use an oscillator, which is made from an energy source and a resonator. Quartz oscillators use a crystal whose resonance frequency depends on its physical dimensions and crystal type. Atomic oscillators use quantized energy levels in atoms; cesium oscillators are primary frequency standards because the SI second is defined from the cesium-133 resonance frequency.
[1] tf.nist.gov › service › pdfAppendix A An Introduction to Frequency Calibrations - NIST [2] Measurement, Instrumentation, and Sensors Handbook ... - NIST
Limits and causes of change
Measured frequency can shift because of environmental effects and aging. Quartz oscillators can be affected by temperature, humidity, pressure, vibration, and long-term aging. Atomic oscillators are much more stable, but they still change slightly over time. Transfer standards can be unsuitable for short-term stability measurements because signal paths may add apparent fluctuations.
[2] Measurement, Instrumentation, and Sensors Handbook ... - NIST [1] tf.nist.gov › service › pdfAppendix A An Introduction to Frequency Calibrations - NIST
SEE THE IDEA
How frequency is defined and checked
A cycle, crest, or other repeating occurrence
Time for one cycle, measured in seconds
How many cycles occur per second, in hertz
A traceable standard used for comparison
The oscillator or signal being checked
Frequency is calculated from the period, then compared against a reference when a device is calibrated.
A repeating event has a period; its frequency is the reciprocal of that period, and calibration compares that value to a traceable reference.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.