Editing Frequency (section)

== Measurement ==
{{unreferenced|section|date=May 2025}}
{{See also|Frequency meter}}

Measurement of frequency can be done in the following ways:

=== Counting ===
Calculating the frequency of a repeating event is accomplished by counting the number of times that event occurs within a specific time period, then dividing the count by the period. For example, if 71 events occur within 15 seconds the frequency is:
<math display=block>f = \frac{71}{15 \,\text{s}} \approx 4.73 \, \text{Hz}.</math>
If the number of counts is not very large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time.{{cn|date=April 2024}} The latter method introduces a [[random error]] into the count of between zero and one count, so on [[average]] half a count. This is called ''gating error'' and causes an average error in the calculated frequency of <math display="inline">\Delta f = \frac{1}{2T_\text{m}}</math>, or a fractional error of <math display="inline">\frac{\Delta f}{f} = \frac{1}{2 f T_\text{m}}</math> where <math>T_\text{m}</math> is the timing interval and <math>f</math> is the measured frequency. This error decreases with frequency, so it is generally a problem at low frequencies where the number of counts ''N'' is small.

{{multiple image
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| image1   = Resonant reed frequency meter.jpg
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| image2   = Czestosciomierz-49.9Hz.jpg
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| footer   = A resonant-reed frequency meter, an obsolete device used from about 1900 to the 1940s for measuring the frequency of alternating current.  It consists of a strip of metal with reeds of graduated lengths, vibrated by an [[electromagnet]].  When the unknown frequency is applied to the electromagnet, the reed which is [[resonance|resonant]] at that frequency will vibrate with large amplitude, visible next to the scale.
}}

=== Stroboscope ===
An old method of measuring the frequency of rotating or vibrating objects is to use a [[stroboscope]]. This is an intense repetitively flashing light ([[strobe light]]) whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down. When the frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integer multiple of the strobing frequency will also appear stationary.

=== Frequency counter ===
{{main|Frequency counter}}
[[File:Frequency counter.jpg|thumb|left|Modern frequency counter]]

Higher frequencies are usually measured with a [[frequency counter]]. This is an [[electronic instrumentation|electronic instrument]] which measures the frequency of an applied repetitive electronic [[signal (electronics)|signal]] and displays the result in hertz on a [[digital display]]. It uses [[digital logic]] to count the number of cycles during a time interval established by a precision [[quartz clock|quartz]] time base. Cyclic processes that are not electrical, such as the rotation rate of a shaft, mechanical vibrations, or [[sound wave]]s, can be converted to a repetitive electronic signal by [[transducer]]s and the signal applied to a frequency counter. As of 2018, frequency counters can cover the range up to about 100&nbsp;GHz. This represents the limit of direct counting methods; frequencies above this must be measured by indirect methods.

=== Heterodyne methods ===
Above the range of frequency counters, frequencies of electromagnetic signals are often measured indirectly utilizing [[heterodyning]] ([[frequency changer|frequency conversion]]). A reference signal of a known frequency near the unknown frequency is mixed with the unknown frequency in a nonlinear mixing device such as a [[diode]]. This creates a [[heterodyne]] or "beat" signal at the difference between the two frequencies.  If the two signals are close together in frequency the heterodyne is low enough to be measured by a frequency counter. This process only measures the difference between the unknown frequency and the reference frequency. To convert higher frequencies, several stages of heterodyning can be used. Current research is extending this method to infrared and light frequencies ([[optical heterodyne detection]]).