Frequency, which is completed in unit timePeriodic changeThe number of times is a quantity that describes the frequency of periodic motion. Commonly used symbolsforνMeans, the unit is one second, and the symbol is s-1。The concept of frequency is not onlyDynamics、opticsApplication inquantum mechanics、electromagneticsAndradioIt is also often used in technology.
To commemorate the contribution of German physicist Hertz, people named the unit of frequency Hertz, or "Hertz" for short, with the symbol of Hz.Every object has a frequency independent of the amplitude determined by its own properties, which is called natural frequency.
[Explanation] The substance is completed in unit timePeriodic changeThe frequency of is called frequency, which is commonly usedfexpress.
Formulaf=1/T
[Physical unit] Hz
Frequency definition
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The number of times a substance completes periodic changes in a unit time is called frequency, which is commonly usedfexpress。
In memory of the contribution of German physicist Hertz, people named the unit of frequency ashertz, referred to as "Hz", with the symbol of HZ.
Frequency is the number of times to complete periodic changes in time, and is the quantity to describe the frequency of periodic movement. It is usually expressed by the symbol f or ν, with the unit of one second and the symbol s-1。
The frequency of alternating current is[1]It refers to the unit timePeriodicityNumber of changes, unit:hertz(Hz), which is inversely related to the period.The frequency of AC in daily life is generally 50Hz or 60Hz, while the frequency of AC in radio technologyAC frequencyIt is generally large, reaching the measurement of kilohertz (KHz) or even megahertz (MHz).
Frequency unit
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The basic unit of frequency in physics ishertz(Hz), referred to as Hz, is also commonly used in kilohertz (kHz) or megahertz (MHz) or gigahertz (GHz).1kHz=1000Hz，1MHz=1000000Hz，1GHz=1000MHz。
Frequency classification
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power frequency
The electricity used in China is[2]one kindsinusoidal alternating current The frequency is 50Hz, that is, 50 periodic changes have been made within one second.The frequency of alternating current is called power frequency in industrial terms.In 2013, there were two kinds of power frequency in the world's power systems, one was 50Hz, and the other was 60Hz.
Tidal frequency
In astrotide, due to the long period of activity of various celestial bodieshertzThe unit of frequency is cph, that is, cycle per hour.If the period of the most common M2 constituent is about 12.42h, its frequency is usually expressed as 0.08051cph.
Angular frequency
The reciprocal of the period is called frequency, which is represented byfexpress,f= 1/T。
In the discipline of control engineering, there are twoAsymptoteThe frequency of intersection point is called turning frequency, also called corner frequency.ωThe value is called the corner frequency.
Statistical frequency
Also called relativityfrequencyThat is, the number of occurrences of an event is divided by the total number of events, that is, the number of occurrences of a data is divided by the total number of data in this group.Frequency is usually expressed in proportion or percentage.
Doppler effect
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Doppler effect
Doppler effect is in memory of Austrian physicist and mathematician Christian John·Doppler(Christian Johann Doppler) in 1842.The main contents are objectsradiationThe wavelength of is changed due to the relative motion of the wave source and the observer.In front of the moving wave source, the wave is compressed,wavelengthIt becomes shorter and the frequency becomes higher（blue shiftblue shift）；When behind the moving wave source, the opposite effect will occur.Wavelength becomes longer and frequency becomes lower（red shiftred shift）；The higher the velocity of the wave source, the greater the effect.According to the degree of wave red (blue) shift, the velocity of the wave source moving along the observation direction can be calculated.
fixed starSpectral lineOfdisplacementDisplay the speed of the star moving in the observation direction, unless the speed of the wave source is very closelight speed, otherwiseDoppler shiftGenerally speaking, the degree is very small.All wave phenomena have Doppler effect.
measuring method
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In order to quantitatively analyze the frequency in physics, frequency measurement must be involved.The general principle of frequency measurement is to convert the periodically changing characteristics into electrical signals through corresponding sensors, and then display the corresponding frequencies by electronic frequency meter, such as power frequency, optical frequency, vibration frequency, etc.In addition, there is also the application of Doppler effect principle to measure the optical frequency.
equipment
There are three methods for measuring frequency: passive frequency measurement, active frequency measurement and electronic counting.
Passive frequency measurement method (which can be divided into resonance method and bridge method) is commonly used for rough frequency measurement, with an accuracy of about 1%.
The active comparison method can be divided into beat frequency method and difference frequency method. The former uses the linearity of two signalssuperpositionTo produce beat frequency phenomenon, and then measure frequency by detecting zero beat phenomenon, which is commonly used for low-frequency measurement, with the error of a few tenths of a Hz;
The latter uses the superposition of two nonlinear signals to generate the frequency difference phenomenon, and then measures the frequency by detecting the homodyne phenomenon, which is commonly used for high-frequency measurement, with an error of about ± 20 Hz.
The electronic counting method has certain shortcomings in the measurement range and accuracy, and the electronic counting method is mainly controlled by a single chip microcomputer.Due to the strong control and operation function of the single chip microcomputer, the electronic counting method has wide measuring frequency range, high precision and is easy to realize.
