Frequency meter is also calledFrequency counter, which is a special method for the measured signalfrequencyMeasuredElectronic measuring instrument。The frequency meter is mainly composed of four parts: time base (T) circuit, input circuit, count display circuit and control circuit.
Frequency, i.esignalPeriodicreciprocalIn other words, the number of signal completion cycles per unit time is generally taken as one second as the basic unit time.
There are many methods to measure frequency, which can be divided into passive frequency measurement, comparison method, oscilloscope method and counting method according to its working principle.The counting method is essentially a comparative method, of which the most commonly used method isElectronic counterLaw.Electronic counter is one of the most common and basic digital measuring instruments.[1]
Passive frequency measurement method
Passive frequency measurement mainly includesResonance method、Bridge methodandFrequency conversion voltage methodEtc.
1) . Resonance method
The principle block diagram is shown in Figure 1:
Fig. 1 Resonance method
2). Bridge method
Any bridge whose balance condition is related to frequency can be used to measure frequency, but the frequency characteristic of the bridge is required to be as sharp as possible.
There are many kinds of frequency measuring bridges, such as Wen's bridge, resonant bridge and double T bridge. Refer to relevant books for some contents.[1]
3). Frequency voltage conversion method
The frequency voltage transformation method is to transform the frequency into voltage or current, and then use a frequency scaled voltmeter or ammeter to indicate the frequency to be measured.
Figure 2 is the principle block diagram of measuring sine wave frequency by frequency voltage transformation method:
The beat frequency method is to directly superimpose the measured signal and the standard signal through linear components (such as earphones and voltmeters) to achieve frequency measurement. Its principle circuit is shown in Figure 3.The beat frequency method is usually only used for audio frequency measurement, not for high-frequency measurement.[2]
Figure 3 Beat method
2) . Difference frequency method
The difference frequency method is to use nonlinear devices and standard signals to carry out the difference frequency transformation of the measured signal to achieve frequency measurement.Frequency measurement in high frequency band is usually conducted by difference frequency method.
The frequency is measured on the oscilloscope according to the Lissajous figure or the number of cycles of the signal waveform.The measuring frequency range of this method is from audio to high-frequency signal.[2]
Counting method
Directly count the number of pulses of the measured signal in unit time, and then display the frequency value in digital form.This method has high measurement accuracy and speed, and is suitable for different frequencies and different accuracy of frequency measurement.Electronic counterThere are two ways to measure frequency: one isDirect frequency measurement, that is, measure the number of pulses of the measured signal within a certain gate time;SecondIndirect frequency measurement, such as periodic frequency measurement.
Due to the rapid development of digital circuits andIntegrated circuitThe application of counter is very wide.Using electronic technology to measure frequency has a series of outstanding advantages, such as high precision, eye-catching and intuitive display, rapid measurement, and easy to realize automatic measurement process. Therefore, this method is the best at present.[2]
Fundamentals
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Figure 4 Basic Principle of Frequency Meter
The basic working principle of the frequency meter is that when the number of cycles of the measured signal in a specific time period T is N, the frequency of the measured signal f=N/T (as shown in Figure 4).
In the process of a measurement cycle, the measured periodic signal is amplified, shaped and differentiated in the input circuit to form a narrow pulse with a specific cycle and sent to an input terminal of the main gate.The other input end of the main gate is the gate pulse generated by the time base circuit generation circuit.When the gate pulse opens the main gate, the narrow pulse of a specific period can pass through the main gate and enter the counter for counting. The display circuit of the counter is used to display the frequency value of the measured signal, and the internal control circuit is used to complete the switching between various measurement functions and achieve measurement settings.[2]
Scope of application
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In the traditional electronic measuring instruments, the oscilloscope has low measurement accuracy and large error when measuring frequency.The spectrometer can accurately measure the frequency and display the spectrum of the measured signal, but the measurement speed is too slow to track and capture the change of the measured signal frequency in real time.It is precisely because the frequency meter can quickly and accurately capture the change of the measured signal frequency, so the frequency meter has a very wide range of applications.
In the traditional manufacturing enterprises, frequency meters are widely used in the production testing of production lines.The frequency meter can quickly capturecrystal oscillator With the change of output frequency, users can quickly find faulty crystal oscillator products by using a frequency meter to ensure product quality.
In the metrology laboratory, the frequency meter is used to calibrate the local oscillator of various electronic measuring equipment.
In the wireless communication test, the frequency meter can be used not only to calibrate the master clock of the wireless communication base station, but also to analyze the frequency hopping signal and frequency modulation signal of the radio station.[2]
Manufacturer introduction
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At present, frequency meter manufacturers in the market can be divided into three categories: manufacturers in mainland China, manufacturers in Taiwan, and manufacturers in Europe and the United States.Among them, European and American frequency meter manufacturers have the largest market share.
European and American frequency meter manufacturers mainly include Pendulum Instruments and Agilent Technology.
Pendulum Instruments is aSwedenCompany, headquartered in the Swedish capitalStockholm。 Pendulum originated from the time and frequency departments of PhilipsTime frequency measurementThe field has more than 40 years of R&D and production experience.The conventional frequency meters of Pendulum Instruments mainly include CNT-91CNT-90、CNT-81、CNT-85。At the same time, Pendulum Instruments also launched rubidium clock time base frequency meter CNT-91R, CNT-85R, and microwave frequency meter CNT-90XL (frequency measurement range up to 60G).
Agilent Technology is an American company headquartered in the United StatesCalifornia。Agilent Technology Co., Ltd. was founded in 1939 and has more than 70 years of R&D and production experience in the field of electronic measurement.The conventional frequency meter signals of Agilent Technology Company mainly include 53181A, 53131A and 53132A.At the same time, Agilent Technology also launched microwave frequency meters: 53150A, 53151A, 53152A (frequency measurement range up to 46G).[2]
