The latest version of the frequency meter measurement method and basic principles
The frequency meter, also known as the frequency counter, is an electronic measuring instrument that measures the frequency of the signal under test. The frequency meter is mainly composed of four parts: a time base (T) circuit, an input circuit, a count display circuit, and a control circuit.
The frequency, that is, the reciprocal of the signal period, that is, the number of cycles per unit time of the signal, generally takes one second as the basic unit time.
Measurement methods
There are many methods for measuring the frequency. According to its working principle, it is divided into passive frequency measurement method, comparison method, oscilloscope method and counting method. The counting method is essentially a comparative method, and the most common method is the electronic counter method. The electronic counter is one of the most common and basic digital measuring instruments.
Folding passive frequency measurement
The passive frequency measurement method mainly includes a resonance method, a bridge method, and a frequency-transform voltage method.
2). Bridge method
Any bridge with balanced conditions and frequency can be used for frequency measurement, but requires that the frequency characteristics of the bridge be as sharp as possible.
There are many types of frequency measuring bridges. Commonly used are Wen's bridges, resonant bridges and double-T bridges. For some contents, please refer to related books.
3). Frequency-voltage conversion method
Frequency-voltage conversion method is to first convert the frequency into voltage or current, and then use the voltmeter or ammeter of the frequency scale to indicate the measured frequency.
Comparative method
The active comparison frequency measurement method mainly includes a beat frequency method and a difference frequency method.
1). Beat frequency method
The beat frequency method is to directly measure the measured signal and the standard signal through linear components (such as earphones and voltmeters) to achieve frequency measurement. The principle circuit is shown in Figure 5.3. The beat frequency method is usually only used for audio measurement and not for high frequency measurement.
2). Difference frequency method
The difference frequency method uses a nonlinear device and a standard signal to perform frequency difference measurement on the measured signal to achieve frequency measurement. High frequency band frequency measurement
Oscillometric method
It is mainly divided into Lishayu graphic method and periodic method.
The frequency is measured on the oscilloscope according to the number of periods of the Lissajous figure or signal waveform. This method measures the frequency range from audio to high frequency signals.
Counting method
The number of pulses of the signal to be measured per unit time is directly counted, and then the frequency value is displayed in digital form. This method is accurate and fast, and is suitable for frequency measurement with different frequencies and different precisions. There are two ways to measure the frequency of the electronic counter: one is the direct frequency measurement method, that is, the number of pulses of the measured signal is measured within a certain gate time; the other is the indirect frequency measurement method, such as the periodic frequency measurement method.
Due to the rapid development of digital circuits and the popularity of integrated circuits, counters are widely used. The use of electronic technology to measure frequency has a series of outstanding advantages such as high precision, eye-catching display, rapid measurement, and easy automation of the measurement process, so this method is currently the best.
Fundamental
The most basic working principle of the frequency meter is: when the number of periods of the measured signal in a certain time period T is N, then the frequency of the measured signal is f=N/T (as shown in the right figure).
During a measurement cycle, the measured periodic signal is amplified, shaped, and differentiated in the input circuit to form a narrow pulse of a specific period and sent to an input terminal of the main gate. The other input of the main gate is the gate pulse generated by the time base circuit generating circuit. During the period when the gate pulse turns on the main gate, a narrow pulse of a specific period can pass through the main gate, thereby entering the counter for counting, the display circuit of the counter is used to display the frequency value of the signal under test, and the internal control circuit is used to complete various measurements. Switch between functions and implement measurement settings.
Application range
In traditional electronic measuring instruments, the oscilloscope has low measurement accuracy and large error when performing frequency measurement. The spectrum analyzer can accurately measure the frequency and display the spectrum of the signal under test, but the measurement speed is slow, and it is impossible to quickly and accurately track the change of the frequency of the signal under test. Because the frequency meter can quickly and accurately capture the change of the frequency of the signal under test, the frequency meter has a very wide range of applications.
In traditional manufacturing companies, frequency meters are widely used in production testing of production lines. The frequency meter can quickly capture the change of the crystal oscillator output frequency, and the user can quickly find the faulty crystal oscillator product by using the frequency meter to ensure the product quality.
In metrology laboratories, frequency meters are used to calibrate the local oscillators of various electronic measuring devices.
In the wireless communication test, the frequency meter can be used to calibrate the main clock of the wireless communication base station, and can also be used to analyze the frequency hopping signal and the frequency modulation signal of the radio station.
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