How to measure the frequency of a square wave

Frequency measurements. In order to quantitatively analyze frequencies in physics, it is necessary to involve frequency measurements. The general principle of frequency measurement is to convert the characteristics of periodic changes into electrical signals through the corresponding sensors, and then display the corresponding frequencies by electronic frequency meters, such as power frequency, sound frequency, vibration frequency, etc.

In addition, there is the application of the Doppler effect principle to the measurement of sound frequency.

There are generally three methods for measuring frequency: passive frequency measurement, active frequency measurement and electronic counting.

1.The passive frequency measurement method (also divided into resonance method and bridge method) is often used for rough frequency measurement, with an accuracy of about 1%.

2.The active comparison method can be divided into beat frequency method and differential frequency method, the former is to use the linear superposition of two signals to produce the beat frequency phenomenon, and then measure the frequency by detecting the zero beat phenomenon, which is often used for low-frequency measurement, and the error is a few tenths of a Hz;

The latter uses the superposition of two nonlinear signals to produce a differential frequency phenomenon, and then performs a frequency measurement by detecting the zero difference phenomenon, which is commonly used for high-frequency measurements with an error of about 20 Hz.

3.The above methods have certain deficiencies in measurement range and accuracy, while the electronic counting method is mainly controlled by single-chip microcomputer. Due to the strong control and computing functions of the single-chip microcomputer, the measurement frequency range of the electronic counting method is wide, the accuracy is high, and it is easy to realize.

Introduction to frequency. Frequency, which is the number of times a periodic change is completed per unit of time, is a quantity that describes the frequency of periodic motion, and is commonly represented by the symbol f or , the unit is one second and a minute, and the symbol is s-1. In order to commemorate the contribution of the German physicist Hertz, people named the unit of frequency hertz, abbreviated as "hertz", and the symbol is hz.

Every object has an amplitude-independent frequency that is determined by its own properties, called natural frequencies. The concept of frequency is used not only in mechanics and acoustics, but also in electromagnetics, optics and radio technology.

Frequency measurement is the most basic measurement in the field of electronic measurement, and there are generally two methods of frequency measurement:

1) Counting. This means that within a certain time interval t, the pulse count of the periodic signal of the loser is: n, then the frequency of the signal is f= n 1'。

The relative error of the measurement is N x100. This method is suitable for high-frequency measurements, where the higher the frequency of the signal, the smaller the relative error.

2) Circumferometry. This method measures the frequency indirectly by measuring the number of pulses n of a standard signal with a frequency of fo in one cycle of the signal being measured, f=f, n. The longer the period of the measured signal (the lower the frequency), the greater the number of pulses n of the measured standard signal, and the smaller the relative error.

You can use an oscilloscope to measure the signal in the NFT operation to find the nth harmonic that needs to be calculated.

The frequency can be calculated by location.

As shown in the red histogram below.

It is to perform FFT (Fast Fourier Transform) on the calibrated square wave of the oscilloscope.

What it will look like in the future. As you can see from the red histogram, the signal component voltage of the signal with a frequency of 0Hz is 0, which means that the signal does not contain a DC component. The first red line is the base band wave of the signal, whose frequency is 1kHz, and the amplitude is the reed value.

By the difference between x-axis cursors x1 and x2, we find that the frequency of the fifth line is 9kHz, which is 9 times that of the fundamental wave, which is the ninth harmonic. From the difference between y1 and y2 of the y-axis cursor, we can see that the amplitude of this subharmonic is 104mV.

1. Periodic method:

1. For any periodic signal, the time t of each cycle can be determined by the measurement method of the time interval described above, and then the frequency f:f=1 t is obtained by the following formula.

2. For example, the measured waveform displayed on the oscilloscope has a period of 8div, the "T div" switch is set to the "1 s" position, and its "fine-tuning" is set to the "calibration" position. The period and frequency are calculated as follows, so the frequency of the measured waveform is 125kHz.

2. Frequency measurement by Li Shayu graphic method:

1. Set the oscilloscope to the X-Y working mode, the measured signal is input into the Y axis, and the standard frequency signal is input into the "X external", and the standard frequency is slowly changed to make the two signal frequencies integer multiples, such as fx:fy=1:2, then a stable Li Shayu pattern will be formed on the phosphor screen.

2. The shape of the Li Shayu figure is not only related to the phase of the two deflection voltages, but also to the frequency of the two deflection voltages. The tracing method can be used to draw the Li Shayu figure at various frequency ratios of UX and UY, and different phase differences.

3. Using the relationship between Li Shayu pattern and frequency, accurate frequency comparison can be carried out to determine the frequency of the measured signal. The method is to lead the horizontal and vertical lines respectively through the Li Shayu figure, and the vertical line of the horizontal line should not pass through or tangent to the intersection of the figure. If the number of points of intersection between the horizontal line and the graph is m, and the number of points of intersection between the vertical line and the graph n, then fy fx=m n

4. When the standard frequency fx is known, the measured signal frequency fy can be found from the above formula. Obviously, in the actual test work, when using the Li Shayu pattern for frequency testing, in order to make the test simple and correct, if conditions permit, the frequency of the known frequency signal is usually adjusted as much as possible, so that the graph displayed on the phosphor screen is a circle or ellipse. In this case, the frequency of the measured signal is equal to the known signal frequency.

5. Due to the different phases of the two voltages added to the oscilloscope, the graphics on the phosphor screen will have different shapes, but this has no effect on determining the unknown frequency. The Lysa pattern method is quite accurate in measuring frequency, but it is time-consuming. At the same time, it is only suitable for measuring signals with lower frequencies.

The methods and principles of frequency measurement are as follows:

There are two commonly used frequency measurement methods: frequency measurement and periodic measurement. The frequency measurement method is to count the number of pulses n of the measured signal in time t, and then find the number of pulses per unit time, that is, the frequency of the measured signal.

The periodic measurement method is to first measure the period t of the measured signal, and then find the frequency of the measured signal according to the frequency f=1 t. However, both of the above methods will produce an error of 1 measured pulse, which has certain limitations in practical application.

One of the biggest features of equal precision measurement is that the actual gating time measured is not a fixed value, but a value related to the signal being measured, which is just an integer multiple of the signal being measured.

In the allowable time of counting, the standard signal and the measured signal are counted at the same time, and then the frequency of the measured signal is deduced by mathematical formula. Since the gating signal is an integer hail multiple of the measured signal, the one-cycle error on the measured signal is eliminated, but the one-cycle error on the standard signal is generated.

Scope of determination

Refers to the number of repetitions in a cycle, or the number of cycles per unit of time. The choice of assay depends on the nature and content of the components to be measured, the influence of coexisting components, and the specific requirements for the accuracy and precision of the method.

The number of times a substance completes periodic changes per unit of time is called the frequency of drying, which is often expressed by f. The unit of frequency in physics is hertz (Hz), abbreviated as hertz, and is also commonly used as a unit of kilohertz (khz) or megahertz (MHz) or GHz. The frequency f is the reciprocal of the period t, i.e., f=1 t.

Frequency signal measurement principle: By measuring the length of the signal period, it is converted into frequency.

Frequency signal generally refers to the frequency of the signal. Periodic signals have their own frequencies and can be converted into sums of sine waves of different frequencies through Fourier series. Most signals (periodic or non-periodic) can be converted into amplitudes and phases at different frequencies using the Fourier transform, which is called the frequency domain when considering the frequency-dependent part of the signal or system.

The characteristics of many physical components change with the frequency of the input signal, for example, the impedance of a capacitor increases at low frequencies and decreases at high frequencies, while the inductance is the opposite, with a higher impedance at high frequencies and a smaller impedance at low frequencies. Some systems are defined in the frequency domain, e.g. a low-pass filter that only allows low-pass signals to pass through at a certain frequency.

The characteristics of a linear non-time-varying system will also change with frequency, so it also has its characteristics in the frequency domain, the frequency response is the input amplitude of the same sine wave, the frequency is different, the output amplitude of each frequency and the relative frequency of the phase can be plotted, you can show the characteristics of a system in the frequency domain.

In the electromagnetic field, the frequency characteristic refers to the relationship between the secondary field of a conductor and the change of the frequency of the secondary field when other conditions are constant. Using the frequency and Zheng characteristic curves measured on anomalous objects, the optimal frequency of anomalies can be determined. Comparison of measured and theoretical frequency characteristic curves allows for a semi-quantitative interpretation of the data obtained.

In the RLC series circuit, the inductive reactance and capacitive reactance change with the change of voltage frequency, so the mode impedance angle, current, voltage and other quantities of the circuit impedance will change with the frequency, and this change relationship is called frequency characteristics.

The problem with signal bandwidth:

1. If the signal and the channel bandwidth are the same and the frequency range is the same, the signal can pass through the channel without loss of frequency components.

2. If the bandwidth is the same but the frequency range is inconsistent, the frequency component of the signal will definitely not be able to completely pass through the channel.

3. If the bandwidth is different and the signal bandwidth is less than the channel bandwidth, but all the frequency components of the signal are included in the passband range of the channel, the signal can pass through without loss of frequency components.

4. If the bandwidth is different and the signal bandwidth is greater than the channel bandwidth, but the main frequency component containing most of the signal energy is contained in the passband range of the channel, the signal through the channel will lose part of the frequency component, but it may still be recognized, just as the baseband transmission of digital signals and voice signals in the ** channel are the same.

The most common frequencies are:

Among the statistical parameters of parameter measurement, there is the method of "rising edge counting", the principle of which is to measure the number of rising edges. In the test, the measurement range can be selected in the cursor area, and the cursor range can be set to 200ms, so that the measured rising edge is multiplied by 5, which is the signal frequency.

When the signal frequency is small, the method of measuring the period will be selected, the period of the signal is measured, the reciprocal of the period is the frequency, the error source of this method lies in the frequency of the clock of the timing of the measurement period, when the signal frequency is large, the method of measuring the number of pulses will be selected, and the number of rising edges of the signal will be measured in the standard time, and the error of this method is the selected problem in the standard time.

Working principle: 1. The digital oscilloscope first samples the analog signal at high speed to obtain the corresponding digital data and stores it. The digital signal processing technology is used to process and calculate the sampled digital signal to obtain various signal parameters (including some electrical parameters of the meta-liquid ripper device that may need to be tested with a multimeter).

2. Draw the signal waveform according to the obtained signal parameters, and carry out real-time and transient analysis of the measured signal, so as to facilitate the user to understand the signal quality and quickly and accurately diagnose the fault. <>