阅读说明：本技术 一种基于小波去噪的长线屏蔽线缆转移阻抗测量系统及方法 (Long-wire shielded cable transfer impedance measurement system and method based on wavelet denoising ) 是由 王世山 丁一波 王娜 郭静 于 2020-04-08 设计创作，主要内容包括：本发明公开了一种基于小波去噪原理的长线屏蔽线缆转移阻抗的测量方法,根据传输线的反射机理,确定内回路的阻抗和外回路的阻抗,并进行阻抗匹配网络的设计；将示波器和阻抗匹配网络连接,阻抗匹配网络与被测屏蔽线缆连接,参考导体平面与阻抗匹配网络连接,振荡电路与信号脉冲发射器连接,置于被测屏蔽电缆旁,夹具固定被测屏蔽线缆；通过信号脉冲发射器产生高频激励信号,再通过振荡电路形成模拟的外部电磁波干扰源；将被测线缆的末端连接至示波器,接受并读取测试系统的响应信号；通过小波去噪后运算获得被测线缆的转移阻抗。本发明通过时域的测量方法得到频域的转移阻抗曲线；扩大了测量的带宽长度,适用于长线屏蔽线缆的转移阻抗测量。(The invention discloses a method for measuring the transfer impedance of a long-line shielded cable based on a wavelet denoising principle, which determines the impedance of an inner loop and the impedance of an outer loop according to the reflection mechanism of a transmission line and designs an impedance matching network; connecting an oscilloscope with an impedance matching network, connecting the impedance matching network with a tested shielded cable, connecting a reference conductor plane with the impedance matching network, connecting an oscillation circuit with a signal pulse transmitter, placing the oscillation circuit beside the tested shielded cable, and fixing the tested shielded cable by a clamp; generating a high-frequency excitation signal through a signal pulse transmitter, and forming a simulated external electromagnetic wave interference source through an oscillation circuit; connecting the tail end of the tested cable to an oscilloscope, and receiving and reading a response signal of the test system; and obtaining the transfer impedance of the tested cable through wavelet de-noising operation. The invention obtains a transfer impedance curve of a frequency domain by a time domain measuring method; the bandwidth length of measurement is enlarged, and the method is suitable for measuring the transfer impedance of the long-wire shielded cable.)

1. A long-line shielded cable transfer impedance test system based on wavelet denoising is characterized by comprising a dual-channel oscilloscope, an oscillation circuit, a signal pulse transmitter, a clamp, an impedance matching network, a reference conductor plane, a potentiometer and an excitation source, wherein the dual-channel oscilloscope is used for receiving response signals, the oscillation circuit is used for generating simulated external electromagnetic field interference, the signal pulse transmitter is used for generating high-frequency excitation signals, the impedance matching network is used for inhibiting signal reflection in a measurement system, the reference conductor plane is used for forming an outer loop, the potentiometer and the excitation source are used for detecting characteristic impedance, the dual-channel oscilloscope is connected with the impedance matching network during measurement, the impedance matching network is connected with a shielded cable to be tested, the reference conductor plane is connected with the impedance matching network, and the oscillation circuit is connected with the signal pulse transmitter, the clamp is arranged beside a tested shielded cable, the clamp fixes the tested shielded cable, the tested cable comprises a shielding layer and an internal core wire, the shielding layer and the core wire of the tested cable form an internal loop, and the shielding layer and the reference conductor plane form an external loop.

2. A method for measuring the transfer impedance of a long-wire shielded cable based on wavelet denoising is characterized by comprising the following steps:

step 1, connecting a cable characteristic impedance measuring circuit, connecting one end of an excitation source to the ground, connecting the other end of the excitation source to one end of a potentiometer, connecting the other end of the potentiometer to the end A of a cable loop, connecting an oscilloscope to the end B of the cable loop, determining the impedance Z of an inner loop according to the reflection mechanism of a transmission line₁And the impedance Z of the outer loop₂Designing an impedance matching network;

step 2, disconnecting the tested cable from the potentiometer in the step 1, connecting an oscilloscope with an impedance matching network, connecting the impedance matching network with the tested shielded cable, connecting a reference conductor plane with the impedance matching network, connecting an oscillation circuit with a signal pulse transmitter, placing the oscillator beside the tested shielded cable, fixing the tested shielded cable by a clamp, forming an inner loop by a shielding layer and a core wire of the tested cable, and forming an outer loop by the shielding layer and the reference conductor plane;

step 3, a high-frequency excitation signal generated by the signal pulse transmitter is used as a signal source of the oscillation circuit, and then a simulated external electromagnetic wave interference source is formed through the oscillation circuit;

step 4, connecting the inner loop and the outer loop in the step 2 to an oscilloscope from the tail end of the tested cable, namely the end far away from the external electromagnetic wave, and receiving and reading a response signal of the test system, namely the voltage U of the inner loop₁(t) and outer loop voltage U₂(t)；

And 5, obtaining the transfer impedance of the tested cable through wavelet de-noising and calculating according to the response signal obtained in the step 4.

3. The method for measuring the transfer impedance of the long-wire shielded cable based on wavelet denoising as claimed in claim 2, wherein in the step 1, the method for determining the characteristic impedance of the inner and outer loops comprises: according to a signal reflection coefficient calculation formula:

wherein Z is_LIs a load impedance, inThe value of the potentiometer, Z, being measured_CIs the characteristic impedance of the loop under test, Z_CIs Z₁Or Z₂When Z is_L＝Z_CWhen the input signal v (t) of the excitation source passes through the point A, no reflection occurs, the potentiometer and the tested cable form a voltage divider at the time, so that the signal of 0.5v (t) reaches the open end B, the voltage measured at the end B is v (t) because the reflection coefficient of the open end is 1, and when Z is detected_L≠Z_CDuring the measurement, the voltage measured at the B end will produce ringing, and the potentiometer Z is slowly adjusted during the measurement process_LUntil the oscilloscope detects that the open-circuit voltage waveform is coincident with the input signal waveform, and the read resistance is the characteristic impedance of the loop.

4. The method for measuring the transfer impedance of a long-wire shielded cable based on wavelet denoising as claimed in claim 2, wherein in step 5, the current on the core wire is ignored since the current on the core wire is almost 0 during the measurement, and the transfer impedance Z is due to_TIs much smaller than the characteristic impedance Z of the outer loop₂，Z_Tl is also much smaller in order of magnitude than Z₂L is the length of the cable under test, using an outer loop voltage U₂(t) calculating the surface current I (t) of the shielding layer, Z_Tl can be ignored, calculated by the following method,

respectively obtaining the voltage U of the inner loop₁(t) and I (t) are respectively subjected to fast Fourier transform to obtain the frequency domain U₁(j omega) and I (j omega), performing multi-level decomposition by utilizing wavelet packets, performing multi-threshold denoising on each layer, and reconstructing the denoised data of each layer to obtain denoised U'₁(j ω) and I' (j ω), and the transfer impedance spectrum Z of the cable under test is obtained by the following equation_T(jω)

Technical Field

The invention belongs to the field of cable performance measurement, and mainly solves the problems that the existing measurement method is too large in transferred impedance noise content and large in experimental error when used for measuring a long-wire shielded cable.

Background

The cable is an essential part of an electric device, the capability of shielding the external interference is very important for the normal and stable operation of a system, the shielding effectiveness of the cable is an important measure for measuring the performance of the cable, the transfer impedance can indirectly reflect the shielding effectiveness of the cable based on a circuit theory, and the measurement is more convenient and accurate, so the transfer impedance of the cable is generally measured and researched to measure the quality degree of the shielding performance of the cable.

The existing transfer impedance measurement method at home and abroad mainly comprises the following steps: three-coaxial method, line injection method, mixing chamber method, etc. The methods are basically measured in a frequency domain, the test result is not intuitive enough, the experimental conditions of the methods are harsh, and large errors exist in long-line cables.

The existing measuring method has larger measuring error and poorer effect on the long-line shielding cable with the length of the cable being larger than the wavelength of the electromagnetic wave. As the application fields of cables in engineering become more and more extensive, it is a necessary trend to design a method for measuring the transferred impedance of a long-wire shielded cable.

Disclosure of Invention

The purpose of the invention is as follows: in order to expand the measurement range of the transfer impedance, the invention provides a method for measuring the transfer impedance of a long-wire shielded cable based on wavelet denoising.

The technical scheme is as follows: a long-line shielded cable transfer impedance test system based on wavelet denoising comprises a dual-channel oscilloscope, an oscillation circuit, a signal pulse transmitter, a clamp, an impedance matching network, a reference conductor plane, a potentiometer and an excitation source, wherein the dual-channel oscilloscope is used for receiving response signals, the oscillation circuit is used for generating simulated external electromagnetic field interference, the signal pulse transmitter is used for generating high-frequency excitation signals, the impedance matching network is used for inhibiting signal reflection in a measurement system, the reference conductor plane is used for forming an outer loop, the potentiometer and the excitation source are used for detecting characteristic impedance, the dual-channel oscilloscope is connected with the impedance matching network during measurement, the impedance matching network is connected with a shielded cable to be tested, the reference conductor plane is connected with the impedance matching network, the oscillation circuit is connected with the signal pulse transmitter and is arranged beside the shielded cable to be tested, the clamp fixes a tested shielded cable, the tested cable comprises a shielding layer and an internal core wire, the shielding layer and the core wire of the tested cable form an internal loop, and the shielding layer and the reference conductor plane form an external loop.

A method for measuring the transfer impedance of a long-wire shielded cable based on wavelet denoising comprises the following steps:

step 1, connecting a cable characteristic impedance measuring circuit, connecting one end of an excitation source to the ground, connecting the other end of the excitation source to one end of a potentiometer, connecting the other end of the potentiometer to the end A of a cable loop, connecting an oscilloscope to the end B of the cable loop, and determining the impedance Z of an inner loop according to the reflection mechanism of a transmission line₁And the impedance Z of the outer loop₂Designing an impedance matching network;

step 2, disconnecting the tested cable from the potentiometer in the step 1, connecting an oscilloscope with an impedance matching network, connecting the impedance matching network with the tested shielded cable, connecting a reference conductor plane with the impedance matching network, connecting an oscillation circuit with a signal pulse transmitter, placing the oscillator beside the tested shielded cable, fixing the tested shielded cable by a clamp, forming an inner loop by a shielding layer and a core wire of the tested cable, and forming an outer loop by the shielding layer and the reference conductor plane;

step 3, a high-frequency excitation signal generated by the signal pulse transmitter is used as a signal source of the oscillation circuit, and then a simulated external electromagnetic wave interference source is formed through the oscillation circuit;

step 4, connecting the inner loop and the outer loop in the step 2 to an oscilloscope from the tail end of the tested cable, namely the end far away from the external electromagnetic wave, and receiving and reading a response signal of the test system, namely the voltage U of the inner loop₁(t) and outer loop voltage U₂(t)；

And 5, obtaining the transfer impedance of the tested cable through wavelet de-noising and calculating according to the response signal obtained in the step 4.

Further, in step 1, the method for determining the characteristic impedance of the inner and outer loops includes: according to a signal reflection coefficient calculation formula:

wherein Z is_LFor the load impedance, the value of the potentiometer, Z, is measured_CIs the characteristic impedance of the loop under test, Z_CIs Z₁Or Z₂When Z is_L＝Z_CWhen the input signal v (t) of the excitation source passes through the point A, no reflection occurs, the potentiometer and the tested cable form a voltage divider at the time, so that the signal of 0.5v (t) reaches the open end B, the voltage measured at the end B is v (t) because the reflection coefficient of the open end is 1, and when Z is detected_L≠Z_CDuring the measurement, the voltage measured at the B end will produce ringing, and the potentiometer Z is slowly adjusted during the measurement process_LUntil the oscilloscope detects that the open-circuit voltage waveform is coincident with the input signal waveform, and the read resistance is the characteristic impedance of the loop.

Further, in step 5, during the measurement, the current on the core is almost 0, so neglecting the current on the core due to the transferred impedance Z_TIs much smaller than the characteristic impedance Z of the outer loop₂，Z_Tl is also much smaller in order of magnitude than Z₂L is the length of the cable under test, using an outer loop voltage U₂(t) calculating the surface current I (t) of the shielding layer, Z_Tl can be ignored, calculated by the following method,

the transfer impedance is defined in the frequency domain, and for the conversion from the time domain result, the data needs to be subjected to Fourier analysis, and the obtained inner loop voltage U is respectively obtained₁(t) and I (t) are respectively subjected to fast Fourier transform to obtain the frequency domain U₁(j ω) and I (j ω), performing multi-level decomposition on the part of data by using wavelet packets and performing multi-threshold denoising on each layerReconstructing the denoised data of each layer to obtain denoised U'₁(j ω) and I' (j ω), and the transfer impedance spectrum Z of the cable under test is obtained by the following equation_T(jω)

Has the advantages that:

the invention obtains a transfer impedance curve of a frequency domain by a time domain measuring method; the bandwidth length of measurement is enlarged, and the method is suitable for measuring the transfer impedance of the long-wire shielded cable. The method of applying the time-varying electromagnetic field to the environment around the tested cable is used, the excitation is not applied to a loop, the working condition of the shielding cable in a complex electromagnetic environment is simulated, the measured transfer impedance can better reflect the shielding characteristic of the cable in the actual application occasion, and the external electromagnetic field interference is generated by introducing a high-frequency excitation source to the oscillation circuit. Performing fast Fourier transform on the response signals obtained by the experiment, and performing wavelet packet decomposition on the part of data and performing layered threshold denoising because the signals have higher noise content in a high-frequency band; after denoising, the signal is reconstructed to obtain denoised experimental data, and the measurement range of the transfer impedance is effectively expanded. The method for denoising by decomposing the wavelet packet and layering the multiple thresholds is adopted, and the problem that dense resonance points appear in measurement response signals of a long-line shielding cable due to the transmission line effect, so that errors occur in measurement data is solved.

Drawings

FIG. 1 is a schematic view of a measuring device;

FIG. 2 is a schematic diagram of an external electromagnetic wave generating device;

FIG. 3 is an equivalent circuit diagram of a cable under test;

FIG. 4 is an equivalent circuit diagram of the measuring device;

FIG. 5 is a schematic diagram of characteristic impedance measurement;

FIG. 6(a) shows the characteristic impedance Z of the inner loop₁The design diagram of the impedance matching network when it is less than 50 Ω, and FIG. 6(b) is the characteristic impedance Z of the inner loop₁And (3) a design diagram of the impedance matching network when the impedance is larger than 50 omega.

Detailed Description

The present invention will be further described with reference to the accompanying drawings.

As shown in fig. 1, the system for measuring transferred impedance of a long-line shielded cable of the present invention includes a dual-channel oscilloscope, an oscillation circuit, a signal pulse transmitter, a fixture, an impedance matching network, a reference conductor plane, a potentiometer and an excitation source, wherein the dual-channel oscilloscope is used for receiving a response signal, the oscillation circuit is used for generating a simulated external electromagnetic field interference, the signal pulse transmitter is used for generating a high-frequency excitation signal, the impedance matching network is used for suppressing signal reflection in the measurement system, the reference conductor plane is used for forming an external loop, the potentiometer and the excitation source are used for detecting characteristic impedance, the dual-channel oscilloscope is connected with the impedance matching network during measurement, the impedance matching network is connected with a measured shielded cable, the reference conductor plane is connected with the impedance matching network, the oscillation circuit is connected with the signal pulse transmitter and is placed beside the measured shielded cable, the clamp fixes a tested shielded cable, the tested cable comprises a shielding layer and an internal core wire, the shielding layer and the core wire of the tested cable form an internal loop, and the shielding layer and the reference conductor plane form an external loop.

The difference between the partial transfer impedance measurement method and the existing partial transfer impedance measurement method is that: the method adopts a method of applying a time-varying electromagnetic field to the environment around the tested cable by using an electromagnetic wave generating device as shown in figure 2, and does not apply excitation in the connection of an outer loop, thereby more truly simulating the working condition of the shielded cable in a complex electromagnetic environment and having more reliable results.

According to the transmission line theory, the shielded cable is equivalent to two parallel transmission lines, so that the shielded cable equivalent circuit shown in figure 3 is obtained, wherein R is shown in figure 3₁Is the source side load impedance, R₂It is known from the equivalent circuit that there are many resonance points in the high frequency band, i.e. the "long line" part, which is the main aspect of affecting the measurement accuracy, and the equivalent circuit diagram of the measurement apparatus shown in fig. 4 can be obtained according to the schematic diagram of the measurement apparatus and the equivalent circuit of the shielded cable.

The measuring method of the invention is carried out according to the following steps:

step 1, connecting a cable characteristic impedance measuring circuit, connecting one end of an excitation source to the ground, connecting the other end of the excitation source to one end of a potentiometer, connecting the other end of the potentiometer to the end A of a cable loop, connecting an oscilloscope to the end B of the cable loop, and determining the impedance Z of an inner loop according to the reflection mechanism of a transmission line₁And the impedance Z of the outer loop₂And designing an impedance matching network, specifically as follows:

according to a signal reflection coefficient calculation formula:

design the measuring circuit as shown in FIG. 5, Z_LFor the load impedance, the value of the potentiometer, Z, is measured_CIs the characteristic impedance of the loop under test, Z_CIs Z₁Or Z₂Within the inner loop Z₁For example, when Z_L＝Z₁When the input signal v (t) passes through the point A, no reflection occurs, and the potentiometer and the cable to be tested form a voltage divider, so that the signal of 0.5v (t) reaches the open end B. Since the reflection coefficient of the open end is 1, the voltage measured at the B end is v (t). When Z is_L≠Z₁During the measurement, the voltage measured at the B end will produce ringing, and the potentiometer Z is slowly adjusted during the measurement process_LUntil the oscilloscope detects that the open-circuit voltage waveform is coincident with the input signal waveform, and the read resistance is the characteristic impedance of the loop. When the characteristic impedance Z is measured₁When the impedance is less than 50 Ω, the impedance matching network is designed as shown in fig. 6(a), and 50 Ω is a common impedance, where R is_sFor series impedance in impedance matching networks, R_pFor parallel impedance in an impedance matching network:

when the characteristic impedance Z is measured₁When the impedance is larger than 50 Ω, the impedance matching network is designed as shown in FIG. 6(b)Wherein R is_s,R_pRespectively as follows:

if it is the characteristic impedance Z of the outer loop₂Then characteristic impedance Z₂The shield layer and a reference ground are connected.

Step 2, disconnecting the tested cable from the potentiometer in the step 1, connecting an oscilloscope with an impedance matching network, connecting the impedance matching network with the tested shielded cable, connecting a reference conductor plane with the impedance matching network, connecting an oscillation circuit with a signal pulse transmitter, placing the oscillator beside the tested shielded cable, fixing the tested shielded cable by a clamp, forming an inner loop by a shielding layer and a core wire of the tested cable, and forming an outer loop by the shielding layer and the reference conductor plane;

and 3, introducing a high-frequency excitation source into the oscillating circuit of the figure 2 by using a signal pulse transmitter, so that the cable to be detected is positioned in the time-varying electromagnetic field generated by the oscillating circuit.

Step 4, connecting the inner loop and the outer loop in the step 2 to an oscilloscope from the tail end of the tested cable, namely the end far away from the external electromagnetic wave, and receiving and reading a response signal of the test system, namely the voltage U of the inner loop₁(t) and outer loop voltage U₂(t)；

And 5, obtaining the transfer impedance of the tested cable through wavelet de-noising operation according to the response signal obtained in the step 4, wherein the current on the core wire is almost 0 in the measuring process according to the equivalent circuit shown in the figure 4, so that the current on the core wire is ignored. Due to transfer impedance Z_TIs much smaller than the characteristic impedance Z of the outer loop₂，Z_Tl is also much smaller in order of magnitude than Z₂Using the outer loop voltage U₂(t) calculating the surface current I (t) of the shielding layer, Z_Tl can be calculated by the method of formula (4) with omission.

The definition of the transfer impedance is given in the frequency domain,to scale from the time domain results, a fourier analysis of the data is required. Respectively obtaining the voltage U of the inner loop₁(t) and I (t) are respectively subjected to fast Fourier transform to obtain the frequency domain U₁(j ω) and I (j ω). Due to the transmission line effect, the measured data has more noise caused by resonance in a high-frequency long line part, the response amplitude of the measured data is small, and the measured data is easily interfered by the noise, so that the data is subjected to multi-level decomposition by utilizing a wavelet packet, each layer is subjected to multi-threshold denoising, and the denoised data of each layer is reconstructed to obtain denoised U'₁(j ω) and I' (j ω), and the transfer impedance spectrum Z of the cable under test is obtained from the equation (5)_T(jω)。