Shielding cable transfer impedance measurement method under impedance mismatching condition

Abstract

The invention relates to the field of shielding cable transfer impedance measurement, in particular to a shielding cable transfer impedance measurement method under the impedance mismatching condition, and the accuracy of shielding cable transfer impedance measurement under the impedance mismatching condition is improved. According to the scheme, the method comprises the steps that an inner loop near-end load Zn and an inner loop far-end load Zf are obtained, an outer loop phase constant beta1 and an inner loop phase constant beta2 are obtained, and internal impedance ZS of a voltage source and an outer loop far-end load ZL are obtained; outer loop characteristic impedance Z1 and inner loop characteristic impedance Z2 are tested and calculated; testing and calculating the ratio V (L) / VS of the coupling voltage at the far end of the inner loop to the amplitude of the output voltage of the voltage source, and the ratio V (0) / VS of the coupling voltage at the near end to the amplitude of the output voltage of the voltage source; and calculating the transfer impedance according to the obtained and calculated parameters. The method is suitable for shielding cable transfer impedance measurement.

Description

Technical Field

[0001] The invention relates to the field of shielded cable transfer impedance measurement, and in particular to a shielded cable transfer impedance measurement method under impedance mismatch conditions. Background Art

[0002] Shielded cables are widely used in electronic systems. Their main function is to reduce the impact of external electromagnetic interference on transmission signals. In order to measure the "shielding effectiveness" of cables, the concept of transfer impedance came into being. It represents the ability of electromagnetic interference to be transmitted from the outside of the shielding layer to the inside of the shielding layer. It is usually used to quantify the protection performance of shielded cables on internal signals under external interference fields.

[0003] The International Electrotechnical Commission (IEC) recommends two methods for measuring the transfer impedance of shielded cables: the triaxial method and the line injection method. The triaxial method measures the transfer impedance by using three coaxial cable systems and is usually used for high-precision shielding effectiveness testing. The line injection method calculates the transfer impedance by injecting current into the shielding layer of the cable under test (CUT) and measuring the coupling voltage of the conductor inside the CUT.

[0004] However, these measurement methods all require that the characteristic impedance of the cable under test matches the impedance of the terminal load to avoid signal reflection caused by impedance mismatch. For cables with standardized characteristic impedance (such as 50Ω coaxial cables), the existing technology can be effectively applied to the testing and calculation of their transfer impedance. However, in fact, in addition to cables with standard characteristic impedance, shielded cables also include non-standard characteristic impedance cables composed of one or more conductors and their corresponding shielding layers. For example, a shielded multi-conductor cable is covered with an insulating sheath, and each conductor has a different characteristic impedance. In this case, the existing technology usually prevents signal reflection by designing an impedance matching circuit. However, this method requires customization of high-precision loads according to different cable characteristics, which is not only costly, but also difficult to implement under complex cable configurations or non-standard characteristic impedance conditions. Therefore, the existing methods have great limitations in flexibility and applicability. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for measuring the transfer impedance of a shielded cable under impedance mismatch conditions, thereby improving the accuracy of measuring the transfer impedance of a shielded cable under impedance mismatch conditions.

[0006] The present invention adopts the following technical scheme to achieve the above-mentioned purpose. The present invention provides a method for measuring the transfer impedance of a shielded cable under impedance mismatch conditions, comprising:

[0007] S1. Get the inner loop near-end load Z n With the inner loop remote load Z f , get the outer loop phase constant β1 and the inner loop phase constant β 2 , get the internal impedance Z of the voltage source S With the external circuit remote load Z L ;

[0008] S2. Test and calculate the external loop characteristic impedance Z 1 And the inner loop characteristic impedance Z 2 ;

[0009] S3. Test and calculate the ratio of the coupling voltage at the far end of the inner loop to the output voltage amplitude of the voltage source V(L) / V S , and the ratio of the near-end coupling voltage to the voltage source output voltage amplitude V(0) / V S ;

[0010] S4. Calculate the transfer impedance according to the parameters obtained in steps S1 to S3.

[0011] Furthermore, step S2 specifically includes:

[0012] For the standardized test configuration of the triaxial method, the external loop characteristic impedance Z 1 is the corresponding standard value;

[0013] For the standardized test configuration of the line injection method, the first test port of the vector network analyzer is connected to the first port of the external loop, and the second test port of the vector network analyzer is connected to the second port of the external loop. The corresponding S 11 and S 21 , and the external circuit characteristic impedance Z is calculated according to the following formula 1 :

[0014]

[0015] In the formula, the calculated Z is the characteristic impedance of the external circuit Z 1 , S 11 and S 21 Indicates the element of the corresponding scattering matrix obtained by testing, Z 0 is the impedance between the first and second ports of the external loop;

[0016] For shielded cables with characteristic impedance standards, the inner loop characteristic impedance Z 2 is the corresponding standard value;

[0017] For multi-core shielded cables, if the characteristic impedance of the inner loop is not uniform, the first test port of the vector network analyzer is connected to the first port of the inner loop, and the second test port of the vector network analyzer is connected to the second port of the inner loop to obtain the corresponding S 11 and S 21, and the inner loop characteristic impedance Z is calculated according to the following formula 2 :

[0018]

[0019] In the formula, the calculated Z is the inner loop characteristic impedance Z 2 , S 11 and S 21 represents the elements of the corresponding scattering matrix.

[0020] Furthermore, step S3 specifically includes:

[0021] If the transfer impedance is calculated by measuring the far-end coupling voltage, a vector network analyzer is used. The first test port of the vector network analyzer is connected to the first port of the outer loop, and the second test port of the vector network analyzer is connected to the second port of the inner loop. The measured result is V(L) / V S ;

[0022] If the transfer impedance is calculated by measuring the proximal coupling voltage, a vector network analyzer is used. The first test port of the vector network analyzer is connected to the first port of the outer loop, and the second test port of the vector network analyzer is connected to the first port of the inner loop. The measured result is V(0) / V S .

[0023] Furthermore, step S4 specifically includes:

[0024] If the transfer impedance is calculated by measuring the far-end coupled voltage, the transfer impedance is calculated according to the following formula:

[0025]

[0026] Where D 1 , D 2 and the constant u 1 、v 1 The expressions are:

[0027]

[0028]

[0029] u 1 =(Z 1 Z 2 β 1 +Z n Z L β 2 )sinβ 1 L-(Z 1 Z 2 β 2 +Z n ZL β 1 )sinβ 2 L

[0030] v 1 =(Z 1 Z n β 2 +Z 2 Z L β 1 )·(cosβ 2 L-cosβ 1 L)

[0031] If the transfer impedance is calculated by measuring the near-end coupling voltage, the transfer impedance is calculated according to the following formula:

[0032]

[0033] In the formula, the constant u 2 、v 2 The expressions are:

[0034]

[0035] The beneficial effects of the present invention are:

[0036] The present invention establishes a universal transfer impedance solution model, which can support the solution of transfer impedance under any CUT characteristic impedance and termination impedance conditions, and solves the problem that traditional methods cannot accurately solve transfer impedance under impedance mismatch conditions.

[0037] The transfer impedance calculation model established in the present invention can accurately measure the transfer impedance without requiring precise impedance matching, thereby significantly simplifying the configuration and operation flow of the experiment.

[0038] The present invention improves measurement flexibility and adaptability: by introducing a non-matching load calculation model in a standardized test configuration, the adaptability of the measurement method is enhanced, and it can be flexibly adjusted according to different tested cable characteristics and test requirements, and is suitable for a variety of different application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the structure of the inner loop and the outer loop in the triaxial method and the line injection method provided in the embodiment of the present invention;

[0040] Figure 2 It is a schematic diagram of the inner loop and outer loop test ports in the triaxial method and line injection method provided in an embodiment of the present invention;

[0041] Figure 3 It is a flow chart of a method for measuring transfer impedance of a shielded cable under impedance mismatch conditions provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0042] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0043] The measuring principle of the present invention:

[0044] like Figure 1 As shown in the figure, the inner loop is formed between the inner conductor and the shield. In the in-line injection method, an additional wire is attached to the entire shield to form the outer loop with the CUT shield. In the triaxial method, the outer loop is formed by the shield of the cable under test and the metal tube of the triaxial unit.

[0045] In the outer loop, the voltage source drives a position-dependent current I(z) through this loop. Imperfect shielding leads to the formation of distributed, position-dependent controlled voltage sources along the inner loop. The cumulative effect of these distributed controlled voltage sources produces corresponding coupled currents in the inner loop.

[0046] Start the analysis from the outer loop. The internal impedance of the voltage source is Zs, and its position is set at Z = 0. At the other end of the cable, the load Z L Set Z = L. Assume that the shielded cable is a lossless transmission line with a length of L.

[0047] Based on the transmission line theory, the external loop current at position z is derived:

[0048]

[0049] In the formula, Z S is the internal impedance of the voltage source, V S is the amplitude of its output voltage. 1 and Z 1 are the phase constant and characteristic impedance of the external loop, Z L is the load at the far end of the external loop, and j represents an imaginary unit.

[0050] The controlled voltage source of the inner loop is derived from the current in the outer loop. The CUT is divided into n segments that are much smaller than the wavelength of the corresponding frequency, which means that the current in the outer loop remains almost constant in the i-th segment (i = 1, 2, ..., n), and I out,i ≈I out,i+1 For the sake of simplicity, the following will be out,i is considered as the current in the i-th segment of the outer loop. For a small differential segment Δz, the voltage source in the inner loop is given by the following formula:

[0051]

[0052] Among them, V i is the controlled voltage source in the i-th segment, Z T is the transfer impedance of the CUT, which is related to frequency but independent of position. Based on the transmission line theory, the coupling voltage formula at the far end of the inner loop is derived as follows:

[0053]

[0054] Among them, D 2 and the constant u 1 、v 1 The expressions are:

[0055]

[0056] u 1 =(Z 1 Z 2 β 1 +Z n Z L β 2 )sinβ 1 L-(Z 1 Z 2 β 2 +Z n Z L β 1 )sinβ 2 L

[0057] v 1 =(Z 1 Z n β 2 +Z 2 Z L β 1 )·(cosβ 2 L-cosβ 1 L)

[0058] In the formula, β 2 and Z 2 are the phase constant and characteristic impedance of the inner loop, Z n and Z f are the loads at the near end and far end of the inner loop respectively. Through similar derivation, the coupling voltage expression at the near end can be obtained:

[0059]

[0060] The constant u 2 、v 2 The expressions are:

[0061] u 2 =(Z f ZL β 1 -Z 1 Z 2 β 2 )cosβ 1 Lsinβ 2 L

[0062] +(Z 1 Z 2 β 1 -Z f Z L β 2 )cosβ 2 Lsinβ 1 L

[0063] v 2 =(Z f Z 1 β 1 -Z L Z 2 β 2 )sinβ 1 Lsinβ 2 L

[0064] +(Z 2 Z L β 1 -Z f Z 1 β 2 )·(1-cosβ 1 Lcosβ 2 L)

[0065] According to the coupling voltage formula of the far end (near end) of the inner loop of the present invention, even if the characteristic impedance of the CUT does not match its termination impedance, the transfer impedance of the CUT can be determined as long as V(0) or V(L) and the characteristic impedance and termination impedance values ​​of the inner and outer loops of the cable are known.

[0066] For the triaxial test configuration, and shielded cables with standardized characteristic impedance, Z 1 and Z 2 Generally straightforward to determine; however, for line injection methods and shielded cables that do not have a standardized characteristic impedance, Z 1 and Z 2 It can be derived from the two-port scattering matrix that describes the transmission characteristics, as follows:

[0067]

[0068] Among them, Z 0 is the port impedance, set to 50Ω, S 11 , S 21 are the elements of the scattering matrix.

[0069] Measurement process:

[0070] If the transfer impedance is calculated by measuring the far-end coupling voltage, the formula for the far-end coupling voltage is as follows:

[0071]

[0072] If the transfer impedance is calculated by measuring the proximal coupling voltage, the formula for the proximal coupling voltage is as follows:

[0073]

[0074] Specifically, Figure 3 As shown in the figure, the transfer impedance measurement method of the shielded cable under impedance mismatch conditions includes:

[0075] Step 1: Determine the known parameters under the current test configuration (triaxial method or line injection method);

[0076] In the present invention, Z S , Z L , Z n , Z f It can be any value. To facilitate the solution, the four termination impedance values ​​are determined in advance before the test, for example, using a common 50Ω termination device or a standard 50Ω impedance instrument port. Phase constant β 1 , β 2 It is related to the test frequency and the medium characteristics on the electromagnetic wave propagation path of the inner and outer loops, and can be directly obtained by looking up the table. The cable length L is directly measured using a ruler.

[0077] Step 2: Test and calculate the external loop characteristic impedance Z 1 And the inner loop characteristic impedance Z 2 ;

[0078] In the present invention, Z 1 , Z 2 It can also be any value.

[0079] For the standardized test configuration of the triaxial method, the external loop characteristic impedance Z 1 is the corresponding standard value, such as the standard 150Ω;

[0080] For the standardized test configuration of the line injection method, the characteristic impedance of the external loop is not uniform, so for Z 1 In unknown cases, a vector network analyzer should be used, such as Figure 2As shown, through the vector network analyzer, the first test port port1 of the vector network analyzer is connected to the first port A of the outer loop, and the second test port port2 of the vector network analyzer is connected to the second port B of the outer loop, and the corresponding S is obtained by testing. 11 and S 21 , and the external circuit characteristic impedance Z is calculated according to the following formula 1 :

[0081]

[0082] In the formula, the calculated Z is the characteristic impedance of the external circuit Z 1 , S 11 and S 21 Indicates the element of the corresponding scattering matrix obtained by testing, Z 0 is the impedance between the first and second ports of the external loop;

[0083] For shielded cables with characteristic impedance standards, such as coaxial cables, the inner loop characteristic impedance Z 2 is the corresponding standard value;

[0084] For multi-core shielded cables, if the characteristic impedance of the inner loop is not uniform, the first test port port1 of the vector network analyzer is connected to the first port C of the inner loop, and the second test port port2 of the vector network analyzer is connected to the second port D of the inner loop, and the corresponding S is obtained by testing. 11 and S 21 , and the inner loop characteristic impedance Z is calculated according to the following formula 2 :

[0085]

[0086] In the formula, the calculated Z is the inner loop characteristic impedance Z 2 , S 11 and S 21 represents the elements of the corresponding scattering matrix.

[0087] Step 3: Test and calculate the ratio of the coupling voltage at the far end of the inner loop to the output voltage amplitude of the voltage source V(L) / V S , and the ratio of the near-end coupling voltage to the voltage source output voltage amplitude V(0) / V S ;

[0088] If the transfer impedance is calculated by measuring the far-end coupling voltage, a vector network analyzer is used to connect the first test port port1 of the vector network analyzer to the first port A of the outer loop, and the second test port port2 of the vector network analyzer to the second port D of the inner loop. The measured result is V(L) / V S ;

[0089] If the transfer impedance is calculated by measuring the proximal coupling voltage, a vector network analyzer is used to connect the first test port port1 of the vector network analyzer to the first port A of the outer loop, and the second test port port2 of the vector network analyzer to the first port C of the inner loop. The measured result is V(0) / V S .

[0090] Step 4: Substitute the parameters from step 1 to step 3 into the corresponding Z T Calculation formula for calculation.

[0091] In summary, the present invention improves the accuracy of measuring the transfer impedance of a shielded cable under impedance mismatching conditions, while simplifying the complex experimental configuration problems caused by achieving impedance matching.

[0092] The above is only a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art shall not deviate from the spirit and scope of the present invention, and shall be within the scope of protection of the claims attached to the present invention.

Claims

1. A method for measuring the transfer impedance of a shielded cable under impedance mismatch conditions, characterized in that: include: S1. Get the inner loop near-end load Z n With the inner loop remote load Z f , obtain the outer loop phase constant β1 and the inner loop phase constant β2, and obtain the internal impedance Z of the voltage source S With the external circuit remote load Z L ; S2, test and calculate the outer loop characteristic impedance Z1 and the inner loop characteristic impedance Z2; S3. Test and calculate the ratio of the coupling voltage at the far end of the inner loop to the output voltage amplitude of the voltage source V(L) / V S , and the ratio of the near-end coupling voltage to the voltage source output voltage amplitude V(0) / V S ; S4. Calculate the transfer impedance according to the parameters obtained in steps S1 to S3.

2. The method for measuring the transfer impedance of a shielded cable under impedance mismatching conditions according to claim 1, characterized in that: Step S2 specifically includes: For the standardized test configuration of the triaxial method, the outer loop characteristic impedance Z1 is the corresponding standard value; For the standardized test configuration of the line injection method, the first test port of the vector network analyzer is connected to the first port of the external loop, and the second test port of the vector network analyzer is connected to the second port of the external loop. The corresponding S 11 and S 21 , and the external loop characteristic impedance Z1 is calculated according to the following formula: In the formula, the calculated Z is the characteristic impedance of the external circuit Z1, S 11 and S 21 It represents the element of the corresponding scattering matrix obtained by the test, and Z0 is the impedance of the first and second ports of the outer loop; For shielded cables with characteristic impedance standards, the inner loop characteristic impedance Z2 is the corresponding standard value; For multi-core shielded cables, if the characteristic impedance of the inner loop is not uniform, the first test port of the vector network analyzer is connected to the first port of the inner loop, and the second test port of the vector network analyzer is connected to the second port of the inner loop to obtain the corresponding S 11 and S 21 , and the inner loop characteristic impedance Z2 is calculated according to the following formula: In the formula, the calculated Z is the inner loop characteristic impedance Z2, S 11 and S 21 represents the elements of the corresponding scattering matrix.

3. The method for measuring the transfer impedance of a shielded cable under impedance mismatching conditions according to claim 1, characterized in that: Step S3 specifically includes: If the transfer impedance is calculated by measuring the far-end coupling voltage, a vector network analyzer is used. The first test port of the vector network analyzer is connected to the first port of the outer loop, and the second test port of the vector network analyzer is connected to the second port of the inner loop. The measured result is V(L) / V S ; If the transfer impedance is calculated by measuring the proximal coupling voltage, a vector network analyzer is used. The first test port of the vector network analyzer is connected to the first port of the outer loop, and the second test port of the vector network analyzer is connected to the first port of the inner loop. The measured result is V(0) / V S .

4. The method for measuring the transfer impedance of a shielded cable under impedance mismatching conditions according to claim 1, characterized in that: Step S4 specifically includes: If the transfer impedance is calculated by measuring the far-end coupled voltage, the transfer impedance is calculated according to the following formula: Wherein, the expressions of D1, D2 and constants u1 and v1 are: u1=(Z1Z2β1+Z n WITH L β2)sinβ1L-(Z1Z2β2+Z n WITH L β1)sinβ2L v1=(Z1Z n β2+Z2Z L β1)·(cosβ2L-cosβ1L) If the transfer impedance is calculated by measuring the near-end coupling voltage, the transfer impedance is calculated according to the following formula: In the formula, the expressions of constants u2 and v2 are:

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