EMC (Electro Magnetic Compatibility) test rectification method based on spectrum superposition and loop transmission characteristics

Abstract

The invention discloses an EMC test rectification method based on spectrum superposition and loop transmission characteristics. The EMC test rectification method comprises the following steps: S1, collecting time domain signals of each module of a tested product; s2, processing the time domain signal to obtain a harmonic frequency spectrum; s3, counting the impedance and physical length of each module on the same loop, and superposing each harmonic spectrum to generate a mixed spectrum; s4, if the filter circuit is arranged in the loop, deducting the attenuation degree of a Baut diagram from the mixed spectrum according to the Baut diagram of the filter circuit; s5, judging whether the harmonic component of the mixed frequency spectrum belongs to a long-line propagation frequency band or not according to the length of the harmonic component of the mixed frequency spectrum in an inherent circuit loop; and S6, according to the judgment result of the mixed spectrum, comparing the judgment result with a CE / RE test limit value, if the judgment result is greater than the test limit value, judging that there is a risk, and performing regression design avoidance.

Description

Technical Field

[0001] This invention relates to an EMC testing and rectification method based on spectrum superposition and loop transmission characteristics, belonging to the field of automotive testing. Background Technology

[0002] Currently, electronic and electrical components constitute a significant portion of a car's interior, and their electromagnetic compatibility (EMC) is closely related to the overall safety of the vehicle. Therefore, EMC testing is essential for vehicles, and this has become one of the most serious challenges facing automakers. EMC testing for vehicles includes EMC testing of the vehicle's onboard receivers based on the CISPR 25 standard. EMC testing is a necessary experiment for automotive-grade products and serves as an indicator for evaluating a product's electrical compatibility.

[0003] In existing technologies, CE (conducted emissions) tests electromagnetic interference (EMI) conducted by electrical equipment to the outside through power lines or signal lines. RE (radiated emissions) tests interference radiated out of space as electromagnetic waves by equipment. CE and RE tests include PWM, step, and pulse signals generated by CNC circuits, DC-DC modules, etc., which have diverse spectral characteristics, making it difficult to quickly pinpoint the interference source and identify the spectrum. High-frequency harmonics propagating along long lines of power buses or branch lines generate alternating magnetic fields in space and eventually radiate. Existing test methods often ignore the superposition effect of loop impedance and physical length. Quantitative comparisons between the transfer function and loop equation of filter circuits and standard limits (refer to CISPR, GB / T 18655—2018) are insufficient, leading to blind and inefficient rectification measures. Especially in CE and RE tests, due to the wide test frequency band and complex interference sources, it is difficult to quickly and accurately locate the source of the problem and the propagation path, resulting in long rectification cycles and high costs. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings and deficiencies of existing technologies and to provide an EMC testing rectification method based on spectrum superposition and loop transmission characteristics. This method aims to solve the problem of low rectification efficiency in the CE / RE testing stage of EMC testing, which cannot quickly and accurately locate the source of automotive parts that do not meet EMC testing standards.

[0005] An EMC testing and rectification method based on spectrum superposition and loop transmission characteristics includes the following steps: S1. Collect time-domain signals from each module of the product under test; S2. Process the time-domain signal to obtain the harmonic spectrum; S3. Calculate the impedance and physical length of each module on the same circuit, and superimpose the harmonic spectra to generate a mixed spectrum; S4. If a filter circuit has been installed in the loop, the attenuation of the Bode plot is subtracted from the mixed spectrum according to the Bode plot of the filter circuit. S5. Determine whether the harmonic components of the mixed spectrum belong to the long-line propagation frequency band based on the inherent circuit loop length. S6. Based on the determination result of the mixed spectrum, compare it with the limit of the CE / RE test. If it is greater than the test limit, it is determined to be risky, and regression design should be used to avoid it.

[0006] Through this technical solution, the modular time-domain signal acquisition and harmonic spectrum extraction described in steps S1 and S2 can completely record the characteristics of PWM, step, and pulse interference signals generated by each sub-module under working conditions, avoiding omissions or misjudgments caused by single-point measurement; Step S3 statistically analyzes the equivalent impedance and physical length of the wires of each module in the loop, and superimposes their respective spectra into a mixed spectrum, effectively reflecting the cumulative gain and phase coupling effect of high-frequency harmonics in long-line propagation, overcoming the problem of neglecting loop parameters in traditional methods, and significantly improving the positioning accuracy of the emission source and propagation path; Step S5 further determines the long-line propagation frequency band based on the inherent length of the circuit, which can distinguish between near-field, far-field, and long-line radiation modes, providing a clear direction for targeted rectification; The overall process can complete the closed loop of measurement → analysis → comparison → positioning → rectification suggestions in a laboratory environment, improving EMC rectification efficiency, shortening the verification cycle, and reducing manpower and equipment investment costs; Using this method, potential electromagnetic emission and propagation problems can be discovered in a timely manner in the early stages of product development and mass production, and the optimization of structural layout, wiring, and filtering schemes can be guided by quantitative and visual means, thereby significantly improving the EMC of the entire vehicle. Performance ensures driving safety and electrical system stability.

[0007] Preferably, in step S5, if it is determined to be a long-line propagation frequency band, the signal strength is used as the value after 1m distance measurement and compared with the RE test limit; if it is determined to be a short-line propagation frequency band, the product under test is used as a spare 50Ω loop impedance and signal strength to establish a loop equation, and the result is compared with the CE test limit.

[0008] This technical solution allows for accurate comparison of far-field radiation levels by directly comparing the field strength value measured at a 1m distance with the RE limit for long-line propagation bands. For short-line propagation bands, a precise loop equation is established by adding a standard 50Ω load to the product under test and combining loop impedance and signal strength, which is then compared with the CE limit to realistically simulate conducted interference environments on power lines. The system automatically selects the corresponding limit based on the propagation mode, providing clear comparison results without the need for separate test benches, saving test switching time. Quantitative gap feedback enables design engineers to specifically optimize filters, wiring, or shielding schemes, significantly shortening the rectification-retest cycle.

[0009] Furthermore, in step S5, the long-line propagation determination condition is that the loop length L ≥ λ / 10, where λ is the harmonic wavelength.

[0010] This technical solution, with its clear and frequency-adaptive threshold formula, helps test engineers quickly determine the propagation mode during the data analysis phase without the need for additional on-site trials or experience-based calibration. In automated analysis software, simply reading the loop length and spectrum and comparing it with L ≥ λ / 10 allows for automatic classification, greatly improving testing efficiency.

[0011] Preferably, in step S2, the harmonic spectrum is obtained by performing a Fourier transform on the time-domain signal.

[0012] This technical solution enables Fourier transform to decompose complex PWM, step, and pulse signals into harmonic components and their amplitudes in a single step, avoiding the inefficiency and omissions of traditional narrowband scanning or frequency jump measurements. The full-spectrum output covers all meaningful higher-order harmonic information, providing complete and accurate spectral data for subsequent superposition and limit comparison. Whether it's static DC ripple, dynamic PWM modulation, or complex pulse groups, Fourier transform can process them within the same framework, improving the versatility of the testing method. After obtaining the accurate harmonic spectrum, it can be directly used for quantitative superposition in step S3 and filtering subtraction in step S4, achieving high accuracy in subsequent loop transmission analysis. It provides traceable data for risk assessment and remediation recommendations, ensuring a good data chain and consistency throughout the entire EMC remediation process, from signal acquisition to result evaluation.

[0013] Preferably, in step S4, the filter attenuation A(ω) is obtained by passing the Bode plot data at a rate of 20 log... 10 Formal calculation.

[0014] This technical solution highlights the significant attenuation of high-frequency harmonics outside the filter passband while preserving minute attenuation variations, allowing engineers to clearly see the filtering effect across each frequency band. The Bode plot directly corresponds to the test report, facilitating the overlay display of test curves, design curves, and limit curves, thus improving communication efficiency. When subtracting from mixed frequencies, the subtraction operation is performed directly in the logarithmic domain, avoiding complex calculations such as amplitude multiplication and square root extraction in the linear domain, simplifying the algorithm. Engineers can directly assess the required filter order or improvement measures based on the "insufficient attenuation in dB," such as adding an LC structure or replacing the inductor with a higher Q value. Quantitative attenuation difference feedback makes the rectification plan more targeted and persuasive, shortening the design verification cycle.

[0015] Preferably, in step S6, when the test limit is exceeded, a risk report is output and design avoidance suggestions are given.

[0016] This technical solution automatically generates a risk report upon detecting any exceedances, including the exceeding frequency point, exceeding amplitude, and corresponding circuit or module information. This helps engineers grasp the full picture of the problem immediately. The report integrates spectral curve comparison charts, data before and after filtering, and propagation mode determination results, achieving multi-dimensional visualization and improving information transmission efficiency. For different types of exceedances (long-line radiation or short-line conduction), the report provides corresponding avoidance measures, such as adding shielding, optimizing trace length, adjusting filter LC parameters, or improving grounding schemes. The recommendations are quantitatively clear (e.g., "add at least 15dB stopband attenuation at 150MHz" or "shorten the total conductor length by 20%), facilitating rapid implementation by the engineering team. Timely and accurate avoidance guidance reduces unnecessary material waste and hardware rework, saving testing and development costs.

[0017] The beneficial effects of this invention are as follows: It allows for the prediction of potentially exceeding frequency limits and loops through simulation or spectrum calculations before laboratory testing, significantly reducing the number of repeated tests and saving development time and costs. Utilizing the refined harmonic spectrum obtained through FFT, combined with loop physical length and impedance models, it is possible to clearly identify which module and which line generates the strongest interference in which frequency band. The risk report not only points out the exceeding frequency points but also provides targeted suggestions (such as adding filters, optimizing wiring, and improving grounding), facilitating the rapid development of rectification plans and verification of their effects. It can be applied to various electronic systems containing PWM, power modules, and long loops; it can be used for EMC pre-assessment of automotive electronics and industrial equipment, and also for auxiliary analysis during laboratory testing. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, obtaining other drawings based on these drawings without creative effort still falls within the scope of the present invention.

[0019] Figure 1 This is a flowchart illustrating the present invention; Figure 2 This is an equivalent circuit diagram of an embodiment of the present invention; Figure 3 This is a fitted spectrum diagram of an embodiment of the present invention; Figure 4 This is a software spectrum diagram of an embodiment of the present invention; Figure 5 This is a fitted spectrum diagram of an embodiment of the present invention, ranging from 0 to 100000000; Figure 6 This is an impedance curve diagram of an embodiment of the present invention; Figure 7 This is the improved fitting spectrum diagram according to an embodiment of the present invention; Figure 8 This is a simulation diagram of the radiation emission test environment according to an embodiment of the present invention; Figure 9 This is a simulation diagram of the conducted emission test environment according to an embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0021] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0022] The directional and positional terms used in this invention, such as "up," "down," "front," "back," "left," "right," "inner," "outer," "top," "bottom," and "side," are merely for reference to the accompanying drawings. Therefore, the directional and positional terms used are for illustrating and understanding this invention, and not for limiting the scope of protection of this invention.

[0023] An embodiment of the EMC testing and rectification method based on spectrum superposition and loop transmission characteristics of the present invention includes the following steps: S1. Collect time-domain signals from each module of the product under test; S2. Process the time-domain signal to obtain the harmonic spectrum; The harmonic spectrum is obtained by performing a Fourier transform on the time-domain signal.

[0024] Using Fast Fourier Transform (FFT), the harmonic amplitudes |F(ω)| and frequency distributions of each signal are calculated. For periodic signals (PWM), the spectrum is a discrete sinusoidal envelope; for short-time step signals, the spectrum exhibits continuous 1 / ω decay.

[0025] |F(ω)| represents the amplitude spectrum at frequency ω, that is, how strong the frequency component is in the original signal.

[0026] This formula can be used to convert the acquired oscilloscope time-domain waveform f(t) into frequency-domain data, and obtain a "harmonic frequency-amplitude" lookup table.

[0027] S3. Calculate the impedance and physical length of each module on the same circuit, and superimpose the harmonic spectra to generate a mixed spectrum; S4. If a filter circuit has been installed in the loop, the attenuation of the Bode plot is subtracted from the mixed spectrum according to the Bode plot of the filter circuit. The filter attenuation A(ω) is obtained by passing the Bode plot data in 20 logarithmic increments. 10 Formal calculation.

[0028] If a filter capacitor or EMI filter has been installed in the circuit, calculate the attenuation A(ω) at each harmonic frequency point based on its Bode plot; S5. Determine whether the harmonic components of the mixed spectrum belong to the long-line propagation frequency band based on the inherent circuit loop length. If the frequency band is determined to be a long-line propagation band, the signal strength is used as the value after a 1m distance measurement and compared with the RE test limit. If the frequency band is determined to be a short-line propagation band, a loop equation is established using a spare 50Ω circuit, loop impedance, and signal strength of the product under test, and the result is compared with the CE test limit.

[0029] The condition for determining long-line propagation is that the loop length L ≥ λ / 10, where λ is the harmonic wavelength.

[0030] The spectrum analyzer sweep rate should be adjusted according to the CISPR band and detection mode.

[0031] The spectrum analyzer can be used for compliance measurements of this standard, and the precautions for use of the spectrum analyzer in CISPR 16-1-1:2015 must be strictly followed. The broadband transmit repetition frequency of the device under test must be greater than 20 Hz.

[0032] The minimum sweep times in Table 1 apply only to transmission measurements where the pulse repetition interval of the signal is less than the minimum observation time for each measurement frequency (when the step size is equal to half the resolution bandwidth Bres). For measurements of signals with pulse repetition intervals greater than the minimum observation time and for intermittent signals, the sweep time should be increased.

[0033] If the pulse repetition interval of the signal is known, the scan time should allow the observation time at each frequency point to be greater than the reciprocal of the pulse repetition frequency of the signal.

[0034] If the total frequency sweep time is not less than the minimum frequency sweep time specified in Table 1, multiple fast frequency sweeps with maximum value hold function can be used as an alternative method.

[0035] For the frequency sweep time Ts, min The settings are as follows: Where Δf is the frequency scan span, B res The resolution bandwidth is designed to meet the requirements of short-interval pulse measurement. For details on interference measurement and determination of minimum sweep time, please refer to CISPR 16-2-1:2014 and GB / T 6113.203—2016.

[0036] S6. Based on the determination result of the mixed spectrum, compare it with the limit of the CE / RE test. If it is greater than the test limit, it is determined to be risky, and regression design should be used to avoid it.

[0037] If the determination result of the mixed spectrum exceeds the corresponding limit, a report will be output indicating: Exceeding frequency limits and corresponding modules; Recommended measures (increase the filter capacitor, adjust the filter bandwidth, optimize the loop length, improve the grounding layout, etc.).

[0038] For example, a certain product contains a DC-DC module circuit; see [reference needed]. Figure 2 Because of the MOS switches inside the DC-DC chip, there may be issues with power line conduction or radiated emissions.

[0039] See Figure 8 While the testing environment and parameters for radiated emission testing equipment vary among vehicle manufacturers, the requirements for experimental equipment and parameters are generally similar. National and international standards are used as a unified reference.

[0040] Figure 8 The testing environment is based on the national standard GB / T 18655—2018.

[0041] Among them, 1 is the device under test (near-end grounded if required in the test plan); 2 is the test harness; 3 is the simulated load (placed and grounded according to 6.5.2.6); 4 is the power supply (location optional); 5 is the artificial network (AN); 6 is the reference ground plane (connected to the shielded room); 7 is the low relative permittivity material support (εr ≤1.4); 8 is the horn antenna; 10 is the high-quality coaxial cable (50 Ω), such as double-shielded; 11 is the wall panel connector; 12 is the measuring equipment; 13 is the RF absorbing material; 14 is the simulation and monitoring system; 16 is the fiber optic feedthrough; and 17 is the optical fiber.

[0042] See Figure 9 The test environment and parameters for conducted emission test equipment. Figure 9 The testing environment is based on the national standard GB / T 18655—2018.

[0043] Wherein, 1 is the power supply (which can be placed on the reference ground plane); 2 is the artificial network; 3 is the test object (the housing should be grounded if required by the test plan); 4 is the simulated load (the metal casing should be grounded if required by the test plan); 5 is the reference ground plane; 6 is the power line; 7 is the low relative permittivity support (εr≤1.4); 8 is the high-quality coaxial cable (50 Ω), such as double-shielded cable; 9 is the measuring equipment; 10 is the shielded room; 12 is the wall panel connector; 13 is the test wiring harness (excluding the power cord).

[0044] Note: If required by the test plan, the grounding wire length of the test piece shall not exceed 150 mm. Based on the operating mode (PWM, step, pulse, etc.) of each circuit module of the product under test, use an oscilloscope or spectrum analyzer to acquire the time-domain waveforms and record parameters such as peak voltage amplitude VPP, period T, and duty cycle.

[0045] Using Fast Fourier Transform (FFT), the harmonic amplitudes |F(ω)| and frequency distributions of each signal are calculated. For periodic signals (PWM), the spectrum is a discrete sinusoidal envelope; for short-time step signals, the spectrum exhibits continuous 1 / ω decay.

[0046] The PWM signal is replaced by a Fourier series. Where n = 0, ±1, ±2... D is the duty cycle, w0 is the fundamental angular frequency, and T is the period.

[0047] R1 is the on-resistance of the MOS transistor, and C2 is equivalent to the filter capacitor at the VPP terminal of the chip.

[0048] The selection of C2 will now be discussed, along with its impact on power line conduction and radiated emissions.

[0049] Substitute the circuit parameters into the above formula to calculate the amplitude of each harmonic. Excel can be used to reduce the computational workload. The calculation accuracy can be ±1%, as shown in Table 2 below, which displays some of the harmonic amplitude data.

[0050] Table 2 See Figure 3-5 The obtained fitted spectrum is very close to the spectrum analyzed by the software. It can be seen that without the addition of a filter capacitor, the power supply conduction test frequency band data will exceed the standard (about 50 dB (µV)).

[0051] For example, C2, with a large capacitor of 4.7uF, has its impedance curve referenced from the datasheet as follows. Figure 6 .

[0052] Further calculations yielded the harmonic amplitude data, as shown in Table 3, which contains some of the harmonic amplitude data.

[0053] Table 3 The spectrum obtained after adding a 4.7uF capacitor is shown in the attached image. Figure 7 It can be concluded that there is a significant improvement in the 250kHz-3MHz range.

[0054] However, after 30MHz, other small-value capacitors are still needed for combined filtering. And before 250kHz, large-capacity electrolytic capacitors are required for improvement.

[0055] The calculation method is the same as above.

[0056] By analyzing the signals in the product circuit, the source of the signal can be located accurately. This requires theoretical calculations and can be done in the early stages of product design.

[0057] Before laboratory testing, potential frequency bands and loops that may exceed limits can be predicted through simulation or spectrum calculations, significantly reducing the number of repeated tests and saving development time and costs. Using the refined harmonic spectrum obtained through FFT, combined with loop physical length and impedance models, it is possible to clearly identify which module and which line generates the strongest interference in which frequency band. The risk report not only points out the frequency points exceeding limits but also provides targeted suggestions (such as adding filters, optimizing wiring, and improving grounding), facilitating the rapid development of remediation plans and verification of their effects. It can be applied to various electronic systems containing PWM, power modules, and long loops; it can be used for EMC pre-assessment of automotive electronics and industrial equipment, and also for auxiliary analysis during laboratory testing.

[0058] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

[0059] While the invention has been described with reference to several specific embodiments, it should be understood that the invention is not limited to the disclosed specific embodiments. The invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. An EMC testing and rectification method based on spectrum superposition and loop transmission characteristics, characterized in that: Includes the following steps: S1. Collect time-domain signals from each module of the product under test; S2. Process the time-domain signal to obtain the harmonic spectrum; S3. Calculate the impedance and physical length of each module on the same circuit, and superimpose the harmonic spectra to generate a mixed spectrum; S4. If a filter circuit has been installed in the loop, the attenuation of the Bode plot is subtracted from the mixed spectrum according to the Bode plot of the filter circuit. S5. Determine whether the harmonic components of the mixed spectrum belong to the long-line propagation frequency band based on the inherent circuit loop length. S6. Based on the determination result of the mixed spectrum, compare it with the limit of the CE / RE test. If it is greater than the test limit, it is determined to be risky, and regression design should be used to avoid it.

2. The EMC test and analysis method based on spectral superposition and loop transmission characteristics as described in claim 1, characterized in that: In step S5, if the frequency band is determined to be a long-line propagation band, the signal strength is used as the value after 1m distance measurement and compared with the RE test limit; if the frequency band is determined to be a short-line propagation band, the product under test is equipped with a 50Ω loop impedance and signal strength to establish a loop equation, and the result is compared with the CE test limit.

3. The EMC test and analysis method based on spectral superposition and loop transmission characteristics as described in claim 2, characterized in that: In step S5, the long-line propagation determination condition is that the loop length L ≥ λ / 10, where λ is the harmonic wavelength.

4. The EMC test and analysis method based on spectral superposition and loop transmission characteristics as described in claim 1, characterized in that: In step S2, the harmonic spectrum is obtained by performing a Fourier transform on the time-domain signal.

5. The EMC test and analysis method based on spectral superposition and loop transmission characteristics as described in claim 1, characterized in that: In step S4, the filter attenuation A(ω) is calculated using the Bode plot data at a rate of 20 log... 10 Formal calculation.

6. The EMC test and analysis method based on spectral superposition and loop transmission characteristics as described in claim 1, characterized in that: In step S6, when the test limit is exceeded, a risk report is output and design avoidance suggestions are given.

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