EMC AND EMI Pretest/Pre-compliance Solution of GW INSTEK

 Solution for Conduction EMI

*Radiation via air & Conduction via power

GKT-008 vs conventional probes                                          Identify EMI source better by small size                                               

ANT-04: higher sensitivity than H probe                                 ANT-04: higher sensitivity than E probe                                             

H probe: Different results with different angles                    ANT-04: Similar results with different angles                                     

Working principle of conventional H probe (I)

H probe detects much more while deviating 1cm                   ANT-04 detects more while aiming the center  

Why H probe aiming at center detects less signal?

H probe’s results differ from E probe

H field & E field are two components of EMI

ANT-04 and 05 directly sense EMI energy

Far field response estimation

● Convert the near field measurement results to far field response (3m or 10m can be selected).

● Helpful to confirm whether the EMI trimming works or not.

Test system configuration

Peak Table: Automatically search 10 peaks                   

Pass/Fail: Inspection result display

* Một số Video hường dẫn Kỹ thuật cơ bản về kiểm tra EMC của hãng GWINSTEK:

1> General introduction of GW Instek EMC pretest solution (3:04)

Briefly explain EMC basics, and introduce GW Instek's EMC pretest solution and its advantages comparing with conventional EMC test products.

2> EMC, EMI and EMS basics . Radiated and Conducted. (1:43)

EMC includes EMI and EMS. EMI is the self-emitted noise poses impact on other electronic devices, and EMS is affecting electronics products by surrounding electronic devices' noise.

3> Challenges of using conventional H probes and E probes. (1:55)

Use H probe with different angle will get different magnetic field measurement result, electric filed measured by E probe is also different from H probe.

4> Peak, Average and QP(Quasi-Peak) detectors. (1:54)
In order to accurately measure different type EMI signals, the regulations require using Peak, QP and Average detectors to conduct measurements to determine if the results meet the regulations.

7> Conducted EMI test solution. (1:29)

GSP-9330 spectrum analyzer collocating with LISN GLN-5040A and an isolated transformer GLN-5060 can conduct conductive EMI.

9> With the GSP-9330, You Only Need To Focus On EMI Measurements. (1:09)

LET ENGINEERS DO WHAT THEY ARE SUPPOSED TO DO!!!

It often takes a lot of time to understand the operation and settings of the instrument when conducting the EMI test. Furthermore, most engineers are interrupted and asked to solve the EMI issues.  

*** Các tài liệu về Giải pháp kiểm tra EMC của hãng GWInstek:

EMC Documents and Resources
	Application Note - The brand new concept of EMI probes Simplify the complicated EMC measurement and debugging!
	Brochure - EMI Solution Brochure of General introduction of EMC pretest solution
	Slide - EMI Solution Slide of General introduction of EMC pretest solution

*** Các câu hỏi thường gặp về giải pháp kiểm tra EMC của hãng GWInstek:

01. Is the equipment used by the Pre-compliance test different from that of the certification lab? GW Instek's EMC pre-compliance solution focuses on enabling engineers to quickly and easily evaluate and debug product’s EMI in a general working environment. The differences of certified laboratories which focus on certification are as follows:  Impact of environmental noise: In the certified laboratory, the noise can be well controlled and regularlly examined; however, the environmental noise cannot be effectively suppressed in the general working environment. The conductive interference measurement is mainly the purity of the power supply in the working environment, and the radiated interference is the existing wireless signal in the environment. For suggestions on how to respond, please refer to “How to eliminate the impact of environmental noise when performing Pre-compliance measurement”?" Anechoic chamber has good electromagnetic wave isolation and grounding structure The instrument used by Pre-compliance is a spectrum analyzer, which is functionally different from the EMI receiver used by the certified laboratory. Spectrum Analyzer (left) and EMI Receiver (right) The probe used for the radiated EMI in the debugging stage is different from the antenna measurement used by the certified laboratory; the former is the near-field measurement of the electromagnetic field, and the latter is the far-field measurement. The two are essentially different. Near-field measurement of the probe on the PCB surface (left) and far-field measurement of the antenna  10m away (right)

02. What should I do if there is a difference between the Pre-compliance and the certified lab test results? It is normal for the pre-compliance and certified lab test results to be different, so it is important to understand the cause of the difference in order to get a reference from the pre-compliance measurement so as to conduct debugging. The difference between the two can be referred to “What is the difference between the Pre-compliance test and the program used by the certified laboratory?”. The most important thing is to establish a test reference for pre-compliance after obtaining the differences between the two. For example, pre-compliance test needs to be tightened and reduced a certain standard (such as 4dB reduction) under regulatory standards that is a margin for testing by certified laboratories. After accumulating and establishing such a test reference, the successful measurement rate in the certification laboratory will be greatly improved.

03. How to remove the effects of environmental noise when performing Pre-compliance tests? The environmental noise includes conduction and radiation EMI. After connecting the DUT, the EMI measurement is performed before the power of the DUT is turned on. The result obtained is the environmental noise, and then the power of the DUT is turned on to distinguish between ambient noise and DUT EMI signals. 1. Use Trace comparison mode Users can compare the difference via the measured Trace under different states to determine whether the noise is caused by background noise or the DUT by analyzing the waveforms. In the following example, users can connect the GSP-9330 with the near field probe or LISN before the DUT is turned on; then set the Trace in the sweep to the View mode. The Trace obtained at this time will be background noise, which is the yellow trace in the following picture. Turn on the power of the DUT and enter the operation mode. At this time, another Trace is used for testing. Now you can see the difference of the signal, such as the pink Trace. The comparison method is used to confirm whether the signal found is from the DUT. As shown in the following display, the signal in the red frame shows that the signals measured before and after the DUT is turned on are almost similar, which means that the signals in this frequency bandwidth are more likely the background noise. Users can eliminate the issue when analyzing. Topographic mode displays noise hidden in the scan signal (indicated by a red arrow) 2. Use Topographic display mode Another mode is to use the Topographic display mode. (This mode cannot be used under the EMI Mode. This mode can be used after leaving the EMI Mode) Topographic mode displays noise hidden in the scan signal (indicated by a red arrow)

04. Differences in EMI signal detection using an EMI receiver and GSP-9330 spectrum analyzer. In the EMI test, to determine whether the DUT can pass the EMI regulations, the signal must be detected by the detection method of QP Detector and EMI-Avg Detector respectively. When testing by an EMI receiver in a laboratory, in order to speed up the test, the entire signal is usually extracted via PK+ Detection, and the signal frequency exceeding the limit regulation is captured, and then the narrow frequency or zero frequency is used to find out the QP and AVG values that exceed regulations. As shown in the figure below, the values of QP and AVG obtained in the EMI Report are the results obtained by the software in the background calculation and processing. The EMI signal detection using the GSP-9330 focuses on the source of possible EMI during the Pre-Compliance phase and the Debug process, so PK+ Detector is usually used at this stage to find all EMI sources and identify larger sources of signals to analyze. If you want to know the value of QP and AVG at the frequency point found by PK+, manually set the Span to narrow or zero frequency and set the detection mode to QP or AVG.

05. How does the GSP-9330 generate an EMI Pretest report? There are three ways to generate EMI test reports for GSP-9330: Save the measurement result screen on the USB flash drive: Use the USB flash drive to store the measurement results. After the test is completed, open the Peak Table to mark the 10 signals with the highest amplitude, or use the Marker Table to mark up to 6 Markers, and then insert the USB flash drive until the spectrum analyzer identification program is completed, then you can save it. The saved file format is a JPG image file. The operation procedures are : first insert the USB flash drive on the GSP-9330 front panel USB slot. When the USB icon appears on the screen, press the Quick Save button to complete the screen storage.   USB slot and icon of the GSP-9330 (left); The screen can be directly saved on the USB flash drive (right) Store the measurement result data in the USB flash drive and then use the PC software (such as MS Excel) to read it out for processing: store the measurement data with USB flash drive, the file name is tra, and the format of the tra file is Text format, the content is the number, frequency and amplitude of 601 points (0~600 points). You can use the PC software such as MS Excel for subsequent processing. Users can enter the regulatory limit line into MS Excel for displaying at the same time. Expressed in MS EXCEL Use PC software SpectrumShot output report provided by GW Instek: Use GSP-9330 dedicated PC software SpectrumShot to read the test data while in connection, or directly read the tra file in USB flash drive offline to print or output an RTF format file. The software can be freely downloaded from the GW Instek website. Reports generated by SpectrumShot   This PC software must be collocated with the NI-VISA driver. Please download it from the NI website.

06. Can the GSP-9330's built-in EMI test regulations be increased? Yes. The GSP-9330 has built-in EN55022A, EN55022B, EN55015, FCCA and FCCB regulations. If other regulations are required, users can use the User Define function (as shown in the figure below) to edit and store the regulatory limit line on the spectrum analyzer. There is no limit to the storage capacity of the regulatory limit line, as long as the capacity is sufficient to store. However, if you need to call the test regulations, up to 5 sets of limit line regulations can be called at a time. In addition, you can use the dedicated PC software SpectrumShot for editing and remote control. The software can be downloaded freely from the GW Instek website. The software has built-in EN55011, EN55012, EN55014, EN55015, EN55022, EN55025, EN55032, FCCA and FCCB commonly used regulations. Users can select the items to be tested according to the test requirements, such as conduction and radiation, Class level, product category, Detector level etc., as shown below. If there are other regulatory requirements, users can also edit the new limit line. This PC software must collocate with the NI VISA driver. Please go to the NI website to download it. User-defined EMI regulation operating menu PC software SpectrumShot EMI limit line menu)

07. GSP-9330 can display two Limit Lines of AVG and QP simultaneously in the conduction bandwidth setting of EMC Pretest Mode, but only one can be displayed when self-defining Limit Line. Can two limit lines be displayed? To display two self-defined Limit Lines, please use the GSP-9330 dedicated PC software SpectrumShot. This PC software must be collocated with the NI-VISA driver. Please download it from the NI website.

08. Why does the GSP-9330 display the Power Over Range warning message under the EMI Mode? When the EMI signal is too strong, the GSP-9330 will send a warning message to avoid damage to the spectrum analyzer. It is recommended to connect an attenuator before the input terminal. Power Over Range is determined by whether the signal is greater than Ref. Level. For example, the Ref Level in the figure below is set to 80dBuV, which means that Power Over Range will appear if the signal is greater than 80dBuV.

09. Can the GSP-9330 set Trace1 to PK+, Trace2 to QP, and Trace3 to AVG? Yes, at this time, the RBW will switch to the EMI bandwidth according to the current frequency bandwidth. Frequency RBW <30MHz 9kHz 30MHz-1GHz 120kHz >1GHz 1MHz After setting, each Trace will simultaneously scan PK+, QP, and AVG according to the settings.

10. What is the difference between the GSP-9330 EMI Test > Correction > Horizontal/Vertical setting and Sensor Probe > Correction > 3m/10m setting? The Correction under EMI Test is used to compensate the signal measurement of the antenna when it is horizontally or vertically polarized; the 3m/10m Correction under Sensor Probe is converted into a 3m/10m anechoic chamber after using the signal measured by the ANT-04 probe. Simulation results. The former is the correction of the antenna far-field measurement, and the latter is the test of the near-field probe simulation in the anechoic chamber.

11. What is the difference between using a digital oscilloscope's FFT technology and a spectrum analyzer to analyze EMI signals? The digital oscilloscope samples the signal of the DUT and uses FFT technology to obtain the spectrum distribution. Using this method to analyze EMI signals will cause large errors in amplitude measurements due to the following factors: The bandwidth of the oscilloscope and probe Input resistance Sampling rate The number of bits in the ADC is insufficient Therefore, in the application of EMI testing, FFT obtained by digital oscilloscope (DSO) cannot replace spectrum analyzer. The bandwidth of the oscilloscope and probe The bandwidth indicated by the oscilloscope is 3dB bandwidth. For example, an oscilloscope with a bandwidth of 100MHz will have 3dB (about 30%) distortion when measuring a true 100MHz signal. In order to reduce the measurement distortion caused by the bandwidth, the general choice for the oscilloscope bandwidth selection is 5 times the highest frequency of the DUT signal, for example, measuring a 100 MHz signal, it is recommended to use a 500 MHz digital oscilloscope. The bandwidth and frequency curves of the oscilloscope and probe The EMI regulations must be at least 1 GHz for electromagnetic radiation measurements. Passing or failing is often determined by 1 to 2 dB. In order to avoid false judgement caused by high frequency EMI or harmonic signal errors, it is recommended to use an oscilloscope with sufficient bandwidth, which means that the oscilloscope needs a bandwidth of up to 5 GHz in order to maintain consistent amplitude measurement accuracy. In addition to the oscilloscope, the probe used also has the same bandwidth problem. The active probe has a larger bandwidth, but the price may increase by 5 to 10 times. The bandwidth indicated by the passive probe is that the probe needs to be in the 10:1 position, otherwise the bandwidth will decrease. Input resistance If an EMI near-field probe is used to connect the oscilloscope for replacing the oscilloscope probe, reflection will occur in the high-frequency part because the input impedance of the oscilloscope is generally 1MW. Hence, the signal will be reflected back to the probe end instead of entering the oscilloscope. This situation will vary with frequency. If you really want to use it, you must use an oscilloscope with an input impedance of 50W. The number of bits in the ADC is insufficient The general oscilloscope's ADC is 8-bit, and the special-purpose DSO will have ADC with more bits. GW Instek's GSP-9330 is 23-bit by patented technology in small signal positions. The difference in the number of these ADC bits will make a huge difference in the results of the measurements. The lowest range step of the 8-bit ADC, linear scale (left) and logarithmic scale (right) Assume that the oscilloscope's vertical level has a minimum level of 1mv/div and 10 graticules represent 10mV. The 8-bit ADC is used for sampling measurement. The minimum measurable voltage is, After converting to dBuV: This value is already higher than the upper limit value(2) of Class B of the CISPR22 electromagnetic radiation ITE product without considering the antenna factor(1), and since the resolution is 40uV, the following step reading value is 80uV. , after the conversion, the value is 38dBuV, which means that the EMI signal between 32 and 38dBuV cannot be measured. Note: (1) See question 5 for antenna factors. (2) Regulation for CISPR22 electromagnetic radiation ITE products Class B@10m are as follows: Frequency, MHz QP limits, dBuV/m 30 ~230 30 230~1000 37 Sampling rate of the oscilloscope ADC According to Nyquist–Shannon sampling theorem, the sampling rate of a digital oscilloscope must be greater than or equal to twice the highest frequency of the DUT signal. In the analysis of high-frequency EMI signals, it is more important to select an oscilloscope with a bandwidth of 5 times, and then the sampling rate needs to be more than twice the highest frequency of the DUT signal that requires a high-priced oscilloscope. When GW Instek GSP-9330 is at 10Hz RBW, the base noise is -133dBm when the preamplifier is turned on. That is equivalent to -26dBuV, which measures signals nearly 58dB less than the oscilloscope. Therefore, the difference between the oscilloscope and the spectrum analyzer's ability in measuring small signals is tremendous.

12. Can GKT-008 be used with other brands of spectrum analyzers? Spectrum analyzer with an input impedance of 50 W can use the GKT-008 probe. However, the functions related to GKT-008 in the EMC Pretest built in GSP-9330 cannot be used. In terms of hardware architecture, the GKT-008 is designed to be used with a spectrum analyzer. Spectrum analyzer with 50 Ω input impedance can use the GKT-008 probe. However, the GKT-008 has some very useful extension functions that must be used in conjunction with the EMC Pretest feature of the GSS-9330 spectrum analyzer. These features include: Far field EMI measurement estimation Estimation of EMI after the chip's pins are added with traces to the PCB Estimation of EMI after the communications interface is connected to the cable After near field probe ANT-04 is connected with TG, simulate signal source for EMS test

13. Is it better to use the H and E probes to measure the electric and magnetic fields of the EMI signal separately? Using the near field electric field probe and the magnetic field probe to measure the near field electric field and the magnetic field respectively can only obtain the two components of the electromagnetic energy in the EMI signal. The GKT-008 can directly and effectively sense the actual electromagnetic energy. The near field electric field probe and the magnetic field probe can only measure two components of the electromagnetic energy of the EMI signal. The actual electromagnetic energy is the vector outer product of the electric field and the magnetic field. Hence, the large near field magnetic field does not mean that it will radiate to become the interference of EMI. It is the same for the electric field. Therefore, measuring the near field electric field and the magnetic field is only a substitute method when the electromagnetic wave energy cannot be directly measured. Moreover, the electric field E and the magnetic field H respectively measured by the spectrum analyzer are difficult for the calculation of the electromagnetic energy vector by using the formula. Moreover, in the process of EMI debugging, the problem of a magnetic field or an electric field must be considered to solve whether for isolation mechanism, filter circuit or grounding design of the circuit in terms of techniques and methods for solving the problem. The main method is to measure the frequency and amplitude (energy) of the EMI noise so as to confirm which circuit or component produces the interference signal source. The interference can be improved by suppressing the current (such as series resistance) or voltage (such as parallel bypass capacitor). Using the near field probe of GKT-008 to directly measure the electromagnetic energy can directly measure the actual near field electromagnetic energy. There is no need to measure the electric field and the magnetic field separately by using GKT-008, which becomes a more efficient tool. In some special applications, separate analysis of the electric and magnetic fields is required such as larger power motor rotation systems.

14. Does the GKT-008 near field probe provide antenna factors? Because the principle of the near field probe is not the same as the working principle of the antenna, there is no antenna factor parameter. The function of the antenna is to convert the electric field strength ES of the received electromagnetic wave into a voltage , which is measured and displayed by the output port connected to the spectrum analyzer, and the relationship with the antenna factor  is as follows:  is the electric field strength of the signal,is the voltage generated at the output port of the antenna. In a linear representation,  But in the logarithmic expression,  Therefore, the electric field strength of the electromagnetic wave is equal to the measured reading value (voltage, dBuV) of the spectrum analyzer plus the antenna factor (dB/m). For example, a spectrum analyzer connected to a Biconical antenna has a signal of 30dBuV at 320MHz and an antenna factor of 28dB/m, then the electric field strength is 30dBuV+28dB/m =58dBuV/m. In the case of the far field of the electromagnetic field described above, the electric field strength Es transmitted in the air is related to the wave impedance Z, which is defined as Z = E / H (the electric field E divided by the magnetic field H). The wave impedance Z of the far field is a constant 377 (120pi) Ohm, which represents that the relationship between the measured value of the electric field and the magnetic field is fixed. The antenna factor can be utilized here. However, the wave impedance Z of the EMI probe in the near field is not fixed. It is related to the radiation source, the test distance and the probe form. Unlike the fixed far field of 377 ohm, the near field probe does not have the antenna factor.

15. What is the SMA Cable specification in the GKT-008? The SMA Cable model in the GKT-008 is GTL-303 and its specifications are: Insertion Loss: Less than 1.5dB, 0 ~ 3GHz. Return Loss: Less than -18dB, 0 ~ 3GHz.

16. What is the difference between the Pretest and Debug options under the AC Voltage Probe for EMI Mode of GSP-9330? The measurement result of Debug Mode is not processed. The Debug Mode is the default mode, and the measurement results are not processed, and the Pretest Mode is the mode after adding the Correction Factor of the PR-01 AC Probe.

17. What are the characteristics of the PR-01 AC probe? The characteristics of the PR-01 are listed below. Users can also find the relevant specifications in the GKT-008 user manual. Frequency Range 150kHz to 30MHz Insertion Loss S21/ Frequency response -2.4dB (Avg.) Deviation of Insertion Loss ±3dB Rated Voltage 300Vac

18. Is the Pre-amplifier preset turned off when using AC Probe PR-01? When users use the function of EMC Pretest > AC Voltage Probe, the Pre-amplifier preset is turned off. PR-01 is mainly used to measure the EMI signal of the AC power supply.  A large signal feedback during the low frequency measurement is likely to occur that will damage the RF end of the GSP-9330. Therefore, when using the function of the AC Voltage Probe, the preamplifier is preset off. Users can still turn on the amplifier according to the requirements of the measurement.

19. PR-01 can test conducted EMI below 30MHz. Its withstand voltage is AC 300Vrms. What is the maximum DC bias value? The highest DC bias of PR-01 should not exceed 50Vdc. It is recommended to test after the power supply of the DUT is connected and stabilized.

20. What is the difference between the 9kHz and 150kHz on the GLN-5040A? Different products will use different regulations for EMC testing according to their electrical characteristics. According to the existing regulations, in the conducted test, 9 kHz is suitable for the conducted test of lighting equipment, and 150 kHz is suitable for the conducted test of general audio-visual equipment, household appliances and information products. The following table is a list of regulations and conducted EMC test frequency ranges for products in various commonly used categories. 9k ~ 30MHz Lighting fixtures and equipment EN55015/CISPR15 150k~30MHz Lighting fixtures and equipment EN55015/CISPR15 Industrial, scientific, medical products (1) EN55011/CISPR11 Audio and video equipment EN55013/CISPR13 Household and similar equipment EN55014-1/CISPR14-1 Information products and office equipment EN55022/CISPR22 (1) Industrial, scientific, and medical products under the FCC's regulations, the frequency range of EMC conducted testing is 9k~30MHz.

21. Why is there a defferent of about 10dB while using GLN-5040A? A 10dB attenuator is designed in the GLN-5040A to protect the spectrum analyzer during testing. In the GSP-9330, the compensation function is also designed. 10dB compensation can be obtained by turning on (EMC Pretest >> EMC ON >> EMI Test >> Band selects the conduction band >> More 1/2 >> GLN-5040A 10dB Comp. ON / OFF >> Select ON to get the real value.

22. Can GLN-5040A use DC Input? The specification supports 0~50Vdc input. GLN-5040A supports DC input and supports up to 50Vdc input. However, since GLN-5040A is designed primarily for AC LISN, it can be used for reference when using DC input, however, it is not the same as DC LISN.

23. Can the DUT and the test instrument be placed on the work table when using the LISN to test conducted EMI? NO. The regulations have clear definitions for the setup and environment of the test equipment, including the height of the test table, the size of the grounding surface, the distance between the DUT and the test equipment, the power outlet, etc. The DUT in the regulation is placed on the test table and LISN is placed on the ground surface. If the test site is not built in accordance with the regulations in the general working environment, the measurement results may cause errors.

24. Is the conducted EMI measurement results of the GLN-5040A and GIT-5060 different from the results measured by the laboratory? When the laboratory is constructed, it usually uses a power filter (with filtering, voltage regulation, etc.) and is built with a standard field that meets the regulations. Therefore, there is less power supply noise after the LISN. GIT-5060 is mainly used to isolate the ground leakage current after LISN and the DUT is connected, and there is no filtering and voltage regulation function, so there will be some differences. Refer to "What do you do when there is a test result difference between the pre-compliance and certified lab?" and "How to remove the effects of environmental noise when performing Pre-compliance measurement?"

25. GLN-5040A has a maximum of 16A, but GIT-5060 has a maximum of only 4A (at 220V), and the test capacity is too small. How do you increase their test capacities? If the test capacity is greater than GIT-5060 requirement, two methods are recommended: first, replace GIT-5060 with AC Source, but it is better to use the Linear Mode and it has passed the CE specification. Second, use EMI power filter. This method requires special EMI manufacturers to construct because site evaluation and further specification evaluation are required to meet your needs.

26. Can I use AC Source instead of GIT-5060 or place it in front of GIT-5060? Yes. But not to choose Switching Mode AC Source to avoid the mixing of PWM signals and try to choose CE certified.  The noise of AC Source will be smaller.

27. What are the characteristics of Transient Limiter GPL-5010? GPL-5010 operates from 9 kHz to 200 MHz with an attenuation of 10 dB to 12 dB. Detailed specifications can be found in GPL-5010 user manual, which can be downloaded from the website.

28. What is the timing of using Transient Limiter GPL-5010? If users concern big signal fluctuation while conducting EMI testing, especially the spectrum analyzer might be damaged when using PR-01 to directly contact AC circuit, use GPL-5010 to protect GSP-9330, as shown in the following diagram.

29. Noise difference between Common mode and Differential mode. Common mode and differential mode noise can theoretically be analyzed in conducted and radiated EMI. However, when it comes to the actual noise suppression technology, it is more meaningful to discuss the conducted noise. It is also possible to seek reference countermeasures through measurement. The following figure shows the common circuit for conducted EMI suppression in power supply. CY, LCM is a common mode noise filtering network, and CX is a differential mode noise filtering network. If we can measure the common mode component and the differential mode component of the conducted noise separately, we know that we should adjust the common mode filter CY, LCM, or the differential mode filter CX to reduce the noise. The following is a brief description.  and  in the figure represent the currents on the two power lines. In theory, we can treat  and  as the sum of the common mode and differential mode noise currents, namely:  and ,  is common mode current,  is differential mode current. Furthermore, we found that  and  can be obtained from  Comparing the two equations, as long as the signal of one of the power supplies can be added to the other power line signal in the forward and reverse directions, the sum and the difference of  and  can be obtained separately, and the  and  can be measured. Therefore, a  and coupler/adder can be designed at the power supply terminal of the DUT to obtain  and . There is an approximation, which uses a current probe with a spectrum analyzer to measure the sum of the two power lines, as shown in the following figures. In Figure (a), run two power cables through a current probe, connect the probe with a spectrum analyzer to obtain . Run one of the power cables around the probe (b) to make the current direction opposite to the direction as shown in. (a). By so doing, the  can be measured from the spectrum analyzer, and the results are shown below. It can be seen that the common mode noise component is higher than that of the differential mode, and we know that the common mode filter  and  need to be adjusted to reduce the common mode noise component. Although this is not an accurate measurement, it is an approximation that is worthy of reference. Common mode and differential mode noise

30. Which power cable should I use for EMI testing? A power supply cable with a magnetic ring can suppress conducted or radiated EMI by the characteristics of the magnetic ring. The better the isolation, the better the power supply wire can suppress the EMI from radiating out of the power supply to form radiated EMI, but the cost is relatively higher.