“The frequency of electromagnetic interference is often very high, so the high frequency characteristics of the interference filter are very important. The three-terminal capacitor cleverly uses the two lead inductances on one electrode to form a T-type low-pass filter, which eliminates the lead in the traditional capacitor The bad influence of inductance improves high-frequency filtering characteristics, so three-terminal Capacitors are more suitable for interference filtering.
“
1. Why do electromagnetic compatibility designs for products?
Answer: Meet the functional requirements of the product, reduce the debugging time, and make the product meet the requirements of the electromagnetic compatibility standard, so that the product will not cause electromagnetic interference to other devices in the system.
2. From what aspects can the EMC design of the product be carried out?
Answer: circuit design (including device selection), software design, circuit board design, shielding structure design, signal line/power line filtering design, circuit grounding method design.
3. In the field of electromagnetic compatibility, why is it always described in decibels (dB)? How much dBmV is 10mV?
Answer: Because the amplitude and frequency range to be described are very wide, it is easier to express with logarithmic coordinates on the graph, and dB is the unit when expressed in logarithm, and 10mV is 20dBmV.
4. Why can’t the spectrum analyzer observe transient interference such as electrostatic discharge?
Answer: Because the spectrum analyzer is a narrow-band sweep frequency receiver, it only receives energy in a certain frequency range at a certain moment. Transient interference such as electrostatic discharge is a kind of pulse interference. Its spectrum range is very wide, but the time is very short. Therefore, when the transient interference occurs, the spectrum analyzer can only observe a small part of its total energy, which cannot reflect the actual Interference situation.
5. When diagnosing electromagnetic interference problems in the field, it is often necessary to use a near-field probe and a spectrum analyzer. How to make a simple near-field probe with a coaxial cable?
Answer: Strip the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small loop (1 to 3 turns) with a diameter of 1 to 2 cm, and weld it to the outer layer.
6. For a piece of equipment, the original electromagnetic radiation emission intensity is 300mV/m. After adding the shielding chassis, the radiation emission is reduced to 3mV/m, then how much dB is the shielding effectiveness of this chassis?
Answer: The shielding effectiveness of this case should be 40dB.
7. When designing a shielding case, what factors should be used to select shielding materials?
Answer: From the perspective of electromagnetic shielding, the main consideration is the type of electric field wave to be shielded. For electric field waves, plane waves or higher frequency magnetic field waves, general metals can meet the requirements. For low-frequency magnetic field waves, materials with higher magnetic permeability should be used.
8. In addition to the shielding material, the shielding effectiveness of the chassis is affected by other factors?
Answer: Affected by two factors, one is the conductive discontinuities on the chassis, such as holes, gaps, etc.; the other is the wires passing through the shielding box, such as signal cables, power lines, etc.
9. What problems should be paid attention to when shielding the magnetic field radiation source?
Answer: Because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding purpose is achieved mainly by absorption loss. Therefore, a shielding material with higher magnetic permeability should be selected. In addition, when designing the structure, keep the shielding layer as far away from the radiation source as possible (to increase reflection loss), and try to avoid making holes, gaps, etc., close to the radiation source.
10. When designing a shielding structure, there is a principle: try to keep the cables in the case away from gaps and holes. why is that?
Answer: Since there is always a magnetic field near the cable, and the magnetic field is easy to leak from the hole (it has nothing to do with the frequency of the magnetic field), when the cable is very close to the gap and hole, magnetic field leakage will occur, reducing the overall shielding effectiveness.
11. Measuring the biomagnetic information of the human body is a new medical diagnosis method. The measurement of this biomagnetism must be carried out in a magnetic field shielded room, which must be able to shield the alternating electromagnetic field from static magnetic field to 1GHz. How should I design this shielded room?
Answer: First of all, we must consider the selection of shielding materials. Since magnetic fields with very low frequencies are to be shielded, materials with high magnetic permeability, such as Permalloy, are used. After the permalloy is processed, the permeability will decrease, and it must be heat treated. Therefore, the shielding room should be made into an assembled type, which is assembled from plates. The plates are processed in advance according to the design, and then heat treated, transported to the site, and carefully installed. The joints of each plate should be overlapped to form a continuous magnetic path. The shielding room constructed in this way has better shielding effectiveness against low-frequency magnetic fields, but the gap will cause high-frequency leakage. In order to make up for this shortcoming, the outer layer of the permalloy shielding room needs to be welded with aluminum plates to form a second layer of shielding, thereby shielding the high-frequency electromagnetic field.
12. What is a cut-off waveguide plate (honeycomb plate)? In what occasions should it be used? What problems should be paid attention to when using?
Answer: The cut-off waveguide board is an array board composed of many cut-off waveguides. It is used in the occasions that require high shielding efficiency and ventilation. When using it, pay attention to the use of electromagnetic sealing gaskets between the honeycomb board and the chassis for installation or welding. .
13. There is an equipment with a plastic case, and the electromagnetic radiation exceeds the standard. In order to meet the requirements of the electromagnetic compatibility standard, the developer sprayed conductive paint on the inside of the case, and the result did not improve significantly. Please analyze what might be the reason?
Answer: There may be too many holes and too large holes in the original plastic case, resulting in too serious leakage, or the gap may not be tight (maybe the contact is not tight, or there may be no conductive paint sprayed on the joint), resulting in leakage. In addition, the cables (signal cables, power cables) on the original chassis often do not have good filtering measures, and these cables may cause electromagnetic leakage of the chassis.
14. What kinds of transparent shielding windows are there? What problems should be paid attention to when using?
Answer: There are two types, one is a shielding window composed of glass sandwiched with metal mesh, and the other is a shielding window composed of a thin metal film plated on the glass. When in use, pay attention to the metal mesh or conductive coating must be in close contact with the basic conductive parts of the shielding case.
15. When using shielding glass with a metal mesh sandwich on the Screen of a CRT monitor, there will be annoying streaks. How to reduce this phenomenon?
Answer: Rotate the direction of the screen to make an angle of 15-20 degrees between the weft line and the scanning line of the picture tube.
16. What are the two key characteristics of electromagnetic sealing gaskets? List as many types of electromagnetic sealing gaskets as possible, and explain the applicable occasions of various products.
Answer: The two characteristics that electromagnetic sealing gaskets must have are elasticity and conductivity. Common types of electromagnetic sealing gaskets are: finger reeds, metal mesh gaskets, conductive rubber, conductive cloth-wrapped foam rubber, spiral tubes, etc. Except for occasions with tangential sliding contact, avoid using finger reeds; when environmental sealing requirements are required, conductive rubber should be used; in other occasions, conductive cloth pads can be used, and screens can also be used when the frequency of shielding is not high. Cushion; to ensure that it will not be over-compressed, a spiral tube can be used.
17. What problems should be paid attention to when using electromagnetic sealing gaskets?
Answer: The thickness of the Panel should be appropriate to prevent deformation under the action of the rebound force of the gasket and cause larger gaps. When the panel thickness is thin, the tightening screws need to use a smaller interval. Set limit structure to prevent excessive compression. Choose appropriate metal materials to reduce electrochemical corrosion.
18. A metal rod must pass through a shielded case. How to deal with it so as not to damage the shielding effectiveness of the case?
Answer: The periphery of the metal rod is reliably overlapped with the shielding base through the beryllium copper reed.
19. What is the main function of the power line filter? What parameters are mainly considered when selecting a model? What problems should be paid attention to when using power line filters?
Answer: The function of the power line filter is to suppress the conduction emission current from propagating along the power line. When selecting the type, consider the insertion loss (common mode and differential mode), rated current, voltage, effective frequency range and other parameters. Pay attention to the installation method when using it, and ensure that the RF grounding and input and output isolation are good to prevent filtered wires. Contaminated again.
20. Why is the high frequency filtering characteristic of the power line filter important?
Answer: If the high-frequency filtering characteristics are not good, it will cause the equipment’s radiation emission to exceed the standard or be sensitive to pulse interference.
21. When designing structural electromagnetic compatibility, there is a principle: The filtered power cord should be as far away as possible from various signal cables. why is that?
Answer: If the power cord is very close to the signal cable, the high frequency signal on the signal cable will be coupled to the power cord (especially the filtered part), causing the conducted emission on the power cord to exceed the standard.
22. Why can’t I just pursue compact size when choosing power line filters?
Answer: The size of the filter is mainly determined by the Inductor in the filter circuit. A smaller Inductor must be used in a smaller filter, and the smaller inductor may also have a smaller inductance, which will cause the filter Low-frequency filtering performance is poor. In addition, the size of the filter is reduced, and it is necessary to keep the internal components close to each other, which will reduce the high-frequency performance of the filter.
23. What is the insertion loss of the filter? What method can be used to measure the insertion loss of the filter to get a very safe result?
Answer: The current and voltage loss caused by the filter being connected to the circuit is called the insertion loss of the filter. The interference filter should have as much insertion loss as possible for the signal of the interference frequency. The insertion loss of the measuring filter should be measured under the condition that the ratio of source and load impedance is 0.1:100 (or vice versa). At this time, the result under very bad conditions can be obtained, that is, the result is very safe.
24. Generally speaking, AC line filters can be used in DC applications, but DC line filters cannot be used in AC applications. Why?
Answer: The bypass capacitor used in the DC line filter is a DC capacitor. It may be damaged by overheating when used under AC conditions. If the DC capacitor has a low withstand voltage, it will be broken down and damaged. Even if these two situations do not occur, due to the large capacity of the common-mode bypass capacitor in the general DC filter, excessive leakage current will occur when used in AC applications, which violates the provisions of safety standards.
25. What is the main function of the signal line filter? What are the types in terms of installation methods? How to determine which installation method of signal line filter to use?
Answer: Reduce unnecessary high frequency components (mainly common mode) on the signal line, thereby reducing the electromagnetic radiation of the cable, or preventing the cable from being used as an antenna to receive spatial electromagnetic interference and conduct it into the chassis. There are two ways to install on the circuit board and the installation on the panel. When the frequency that needs to be filtered is low, use the structure installed on the circuit board; when the frequency that needs to be filtered is higher, use the structure installed on the panel.
26. The signal transmitted on a signal line has a very high frequency of 30MHz. The measurement shows that there is a common mode interference current of 120MHz on this wire. Using the common mode radiation formula to predict, as long as the common mode current is suppressed by 30dB, it can be satisfied According to the requirements of electromagnetic compatibility standards, how many low-pass filter circuits are needed at this time?
Answer: According to the meaning of the question, the cut-off frequency of the low-pass filter is 30MHz, and the insertion loss at 120MHz is greater than 30dB. Since the insertion loss of the N-order filter increases at a rate of 6N (dB) per octave, and 30MHz to 120MHz is two octaves, if the cut-off frequency of the N-order filter is 30MHz, the insertion loss at 120MHz is The range is 12N (dB). To make the process 12N> 30, N=3 can be used, that is, the order of the low-pass filter is at least 3.
27. Why are three-terminal capacitors more suitable for interference filtering?
Answer: The frequency of electromagnetic interference is often very high, so the high frequency characteristics of the interference filter is very important. The three-terminal capacitor cleverly uses the two lead inductances on one electrode to form a T-type low-pass filter, which eliminates the traditional capacitor. The bad influence of the inductance of the middle lead improves the high-frequency filtering characteristics, so the three-terminal capacitor is more suitable for interference filtering.
28. Why is the through-core capacitor an ideal device for interference filtering?
Answer: The feedthrough capacitor is a three-terminal capacitor, but compared with the ordinary three-terminal capacitor, because it is directly mounted on the metal panel, its grounding inductance is smaller, and there is almost no influence of lead inductance. In addition, its input and output terminals are isolated by metal plates, eliminating high-frequency coupling. These two characteristics determine that the feedthrough capacitor has a filtering effect close to an ideal capacitor.
29. What is the difference between the magnetic core used for electromagnetic interference suppression and the traditional magnetic core used as an inductor? What happens if the two are used incorrectly?
Answer: The material traditionally used as the magnetic core of the inductor has very small loss, and the loss of the inductor made with this kind of magnetic core is very small. The magnetic core used for electromagnetic interference suppression has a large loss, and the inductor made with this kind of magnetic core has a large loss, and its characteristics are closer to resistance. If the two are used incorrectly, neither will achieve the intended purpose. If the magnetic core used for electromagnetic interference suppression is used in an ordinary inductor, the Q value of the inductor will be very low, which will make the resonant circuit fail to meet the requirements, or cause excessive loss of the signal to be transmitted. If the magnetic core used to make an ordinary inductor is used for electromagnetic interference suppression, the inductance and the parasitic capacitance in the circuit will resonate, which may increase the interference at a certain frequency.
1. Why do electromagnetic compatibility designs for products?
Answer: Meet the functional requirements of the product, reduce the debugging time, and make the product meet the requirements of the electromagnetic compatibility standard, so that the product will not cause electromagnetic interference to other devices in the system.
2. From what aspects can the EMC design of the product be carried out?
Answer: Circuit design (including device selection), software design, circuit board design, shielding structure design, signal line/power line filtering design, circuit grounding method design.
3. In the field of electromagnetic compatibility, why is it always described in decibels (dB)? How much dBmV is 10mV?
Answer: Because the amplitude and frequency range to be described are very wide, it is easier to express with logarithmic coordinates on the graph, and dB is the unit when expressed in logarithm, and 10mV is 20dBmV.
4. Why can’t the spectrum analyzer observe transient interference such as electrostatic discharge?
Answer: Because the spectrum analyzer is a narrow-band sweep frequency receiver, it only receives energy in a certain frequency range at a certain moment. Transient interference such as electrostatic discharge is a kind of pulse interference. Its spectrum range is very wide, but the time is very short. Therefore, when the transient interference occurs, the spectrum analyzer can only observe a small part of its total energy, which cannot reflect the actual Interference situation.
5. When diagnosing electromagnetic interference problems in the field, it is often necessary to use a near-field probe and a spectrum analyzer. How to make a simple near-field probe with a coaxial cable?
Answer: Strip the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small loop (1 to 3 turns) with a diameter of 1 to 2 cm, and weld it to the outer layer.
6. For a piece of equipment, the original electromagnetic radiation emission intensity is 300mV/m. After adding the shielding chassis, the radiation emission is reduced to 3mV/m, then how much dB is the shielding effectiveness of this chassis?
Answer: The shielding effectiveness of this case should be 40dB.
7. When designing a shielding case, what factors should be used to select shielding materials?
Answer: From the perspective of electromagnetic shielding, the main consideration is the type of electric field wave to be shielded. For electric field waves, plane waves or higher frequency magnetic field waves, general metals can meet the requirements. For low-frequency magnetic field waves, materials with higher magnetic permeability should be used.
8. In addition to the shielding material, the shielding effectiveness of the chassis is affected by other factors?
Answer: Affected by two factors, one is the conductive discontinuities on the chassis, such as holes, gaps, etc.; the other is the wires passing through the shielding box, such as signal cables, power lines, etc.
9. What problems should be paid attention to when shielding the magnetic field radiation source?
Answer: Because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding purpose is achieved mainly by absorption loss. Therefore, a shielding material with higher magnetic permeability should be selected. In addition, when designing the structure, keep the shielding layer as far away from the radiation source as possible (to increase reflection loss), and try to avoid making holes, gaps, etc., close to the radiation source.
10. When designing a shielding structure, there is a principle: try to keep the cables in the case away from gaps and holes. why is that?
Answer: Since there is always a magnetic field near the cable, and the magnetic field is easy to leak from the hole (it has nothing to do with the frequency of the magnetic field), when the cable is very close to the gap and hole, magnetic field leakage will occur, reducing the overall shielding effectiveness.
11. Measuring the biomagnetic information of the human body is a new medical diagnosis method. The measurement of this biomagnetism must be carried out in a magnetic field shielded room, which must be able to shield the alternating electromagnetic field from static magnetic field to 1GHz. How should I design this shielded room?
Answer: First of all, we must consider the selection of shielding materials. Since magnetic fields with very low frequencies are to be shielded, materials with high magnetic permeability, such as Permalloy, are used. After the permalloy is processed, the permeability will decrease, and it must be heat treated. Therefore, the shielding room should be made into an assembled type, which is assembled from plates. The plates are processed in advance according to the design, and then heat treated, transported to the site, and carefully installed. The joints of each plate should be overlapped to form a continuous magnetic path. The shielding room constructed in this way has better shielding effectiveness against low-frequency magnetic fields, but the gap will cause high-frequency leakage. In order to make up for this shortcoming, the outer layer of the permalloy shielding room needs to be welded with aluminum plates to form a second layer of shielding, thereby shielding the high-frequency electromagnetic field.
12. What is a cut-off waveguide plate (honeycomb plate)? In what occasions should it be used? What problems should be paid attention to when using?
Answer: The cut-off waveguide board is an array board composed of many cut-off waveguides. It is used in the occasions that require high shielding efficiency and ventilation. When using it, pay attention to the use of electromagnetic sealing gaskets between the honeycomb board and the chassis for installation or welding. .
13. There is an equipment with a plastic case, and the electromagnetic radiation exceeds the standard. In order to meet the requirements of the electromagnetic compatibility standard, the developer sprayed conductive paint on the inside of the case, and the result did not improve significantly. Please analyze what might be the reason?
Answer: There may be too many holes and too large holes in the original plastic case, resulting in too serious leakage, or the gap may not be tight (maybe the contact is not tight, or there may be no conductive paint sprayed on the joint), resulting in leakage. In addition, the cables (signal cables, power cables) on the original chassis often do not have good filtering measures, and these cables may cause electromagnetic leakage of the chassis.
14. What kinds of transparent shielding windows are there? What problems should be paid attention to when using?
Answer: There are two types, one is a shielding window composed of glass sandwiched with metal mesh, and the other is a shielding window composed of a thin metal film plated on the glass. When in use, pay attention to the metal mesh or conductive coating must be in close contact with the basic conductive parts of the shielding case.
15. When using shielding glass with a metal mesh sandwich on the screen of a CRT monitor, there will be annoying streaks. How to reduce this phenomenon?
Answer: Rotate the direction of the screen to make an angle of 15-20 degrees between the weft line and the scanning line of the picture tube.
16. What are the two key characteristics of electromagnetic sealing gaskets? List as many types of electromagnetic sealing gaskets as possible, and explain the applicable occasions of various products.
Answer: The two characteristics that electromagnetic sealing gaskets must have are elasticity and conductivity. Common types of electromagnetic sealing gaskets are: finger reeds, metal mesh gaskets, conductive rubber, conductive cloth-wrapped foam rubber, spiral tubes, etc. Except for occasions with tangential sliding contact, avoid using finger reeds; when environmental sealing requirements are required, conductive rubber should be used; in other occasions, conductive cloth pads can be used, and screens can also be used when the frequency of shielding is not high. Cushion; to ensure that it will not be over-compressed, a spiral tube can be used.
17. What problems should be paid attention to when using electromagnetic sealing gaskets?
Answer: The thickness of the panel should be appropriate to prevent deformation under the action of the rebound force of the gasket and cause larger gaps. When the panel thickness is thin, the tightening screws need to use a smaller interval. Set limit structure to prevent excessive compression. Choose appropriate metal materials to reduce electrochemical corrosion.
18. A metal rod must pass through a shielded case. How to deal with it so as not to damage the shielding effectiveness of the case?
Answer: The periphery of the metal rod is reliably overlapped with the shielding base through the beryllium copper reed.
19. What is the main function of the power line filter? What parameters are mainly considered when selecting a model? What problems should be paid attention to when using power line filters?
Answer: The function of the power line filter is to suppress the conduction emission current from propagating along the power line. When selecting the type, consider the insertion loss (common mode and differential mode), rated current, voltage, effective frequency range and other parameters. Pay attention to the installation method when using it, and ensure that the RF grounding and input and output isolation are good to prevent filtered wires. Contaminated again.
20. Why is the high frequency filtering characteristic of the power line filter important?
Answer: If the high-frequency filtering characteristics are not good, it will cause the equipment’s radiation emission to exceed the standard or be sensitive to pulse interference.
21. When designing structural electromagnetic compatibility, there is a principle: The filtered power cord should be as far away as possible from various signal cables. why is that?
Answer: If the power cord is close to the signal cable, the high frequency signal on the signal cable will be coupled to the power cord (especially the filtered part), causing the conducted emission on the power cord to exceed the standard.
22. Why can’t I just pursue compact size when choosing power line filters?
Answer: The size of the filter is mainly determined by the inductor in the filter circuit. A smaller inductor must be used in a smaller filter, and the smaller inductor may also have a smaller inductance, which will cause the filter Low-frequency filtering performance is poor. In addition, the size of the filter is reduced, and it is necessary to keep the internal components close to each other, which will reduce the high-frequency performance of the filter.
23. What is the insertion loss of the filter? What method can be used to measure the insertion loss of the filter to get a very safe result?
Answer: The current and voltage loss caused by the filter being connected to the circuit is called the insertion loss of the filter. The interference filter should have as much insertion loss as possible for the signal of the interference frequency. The insertion loss of the measuring filter should be measured under the condition that the ratio of source and load impedance is 0.1:100 (or vice versa). At this time, the result under very bad conditions can be obtained, that is, the result is very safe.
24. Generally speaking, AC line filters can be used in DC applications, but DC line filters cannot be used in AC applications. Why?
Answer: The bypass capacitor used in the DC line filter is a DC capacitor. It may be damaged by overheating when used under AC conditions. If the DC capacitor has a low withstand voltage, it will be broken down and damaged. Even if these two situations do not occur, due to the large capacity of the common-mode bypass capacitor in the general DC filter, excessive leakage current will occur when used in AC applications, which violates the provisions of safety standards.
25. What is the main function of the signal line filter? What are the types in terms of installation methods? How to determine which installation method of signal line filter to use?
Answer: Reduce unnecessary high frequency components (mainly common mode) on the signal line, thereby reducing the electromagnetic radiation of the cable, or preventing the cable from being used as an antenna to receive spatial electromagnetic interference and conduct it into the chassis. There are two ways to install on the circuit board and the installation on the panel. When the frequency that needs to be filtered is low, use the structure installed on the circuit board; when the frequency that needs to be filtered is higher, use the structure installed on the panel.
26. The signal transmitted on a signal line has a very high frequency of 30MHz. The measurement shows that there is a common mode interference current of 120MHz on this wire. Using the common mode radiation formula to predict, as long as the common mode current is suppressed by 30dB, it can be satisfied According to the requirements of electromagnetic compatibility standards, how many low-pass filter circuits are needed at this time?
Answer: According to the meaning of the question, the cut-off frequency of the low-pass filter is 30MHz, and the insertion loss at 120MHz is greater than 30dB. Since the insertion loss of the N-order filter increases at a rate of 6N (dB) per octave, and 30MHz to 120MHz is two octaves, if the cut-off frequency of the N-order filter is 30MHz, the insertion loss at 120MHz is The range is 12N (dB). To make the process 12N> 30, N=3 can be used, that is, the order of the low-pass filter is at least 3.
27. Why are three-terminal capacitors more suitable for interference filtering?
Answer: The frequency of electromagnetic interference is often very high, so the high frequency characteristics of the interference filter is very important. The three-terminal capacitor cleverly uses the two lead inductances on one electrode to form a T-type low-pass filter, which eliminates the traditional capacitor. The bad influence of the inductance of the middle lead improves the high-frequency filtering characteristics, so the three-terminal capacitor is more suitable for interference filtering.
28. Why is the cross-core capacitor an ideal device for interference filtering?
Answer: The feedthrough capacitor is a three-terminal capacitor, but compared with the ordinary three-terminal capacitor, because it is directly mounted on the metal panel, its grounding inductance is smaller, and there is almost no influence of lead inductance. In addition, its input and output terminals are isolated by metal plates, eliminating high-frequency coupling. These two characteristics determine that the feedthrough capacitor has a filtering effect close to an ideal capacitor.
29. What is the difference between the magnetic core used for electromagnetic interference suppression and the traditional magnetic core used as an inductor? What happens if the two are used incorrectly?
Answer: The material traditionally used as the magnetic core of the inductor has very small loss, and the loss of the inductor made with this kind of magnetic core is very small. The magnetic core used for electromagnetic interference suppression has a large loss, and the inductor made with this kind of magnetic core has a large loss, and its characteristics are closer to resistance. If the two are used incorrectly, neither will achieve the intended purpose. If the magnetic core used for electromagnetic interference suppression is used in an ordinary inductor, the Q value of the inductor will be very low, which will make the resonant circuit fail to meet the requirements, or cause excessive loss of the signal to be transmitted. If the magnetic core used to make an ordinary inductor is used for electromagnetic interference suppression, the inductance and the parasitic capacitance in the circuit will resonate, which may increase the interference at a certain frequency.
