The field of electromagnetic interference (EMI) was first quietly officially recognized in 1933 under the leadership of a subcommittee of the International Electrotechnical Commission (IEC) in Paris. The subcommittee was established in the name of CISPR (Special Committee on International Radio Interference) to better understand the long-term complications that may be caused by radio frequency technology. Since it was founded by Morse, Henry and Vail in 1820, the popularity of radio has exploded and become a must-have household appliance during the Great Depression. It was soon determined that intentional and unintentional RF transmissions began to affect other electrical systems, leading to an ever-increasing awareness of EMI in the electronics community. In 1934, CISPR began to create and distribute requirements for the recommended allowable emission and immunity limits of electronic equipment. These requirements evolved into most of the current EMC regulations in the world.
Throughout the 1960s, 1970s, and 1980s, researchers became increasingly concerned about the increase in electromagnetic radiation interference. In 1967, the US military issued Military Standard 461A, which established the testing and verification requirements for electronic devices that have been used in military applications, as well as emission and sensitivity limits for new military electronic devices.
In 1979, the U.S. Federal Communications Commission (FCC) imposed legal restrictions on the electromagnetic radiation of all digital devices in the United States. As systems become faster, smaller, and more powerful, as emerging technologies show a tendency to interfere with other electrical systems. run.
In order to better understand how noise is generated, avionics and aerospace engineers have studied EMI-related issues and determined a new system design method to minimize transmission noise while also being able to withstand external sources A certain amount of noise.
Initially, most companies opted for fast, awkward shielded enclosure designs, which were nothing more than the least effective Faraday cages. More savvy researchers, from companies seeking better long-term solutions to eliminate the sensitivity of their sensitive electronic equipment to EMI, tend to adopt more professional and focused methods, combined with better electronic design and layout, and at the same time Add additional shielding and filtering elements when needed.
The creation of multiple certification levels helps ensure the compatibility of electrical systems with respect to radiated and conducted emissions and susceptibility. The introduction of these standards allows professionals to easily identify electrical systems that can be integrated into their own components without worrying about EMI issues. Today, as these stricter regulations continue to be integrated into the expanding electronic field, all types of equipment, especially highly sensitive reconnaissance, medical and avionics equipment, are safer and will not be catastrophic due to "noisy" EMI Risk of failure. .
Although EMC regulations are very helpful in simplifying the creative process and ensuring overall design safety, they will not automatically repair systems created without pre-consideration of external radio frequency fluctuations. Many complex systems invented today are still affected by EMI due to the generation or reception of interference that can cause malfunctions. These problems are usually solved by a combination of shielding the system from electromagnetic fluctuations and filtering to reduce unwanted and potentially harmful energy.
Shielding is a technique used to control EMI by blocking the transmission of radio frequency (RF) noise from the source, similar to the way lead and concrete are used for nuclear radiation shielding. Shielding can be deployed at the RF source, receiver, or anywhere in between in the circuit. In the case of a successfully shielded electric field, the effectiveness is a function of the thickness, conductivity, and continuity of the shielding material. Adding shielding to the cable can protect the inner conductor from the influence of the electric field, thereby reducing the interaction force of the magnetic field.
It is important to remember that when an electric field interacts with a conductor, it generates a voltage, which appears as electrical "noise" on the circuit. When a properly grounded shield is placed around a conductor, the electric field energy is usually discharged without affecting it, thereby reducing or eliminating noise. This is the basic principle behind driving general electric shields (called Faraday cages).
When electromagnetic waves passing through space encounter shielding, the following situations will occur:
Most high-frequency shielding problems are not caused by the material's inability to eliminate noise, but by physical gaps and openings in the shielding material. Although shielding is the main option for many EMI-related issues, filter options can solve problems related to penetration of shielded enclosures and the input and output of electrical systems, which are usually the most vulnerable point in a shielded system.
No different from the hull, any hole in the shielding system can easily lead to catastrophic failure. Since input and output are the weakest points associated with shielded enclosures, these are the most effective locations for filtering due to the inherent fragility.
In these locations, filtering and transient suppression at the interface of the shielded enclosure are the most effective ways to protect the system from compatibility issues. By using filters and filter connectors at the input/output interface of the system, the design team can eliminate EM and RF noise from internal or external sources at the connector interface, and shunt unwanted energy to the grounded shielded enclosure. Therefore, this is the best place to eliminate high frequency noise and alleviate any EMI related issues.
Figure 1: Various filter types designed to eliminate EMI in the system
When considering filters and filter connectors to help eliminate noise problems in the system, determining the best EMI solution is based on the mechanical configuration of the system. There are multiple filtering options, and these filters have many different configurations, as shown in Figure 1.
Types of low-pass filtering options
The filter solution shown has a variety of configurations designed to mitigate EMI issues based on specific system requirements. Each of these products belongs to a subset of electrical engineering, specifically, they are "low pass" filters. In other words, these filters allow low-frequency data to pass, while blocking high-frequency signals.
There are many ways to implement low-pass filters, as shown below:
The C filter is the simplest and most direct solution. They consist of a decoupling capacitor from the pin or signal line to ground.
Figure 1a: Schematic diagram of C filter
CL or LC filters provide additional filtering because they consist of a single grounded capacitor and are coupled with an inductor or "choke." This improves the filtering efficiency of the decoupling capacitor and the filtering effect related to the impedance of the inductor.
When the source impedance is less than the load impedance, the LC filter circuit is most effective. When the load impedance is less than the source impedance, the CL filter is most suitable for use in the circuit.
Figure 1b: Schematic diagram of CL or LC filter
The Pi filter consists of two decoupling capacitors separated by an inductor, which can effectively capture the target noise and provide excellent high-frequency filtering performance.
Figure 1c: Schematic diagram of Pi filter
It should be noted that grounding is the most important aspect of any good electrical system. Filters and filter connectors need good grounding conductivity to work properly. These designs assume that the enclosure/partition in which they are installed is "well grounded" and can provide a low impedance connection path to the system ground. 1
The filter must be selected based on the noise frequency in the system that is directly related to the target frequency of the data signal transmitted through the interface. The capacitor of the filter must be chosen so that it does not interfere with or "clipping" the edges of the data signal. In addition, the selected filter type must conform to the frequency spectrum of the noise being processed. In other words, does the filter only need to eliminate a narrow range in the frequency band where noise exists, or does it need to be more like a wideband filter, blocking a wide frequency (for example: GHz) range?
Choosing the filter capacitance depends on the calculated filter's -3dB cutoff frequency, which represents the frequency at which the filter response amplitude is 3dB lower than the passband level.
The following is the formula used to find the cutoff frequency of the system:
In this formula, fc is the cutoff frequency of the filter, as shown in the performance and frequency diagram in Figure 2, and the R (resistance) and C (capacitance) of the equivalent circuit are shown in Figure 3.
Figure 2: Description of different filter types and their corresponding effective ranges
Figure 3: RC circuit with corresponding power supply frequency diagram
As the systems of electrical engineers continue to compound and evolve, this information may be difficult to determine, let alone other experts working in related fields. Fortunately, most major filter manufacturers provide insertion loss graphs that clearly show the -3dB cutoff frequency associated with the various filters they provide. This data is usually presented in tabular form and standardized for a standard 50 ohm load. Therefore, by viewing the published insertion loss or filter performance data of different capacitors at different frequencies, it is easier to select a filter.
It is important to note that the performance of chip capacitor filters is not perfect. Due to the self-resonance of chip capacitors, they are more like a "notch" filter than a high-pass filter. Although this behavior is negligible in some applications, these notches of filtering capabilities are crucial, and they must be understood to effectively reduce high-frequency noise in electrical systems. Compared with the predicted filter performance of "ideal" capacitors, these chip capacitor filters have limited high-frequency performance, as shown in Figure 4.
Figure 4: Comparison of the insertion loss diagram of an ideal capacitor and a 1,000pF capacitor
In addition, the mechanical packaging and circuit layout of the filter using chip capacitors greatly affect the performance of the filter. As shown in Figure 5, the equivalent series inductance directly affects the filter performance. When connecting these devices to the signal and ground, the design team should pay special attention to the layout and conductivity of the traces used to minimize this effect, so that the chip capacitors can achieve the best filtering performance.
Figure 5: Expanded view of capacitor showing internal residual inductance
These low-pass filtering devices come in many physical forms, but also have different performance levels. The choice of these devices depends on the frequency or frequencies of the noise problem and the severity of the problem.
Figure 2 also shows the various filter types and the relative filter performance of simple chip capacitor filters. It can be clearly seen from these data that the role of chip capacitors is similar to the aforementioned notch filter, while C, CL, LC and Pi filters are composed of planar arrays, disk capacitors or ceramic tubes, which can provide better Broadband and higher overall filter performance level.
With a basic understanding of the field and its related components, the next step is to study some real-time applications and analyze the filter selection process from the perspective of an EMI expert.
In Figure 6, please note the noise measured in the system and the allowable limit shown by the solid red line.
Figure 6: Example of measurement noise associated with a system without EMI filters
Think of the curve in Figure 7 as the noise output of a device that was not measured with an EMI filter, which resulted in costly production shutdowns. It should be noted that the frequency between 30MHz and 70MHz is beyond the allowable range, which means that the design cannot be brought to the market regardless of whether the actual product is successful or not. Even if the device has a shielded enclosure, this area requires extremely expensive redesign or additional filtering to reduce this noise to an acceptable level.
Figure 7: Performance graph provided by the manufacturer
The largest interruption of this device occurred at 39MHz, as shown in Figure 6 above, with a green hash mark. Based on this observation, it is necessary to choose a filter connector that will allow low-frequency data (in this case, below 1.0MHz) to be transmitted without being affected or degraded, while filtering out high-frequency noise in the system .
With this information, coupled with looking at the filter manufacturer's performance data shown in Figure 7, it can be determined that filtering in the 10,000pF to 30,000pF range is most likely to be the best EMI solution for this application.
Next, it is recommended that the design team determine whether it is suitable to block the relevant frequency according to the low cost and high reliability of the filter system of the chip capacitor filter type. With this in mind, the test was conducted using two general system solutions, namely 10,000pF capacitor filter plug-in and 22,000pF capacitor filter plug-in.
Figure 8 shows the result of testing the 10,000pF chip cap insert. As a result, it was found that the device was acceptable at higher frequencies, but did not provide sufficient filtering at lower frequencies (between 10MHz and 60MHz) to reduce noise below the allowable test limit.
Figure 8: The system in Figure 6 has 10,000pF capacitor filtering
Then use the 22,000pF chip cap filter plug-in to test, try to adjust the maximum filter performance to the lower frequency area where noise persists. As shown in Figure 9, the performance is improved, with only small spikes at or near the noise limit at lower frequencies. But this filter causes the energy to "expand" to higher frequencies around 105MHz, exceeding the allowable limit.
Figure 9: The system in Figure 7, with universal 22,000pF capacitor filtering
These test results indicate that the universal chip capacitor filter plug-in cannot provide adequate filtering. In order to permanently solve this problem, a more professional method using additional filtering is needed.
In order to achieve the expected results, the customer tested a special filter plug-in, which was designed to minimize the equivalent series inductance. This effect is achieved by mounting chip capacitors on isolation channels that separate the pad contact area from the signal pins and solid ground plane.
This filter plug-in design provides maximum shielding efficiency and minimizes equivalent series inductance, provides the best performance of chip capacitors, and also utilizes the existing shielding in the system. Combined with a dedicated 22,000pF filter, the filter element achieves the results shown in Figure 10, thereby meeting EMC regulations with a cost-effective long-term solution.
After considering the results of these tests, a broadband filter (Figure 10) was used to complete the device design because there is not much margin in the higher frequencies, where chip capacitors only provide minimal filtering. This special 22,000pF disc filter connector (a C filter constructed with disc capacitors) provides a higher level of filter performance and the wider frequency performance required.
Figure 10: The system of Figure 7 when using a custom filter
Compared with other filters tested (including filters with the same capacitance), this method improves overall EMI by using shielding that already exists in the design instead of trying to filter the system as a "magic bullet" solution performance. It complements the successful parts of the system and enhances the overall performance of the entire system, rather than treating the system as a separate entity.
All in all, solving the EMI noise problem is more like doctors contacting patients than most forms of remedial engineering solution testing. The first steps include gathering information about the obvious "symptoms" of the problem. In the evaluation phase, using a spectrum analyzer with a near-field probe to evaluate the inside and outside of the system can help identify the source of noise, just like a doctor takes an X-ray of an injury. Once the problem has been well evaluated or diagnosed, simple remedial measures can be applied first, and then more complex solutions can be evaluated.
The real-time examples illustrated here are related to the core theoretical issues related to grounding. Most EMI problems come from improperly grounded electrical connections, becoming accidental EMI transmitters or receivers, or becoming points where EMI may leak into or from the system. To resolve such issues, first try to resolve any grounding issues. Next, try to patch the system with shielding at the board or housing level to control the transmission of noise in the area found during the sniffing test in the system.
If the problem persists, start looking at the filtering solution, as described in this article. And don't forget to seek the advice of qualified EMI consultants, they can help you determine a reliable EMI filtering solution.
This article does not specifically describe shielding and grounding. We personally recommend "Although blocking is indeed effective, but filtering is the best" on www.4emi.com as a more in-depth overview.
When it enters the PCB, why use a filter connector instead of placing a filter directly on the pin? The historical problem is that the filter connector is expensive and the delivery time is too long. I understand the retransmission problem here, but it seems a bit overkill in practice.
We often see EMI/EMC requirements imposed on units as if they are independent and no physical system is wrapped around them. This seems too much.
I would like to know my opinion on this.
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