Detecting the possibility and sources of electromagnetic interference is necessary for the normal functioning of any electronic and electrical device. This needs to be done from the board level to external sources within or outside a network. Various devices such as spectrum analyzers display levels of RF signals at a given site. The device you use depends on the application and the accuracy required. Once the EMI sources and levels are detected, it is equally important to select the right method to mitigate or suppress this EMI or RFI. There are several methods for controlling EMI, such as EMI cans, shields, and more. This post offers some guidelines on EMI detection challenges, detecting electromagnetic interference accurately, and mitigating the same, especially at the board level.
Common EMI Detection Challenges
While it is crucial to choose the right method and device to detect EMI, detecting certain types of EMI and their exact source can be challenging. Here are some common challenges when detecting EMI.
- High ambient noise: This factor makes it difficult to detect and analyze EMI. EMI generates a buzzing sound that causes disturbance, which further adds to the noise level if the ambient noise of the system or device is naturally high. This may be the case with certain types of transformers, meters, and more, depending on the application system. In such cases, one needs to reduce the system noise as an initial step.
- Identifying hidden or intermittent EMI sources: In complex networks and integrated systems, there are too many connected devices and components, some of which may produce EMI. At times, some components mounted on PCBs produce EMI, which may be challenging to detect. This makes it difficult to identify exact or hidden sources. Use a combination of probes such as near-field probes, current measuring devices, and nearby antennas to identify sources, measure high-frequency cable currents, and identify exactly which signals radiate electromagnetic emissions, respectively.
- Differentiating between internal and external EMI: Here, one needs to identify if the emissions are generated within the device, at the board level, or from external nearby elements. The type of EMI, such as conducted or radiated, also needs to be considered. Conducted EMI requires physical connectivity, while radiated is indirect and may travel via air. Radiated EMI is likely to be from external yet nearby electronic devices.
Tools and Techniques for Detecting EMI
There can be many causes of electromagnetic interference and sources such as antennas, I/O cables, and board level components among many others. Here are some devices and techniques that help you measure the levels of EMI coming from a particular source.
- Spectrum analyzers:Spectrum analyzers are devices that measure waves in a given frequency range, and display the level of RF signals on the screen. It shows different frequencies within a signal in spikes or bands plotted on a graph. Each model is designed for specified frequency ranges. It has a range of band filters that allow the specified frequency band. This output of each band is passed to the detector outputs connected to the switch, which facilitates the outputs to the CRO deflection plate. CRO displays the signal frequency on the screen.
- Time-domain reflectometers: TDR uses reflected waveforms to analyze and measure impedance in circuit boards. It checks electrical or wiring issues, discontinuities in connectors or the circuit flow path. Here, we need to know the speed of the signal passing through the traces. This velocity factor calculates the time taken by the reflected signals to cross the entire distance. While you may set your frequency range, there are limitations in terms of resolution, wherein some small discontinuities may be seen as a single form. This may lead to inaccurate reading.
- EMI receivers: These are devices that offer accurate output in terms of measuring electromagnetic emissions. These are used to check electromagnetic compatibility of an electronic product or EMC and if the device adheres to the test standards. The electromagnetic disturbances are measured through either stepped scans or FFT-based time-domain scans. These devices combine the features of signal and spectrum analyzers, and can analyze signal spectrums in real-time.
- Near-field probes: These are antenna-like electric devices that capture electromagnetic emissions within small ranges. They do not have precision and may miss detecting very small signals. These probes are of varied types in terms of the shape and size of their tips, and the measuring ability of these devices depends largely on this one factor.
- Oscilloscopes: These devices measure EMI on power lines and high-voltage signals. Here signals are converted into analog through amplification, then again converted to digital signals by ADU chips and sent for storage and further processing. Once the processing is complete, the signal is converted back into analog by DAC chip. These devices measure signal amplitude or voltages vs. time domain, and the waveforms are displayed on the screen in real-time after the processing by the DAC chip.
Mitigation Strategies After EMI Detection
Filtering, shielding, and grounding are widely adopted to mitigate or suppress EMI using different tools such as EMI filters, cans, enclosures, shields, and more. Here are some effective EMI mitigation methods.
- Enclosures: Often, devices come with metallic or plastic housings or enclosures that help protect the exchange of internal and external signals. However, if the EMI is internal or at the board level, this may not help unless the board is shielded.
- Cable shielding: Highly flexible copper braided shielding with tin plating is widely used to mitigate EMI from cables. Based on your requirements, you need to select the right size and diameter. Conductive metals work well when blocking EMI signals.
- EMI filters: EMI filters are widely used to protect sensitive electronic components. They can be designed to be mounted at the point of entry on panels and various surfaces, and are either active or passive.
- Grounding: Grounding of any circuit or system is essential for unwanted EMI, shorting, transient errors, and more. It offers a low impedance for circuits, especially in high-frequency applications, and hence controls radiated emissions and noise currents in the circuit. In fact, the shields also must be grounded, or else they may work like an antenna attracting external signals. While grounding in terms of earthing is necessary to ensure safety, controlling EMI can be done on any plane or surface.
- Board-level shields: Internal board-level EMI can be best suppressed by using PCB EMI shields. These are super flexible, thin shields made of plasticized metal that conform to the shape and size of the board. They can cover the boards fully or in parts, protecting sensitive components, wiring, vias, and more. These shields not only block the EMI produced by mounted components but also from external sources to the circuit board.
After detecting EMI sources, its mitigation is crucial for any electronic or electromechanical equipment to function normally, whether for a device or at the printed circuit board level. Effective shielding is necessary at the PCB level because of the complexities that arise due to densely mounted components and complex designs, and small form factors. While most devices nowadays are designed to be EMC compatible, EMI suppression or filtration is an essential step. The best way to shield board level EMI is by using plasticized EMI shields with one conductive side. XGR Technologies offers SnapShot™ EMI shields made from a thin polyetherimide film on the interior and conductive tin on the exterior surfaces. These are flexible and lightweight shields that help reduce electromagnetic inference. If you need further information on these EMI shields, you can contact the team at XGR Technologies via phone or email.
