As electronics professionals or those in related fields, a term commonly heard is EMI. It is generally believed that its presence can lead to issues such as signal quality degradation, reduced device performance, or failure of communication systems. Therefore, in circuit design, considerable effort is put into brainstorming ways to minimize or prevent EMI. So, what is EMI?
Definition of EMI
The ideal operating conditions for a device involve stable operation within its own electromagnetic environment without being affected by external interference. When these two conditions are met, the device is considered to have Electromagnetic Compatibility (EMC). Derived concepts of EMC include Electromagnetic Sensitivity (EMS) and Electromagnetic Interference (EMI).
EMI refers to the interference caused by electromagnetic radiation emitted by one electronic device on the functionality of another nearby device. This means that a device can act as both the source and the recipient of interference?
Yes! Here’s an example:
In high-speed circuit design, various components, wires, and signal lines of the circuit may act as antennas, generating electromagnetic radiation. This radiation can lead to electromagnetic interference, affecting the normal operation of the internal system or other devices.
Therefore, EMI is a common phenomenon in the field of electronics, and almost all electronic devices have the potential to generate or be affected by EMI interference. This interference can manifest in various forms, ranging from minor interruptions in audio or visual signals to severe malfunctions in sensitive electronic devices.
Firstly, there is electrical noise, which occurs due to reasons such as voltage fluctuations or harmonic distortion, leading to unexpected and unwanted electrical signals or voltages in circuits or electronic systems. It typically appears in a scattered form and is eventually radiated in the form of electromagnetic waves, affecting surrounding electronics.
Secondly, during circuit design, issues related to grounding or connections may prevent the efficient flow of current back to the ground, increasing the possibility of electromagnetic radiation. If the device lacks sufficient shielding design, with no enclosure, shielding cover, or electromagnetic protection, electromagnetic radiation may escape into the surrounding environment.
Additionally, according to Ampere’s Law and Faraday’s Electromagnetic Induction Law, changing currents cause changes in the magnetic field in the surrounding space. This changing magnetic field induces changes in the electric field, forming an electromagnetic field. In other words, in devices such as power supplies, motors, relays, etc., the rapid switching of current can lead to instantaneous changes in current, ultimately propagating EMI through electromagnetic radiation, sometimes manifesting as radiofrequency signals.
Finally, in the modern societal environment, due to the proliferation of wireless technologies and communication devices around us, RF signals emitted by external sources such as broadcasting stations, mobile phones, or wireless networks may interfere with susceptible electronic devices.
EMI Coupling Paths
Based on the propagation method, EMI coupling paths can be classified into two types: conducted coupling and radiated coupling.
Conducted coupling refers to the way interference noise propagates along a conductive medium (wires, cables, PCB) from one circuit to another. This coupling method requires direct contact to occur.
Radiated coupling involves the emission of electromagnetic waves into space, and then coupling with another circuit. It occurs without the need for direct contact between two objects. For instance, high-speed signal lines, power lines, and other conductors may radiate an electromagnetic field into space, which can be captured by nearby conductors or circuits.
EMI Sensitive Devices
Devices prone to electromagnetic interference are referred to as sensitive devices, whether individual components or entire devices. Typically, the following types of devices are considered more susceptible to interference:
Low-voltage devices: These devices usually generate or process weaker electrical signals, making them more susceptible to the influence of external electromagnetic fields. Additionally, they are more likely to be submerged in the electromagnetic noise present in the environment. Low-voltage signals also require longer transmission distances, and longer conductors may act as antennas, introducing more electromagnetic radiation or capturing external noise.
Communication equipment: Typically operating at higher frequencies, requiring antennas for signal transmission and reception. Moreover, due to the need to accurately receive and decode signals in a complex electromagnetic environment, communication devices demand a high signal-to-noise ratio.
Other EMI-sensitive devices include small signal devices, precision instruments, medical equipment, etc.
Measures to Prevent EMI Interference
Regardless of whether it’s electromagnetic interference generated by the device itself or interference from external sources, designers need to take necessary measures to prevent it, ensuring the stability and reliability of the system.
Firstly, there is the option of metal shielding protection. Based on the electromagnetic shielding effect, when electromagnetic waves propagate, a metal enclosure can provide a shielding layer that impedes the propagation of electromagnetic waves, thereby reducing or preventing the impact of external electromagnetic fields on internal devices. This protective measure not only avoids interference from external radiation sources but also prevents the overflow of internal radiation, providing comprehensive shielding protection.
Secondly, during the component layout phase, considerations can be given to elements such as filters, ferrite beads, and suppressors. Taking filters as an example, they are designed to block electromagnetic waves within specific frequency ranges. Thus, when the equipment is interfered with from a particular frequency band, the filter can selectively weaken or block the electromagnetic waves from this band without affecting the transmission of signals in other frequency bands.
Although the above two measures can effectively prevent EMI interference, they inevitably lead to increased costs. To provide good interference prevention effects for your project at an affordable cost, it’s essential to optimize the design. For example:
- Ensure the existence of a low-impedance, low-inductance ground wire path to avoid forming ground loops and eliminate unnecessary currents.
- Implement appropriate impedance matching and terminal matching to reduce signal line reflections and ensure signal transmission quality.
- Shorten the length of signal paths, keep sensitive signal lines and high-frequency signal lines away from potential noise sources to reduce the likelihood of interference.
- Segment the ground plane and minimize return path inductance to enhance the system’s anti-interference capability.
- Use differential signal transmission to suppress common-mode noise and enhance the system’s anti-interference ability.
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