Electromagnetic Compatible System Requirements

Electromagnetic compatibility (EMC) ensures devices operate without causing or experiencing interference. This guide outlines critical system requirements for achieving EMC compliance, focusing on shielding, grounding, and filtering strategies.

We will explore electromagnetic compatible system (EMC) requirements and discover design aspects of electromagnetic compatible systems. Think about when you connect a DC motor or mixer to the AC mains while watching TV – it often creates noise on the TV screen. Similarly, lightning can cause noise in the TV receiver. The DC motor, lightning, and fluorescent lights are all sources of electromagnetic interference.

There are two types of electromagnetic emissions: radiated and conducted.

• Radiated emission refers to undesired EM wave radiations that can cause nearby systems to malfunction.
• Conducted emission refers to undesired radiations through electric power lines, potentially disturbing other systems connected to the same power line.

Electromagnetic Interference (EMI) is the term used to describe the interference caused to one system by undesired EM radiations from another system.

Electromagnetic compatible (EMC) refers to the ability of one system not to radiate undesired EM waves, allowing other systems to function as intended without any problems. In other words, an electromagnetic system is considered electromagnetically compatible with its environment if it can function compatibly with other electronic systems and does not produce or receive unintentional radiation.

The essential requirements to make a system electromagnetic compatible are as follows:

• The system itself will not radiate any unintentional radiation (i.e., it will not cause interference to other systems).
• The system will not receive any unintentional radiation (i.e., it should not be susceptible to the emissions of other systems).
• The coupling paths between the systems will be as inefficient as possible.
• Any component or module of the system will not produce interference within the same system itself.

The same effect can be observed when oscillators are not properly shielded, or DC supply lines to RF circuits are not properly designed. Due to the increase in interference in digital systems, the FCC in the US has specified limits for electromagnetic emissions. If a system fulfills these limits, it is said to be EMC compliant.

There are two classes of devices based on their applications:

• Class A: For business use
• Class B: For residential use

The FCC has defined conducted emission and radiated emission limits at different frequency ranges for both Class A and Class B devices. If these undesired radiations are not minimized, they can couple with other nearby circuits and create various problems such as crosstalk, propagation delay, parasitic effects, and false triggering.

Therefore, when designing PCBs based on microstrip as well as other RF structures, the following needs to be considered.

Electromagnetic Compatible System Design

The following are important aspects to consider when designing an electromagnetic compatible system:

• Grounding problem: Conductors used for grounding have a certain amount of impedance. Any current passing through an imperfect ground conductor will result in different potentials at different points on the ground conductor. A ground grid structure is used to minimize this problem. The grid must be connected at all points using via holes to make it more effective.

• Emission Models: The unintentional antennas in a system are the wires, PCB lands, and other metallic structures. Depending on the current flow through two close wires or lands, there are two modes: common mode and differential mode.

  Differential mode emissions typically have less radiated field compared to common mode emissions, assuming the same frequency and line length. However, differential mode emission field depends on frequency and increases with frequency. Differential mode emissions can be minimized by reducing the current level and loop area. Common mode emission level is minimized by reducing line length and current level.

• Clock pulse implementation: In mixed circuit designs consisting of both digital and analog parts, clock pulse implementation is very important. Clock signals are periodic and deterministic signals. A pure square wave has many harmonics of the sine wave, consisting of high-frequency spectral components. By changing the shape of the square wave to a trapezoidal pulse, these high-frequency emissions can be greatly reduced. This is achieved by introducing rise and fall times to the square wave pulse.

• Crosstalk reduction: This can be done by providing shielding ground lines adjacent to the active interconnections.

• Substrate material selection: A substrate of MMIC (Monolithic Microwave Integrated Circuit) is a piece of material on which the RF circuit layout is etched. Different RF components are then soldered onto this layout. The ideal substrate material should have a high dielectric constant, low dissipation factor, and no variation of DC voltage over the entire frequency range of operation.

• Conductor material selection: The ideal conductor material should have high conductivity, a low-temperature coefficient, and good adhesion with the substrate.

• Impedance mismatch: Transmission lines should be perfectly terminated with the proper characteristic impedance. If not, it can result in reflections and damped oscillations.

• Power supply: Variations and transients should be minimized at the supply source using a ferrite bead and/or capacitor.

Conclusion

Meeting EMC requirements is crucial for device reliability and regulatory compliance. By following the guidelines above, you can create systems that are resilient to electromagnetic interference.