Electromagnetic interference phenomena inside the circuit

Electromagnetic Interference (EMI) Phenomena Inside the Circuit

1. Conducted EMI

• Power Line Interference:
  This is a common form of conducted EMI where unwanted electrical signals are transmitted through the power supply lines. For example, in a circuit powered by the mains electricity (110V or 220V AC in different regions), electrical appliances connected to the same power outlet or on the same power distribution network can introduce disturbances. High-power devices like motors or electric heaters, when turned on or off, can cause voltage spikes or surges on the power line. These spikes can then travel through the power supply lines into the circuit under consideration, potentially affecting sensitive components such as microcontrollers, integrated circuits, or analog amplifiers.
• Signal Line Interference:
  Signals traveling along the interconnecting wires within a circuit can also experience interference. For instance, in a printed circuit board (PCB), data lines carrying digital signals from one component to another can pick up unwanted noise. This can happen if the signal lines are routed close to high-current carrying traces or power lines. The electromagnetic field generated by the current in these nearby lines can couple with the signal lines through capacitive or inductive coupling mechanisms. As a result, the integrity of the digital signals can be compromised, leading to errors in data transmission, such as bit flips in digital communication protocols.

2. Radiated EMI

• Near-Field Radiation:
  In close proximity to active components within a circuit, there is near-field radiation. For example, when a high-speed switching component like a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is operating, it generates rapidly changing electric and magnetic fields in its immediate vicinity. These fields can couple with nearby components or traces, causing interference. If a sensitive analog amplifier is placed too close to such a switching device, the amplifier's input signal may be distorted by the radiated electromagnetic fields. This is especially problematic in high-frequency circuits where the dimensions of the components and their spacing can be comparable to the wavelength of the electromagnetic waves being generated.
• Far-Field Radiation:
  At larger distances from the circuit, there can still be radiated EMI in the form of far-field radiation. This occurs when the electromagnetic waves generated by the circuit components are able to propagate over significant distances. For example, in a wireless communication device like a Wi-Fi router or a Bluetooth module, the radio frequency (RF) signals it transmits can interfere with other nearby electronic circuits if proper shielding and filtering are not in place. Even in non-wireless circuits, if high-frequency signals are present and not properly contained, they can radiate out and cause problems in neighboring equipment.

3. Coupling Mechanisms

• Capacitive Coupling:
  Capacitive coupling occurs when there is an electric field interaction between two conductors separated by an insulator. In a circuit, this can happen between adjacent traces on a PCB. For example, if a trace carrying a high-frequency clock signal is routed close to a sensitive analog input trace, the electric field from the clock signal can induce a small voltage across the gap between the traces (due to the capacitance between them). This induced voltage appears as noise on the sensitive trace and can disrupt the proper functioning of the associated component.
• Inductive Coupling:
  Inductive coupling is based on the magnetic field interaction between conductors. When current flows through a wire, it generates a magnetic field around it. If another wire is in the vicinity of this magnetic field, a voltage can be induced in the second wire according to Faraday's law of electromagnetic induction. In a circuit with multiple coils or inductive components, such as in a power supply transformer or inductor-based filters, mutual inductance between these components can cause unwanted coupling of signals. For instance, if the magnetic field from a switching power supply's inductor leaks and couples with a nearby signal line, it can introduce noise into the signal being carried by that line.

4. Effects of EMI on Circuit Performance

• Malfunction of Digital Components:
  In digital circuits, EMI can lead to incorrect operation of logic gates, flip-flops, and microcontrollers. For example, a voltage spike on a power supply line due to conducted EMI might cause a microcontroller to reset unexpectedly or execute incorrect instructions. Bit errors in digital communication can also occur due to interference on the data lines, resulting in corrupted data being received or sent.
• Degradation of Analog Signals:
  For analog circuits, EMI can distort the input or output signals of amplifiers, filters, and other analog components. Noise added to an analog signal can reduce the signal-to-noise ratio, making it difficult to accurately detect or process the desired signal. In audio circuits, for example, EMI can cause audible hum or crackling sounds in the output.

5. Mitigation Strategies

• Shielding:
  Using conductive enclosures or shields around sensitive components or the entire circuit can help reduce radiated EMI. For example, in a circuit with a sensitive radio receiver, enclosing it in a metal box can block external electromagnetic fields from interfering with its operation. Similarly, shielding cables by using braided shields or foil wraps can prevent external fields from coupling with the signals carried by the cables.
• Filtering:
  Installing filters at appropriate points in the circuit can attenuate unwanted frequencies. For example, a low-pass filter can be used to block high-frequency noise from reaching sensitive components. In a power supply, capacitors and inductors are often combined to form filter circuits that smooth out voltage ripples and remove high-frequency spikes.
• Proper Layout and Routing:
  On a PCB, careful layout of components and routing of traces can minimize coupling between different parts of the circuit. Keeping high-current and high-frequency traces separated from sensitive signal lines, using ground planes to provide a stable reference and reduce electromagnetic field emissions, and minimizing loop areas in signal paths can all help in reducing EMI.