What is Electrical Noise?
Electrical noise is a phenomenon that occurs when a system is affected by stray EM fields. These fields typically originate from current and voltage variations in associated components and can be transferred via radiation or through physical contact with a conductor.
Electrical noise causes several challenges in electronics and electrical installations that can impair their function and cause damage to internal components or even entire devices. Therefore, it is necessary to minimise its effects to ensure that systems function as intended.
For noise to occur, the following three things must exist:
- A source of electromagnetic radiationâthe component or system producing EMI;
- A propagation pathwayâEMI can be propagated via radiation or physical contact;
- The receiving mediumâthe circuit that suffers the disturbance.
As electronics are becoming increasingly complex and miniaturised, the number of components integrated into PCBs are increasing. Some of these components will often emit EM energy during operation. Also, power systems that do not have noise-filtering systems may experience the harmful effects of EM noise. The most common EMI problems are radio frequency (RF) noise, noise in power lines, and electrostatic discharge (ESD).
Power line interference is associated with high voltage installations. At high voltages, power lines will emit an audible noise when the high electric field strength of the conductor is not filtered properly. This sound is due to a 'Corona discharge': a phenomenon that produces a visible glow that can be observed at close proximity to a bare conductor.
An electrostatic discharge (ESD) induces EMI when a static charge is transferred into an electrical system. It can cause damage to components or degrade a signal.
RF noise produced by electronic components, such as transmitters, LED lamps, bi-directional, and amplifiers also produce EM waves that can compromise signal integrity in radio/data transmission.
Inductive coils on a circuit board. Image courtesy of Pixabay.
Techniques for Minimising Electromagnetic Interference
We will now discuss some of the best practices for minimising EMI in electronic circuits. Engineers can consider using the following techniques in their PCB designs.
Proper Layout or Component Arrangement
How components are arranged on a PCB can affect the amount of electromagnetic energy that will be produced.
Best practices for designing PCB component layouts involve the following:
- The separation of analogue and digital circuits to prevent crosstalk during signal transmission;
- The separation of signal cables from power cables to preserve signal integrity;
- High-speed traces that are kept separate from low-speed traces in the trace layout; and
- Return paths that are kept as short as possible.
One of the most effective ways to minimise the noise in electronic circuits involves the use of EMI shielding.
A common shielding device is the Faraday cage: an enclosure made of conductive materials of a suitable thickness, which helps to block the passage of RF waves. The shielding effectiveness of the device used is proportional to the conductivity of the material. Therefore, engineers must utilise high-conductivity materials with low resistances, in order to significantly attenuate the EM radiation.
EMI Filters are devices used to attenuate noise that is generated in power systems and electronic circuits. Filtering allows low-frequency signals to pass through, while blocking the high-frequency signals that cause electrical disturbances. For example, an EMI filter that comprises an inductor and capacitor can minimise the noise generated in DC motors.
Passive components on a PCB. Image courtesy of Pixabay.
Ensuring proper grounding is essential for minimising EMI in circuit designs. In PCBs, the ground plane provides a 0-volt line to the power supply ground terminal. Typically, the larger the cross-sectional area of the ground plane, the more effectively EMI will be reduced across the board.
Engineers can employ the following grounding techniques to minimise EMI in PCBs:
- Using a multi-layer board where the ground plane is too small
- Separating the ground plane into analogue and digital sections to prevent noise coupling;
- Integrating decoupling capacitors in the ground plane to minimise the return current path; and
- Not creating too many ground connections that can radiate more EMI across the board.
Differential Pair Routing
Proper differential pair routing can help designers minimise noise and preserve signal integrity in high-speed circuits. The technique is used to design a balanced transmission system that prevents common mode noise and signal loss while improving the signal-to-noise ratio.
Some best practices for creating differential pairs involve the following:
- When routing differential pairs, the conductive traces should be kept parallel to each other;
- Differential pairs should be given adequate clearance to prevent EM induction;
- When routing differential pairs, sharp turns should be avoided as they tend to generate EMI. Where curves are necessary, they should be made smooth and deviate by no more than 45Â°; and
- Trace lengths of the differential pairs should be evenly matched or stay within permissible mismatch tolerances. Widely mismatched lengths can generate EM radiation that will negatively impact signal integrity.