Someone wise once said, “Noise makes no good; good makes no noise.” Unwanted noise is never pleasant, be it your neighbor running a power tool at 7 AM on a Sunday or the car horns while you are stuck in traffic. Similarly, uninvited noise on a printed circuit board (PCB) is not good for the final product. In electronics, noise leads to low efficiency and decreased productivity.
So, where does noise emerge in a PCB, and how can you avoid it in the PCB design phase? This post contains all the information you need to know.
(CompileIdeas / pixabay)
What Is PCB Noise?
In the electronic world, noise generally refers to an unwanted signal. In an electrical circuit, too much noise can disrupt the characteristics or measurements of a signal. This is true for circuits that operate at high frequencies. In these frequencies, electromagnetic traces built around the PCB traces and components interfere with the signals running along the other traces inside the same PCB. The resultant effect produced by these signals leads to poor circuit performance and signal integrity.
While you can’t eliminate noise, there are ways to reduce its level significantly. The first step to deal with any noise in the PCB is first to identify its type.
Causes of Noise in PCB
In a PCB, noise commonly occurs due to current spikes on electrical signals. Noise can come from anywhere: air, power supply, resistor, switch regulator, etc. Here’s a closer look at different types of PCB noise.
Johnson Noise
Designers will always have Johnson Noise (also known as thermal noise) unless they start designing PCBs with superconductors. That is because every component contains at least a little resistance. Thermal noise is related to temperature, bandwidth, and resistance. The higher the temperature and resistance, the higher will be the noise in a PCB. If there are more frequencies in your analysis, there will be more thermal noise.
Shot Noise
When electrons go through a conductor, they bump along and cause energy accumulation which turns into kinetic energy every time the electron crosses a barrier. These random variations in movement between electrons lead to variations in current. This is what’s referred to as shot noise, and it’s more prominent in semiconductors because they have more barriers. High currents will face more shot noise.
Flicker Noise
Scientists are still studying this type of noise, but it is the noise created by almost every electronic component and decreases in amplitude as the frequency rises. This relationship between amplitude and frequency is a defining characteristic of flicker noise.
Popcorn Noise
This noise only occurs in semiconductors, and semiconductors are housed in every electronic these days. The defective material of a semiconductor leads to abrupt voltage and transition in current. If you amplify a contaminated signal and put it on speaker, you’ll hear the popcorn noise. If the semiconductor material is improved, the noise will reduce.
PCB Design Considerations for Noise Reduction
To reduce noise in a circuit board, you need to consider all the aspects of PCB design. Here is how you can get started:
Layer Stackup:
Layer stackup in a PCB plays a huge role in maintaining signal integrity and reducing the potential for noise buildup. Designers should stack the layers in microstrip or stripline. These are arrangements that put layers with high-speed signals either next to or between two planes. Layer arrangement can control plenty of EMI generated in the PCB and protect external EMI from other sources.
Placement of Components:
In the design phase, how you place the board components has a critical impact on noise levels. For example, placing the power components together on the same layer can help reduce inductance from nearby traces. Make sure to place high-speed components in a way that their traces are routed along a path that is as short as possible. For bypass capacitors, place them together with each power component. This helps reduce the current spike during signal switching.
Trace Routing:
The shorter and thicker your traces, the better it is for the PCB design. It leads to low inductance in the traces and reduces noise. Signal traces should also be short except for lines that require specific lengths. It’s best to place controlled impedance traces with specific width calculated with the help of an impedance calculator. When there are two signal layers, the routing should be horizontal on one and vertical on the other to reduce the chances of ‘crosstalk’ between these neighboring layers.
Ground Planes:
In PCB design, there must be a clear return path for ground planes to manage sensitive signals that avoid plane splits. In working with onboard power supplies, you can use a ground plane separate from the main ground plane. By separating and isolating the power supply ground, you’ll save the rest of the PCB from power supply noise. Also, connect each ground pin to its plane separately and not chain them together, as it’ll help the return path.
Angle:
Do not use 90-degree turns on PCB traces. If you must use a 90-degree turn, use two turns of 45° each. Keep the traces away from oscillators, and do not run a trace under an oscillator.
Circuitry:
Place the analog and digital circuitry separately on a PCB. Digital circuitry gives birth to digital noise, creating errors on the digital and analog sides of the circuits that are not suitably separated.
Using PCB Design Tools
The rule of thumb here is first to follow all the rules, constraints, and setup instructions available in your CAD program. The constraints will help you set trace width, trace length, and rules for spacing components. This program also has layer stack-up generators, impedance calculators, and previews to help you set up the PCB layers correctly before you finalize anything. If you’re doing design collaborations to reduce PCB noise, CAD programs should still come in handy.
Advanced CAD programs also include placement features and help you align each component in an organized manner. You can also route differential pairs, which is a great help in designing a printed circuit board. In short, there are design tools out there that take care of the nitty-gritty for you with the help of machine learning and calculations. As you follow the instructions in these programs, you will design a PCB that does not create much noise when used.
The Takeaway…
Noise is a constant pitfall in any electronic device, so it must be managed and not eliminated. By following the techniques we’ve discussed, you’ll be able to design a PCB that creates the least noise and doesn’t interfere with system performance. Need help with PCB assembly? Contact our experienced team at EMS Solutions.