What are some of the potential noise problems on a printed circuit board?
Some PCB layout tips that can help reduce noise in your design.
A look at some PCB design CAD tool features that can help.
The last thing that you want is a circuit board that makes noise
It is early morning and my backyard neighbor has decided to continue the construction project that he started yesterday. I’m not sure exactly what it is that he is doing, but it sounds like it involves a high-powered tool ratcheting something down on a sheet of plywood-–at 7:00 in the morning. Even with the doors and windows closed, I can easily track his progress, and I really don’t appreciate the noise.
Uninvited or unplanned noise is usually not a pleasant experience, especially on a printed circuit board. The difference is that while a noise problem in electronics won’t wake us up early in the morning as my neighbor did, it can bring the performance of a PCB to its knees. Here are some ideas on PCB design for noise reduction that can help you turn down the volume on your next design project.
Potential Noise Problems on a Circuit Board
With all the different circuits in operation on a PCB, there are many opportunities for undesirable noise to be created. Let’s look at some of the more common noise problems that can occur.
Ground bounce: As the switching speeds of high-speed digital circuits continue to increase, there is less time for the signals to return back to their reference ground level. This can cause the signal to “bounce” above that ground level resulting in higher than expected amounts of current and create noise in the output signal. With a lot of switching happening at the same time, the amount of noise from the ground bounce can create false or double switching. This effect can cause failures in the normal operations of the circuits.
Crosstalk: With multiple circuits operating in a PCB, the more active circuits can influence the less active circuits if their traces are running side by side. This noisy unintentional electromagnetic coupling between the two is known as crosstalk. It can happen with traces that are too close to each other horizontally (side by side) or vertically on another layer.
EMI: Electromagnetic interference can come from many sources, with one of the chief problems being poorly designed return signal paths. Ideally, the signal layers of PCB will be sandwiched between power and ground planes allowing for a clear return path on the ground plane. If the plane is blocked by holes or split planes then the signal return will wander around trying to find its way back creating noisy interference as it goes.
Power supply: To work effectively, the onboard power supplies need to be laid out with the shortest paths possible for the current. Without care in how the parts are placed, the power supply could create a ringing signal that will add to the noise level of the board.
These are some of the most prevalent sources for noise in a printed circuit board. Next, we’ll take a look at some ways to combat this noise in the layout of the board.
The correct layout of an onboard power supply in a PCB design can help reduce noise
PCB Design for Noise Reduction: Place and Route Considerations
To reduce the possibility of noise being generated in a circuit board, you need to look at all aspects of the board design:
Layer stackup: The configuration of the layer stackup in a circuit board plays a big part in its signal integrity as well as reducing its potential for generating noise. The layers should be configured in a microstrip, or stripline, arrangement that places layers of sensitive high-speed signal traces next to or between two planes. How these layers are arranged can end up controlling a lot of the EMI that is generated by the board as well as protecting the circuitry from EMI coming in from external sources.
Component placement: How the components are placed on the board will also play a huge role in reducing noise. Power components should be placed closely together and on the same layer to reduce inductance that can be created in the traces and vias. High-speed components should be placed so that their traces can be routed as short as possible. Bypass capacitors need to be plentiful and placed as closely as possible to each power pin of active components. This will help to reduce the current spikes during signal switching to control ground bounce.
Trace routing: In general, the shorter and fatter you can make your power traces, the better. This keeps their inductance low and helps to control the noise. For signal traces you also need to keep those as short as possible, except for measured lines that require specific trace lengths. Differential pairs need to be routed with their traces together and with sufficient clearance to any other routing. Controlled impedance traces need to be routed at specific widths determined by an impedance calculator. Lastly, make sure that when there are two signal layers next to each other in the stackup that the routing is horizontal on one and vertical on the other. This will lessen any chance of broadside coupling (crosstalk) between the traces on the two layers.
Ground planes: Make sure that you design your ground planes so that there is a clear return path for sensitive high-speed signals that avoid plane splits, slots, or an abundance of holes. You will also want to connect each ground pin to the plane separately instead of daisy-chaining them together, which will help the return path. When working with onboard power supplies, it is best to use a ground plane that is isolated from the main ground plane of the board. These planes can be tied together at a single point, but by isolating the power supply ground you will help to shield the rest of the board from the power supply noise.
By following these recommendations, you will help yourself a lot in reducing the noise in your printed circuit board design. You can help yourself even further by using the full resources of your PCB design tools as well.
Measured lines with tuned traces at the correct length all go towards helping to reduce PCB noise
How Your Design Tools Can Help
The first thing that you need to do is make sure that you are setting up all of the design rules and constraints that are available to you in your CAD tools. These constraints can help you by setting trace width, length, and spacing rules for critical nets as well as governing trace routing topologies. You also could benefit from layer stackup generators and impedance calculators to help you configure your board layers correctly before you start layout. Many advanced CAD systems also have numerous placement features that will help you organize and align your component placement. And lastly, having the ability to automatically route differential pairs together as well as measured traces at the correct lengths is a great help too.
Thankfully there are PCB design tools already available that can handle this level of design expertise. One such PCB design system with all of the features and functionality that we’ve talked about here is OrCAD from Cadence. With OrCAD PCB Designer, you will have the capabilities that you need to reduce the noise on your next PCB design.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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