EMI Reducing PCB Shielding Techniques to Incorporate in Your Designs

May 28, 2018 Altium Designer

Metal shielding on a PCB

 

I spent most of my twenties producing and mastering music for local artists. As much as I love older consoles, it gets annoying to listen to audio from one channel bleeding over to other channels. Newer consoles are built with proper shielding and crosstalk reduction measures, and the analog signals in these newer consoles are much cleaner.

 

Digital devices are not immune either. Modern computers and smartphones continue to pack more silicon-based components in less space. But sensitivity to noise increases as silicon geometries get smaller over time. Newer electronic systems tend to use smaller silicon-based ICs that could be more susceptible to EMI. Packing devices closer together also increases the potential of noise coupling between components on a PCB.

 

The conscientious designer should take time to study shielding techniques in order to protect their devices against interference and maintain good signal integrity. Proper component layout and trace routing can greatly reduce noise coupling between components. Other more extreme measures, like metal can shields, are more effective in combating EMI.

Shielding With Metal Enclosures

If you remember you basic electromagnetics classes, you should know that a grounded metal plate greatly attenuates electric fields. Stray electric fields, including RF fields, can be effectively shielded using a metal enclosure that surrounds sensitive components. When various components are separated into different enclosures, the shielding also prevents interference between components.

 

Once the shielding is grounded, it can provide a conducting plane for ESD, RF fields, internal fields, and noise carried on the cables that enter and exit the enclosure. The shielding must provide a low impedance path for external RF sources to reach ground reference point in order to quickly dissipate any currents induced by stray fields.

 

Shielding on a digital TV PCB

Shielding on a digital TV PCB

 

The ground point that connects back to the power supply should also make a single connection with all shielding cans in order to prevent ground loops. In other words, connect ground planes in a star or single-point power topology. Multi-point ground connections are fine for protection against low-frequency signals, but this is not recommended if you need to shield against higher frequency (>30 MHz) signals.

 

Some signals will be able to radiate from the enclosure unless a solid enclosure is used. Slots or gaps in a metal enclosure should be made as small as possible. If slots are present in the enclosure, the enclosure can act as a slot antenna. When ventilation of components is critical, small circular shielding should be used instead of longer slots.

Reducing Analog Signal Crosstalk With Guard Traces

Crosstalk is a well-known issue in parallel traces, and proper spacing is the most common design rule prescribed to prevent crosstalk. In analog traces, the oscillating signal can induce a current in a nearby trace, called crosstalk. Analog crosstalk becomes worse at higher frequencies. When signals in parallel traces run in the same direction, the traces induce noise in each other, resulting in signal degradation.

 

Your PCB might not have enough available space to separate two parallel analog traces. One convenient way to reduce crosstalk between two parallel analog traces is to run traces as differential pairs. As this isn’t always possible, a grounded trace can be routed between the two signal traces. This grounded trace is called a guard trace. Crosstalk between two differential pairs can also be reduced using a guard trace.

 

PCB vias

Segregate your functional blocks using vias fences

 

Guard traces are especially effective in shielding crosstalk between analog traces, and between an analog and a digital trace. But guard traces are less effective in reducing crosstalk between two digital traces. Guard traces do not provide much benefit if your digital PCB already has a solid ground plane.

Via Fences

RF circuits with antennas should already separate the antenna and ground plane so that they do not overlap. Another measure that can be used to decouple noise between the antenna and the ground plane is to use a via fence. Via holes lined up along the border between the antenna and its ground plane segregates the antenna and the ground plane into separate functional blocks.

 

Via fences are not limited to use with antennas. They are one method that is commonly used to provide shielding for microstrip and stripline transmission lines, as well as shield functional circuit blocks from each other. Via fences are easy to implement, but they do use up board space and are not as effective metal cans.

 

Reducing EMI in your PCB always begins with a good layout. PCB design software like Altium Designer has a range of layout tools to help you maximize the chances of your design operating properly, even in the harshest EMI environments. Analysis and simulation tools, rule checking features, and CAD tools will help ensure your design comes out flawless.

To learn more about how you can use Altium Designer to bring your ideas to life, talk to an Altium expert today.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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