Simple EMI Filter and Shielding Design Choices to Remove Noise
EMI, EMS, and EMC tests are important parts of device qualification, especially if you want to ensure FCC/IC/CE compliance. EMI emission should be suppressed from a device under test, and it should have a minimum required level of immunity to external EMI. The last thing you need is for a device to fail to operate properly when brought near your cell phone.
Placing an EMI filter on each critical circuit or component is one strategy for providing EMI immunity. However, this is not the whole story, as you also need to suppress EMI emission from these circuit blocks. That being said, an EMI filter is still an important part of an EMC compliance strategy for new products. Here’s how you can incorporate an EMC filter and other EMI suppression strategies in your next PCB.
Passive Components as EMI Filter Circuits
Passing EMC certification and ensuring a device works properly requires taking steps to reduce EMI in your PCB. The most common type of EMI filter is a passive component, which will provide higher impedance against noise at certain frequencies. Noise that is received from EMI will typically be inductive, which passes a back EMF into the circuit, and higher frequencies will pass a stronger EMF. Using a passive that presents higher impedance to noise signals will reduce their impact on circuit performance.
As high frequency noise induces a stronger back EMF than low frequency noise, the typical strategy is to use low-pass filtering on the input in a critical circuit with high cutoff frequency. With analog signals, one strategy is to use a bandpass filter that is centered at the signal’s frequency. There are three primary methods to placing low-pass filters from passives on critical circuits; these are called C, LC, CL, Pi, and T filters. A C filter is just a bypass capacitor that is shunted to ground.
These filters are shown in the image and table below. The circuits shown below are intended to provide multiple functions. In particular, they are intended to prevent filtering around a desired signal bandwidth while filtering noise at other frequencies. Similarly, they are intended to ensure impedance matching along an interconnect if needed. This is one reason that LC circuits are the typical networks used for impedance matching in antennas; they can provide impedance matching in narrow bandwidths while providing strong filtration of EMI simultaneously.
Common EMI filter designs
An important point to note here is the use of these filter circuits will be affected by the source and load impedances. Be sure to use some basic frequency sweep simulations to check the filtration provided by your EMI filter.
This use of passives as filtering elements should illustrate the advantage of differential pairs. Except for differential crosstalk, EMI typically manifests itself as common-mode noise, and differential receivers are nearly immune to common mode noise as long as each trace in the pair is length matched. Your routing tools in your PCB design software should include length tuning tools to ensure your differential pairs are length matched and that groups of signals on a bus do not have excessive skew.
Shielding and Isolation Structures
Rather than placing EMI filters on every critical circuit and component, a better choice is to design the board to have better noise immunity. There are two principle ways to do this: placing shielding cans over critical circuit blocks, and designing conductive isolation structures in your board.
A quick and dirty way to provide EMI immunity is with shielding. This involves placing a shielding can over critical circuit blocks or components, which is then connected to ground. This shield effectively acts like a Faraday cage that prevents stray electromagnetic fields from impinging on the component. This also prevents noise from propagating between components in a board.
While shielding is effective, it is not always practical or cost effective. Shielding usually needs to be custom-designed and fabricated, which adds to costs. However, many connectors for use in high speed/high frequency designs are already shielded against EMI, and you won’t need to take extra measures to protect against shielding with these components. Some component manufacturers also provide shield cans for use with their components. If you know your board will be deployed in an electrically noisy environment, using shielding is a great way to ensure immunity to EMI and can help you pass EMC certification.
Shielding as an EMI filter for a PCB
As shielding is not always the best choice for EMI immunity, a better option is to implement proper isolation techniques. These techniques will help prevent interference between critical circuit blocks and from external EMI. Perhaps the most common and simplest isolation structure is a via fence; these vias are typically placed along antenna feedlines or other transmission lines that contain high frequency analog signals. The spacing between vias determines the frequencies that can be isolated in these structures.
Another common isolation structure in RF PCBs is to place copper grids around different circuit blocks on the surface layer. The walls of these grids contain a grounded via fence. In addition, grounded interstitial vias throughout the surface layer can provide strong RF shielding. This is equivalent to the strategy used to isolate a printed microstrip antenna on the surface layer; the antenna is placed at one edge of the board, and a via fence is placed along the edge of the antenna to provide isolation.
Some of these through-hole vias are used to provide shielding between these components.
These vias and the copper to which they are connected provide a static image charge that opposes incident EMI from components. They provide broadband noise suppression from within critical circuit blocks and the board edge, but due to the complex geometry of these structures, they have a limiting cutoff frequency and provide weaker EMI suppression at certain harmonics, just like any other resonant structure. More complex isolation structures are still a subject of open research. When used alongside EMI filters on important components, shielding and isolation structures provide major steps towards a comprehensive EMI suppression strategy.
Broadband EMI filter design is as much an art as it is a science, and you can implement the delicate circuit and board design choices with the right PCB layout and design software. Allegro PCB Designer and Cadence’s full suite of design tools can help you analyze and simulate your board’s behavior in the presence of noise and layout your new product to have noise immunity. You’ll also have access to a set of tools to generate 3D models, for MCAD design, run electrical simulations, and preparing for manufacturing data.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.