Electrical filters, which are typically constructed using capacitors and inductors are reasonably easy to design and should be easy to predict the attenuation of the filter. However, it can be disappointing and frustrating when, during an EMI compliance test, these filters are not performing as hoped. What types of issues can cause that?

First it should be noted that the filter has more components than the individual capacitors and inductors. Although it is easy to design filters as ideal devices, there are parasitic and physical aspects of the circuit that must be accounted for, especially in the higher frequency range of the filter. One neglected issue is the path of the trace to the component. If a long trace is used to route from the line to be filtered to the capacitor, that trace has both an inductance and a resistance. For this reason, it is recommended that capacitors are placed as close as possible to the trace being filtered.

But the next component is the trace from the capacitor to the reference plane or return path. Included with this impedance is that of the whole path back to the source of the noise. This may be another long trace, or a via to a reference plane. But this plane is not the source, only the next path back to the source of the noise. This may be through another set of vias and traces, or it may be to standoffs or other chassis connections. The point is that the return path can be complex and arduous, and the path impedance, both before and after the capacitor, reduces the effectiveness of the component.

Where capacitors (C) will decrease in impedance with higher frequency, inductors (L) will increase.  Above an LC resonance frequency, the impedance will be dominated by the inductance of the current path, which will be greater than the impedance of the capacitor. This means the filter performance will degrade as a result.  It is common for this to occur at and above 10 MHz, sometimes at lower frequencies than that.

If the layout is carefully designed to avoid impedance buildup due to this inductance, there may still be issues with location and cross-coupled noise. Cross-coupled noise is caused by fields — typically magnetic fields from traces, wires, and inductors or other magnetic components — which couple into other traces and wires. If the second set of traces and wires is after the filter but before the connector, the lines which have been filtered can be contaminated with more noise from these fields. The filter is rendered ineffective, and compliance becomes a problem.

The solution is to keep line-to-chassis capacitance connected as close as possible to the line being filtered, and have a very short path to chassis, minimizing the inductance and impedance of the filter. This is what is accomplished with the EESeal filter products. When series impedance with the capacitor is kept as low as possible, attenuation can be found up and over 1 GHz. Compare this performance to the filter which degrades due to trace and wire inductance.

To protect against energy being coupled back onto filtered lines, both shielding and location of the filter are important. Having the filter mounted inside the connector provides isolation from fields which can cross-couple onto the cables that are filtered. Also, with the filter at the point of penetration, any energy on the cable as it enters the equipment can be reduced before it couples into sensitive circuitry inside the chassis. This provides an additional level of immunity for the equipment.

High quality filters can be designed which perform extremely well on paper but are degraded in actual use. Filtering at the connector pins is easy to perform, easy to install, and can give reliable results to your product.