Depending on the frequency spectrum of the interference (broadband, pulse or transient, ESD, etc.) and the coupling mechanism, designers can draw on many different techniques to reduce the effects of EMI. These techniques include grounding, shielding, filtering, careful PCB layout, and more.
Stopping the noise at the point at which it enters or leaves the module (at the connector) is a very effective way to reduce conducted emissions or susceptibility caused by unwanted interference propagating via a wiring harness or power cable.
If an EMC issue manifests itself early enough in the development process, the simplest approach is to add filter circuits on the PCB at the input/output pins. Figure 1 shows some common filter configurations.
Figure 1: Common filter circuits for EMI suppression
The simplest filter is a decoupling capacitor from the signal line or pin to ground. The capacitor C combines with the output impedance of the source ROUT to give a 1-pole low-pass filter with a roll-off of 20 dB per decade above the 3-dB cutoff frequency f C given by:
Adding an inductor gives an LC or CL filter, which provides both capacitive and inductive filtering. These filters have two frequency-dependent components and provide two-pole roll off (40 dB per decade).
Adding a third component gives the Pi or T configurations, so named based on their schematic appearance. These filters provide three-pole roll off (60 dB per decade). The Pi filter has capacitors on the input and output side with an inductor between them to form a CLC configuration. The Pi configuration has low input and output impedance at high frequencies as the capacitors shunt high frequencies to ground.
The T configuration is the inverse: two inductors with a center capacitor to ground in an LCL arrangement. The T filter has high impedance at high frequencies due to the series inductive chokes.
The formulas above assume ideal components, but real-world devices have additional elements that turn them into L-C-R combinations (a primary attribute plus two parasitic elements). These parasitic elements change the real component behavior compared to the ideal version and affect the filter performance.
A real-world inductor, for example, has parasitic capacitance between the inductor turns and parasitic resistance in the wire of the inductor and the leads. While a real-world capacitor has an equivalent series inductance (ESL) due to the leads and package. As the frequency increases, the ESL also increases, whereas the impedance of the capacitive element decreases with increasing frequency. At a certain frequency (the self-resonance frequency) the two quantities cancel and the capacitor has zero effective impedance. At frequencies above the self-resonance frequency, the ESL dominates and the capacitor impedance begins to increase. Precision models of real-world devices include the effects of parasitic elements.