Design for Electromagnetic Compatibility in PCB: Let Your Revels Never End
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What if William Shakespeare worked as an electrical engineer instead of a poet and playwright? If electronics existed during the mid-to-late sixteenth century, the Bard may have studied the impact of electromagnetic interference (EMI) on circuits and the need for electromagnetic compatibility (EMC). After all, he wrote about his fascination with noise in The Tempest:
“Be not afeard; the isle is full of noises,
Sounds, and sweet airs, that give delight and hurt not.
Sometimes a thousand twangling instruments
Will hum about mine ears; and sometime voices…"
As with many instances of attempting to solve signal EMI, Caliban—a character in the play—cannot describe the source of the noise.
EMI occurs when a component or system degrades after the coupling of an additional voltage creates an electromagnetic disturbance. Degradation causes the component or system to become an EMI source that spreads interference to other susceptible systems, subsystems, circuits, or the source component. You may wish for “Sounds, and sweet airs, that give delight and hurt not.” Instead, EMI causes me to think of a “thousand twangling instruments” and the spirits that appear at Prospero’s beck and call. Just as Sebastian and Antonio scheme and plot in “The Tempest,” several noisy villains work to ruin circuit performance.
After all, EMI occurs from within the circuit and always manifests itself on outputs, inputs, the power supply, and ground. Noise moves through coupling paths created through inductive, capacitive, or direct conducted coupling. As a result, current loops form between traces and the ground plane. To make everything even more interesting, ground noise injected into longer I/O traces results in mode radiation. Traces and cables that carry high speed clock and data signals secretly enjoy their double-life existence as large, efficient antennas that emit EMI.
Routing channel lines over a segmented ground plane also actively increase EMI signal integrity. As the lines cross over the openings between the segments, each gap joins with its trace and cable brethren and happily serves as an RF antenna. A power supply feeds current and RF energy to clocked circuits that—in turn—may switch synchronously and generate a large noise spike. The reflected high frequency current created within the switching regulator can exit the circuit through the through supply lines or couple out through other paths.
Reducing EMI doesn’t have to be a tremulous journey, and you certainly won’t have to be a playwright to do so. The Tempest has five acts to it, and there are three main practices which can reduce your EMI. You choose which seems less complicated:
Use EMC to establish the capability of a circuit to eliminate or substantially lessen EMI and to reduce any noise impact on other circuits. Much of the effort begins with reducing signal current loop areas and providing low impedance paths that return the signal current to its source. EMC also describes how the circuit increases its immunity against EMI and defines the capability of a circuit board, any enclosures, any attached cables, and the power supply to protect against EMI.
Selecting a grounding system for the power supply has a direct impact on a EMC PCB design. Because partitioned ground planes open the opportunity for current loops, you should use multi-layer PCBs with solid ground planes. Furthermore, you can also place aluminum or tantalum decoupling capacitors at the edge of the circuit to reduce noise injected at the power supply and ground reference planes.
High-frequency circuits require special attention to grounding. Fast slew rates can increase the harmonics in digital signals and produce EMI. Using larger, solid ground planes creates areas that have very low impedance and maximize EMI suppression. You may want to consider the use of multi-point grounding to reduce ground loop currents and the ground impedance of planes.
Decoupling capacitors connected between the power and ground planes reduce electromagnetic interference. 0.01 µF or 0.1 µF ceramic capacitors supply additional current to an IC that switches state through a low inductance path and the IC power pins. In addition, decoupling capacitors create a low-inductance path from the IC ground reference pins to the ground reference plane.
With multi-layer Printed Circuit Boards, you can place the signal layers between the DC power and ground reference planes. Creating space between high speed signal layers and DC voltage layers and between the high-speed signals and the top and bottom layers causes traces to exist only as traces. From there, you should consider using multiple power distribution busses to isolate sensitive circuits from one another.
Achieving good EMC occurs with the intelligent selection and placement of switching regulators, microcontrollers, oscillators, and digital ICs. First, view the entire circuit as functional sub-units or circuit partitions. Partitioning focused on electromagnetic compatibility separates components according to their potential as EMI sources, victims, paths, and passive circuits. After partitioning the components into functional blocks, incorporate shielded EMI filters and decoupling capacitors for additional EMI suppression.
Observe Best Practices with Signal Traces
Ignoring best practices with trace layout creates coupling paths that easily move noise from the source to the victim. You can achieve electromagnetic compatibility by:
- Routing clock traces in the layer between the ground and source voltage plane.
- Using wider traces for clock signals to decrease high frequency damping and the possibility of capacitive coupling between traces.
- Avoiding trace capacitance by eliminating right angles or crosses.
- Eliminating long, parallel clock traces that contribute to crosstalk.
- Ensuring that consistent spacing exists between the traces and that the spacing equals at least one trace width.
Unfortunately, all systems and circuits can generate EMI and have differing levels of EMI susceptibility. As consumers purchase more smart devices and appliances, the need to analyze EMI risks becomes even greater. Left unchecked, EMI can cause dramatic failures.
In The Tempest, Caliban wondered if magic caused the noise that he experienced. While some may believe that controlling EMI requires magic dust, following good practices solves interference problems. Using the right PCB design software, too, can seem like magic—especially when it helps you to plan out trace routing, distribute components efficiently, and check for signal integrity.