With the continuous shrinking of circuit board design and component packaging, and OEMs' demand for higher-speed systems, two major issues, electromagnetic compatibility (EMC) and related electromagnetic interference (EMI), are particularly troublesome for PCB layout and design engineers.
With the continuous shrinking of circuit board design and component packaging, and OEMs' demand for higher-speed systems, two major issues, electromagnetic compatibility (EMC) and related electromagnetic interference (EMI), are particularly troublesome for PCB layout and design engineers.
EMC is closely related to the generation, propagation and reception of electromagnetic energy, which is undesirable in PCB design. Electromagnetic energy comes from multiple sources and mixes together. Therefore, extreme care must be taken to ensure that different circuits, traces, vias and PCB materials work together to make various signals compatible without interfering with each other.
Grounding
An important way to reduce EMI is to design the PCB ground plane. The first step is to maximize the ground area within the total area of the PCB, which can reduce emission, crosstalk and noise. Special care must be taken when connecting each component to the ground point or ground plane, as failure to do so will not fully utilize the neutralization effect of a reliable ground plane.
An especially complex PCB design has several stable voltages. Ideally, each reference voltage has its own corresponding ground plane. However, too many ground planes will increase the manufacturing cost of the PCB, resulting in an excessively high price. A compromise is to use ground planes at three to five different locations respectively, and each ground plane can contain multiple grounding sections. This not only controls the manufacturing cost of the circuit board, but also reduces EMI and EMC risks.
A low-impedance grounding system is critical to minimize EMC. In a multilayer PCB, it is optimal to have a solid ground plane, rather than copper thieving or scattered grounding sections, as it features low impedance, provides a current path, and serves as the optimal return signal source.
The time for the signal to return to the ground is also critical. The round-trip time of the signal to and from the signal source must be matched; otherwise, an antenna-like effect will occur, making the radiated energy a part of EMI. Similarly, the traces carrying current to/from the signal source should be as short as possible. If the length of the source path and the return path are not equal, ground bounce will occur, which will also generate EMI.
If the time for the signal to enter and exit the signal source is not synchronized, an antenna-like phenomenon will occur, thereby radiating energy and causing EMI.
EMI Isolation
Due to the different characteristics of EMI, a good EMC design rule is to separate analog and digital circuits. Analog circuits with high amperage, or high current, should be kept away from high-speed traces or switching signals. If possible, they should be protected with ground signals. On a multilayer PCB, analog traces should be routed on one ground plane, while switching or high-speed traces should be routed on another ground plane. In this way, signals with different characteristics are separated.
Sometimes a low-pass filter can be used to eliminate high-frequency noise coupled with surrounding traces. The filter can suppress noise and return a stable current. It is critical to separate the ground planes of analog and digital signals. As analog and digital circuits have their own unique characteristics, separating them is essential. Digital signals should have a digital ground, and analog signals should be terminated to an analog ground.
In digital circuit design, experienced PCB layout and design engineers pay special attention to high-speed signals and clocks. At high speeds, signals and clocks should be as short as possible and adjacent to the ground plane, because, as mentioned earlier, the ground plane can keep crosstalk, noise and radiation within a controllable range.
Digital signals should also be kept away from the power plane. If they are too close, noise or induction will be generated, thereby weakening the signal.
Crosstalk and Traces
Traces are particularly important to ensure the normal flow of current. If the current comes from an oscillator or other similar equipment, it is especially important to isolate the current from the ground plane, or to prevent the current from running parallel to another trace. Two parallel high-speed signals will cause EMC and EMI issues, especially crosstalk. The resistance path must be the shortest, and the return current path must also be as short as possible. The length of the return path trace should be the same as that of the transmit trace.
For EMI, one trace is called the "aggressor trace", and the other is the "victim trace". Inductive and capacitive coupling will affect the victim trace due to the presence of electromagnetic fields, thereby generating forward and reverse currents on the victim trace. In this case, ripples will be generated in a stable environment where the transmit length and receive length of the signal are almost equal.
In a well-balanced environment with stable traces, the induced currents should cancel each other out, thereby eliminating crosstalk. However, we live in an imperfect world, and this does not happen. Therefore, our goal must be to keep the crosstalk of all traces to a minimum. If the spacing between parallel traces is set to twice the trace width, the impact of crosstalk can be minimized. For example, if the trace width is 5 mils, the minimum spacing between two parallel traces should be 10 mils or more.
Decoupling Capacitors
Decoupling capacitors can reduce the adverse effects of crosstalk. They should be placed between the power supply pin and the ground pin of the device, which can ensure low AC impedance and reduce noise and crosstalk. To achieve low impedance over a wide frequency range, multiple decoupling capacitors should be used.
An important principle for placing decoupling capacitors is that the capacitor with the smallest capacitance value should be as close to the device as possible to reduce the inductive effect on the traces. This specific capacitor should be placed as close to the power supply pin or power trace of the device as possible, and the pad of the capacitor should be directly connected to the via or ground plane. If the trace is long, multiple vias should be used to minimize the grounding impedance.
Trace Angle
To reduce EMI, 90° angles formed by traces, vias and other components should be avoided, as right angles will generate radiation. At the corner, the capacitance will increase, and the characteristic impedance will also change, resulting in reflections, which in turn cause EMI. To avoid 90° angles, traces should be routed to the corner with at least two 45° angles.
Vias
In almost all PCB layouts, vias must be used to provide conductive connections between different layers. PCB layout engineers need to be particularly careful, as vias will generate inductance and capacitance. In some cases, they will also cause reflections, because the characteristic impedance will change when a via is made in the trace.
It should also be kept in mind that vias increase the trace length, which requires matching. In the case of differential traces, vias should be avoided as much as possible. If they cannot be avoided, vias should be used in both traces to compensate for the delay in the signal and return paths.
Shielding
Cables carrying digital circuits and analog currents will generate parasitic capacitance and inductance, causing many EMC-related problems. If twisted pair cables are used, a low coupling level will be maintained and the generated magnetic field will be eliminated. For high-frequency signals, shielded cables must be used, with both the front and back sides grounded, to eliminate EMI interference.
Physical shielding uses a metal package to enclose the entire or part of the system to prevent EMI from entering the PCB circuit. This shield acts as a closed grounded conductive container, which can reduce the antenna loop size and absorb EMI.
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