1. Focus on Crosstalk and Routing
Proper routing is critical to ensure smooth current flow. For oscillators or similar components, it's essential to separate signal traces from the ground plane and avoid parallel routing with other traces. Parallel high-speed signal traces are particularly prone to EMI (electromagnetic interference) and crosstalk. The return path should be as short as possible and equal in length to the signal trace.
For EMI management, there are two types of traces: the "aggressor" and the "victim." Inductive and capacitive coupling from electromagnetic fields can induce forward and backward currents on the victim trace, causing ripple effects. In an optimized and stable PCB environment, induced currents should cancel out, minimizing crosstalk. However, real-world conditions often fail to achieve this ideal.
Minimizing crosstalk requires maintaining a minimum spacing between parallel traces of at least twice the trace width. For instance, if a trace is 5 mils wide, a 10-mil or greater separation is advised. With new materials and components constantly emerging, PCB designers must remain vigilant against EMI and EMC (electromagnetic compatibility) challenges.
2. Decoupling Capacitors
Decoupling capacitors mitigate the adverse effects of crosstalk by ensuring low AC impedance between the power supply and ground pins, reducing noise and interference. Using multiple decoupling capacitors across a wide frequency range achieves better impedance performance.
Place capacitors with the smallest values closest to the device to minimize inductive effects. The capacitor should be directly connected to the power pin or power trace, with pads linked to ground planes or vias. For longer traces, multiple vias are necessary to reduce grounding impedance.
3.Implement a Proper Grounding Strategy
A well-designed ground plane is key to reducing EMI. Maximizing the ground area on the PCB minimizes emissions, noise, and crosstalk. Each component must have a reliable connection to the ground plane; otherwise, its effectiveness will be compromised.
Ideally, complex PCBs with multiple stable voltages require a dedicated ground plane for each voltage. To balance cost and performance, consider grouping related circuits into shared ground planes, reducing EMI while controlling manufacturing costs.
Low-impedance grounding systems are vital for EMC reduction. A solid ground plane is preferred over isolated copper pours in multilayer PCBs to ensure a low-impedance return path. Signal return paths should be as short as possible to prevent ground bounce and EMI.
4. Avoid 90° Angles in Traces
To minimize EMI, sharp 90° angles in traces and vias should be avoided. Right angles introduce capacitance changes and impedance discontinuities, leading to reflections and additional radiation. Instead, route traces with two 45° angles at corners.
5. Careful Use of Vias
Visas are essential for inter-layer connections in PCB layouts but can introduce unwanted inductance, capacitance, and impedance changes, potentially causing reflections.
In differential traces, vias should be avoided when possible. If unavoidable, ensure symmetry by applying vias to both traces to maintain signal timing and balance the return path. Additionally, adjust trace lengths to match any added delays from Vias.
6. Cabling and Physical Shielding
Cables carrying digital and analog signals can introduce parasitic capacitance and inductance, leading to EMC problems. Using twisted-pair cables reduces coupling and mitigates magnetic field effects. For high-frequency signals, shielded cables grounded at both ends are necessary to suppress EMI.
Physical shielding involves enclosing systems or components in conductive enclosures to block EMI. Such shielding acts as a grounded conductive container, minimizing antenna loop sizes and absorbing interference.
