In the layout and routing of high-speed transmission lines, excessive electromagnetic radiation is a critical issue affecting signal integrity and system reliability. This stems from the electromagnetic field energy generated during high-speed signal transmission radiating outwards through unexpected paths, leading to signal attenuation, increased crosstalk, and even interference with other equipment. To avoid such problems, a comprehensive approach is needed, encompassing signal path optimization, shielding design, impedance control, topology selection, and simulation verification.
The signal path design for high-speed transmission lines must adhere to the principle of "shortest path, minimum loop." Signal lines should be as short as possible to reduce the accumulation of electromagnetic radiation at its source. Simultaneously, the return path must be tightly coupled to the signal path, forming a low-impedance channel to prevent increased radiation due to excessive loop area. For example, in multilayer PCB design, high-speed signal lines should be preferentially placed on layers close to the complete reference plane, with the return path connected to the ground plane via vias to ensure the shortest current path and lowest inductance. Furthermore, avoid placing high-frequency clock or power lines near high-speed signal lines to prevent radiation caused by strong electromagnetic field coupling.
Shielding design is the core means of suppressing electromagnetic radiation. For high-speed signal lines, electromagnetic field leakage can be limited by using shielding layers or shielded cavities. The shielding layer needs to be connected to the reference plane via dense vias to form a continuous conductive path, preventing electromagnetic waves from radiating from gaps or holes. For example, in differential signal transmission, a grounded shielding layer can be placed around the differential pair to confine electromagnetic energy within the shielded area; for single-ended signals, grounding can reduce radiation. Simultaneously, the shielding layer should be grounded nearby to avoid a decrease in shielding effectiveness due to excessive inductance in the grounding loop.
Impedance control is crucial for ensuring the quality of high-speed signal transmission. The characteristic impedance of a high-speed transmission line must be matched to both the source and load ends to avoid signal reflection caused by impedance discontinuities. The superposition of reflected and incident waves enhances electromagnetic radiation. During design, impedance must be accurately calculated based on parameters such as dielectric material, linewidth, line thickness, and reference plane spacing, and impedance matching can be achieved by adjusting the linewidth or dielectric thickness. For example, the characteristic impedance of a microstrip line can be reduced by controlling the ratio of linewidth to reference plane spacing (e.g., W/H < 1:3); for striplines, it is necessary to ensure symmetry and uniform spacing between the upper and lower reference planes. Furthermore, impedance continuity must be maintained during inter-layer switching to avoid reflections caused by dielectric changes.
The choice of topology directly affects signal transmission stability and radiation levels. For low-frequency signals, daisy-chain topologies simplify routing and control costs, but high-speed signals are prone to reflection and radiation due to inconsistent branch lengths; in such cases, star topologies should be used to balance the signal load through the central node, reducing branch length differences. For example, in DDR memory routing, T-type or fly-by topologies can balance signal arrival time and reduce radiation risk. Simultaneously, high-speed signal lines should avoid forming closed or open loop structures. Closed loops may produce a loop antenna effect, while open loops form a linear antenna, both of which enhance radiation.
Simulation verification is a necessary step in optimizing layout and routing. Electromagnetic simulation tools (such as SIwave and HyperLynx) can simulate the transmission characteristics of high-speed signals, analyze signal integrity, impedance matching, and radiation levels, and identify potential problems in advance. For example, simulations can identify whether signal line lengths are close to resonant points (such as integer multiples of 1/4 of the wavelength), avoiding resonance that could lead to enhanced radiation. They can also evaluate shielding effectiveness and optimize via density and grounding methods. Based on simulation results, layout and routing parameters, such as line width, spacing, and stack-up structure, can be adjusted to ensure the design meets electromagnetic compatibility (EMC) requirements.
The layout and routing of high-speed transmission lines requires comprehensive optimization from multiple aspects, including signal paths, shielding design, impedance control, topology, and simulation verification. By shortening signal paths, strengthening shielding, maintaining impedance continuity, selecting appropriate topologies, and iterative simulations, electromagnetic radiation can be effectively reduced, signal integrity and system reliability improved, meeting the stringent EMC requirements of high-speed communication.
