Signal Integrity: From High Speed to Radiofrequency Applications

7

Interference and Cross-Talk in Interconnects

Cross-talk results from the parasite coupling between interconnect lines in high-speed digital or analog radiofrequency applications. The parasite voltages resulting from cross-talk are superimposed on useful signals and can disturb the functioning of digital circuits depending on switching thresholds as well as radiofrequency receiver circuits. For example, in LVDS logic circuits, the switching margins are as low as 150 mV, and cross-talk impulses can exceed 1 V for 1 Gbit/s transmissions, when parallel interconnect lines are very close to each other over long distances.

Coupling voltages depend on the intrinsic coupling coefficient of the lines, the rise time of the signal and the line load conditions. We distinguish isolated coupling between lines that cross from coupling distributed between parallel lines. It is important to understand the coupling mechanisms between lines in order to be able to understand cross-talk measurements.

Each level of interconnect has a dominant specific coupling, which is a source of cross-talk:

– electromagnetic coupling by close lines (PCB, MCM);

– coupling by common-mode voltage (electronic packages);

– coupling by substrate (integrated circuits).

7.1. Coupling and interferences due to substrate

In integrated circuits on silicon, for example, the characteristics of the substrate lead to the generation of parasite voltages when currents circulate between the different parts and the ground through ...