Noise

 

Noise in general can be defined as unwanted disturbances superposed upon a useful signal, which tend to obscure its information content. In high performance systems noise management is a major design issue. Noise can limit the achievable system performance by degrading edge rates, increasing delays, and reducing noise margins and can cause false switching of logic. The noise magnitude and frequency are closely tied to the packaging and interconnect scheme used. In a digital system four major sources of noise can be identified as follows [18]:

• Reflection noise: This noise results from the reflection of part or all of the signal due to non constant impedance along the interconnect line. This noise is a potential problem when the time of flight becomes comparable with the rise time of the signal.
• Crosstalk noise: This noise arises whenever signal lines or chip connect leads run parallel to each other. The signal on an active line couples onto a quiet line as noise. This is a consequence of mutual inductance and capacitance between neighbouring transmission lines.
• Simultaneous switching noise: This noise occurs whenever a large number of off-chip or on-chip drivers switch at the same time. This switching activity causes a large current spike to flow through the ground and supply voltage connections. This noise may cause false switching inside the chip that appears as noise on the output leads of any quiet buffers connected to the ground or power rail.
• Electromagnetic interference (EMI): This noise is produced by the package circuit and affects the operations of the surrounding circuits. Whenever a current flows through the package leads, the leads act as antennas radiating electromagnetic signals.

The magnitude of all of these noise sources depends on the rise time of the signals passing through the interconnect. The faster the rise time, the worse the noise. The role of 3D technology in reducing noise is in the reduction of interconnection length, and hence reduction of the associated parasitics which translate into performance improvements. For example, simultaneous switching noise is reduced by shortening the interconnect's length and the associated parasitics, hence producing less simultaneous noise for the same number of interconnections.