Trade.Information

Product Name: Signal Path Modeling

Product Description

To ensure adequate signal integrity many factors must be considered. In some cases, the signal paths may have multiple sources and/or destinations or loads. At high speeds the media interconnecting the sources and destinations will behave as transmission lines instead of lumped capacitive loads and effects such as voltage overshoot and impedance mismatch become important. For optimal speed, net topologies are usually point-to-point, i.e., one source and one destination. The data rate over point-to-point nets is typically limited by frequency-dependent losses in the interconnecting media. For example, in coaxial cables, skin-effect losses will typically dominate whereas in printed circuit boards losses can also be from lossy dielectric materials. Other factors to consider include crosstalk between adjacent signaling paths. Crosstalk is the term used where one digital signal path couples noise to an adjacent path.. Crosstalk is usually easy to predict with the use of electromagnetic simulation and thus a pcb board or an electronic package can be designed to minimize crosstalk effects. Another important consideration in modeling the signal path is to analyze the sources, or output buffers, that will drive the signal path. In many cases the net topology must be designed to work with the output buffers. Additional components, such as series damping and/or pull-down and/or pull-up resistors may be required. Furthermore, the voltage and current requirements for output buffers may not be compatible with input buffer specifications. This incompatibility may be overcome with techniques such as resistive termination and ac coupling. Another source of coupled noise in addition to crosstalk is that termed "simultaneous switching noise", or SSN. SSN is also referred to as "simultaneous switching output" (SSO) noise and "ground bounce". SSN is the response to output buffers switching through the packaging paths which are typically overly inductive. All the factors described above have implications on noise and timing margin - both of which must be adequate to ensure that there are no missed bits in a digital system or that an adequate bit-error-rate (BER) is obtained in a communication system. A comprehensive simulation methodology implemented at ASID involves the simulation of eye diagrams. Eye diagram simulations can predict the noise and timing margins for a particular signal path while taking into account all the factors described above.