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Signal Integrity Issues

Signal Integrity Issues :

In Recent days signal integrity is a critical issue to the VLSI designers. The signal integrity problems are related to electromagnetic and are interms of EMI/EMC. In digital system applications, signal is transmitted from one component to other in the form of logic high or low. At the input of receiver, voltage above reference value is taken as logic high, whereas the voltage below the reference value is taken as logic low. Furthermore, it also depend up on the waveform. Signal waveform distortions are having the various mechanisms. And noise problems that are, Reflection Noise, Crosstalk Noise and Power/Ground Noise.

Rise Time and Signal Integrity :

Due to the significant improvements of the chip fabrication technology, the silicon size is shrinking and the transistor channel length is reduced into sub-micron range. Hence the speed of the logic circuits increases. In deep-sub-micron technology is related with dV/dt or dI/dt, ringing, crosstalk, and power/ground switching noise are existing. Since the systems with faster clock frequency has lower rise time, signal integrity challenges are increasing.

Cross Talk Noise :

In chip packages, a pair of parallel conducting wires separated by a uniform distance, and wires in a socket are used. A pair of metal planes with an attached via another type of transmission line is used. The transmission lines are used for transmitting date from point A to point B. The transmission lines parameters are per-unit-length R (resistance), L (inductance), G (conductance) and C (capacitance). For transmission line structures the parameters are found by numerical techniques. The electromagnetic coupling between the various transmission lines leads to the Crosstalk. Hence, noise picks up on the adjacent signal lines lead to negative logic switching. When the active multiple lines are switching, Crosstalk also involves timing. Depending upon the switching direction, the delays significantly increase/decrease. Because of spacing between the lines, the crosstalk related to signal rise time changes. In order to limit the crosstalk the line space has to be increased and ground guarding band is to be added.