Analysis of Via to Via Crosstalk for Single Ended Signals in the Time & Frequency Domains

At today’s fast switching data rates via to via crosstalk through the PCB cavities can be more profound than trace to trace coupling.This work offers an analysis of the physical basis of the root cause behind via to via coupling using simulations and measurements of test boards to illustrate the principles. Based on the root cause, methods to mitigate the noise are explored and demonstrated.The return current of a signal passing through a via will generate a radial wave that propagates through the cavity and as a result encounter the impedance of the cavity. With this root cause identified, the coupling between the vias is broken into three constituents: the radial wave coupling, excitation of cavity structural resonances and excitation of lumped resonances. Lumped resonances result from the interaction of the cavity with components such as DC blocking capacitors and return vias.

Each of the three mechanisms is analyzed to provide an understanding of its electrical signature and contribution to the overall crosstalk.Novel designs for physical test boards and SMA launch fixtures are used to isolate the effects of the via to via coupling. To explain the concepts and demonstrate the validity of the results, S-Parameters are captured using an Anritsu Shockline MS46524B network analyzer (VNA) and time domain measurements are collected using a Teledyne LECroy WavePro HD 12-bit scope. Correlation to simulations using HyperLynx is also provided to further validate the results.

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