Reconstruction of a single square pulse originally having 40 ps width coming from a lossy and noisy channel in a point to point interconnect

Authors: ALAK MAJUMDER, BIDYUT BHATTACHARYYA

Abstract: The fundamental problem in high speed communication is that it suffers a lot of signal integrity issues due to dispersion (caused by dielectric variation with angular frequency), reflection (S$_{11})$, and insertion losses (S$_{12})$ of the channel made of copper. When a pulse width $\tau $ with magnitude V$_{0}$ is driven through a lossy channel, we observe a reduction in magnitude (due to S$_{12}$ and S$_{11})$ and an increase in pulse width (due to dispersion). It causes different values of skin effect and dielectric loss leading to different effective resistance at each segment as the pulse moves through the channel. This impedance mismatch generates reflection noise, which makes the identification of the received signal difficult at the receiver. Modeling of such a complex situation and reconstruction of a high speed signal driven through a lossy channel remain an open problem for the research community. This work unveils a method of designing a system that can renovate a square wave pulse of 40 ps or less (corresponding to a data rate of 25 Gbit/s or more) after sending the same over a lossy channel from transmitter to receiver. The received noisy signal (Signal-A) is sent through a RC circuit to obtain a different delayed signal (Signal-B). Both the signals are then applied to the two terminals of a comparator. The difference, $\Delta $(t), between Signal-A and Signal-B is measured and it is witnessed that the voltage difference ($\phi )$ of two consecutive maximum peaks of $\Delta $(t) actually provides us with a better way to determine the design criteria of threshold voltage, V$_{T}$, of the comparator for the reconstruction of the square pulse. It helps to eliminate the needless oscillations at the output of the comparator. The design of a threshold voltage depends fully on the channel properties.

Keywords: Driver, receiver, printed circuit board, inter symbol interference, delay line, reconstruction, decision feedback equalizer, feed forward equalizer

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