Analysis of Crosstalk Effects on Jitter in Transcievers

As data rates increase, crosstalk becomes an increasingly important issue. Crosstalk aggressors can attack both amplitude and timing characteristics of the victim signal. The effects of crosstalk on the jitter of a victim signal are not well understood because crosstalk does not fit neatly into the standard definitions of jitter components. As a result, most commercially available jitter analyzers misinterpret crosstalk. We investigate the effects of various types of crosstalk aggressors on a victim signal in a transceiver, with particular emphasis on jitter. We also demonstrate cases where crosstalk is misinterpreted as random jitter (RJ), periodic jitter (PJ), or intersymbol interference (ISI). Author Biography Dr. Daniel Chow is a senior member of technical staff in the characterization group at Altera Corporation. His responsibilities include testing and validation of high-speed components. Specifically, he is responsible for developing Altera’s knowledge base on jitter measurement issues. Dr. Chow received his Ph.D. from the University of California, Davis. Background and Motivation In the field of high-speed data communications, crosstalk becomes an increasingly important issue as data rates increase. Crosstalk can occur anywhere in the system, such as the silicon die, package, or printed circuit board. Crosstalk can affect the voltage and/or timing characteristics of the victim signal, depending on the nature of the coupling between the aggressor and victim. The effects of crosstalk on the victim’s voltage are straightforward [1]. However, the effects of crosstalk on the victim’s timing jitter are less understood. The theories and practices of high-speed timing jitter are well established. Quantities such as random jitter (RJ), periodic jitter (PJ), data-dependent jitter (DDJ), deterministic jitter (DJ), and total jitter (TJ), are well-understood and widely accepted [2]. However, only minimal consideration has been given to crosstalk phenomena in the form of bounded, uncorrelated jitter (BUJ) [3,4]. Since jitter caused by crosstalk is not clearly addressed by the standard definitions of jitter components, most commercially available jitter analyzers are unable to measure it and may misinterpret it as one of the common components of jitter, such as RJ, PJ, or DDJ. In particular, if the victim’s RJ rms value is contaminated, the estimated total jitter is even more severely impacted. We examine real-life situations for each of these cases. This knowledge will help engineers diagnose crosstalk issues by recognizing its effects. What is Crosstalk? Crosstalk, and noise coupling in general, is defined as an interference between two signals in such a way that one signal creates an undesired effect in another signal [5]. Within this broad definition, we can define two categories of crosstalk. In one category, the aggressor signal primarily affects the voltage characteristics of the victim signal. In the other category, the aggressor signal primarily affects the timing characteristics of the victim signal. For an aggressor attacking a victim’s voltage, the coupling mechanisms are generally inductive and/or capacitive between the two signals [5]. Typically, this is caused by two signal traces being in close proximity to each other. Naturally, any impact on the voltage of the victim will have a secondary effect on the timing of the victim due to slopes of the transition edges. This type of coupling has been explored extensively and is generally well-understood [1]. For an aggressor attacking a victim’s timing, the coupling is usually between the aggressor and the victim signal’s power supply or clock. The result is that the aggressor corrupts the timing of the victim signal. This type of coupling is often discussed, but not always well understood. In addition to the coupling mechanism, the effects of crosstalk strongly depend on the relationships between various properties of the aggressor and victim signals such as data rate, phase, and data pattern. Signal Spectra vs. Jitter Spectra When considering issues in signal integrity and jitter, it is important to clarify concepts such as power spectrum and jitter spectrum. Figure 1 shows plots for a voltage waveform, power spectrum, jitter time interval error (TIE) [6], and jitter spectrum.