cross talk issues

Key terms:

interconnect delay
crosstalk delay (speed up or slow down): occures when input to both the lines are undergoing voltage transition
crosstalk pulse: occurs when one line is maintained at a constant voltage leval and the other line is undergoing a step transition.
Process variation
Signal intigrity
Crosstalk Slowdown :It occurs when the Aggressor as well as the victim line has voltage transition in opposite directions
Crosstalk speedup:It occurs when Both the aggressor and the victim lines undergo same kind of transiton
If we look into the details of the crosstalk pulse :
It propagates into the victim line in both the directions forward as well as backward.
Forward crosstalk :A voltage pulse in the victim line in forward direction
Backward crosstalk : A voltage pulse in the victim line in the backward direction

(X)
<------------------------(2)---------------------(backward crosstalk pulse due to inductive coupling)
_________________________.____________________________ Victim line
<----------------(1)-------------- ---------(1)-------------->(forward crosstalk pulse due to capacitive coupling) (backward crosstalk pulse due to capacitive coupling)



_________________________.

':.________________________________(signal transition)

_________________________._______________________________ Aggressor line.
(X')
Fig: Illustration of crosstalk effects in a victim line due to capacitive as well as inductive coupling


Let us consider the situation in the above parllel lines .The line at the bottom, which is driven by the input and undergoing a high to low transition is called the aggressor line or the driven line .The line at the top is the victim line .Let us consider the plane XX' as the starting point for the transition of the signal in the driven line .Let us first analyze the capacitive coupling effect.we observe the forward as well as backward going pulse in the victim line .This may be understood by considering the electron at the plane XX' in agressor line which repels the electron in the victim line and the electron in the victim line moves almost randomly in either direction .This leads to a current in both the direction shown in the figure as (1).This effect is called capacitive coupling effect because this phenomenon is similar to what occurs in the
capacitor ,as the electron moved in to the positive plate repels the electron coming from the negative plate.
The inductive effect produce a pulse in backward direction only .As the current flows in the aggressor line in the forward direction ,the magnetic lines of force passes through the victim line leading to the current flow in the opposite direction in the victim line.
Hance capacitive coupling contributes to both forward as well as backward crosstalk but inductive coupling only contributes to backward crosstalk pulse.

Backward crosstalk pulse is
(1) purely the function of the coupling length* .
(2) Its amplitude does not depend upon the coupling strength but its a constant throughout the region.
(3) the backward crosstalk pulse duration is just the double of the propagation time of the signal transition(step transition) in the coupled region*.

Forward crosstalk pulse amplitude is dependent on the length of the coupling region.Its amplitude increases as the input step signal travels in the cupled region*

*Though there is a physical limit on the amplitude ,but practically the length of PCBs are not adquate enough to cross the limit.
(source: mentor crosstalk paper)