Crosstalk is a fact of life in modern digital systems. We can't eliminate it, but it's our job to figure out how to control it, manage it, and just plain live with it.
Consider the circuit in Figure 1. In the terminology of crosstalk, the gate at bottom left in this particular diagram is the aggressor, and the gate at top right is the victim. Whenever the aggressor changes state, we observe a pulse of crosstalk at the victim. For those of you doing dense, high-speed designs, this is probably an all-too-familiar scenario.
One of the fascinating things about crosstalk is its directionality. Crosstalk waveforms are a function of the direction of current flow. For example, in Figure 1, if we reverse the direction of current flow on the aggressive trace, the crosstalk received on the victim will reverse polarity.
Directionality is an important concept, so I'll go over it step-by-step. First, set up the circuit as in Figure 1. Measure the crosstalk at victim 1 when the aggressor switches from low to high. You will see a blip of crosstalk at victim 1 that initial goes negative. It happens coincident with the arrival at the triple loads of the aggressor signal.
Next, wire a second victim in a direction opposite the first (Figure 2). Repeat the crosstalk measurement, this time observing both victim signals. This time, you will see an initial positive blip of crosstalk on victim 2, a complete reversal of polarity as compared to the blip on victim 1.
The polarity reversal tells us that we are not dealing with capacitive crosstalk. Many digital engineers assume that crosstalk is primarily a capacitive effect. It isn't. Mutual capacitance between two single-ended circuits can only cause positive crosstalk.
The polarity reversal indicates that the interference is due in great measure to mutual inductive coupling. That's the same kind of coupling you get in a transformer. It is well-known that reversing the leads on the primary winding of a transformer will reverse the polarity of the voltage on the secondary. Coupled pc-board traces act in much the same way. If you think of each PCB trace as a little loop of current, you can see how the "crosstalk" transformer works.
First, imagine current from the aggressor flowing out through the aggressor trace to the triple loads. From there the current returns, along the power and ground system, back to the aggressor. That aggressive current makes a loop. Think of this loop as the primary winding of a transformer.
The secondary winding of that same transformer lies nearby. It is the loop formed starting with the gate stuck low, drawn at the top left of the diagram. The loop moves out along its trace to the first victim, and returns back along the power and ground system to the stuck-at-low gate.
These two loops behave in many ways almost exactly like a weakly-coupled, single-turn transformer.
The existence of transformer-type mutual inductive coupling between traces has profound implications for digital designs. For one thing, it implies that crosstalk may vary depending on the applied load in our circuits.
For example, in the figure assuming we are working with a short PCB trace (meaning short compared to the rise or fall time of the aggressor), the aggressor current will be a strong function of the total applied load. The heavier the load, the more aggressor current we will draw, and the more crosstalk we will generate. The triple-load network in the figure will generate nearly three times as much crosstalk as a similar net, with a similar topology, having only one load.
This loading effect is particularly acute when driving banks of SIMM memory modules. Such traces tend to be very short, but heavily loaded, so that the drive currents are almost totally dominated by the load capacitance of the SIMM receivers. As we plug in more SIMM modules, crosstalk goes up.
If you are trying to debug a crosstalk problem on a dense multi-layer board, knowledge of how trace loading affects crosstalk can help you uncover, and fix, the problem.
If you are trying to manage crosstalk from first principles, so it comes out right on the first spin, look into the new crosstalk prediction tools that feature IBIS I/O modeling. Many of these new tools are capable of calculating crosstalk, including the loading effects, in an automated, highly efficient manner.