The Real Truth About Crosstalk
(Originally
published in
Electronic Design
Magazine, August, 1997)
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 the figure below. In the terminology of
crosstalk, the gate at position (A) is the aggressor, and
the gate at position (D) 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 (D) when the gate at (A) switches from low
to high. You will see a negative blip of crosstalk at (D)
coincident with the arrival at position (B) of the
aggressor signal.
Next, reverse
the direction of current flow on the aggressive signal.
This means rearranging the circuit to place the
aggressive driver on the right at position (B) and its
triple load on the left at position (A). Repeat the
crosstalk measurement, again observing at position (D).
This time, you will see a positive blip of
crosstalk, (a reversal of polarity).
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 (at least
in part) 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 gate at position (A) flowing out through
the aggressor trace to the load at (B). From there the
current returns, along the power and ground system, to
the gate at (A). The 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 at position (C),
moving out along the victim trace to the load, and back
along the power and ground system returning to the gate
at (C).
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, 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.
|
