Crosstalk. Crosstalk is the electromagnetic coupling between conductors that are close to each other. Crosstalk is an EMC concern because it deals with the design of a system that does not interfere with itself. Crosstalk is may affect that radiated/conducted emission of a product
Crosstalk is the electromagnetic coupling between conductors that are
close to each other.
Crosstalk is an EMC concern because it deals with the design of
a system that does not interfere with itself.
Crosstalk is may affect that radiated/conducted emission of a product
if, for example, an internal cable passes close enough to another cable
that exists the product.
Crosstalk occurs if there are three or more conductors; many of the
notions learnt for two-conductor transmission lines are easily
transferred to the study of multi-conductor lines.
Crosstalk in three-conductor lines
Consider the following schematic:
near end terminal
far end terminal
The goal of crosstalk analysis is the prediction of the near and far end
terminal voltages from the knowledge of the line characteristics.
There are two main kinds of analysis
This analysis applies to many kinds of three-conductor transmission
lines. Some examples are:
reference conductor (ground plane)
As in the case of two-conductor transmission lines, the knowledge
of the per-unit length parameters is required. The per-unit length
parameters may be obtained for some of the configurations shown
as long as:
1) the surrounding medium is homogeneous;
2) the assumption of widely spaced conductors is made.
Assuming that the per-unit-length parameters are available, we can
consider a section of length of a three-conductor transmission line
and write the corresponding transmission line equations.
It turns out that by using a matrix notation, the transmission line
equations for a multi-conductor line resemble those for an ordinary
two-conductor transmission line.
Let us consider the equivalent circuit of a length of a three-conductor
The transmission line equations are:
The meaning of the symbols used in (1) and (2) is:
We will consider only structures containing wires; PCB-like structures
can only be investigated using numerical methods.
The internal parameters such as rG, rR, r0 do not depend from the
configuration, if the wires are widely separated. Therefore we only
need to compute the external parameters L and C.
It is important to keep in mind that for a homogeneous medium
surrounding the wires, two important relationships hold:
The elements of the L matrix are found under the assumption of wide
separation of the wires. In this condition the current distribution around
the wire is essentially uniform.
We recall a previous result for the magnetic flux that penetrates a
surface of unit length limited by the edges at radial distance and
as in the following.
Then we consider a three-wire configuration:
For this configuration we can write:
Using the result of (11), we obtain
And from these elements, we obtain the capacitance using
Consider the following circuit:
The closed form expression for the near and far end voltages and
are very complex so we will introduce additional simplifications:
1) the line is electrically short at the frequency of interest;
2) the generator and receptor circuits are weakly coupled, i.e.:
Under these assumptions, the near end voltage simplifies to:
and the far end voltage becomes:
In (19) and (20)
The meaning of (19) and (20) is that for electrically short and weakly
coupled lines the voltage due to the crosstalk are a linear combination
of the inductance lm and capacitance cm between the two circuits.
In addition, inductive coupling is dominant for low-impedance loads
(high currents), whereas capacitive coupling is dominant for high-
impedance loads (low currents).
It turns out that (we skip the proof) if losses are included a significant
coupling results at the lower frequencies. This phenomenon is called
Time-domain solution: exact solutions are more difficult to derive for
multiple transmission lines, so we will not consider them.