Differential Signaling - Understanding Signal Integrity - page 172

to pass and do not allow the receiver to be influenced by the transmitter's different
common-mode voltage. This idea is explored in the Problems.
From the Problems we find that (13.4) and (13.5) can be checked by noting
that when the traces are very loosely coupled, R 2 becomes infinite and R 1 becomes
equal to Z o . This makes intuitive sense: Very loosely coupled lines can be treated as
isolated transmission lines and are properly terminated with a resistor equal to Z o .
Some field solvers directly report the even- and odd-mode impedances, which
makes using (13.4) and (13.5) easy. Alternatively, the even and odd impedances can
be calculated by using (9.4) and (9.6) from Chapter 9, as we do in this chapter's
Problems when finding R 1 and R 2.

13.4.2 What Prevents Creating Odd-Mode Signals?
As we have seen, the diff-pair signals must be exactly out of phase for odd-mode
signaling. In fact, anything that causes the two signals to lose this phase relation-
ship (even for a short distance) creates an even-mode voltage that, if large enough,
requires the three-resistor scheme shown in Figure 13.11 for proper termination.
Besides being a signal integrity problem, even-mode currents are a serious electro-
magnetic interference (EMI) concern that can hinder a product from passing radi-
ated emission type testing [24].
Even mode signaling can arise when:

The diff-pair trace lengths are not exactly the same length.

•

The diff-pairs are routed in different dielectrics (microstrip versus stripline)
or on different routing layers.

•

The transmitter output drivers do not switch at precisely the same time.

•

The output drivers have different rise and fall times.

•

Layout techniques (presented in the next section) can help minimize length
matching problems, and proper design of the transmitter output driver (along with
the logic that drives it) can minimize the remaining two items.
Circuit board signal integrity engineers perform simulations or electrical mea-
surements to determine how closely the driver output impedance and rise times are
matched, and the results from field solvers are used to determine if a significant
even-mode component is present in the diff-pairs. If so, it is necessary to use the
three-resistor scheme shown in Figure 13.11. By building detailed circuit models,
the effects that the imperfect driver and unequal trace lengths have on the overall
circuit operation can be determined.

13.5 How Are Differential Transmission Lines Created?

The two traces forming a diff-pair can either be routed side by side on the same
layer ( edge-coupled ) or on top of each other on separate layers ( broadside-coupled ).
Cross-sections of these two configurations ( topologies ) are shown in Figure 13.12.
They may be formed as tightly or loosely coupled pairs, but, as we will see, these
topologies have different electrical characteristics.