Digital Signal Processing Reference
In-Depth Information
traces must be reduced in width (necked down) to fit through the pin field, which
increases the trace impedance through that region.
In practice the communication link may be robust enough to operate satisfac-
torily when the signal is exposed to any of these types of discontinuities. This is
particularly true if the data rate is low enough to make the period of the discontinu-
ity small compared to the signal bit time. However, even a low data rate link can
fail if many of these discontinuities are simultaneously present.
13.6.2 Signal Trace Between Diff-Pairs
When routing edge-coupled diff-pairs, insure that there are no signal traces routed
between the traces forming the diff-pair. It is not usually possible to violate this rule
when the traces are tightly coupled, but violations can arise when the traces are
widely separated (loosely coupled).
Violation of this rule usually comes about when space between the diff-pairs
is large enough for the autorouter to place a signal trace in the area between the
diff-pairs. Autorouting software is a tool often used by layout designers because
it speeds the layout process by working from a list of nets to automatically place
and connect traces to components. If there is enough space, the autorouter may
route signals between the traces forming the loosely coupled diff-pair unless it is
purposely restricted from doing so. This type of error can be hard to find on dense
boards because the errant trace (or traces) may only run for a short distance.
13.6.3 Keeping Signal Traces Far Away from Diff-Pairs
The top view of either a stripline or microstrip edge coupled diff-pair is shown in
Figure 13.14. Traces
A
1 and
A
2 are aggressors routed alongside the diff-pair (
DO
+
and
DO
). It is apparent that trace
A
1 will preferentially couple onto
DO
+ and
will couple much less to
DO
−
. This asymmetric coupling changes the impedance of
DO
+ and makes the noise common mode, which will not be rejected by the receiver.
For these reasons the separation from the aggressor to the diff-pair (
sn
) must
always be much greater than the separation between the traces forming the diff-
pair (
sd
). This rule applies to either broadside or edge-coupled pairs.
−
A1
sn
DO
+
DO
−
sd
A2
Figure 13.14
Top view showing four traces. Prevent common mode noise by making the spacing
between the diff-pair (
sd
) smaller than the spacing to other traces (
sn
).
