Digital Signal Processing Reference
In-Depth Information
are tightly packed together, laddering can be reduced by increasing the separation
between segments.
14.4.4 Guard Traces
Placing a properly grounded guard trace between segments in the serpentine can
reduce or eliminate laddering since this reduces crosstalk between the segments.
Chapter 10 describes how to properly ground a guard trace. Adding them slightly
reduces the serpentine delay [13, 17], but makes the delay more uniform across a
broad range of frequencies [17].
14.5 Vias
A 3D rendering of a through hole via extending through a plane in a circuit board
is shown in Figure 14.7.
On each end the via, a circular ring of metal (“the pad”) connects to the signal
trace, and a hole (the antipad) in the plane permits the conducting barrel to pass
through without making a connection.
14.5.1 Via Circuit Model
Figure 14.8 shows how capacitance is present from the via barrel and the pads to
the plane. All of these capacitors are summed into a single capacitance,
Ct
. An in-
ductor (
Lt
) models the inductance of the barrel and its return path.
Pad
Trace
Barrel
Antipad
Plane
Figure 14.7
A through hole via passing through a plane. The antipad prevents the barrel from con-
necting the plane. A pad connects the signal trace to the barrel. (Figure generated by the Simbeor
fi eld solver [18].)
