IEEE Circuits and Systems Magazine - Q3 2020 - 40

D. Stubs
In case a signal needs to be routed to more than one
destination, stubs should be avoided whenever possible.
Fig. 10(a) shows a matched line with a stub, where the stub
is causing reflection that disturbs the rising edge at the
load. By changing the routing in a way that eliminates the
stub, the rising edge is nearly intact, as shown in Fig. 10(b).
Similarly, stubs can also disturb analog signals.

RS

(a)

(b)

(c)

E. Differential Lines
Differential lines are frequently used for high-speed designs due to their known advantages. To sustain those
advantages, the two lines are routed symmetrically. This
means that the distance between the two lines and the
electrical lengths should be the same. Asymmetrical lines
result in the differential signals not arriving out-of-phase,
and they add up to a finite voltage rather than zero. This
finite voltage produces a return current, which in turn can
result in ground bouncing and radiation.
If, after routing, the two lines have different lengths,
length matching is typically restored by adding a zigzag
near the source to the shorter line of the differential pair
[30]. This way, the differential signal will maintain its
out-of-phase relationship throughout the PCB.

(d)

Figure 6. (a) Source matching. This increases the effective
impedance of the source. It also increases the rise time of
the output signal, reducing high frequency contents. (b)-(d)
Various load matching circuits which can provide different
bias conditions [1]. The matching is added in shunt to reduce
the effective impedance of the load, bringing it closer to Z0.

different layers can limit the coupled length between
the lines.
Crosstalk is classified into two types: 1) Backward
(near-end): where the input signal is coupled at the terminal closer to the source. This type is characterized by
a wide pulse crosstalk (2 # x LINE) and an amplitude that
depends on the distance d. 2) Forward (far-end): where
the input signal couples at the terminal further from the
source. The pulse width is comparable to xRISE , and the
amplitude depends on xRISE, d, in addition to the length
of the coupling l.
Fig. 9(a) shows a PCB with two adjacent lines, resulting
in a coupling between them. The effect of changing the
coupling distance (d) and length (l) on forward and backward crosstalk are shown in Fig. 9(b). The structure in
Fig. 9(a) is measured and the crosstalk values are shown
in Fig. 9(c) versus frequency. The effects of the length and
width effect, at 4 GHz, are shown in Fig. 9(d) and (e).

F. Fiber Weave Effect
The fiber weave effect rises when a signal is routed on
a fiber-based substrate (such as FR-4). A trace might
be above the fiber material, while another trace might
be above epoxy. Since the two materials have different
dielectric constants, the electrical length might differ
from the simulation results, which is problematic in
-differential lines and buses routing. This problem becomes prominent at frequencies higher than approximately 5 GHz [30].
To resolve the fiber weave effect, the lines can be routed in a low-angle zigzag (around 10°). This averages out

Signal Path

Signal Path

Top Copper Layer

+
-

Common Mode
Path

Reference for Top Layer
Return Path
Reference for
Bottom Layer
Bottom Copper Layer

+
-

(a)

(b)

Figure 7. (a) When transitioning from one layer to another, the reference ground return paths should be connected to minimize
impedance disturbance. (b) A similar concept applies to differential lines. The reference grounds have to be cleared between the
transitioning vias to maintain the coupling between the differential lines.

40 	

IEEE CIRCUITS AND SYSTEMS MAGAZINE 		

THIRD QUARTER 2020
IEEE Circuits and Systems Magazine - Q3 2020

Table of Contents for the Digital Edition of IEEE Circuits and Systems Magazine - Q3 2020
