Digital Signal Processing Reference
In-Depth Information
s
s
W
b
h
(a)
(b)
(c)
Figure 13.12
(a, b) Edge-coupled and (c) broadside-coupled diff-pairs in cross-section.
13.5.1 Manufacturing Trade-Offs Between Loosely and Tightly Coupled Pairs
Loosely coupled pairs are attractive to fabricate because the alignment and spac-
ing between traces and to the return planes is not as critical as with tightly coupled
traces. Also, the differential impedance of loosely coupled traces is not affected as
much by over etching. This means that it is easier for the manufacturer to obtain the
specified differential impedance even if the traces are not rectangular (see Chapter
8). Both of these factors reduce scrap and so makes manufacturing less costly.
Another consideration is that by making edge coupled traces loosely coupled
the individual routing layers are thinner than they would be if the traces were
tightly coupled. As illustrated in Table 13.1, this makes it possible to include more
routing layers for a given overall board thickness.
The microstrip traces are covered with 1 mil of solder mask having the same
dielectric constant as the laminate.
In Table 13.1 the tightly edge-coupled microstrip and stripline traces are de-
fined as having a trace separation of (
s
) of equal to the trace width, while
s
is three
times the trace width for loosely coupled traces. This designation is arbitrary, but it
allows us to see how choosing the amount of coupling can affect the routing layer
thickness.
On FR4 separating the 4-mil-wide broadside traces by less than about 5 mils
requires impractical board dimensions, so for the broadside case tight coupling is
defined in Table 13.1 as 7 mils. This requires in a plane separation (
b
) of 31 mils
to obtain a 100
differential impedance. Loosely coupling the traces by increas-
ing the trace separation to 20 mils results in a slight reduction in the overall layer
thickness.
Ω
Table 13.1
Approximate Values for
h
and
b
(in Mils) for Tightly and Loosely Coupled
4-Mil-Wide, Half-Ounce 100
Diff-Pairs on
Ω
FR4 (
4.2)
Topology Tight Loose
h or b s h or b s
Microstrip 4 4 3 12
Stripline 42 4 11 12
Broadside 31 7 29 20
The separation between traces is
s
(in mils). Figure 13.12
defines
h
,
b
, and
s
.
ε
r
=
