Digital Signal Processing Reference
In-Depth Information
13.5.2 Electrical Trade-Offs Between Loosely and Tightly Coupled Pairs
There are three electrical characteristics to consider when choosing between loosely
and tightly coupled traces: signal loss, EMI, and noise.
In general, tightly coupled diff-pairs have lower signal losses than loosely cou-
pled pairs when fabricated on the same laminate and with the same trance width.
Losses at 1 GHz are shown in Table 13.2 when the dielectric thicknesses are as
illustrated in Table 13.1.
RF emissions will be reduced with tightly coupled traces [27, 28], so loosely
coupled diff-pairs should be avoided (especially for microstrip diff-pairs) in those
circumstances where EMI is a concern.
13.6
What Are Some Suggested Diff-Pair Layout and Routing Rules?
The electrical requirements for proper differential signaling are:
The differential impedance must be consistent and of the correct value.
•
The traces forming the diff-pairs must be the same length.
•
Noise must be equally coupled to both traces.
•
These three electrical rules are satisfied with the following rules:
1. Use the trace width and spacing specifi ed by the circuit board manufacturer
to obtain the proper differential impedance.
2. Do not allow a signal trace to come between the two traces forming a
diff-pair.
3. Keep signal traces (and vias) much further from the diff-pair than the spac-
ing between the pairs themselves.
4. Avoid routing over splits in the return path.
5. Route the diff-pair traces on the same layer.
6. Minimize layer hopping.
7. Use differential vias when layers must be changed.
8. Match the overall trace lengths.
9. Match the length on each layer.
10. Route traces passing through pin fi elds and near vias so that noise is equally
coupled on both pairs.
Table 13.2
Loss (in Decibels Per Meter)
Simulated [25, 26] at 1 GHz for Tightly and
Loosely Coupled 4-Mil-Wide 100
Ω
Diff-Pairs
on FR4 (
ε
r
=
4.2)
Topology
Tight
Loose
Microstrip
7.9
8.5
Stripline
8.3
8.4
Broadside
8.2
8.9
