3.7. CROSSTALK TRENDS
As described in
Section 3.6
, the impedance variations due to crosstalk depend on the
magnitude of the mutual capacitance and inductance. In turn, since the mutual parasitics will
depend heavily on the cross-sectional geometry of the trace, the impedance variation will
also. Lower-impedance traces will tend to exhibit less crosstalk-induced impedance variation
than will high-impedance traces for a given dielectric constant. This is because lower-
impedance traces exhibit heavier coupling to the reference planes (in the form of a higher
capacitance, see
Chapter 2
). A transmission line that is strongly coupled to the reference
planes will simply exhibit less coupling to adjacent traces. Lower-impedance lines are
usually achieved either by widening the trace width or using a thinner dielectric. However,
there are negative aspects to each of these choices. Wide traces will consume more routing
area, and thin dielectrics can be very costly (thin dielectrics tend to have shorts between
signal and reference plane layers, so the production yield is low).
Figure 3.13a and b
show how even- and odd-mode impedance varies with trace-to-trace
spacing. Notice that both the odd- and even-mode impedance values asymptotically
approach the nominal impedance. The target line
Figure 3.13: Variations in impedance as a function of spacing: (a) typical stripline two
conductor system; (b) typical microstrip two-conductor system.
Summary :
Lower-impedance traces will tend to exhibit less crosstalk-induced impedance variation than will high-impedance traces for a given dielectric constant. This is because lower- impedance traces exhibit heavier coupling to the reference planes (in the form of a higher capacitance, see Chapter 2 ).
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