Understanding Crosstalk - Understanding Signal Integrity - page 111

FEXT amplitude as a 1-ns rise time signal switching from 0V to 1V since they both
have a 1V-per-nanosecond swing.
It is apparent from (10.1b) that the FEXT amplitude grows as Kf is made
larger, as the length of the coupling region increases, as the voltage swing ( dv ) in-
creases, and when rise time ( dt ) is small. Therefore, FEXT is combated by lowering
the coupling between traces since that lowers Kf , keeping the coupled length short,
reducing the voltage swing of the aggressor, and lengthening its rise time.

10.3.2 What Are Typical Kf Values?
To find the forward crosstalk for a set of traces, a 2D field solver is used to deter-
mine the mutual capacitance and inductance, and Kf and then FEXT are found with
(10.1a) through (10.1c).
Figure 10.4 shows typical Kf values for a pair of 5-mil (0.13-mm)-wide, half-
ounce 65 ε r = 4.2). Since for-

ward crosstalk is not present on low loss stripline, the graph only shows microstrip.
The separation is given as multiples of the trace width. A separation of 3 rep-
resents 15 mils since the trace is 5 mils wide (or 0.39 mm for a 0.13-mm trace).
We find from Figure 10.4 that Kf is

Ω and 50 Ω solder mask-covered microstrips on FR4 (

− 160 pS/m for a 5-mil (0.13-mm)-wide,

50 microstrip separated by 5 mils (“separation” of 1) from its aggressor. Using

this in (10.1b) and assuming that the launched pulse has a 2V swing ( dv in the
equation) and a 1-ns rise time ( dt ), the forward crosstalk pulse is found to be

Ω

− 320

mV if the traces are coupled for a length of 1 meter. The crosstalk would be − 160

mV if the coupled region was a half-meter in length.

0s/m

− 20 ps/m

− 40 ps/m

− 60 ps/m

− 80 ps/m

− 100 ps/m

65 Ω

50 Ω

− 120 ps/m

− 140 ps/m

− 160 ps/m

− 180 ps/m

2 4 6 8 10

Separation (trace width multiples)

Figure 10.4 Kf for half-ounce, 5-mil (0.13-mm)-wide 50 Ω and 65 Ω solder mask-covered mi-
crostrips on FR4. (Raw data from the Polar Si9000 2D fi eld solver [11].)