TOPICS IN HIGH-SPEED CHANNEL MODELING - Advanced Signal Integrity for High-Speed Digital Design - page 427

traversing a split in the reference plane, the coupling mechanism is the slot
itself. A voltage is established between each side of the slot and propagates via
the slotline transmission mechanism from the sourcing line to the victim, where
it couples onto the line with much more efficiency than conventional crosstalk.
Figure 10-12a shows an example of two transmission lines routed over a
slot in the ground plane, and Figure 10-12b shows a measured response of
a test board with a pair of 65- microstrip transmission lines 1.4 in. apart
traversing a 25-mil gap. Note that the coupling coefficient is approximately
20% ( 0 . 05 mV / 0 . 25 mV ) , which is much higher than what would be expected
between the two microstrips routed over a solid reference plane.
The structure shown in Figure 10-12a can be modeled similar to the way
it was done for the single line crossing a slot, as shown in Figure 10-13. The
top circuit line represents the sourcing transmission line that crosses a gap. The
voltage-controlled voltage source (VCVS) represents the physical location where
the transmission line crosses the slot. The middle circuit is a transmission line
that represents the slot and the current-controlled current sources (CCCSs) are
placed at the physical location on the slot where the transmission lines cross. The
current flowing through the VCVS on the sourcing line is mirrored into the CCCS
on the slotline, which represents the current diverging around the gap. The same
circuit is duplicated for the victim transmission line, which effectively models
the coupling by means of the slotline. Figure 10-14 shows a simulated example
of loss-free slotline coupled crosstalk, where D 1 . 0 in. with

50- microstrips. Note that the coupled voltage is about 22% of the voltage

= 0 . 462 in. and S =

Slot

Driving line

D

50

50

Victim line

S

A

500 mV

50 Near end Far end

50

D

(a)

0.1

0.0

− 0.1

0 2 4 6 8 10

Time, ns

(b)

Figure 10-12 (a) Two transmission lines passing over a slot in the reference plane used
to develop an equivalent circuit; (b) measurement of coupled voltage when S = 1.4 in.,

slot width = 25 mils , and 65 - microstrip transmission lines on FR4.