HP 54753A User Manual page 262

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
The other, Z , is due to the strips being at equal but opposite potentials and
oo
carrying equal currents in opposite directions. The backward crosstalk voltage,
V , on a passive line is:
B
Z

V = ---------------------- -

B
Z
where E is the signal propagating down the active line. The backward crosstalk
1
voltage shown in Figure 11-17B at point 2 is equal to 8% of the incident voltage
. Since both lines are active, the crosstalk due to one active line is 4% of E
E
1
for a spacing of 80 mils.
Crosstalk is not ordinarily a problem when using MECL III on microstrip or strip
line circuit boards, when line spacings are greater than 30 mils. The mutual
inductance and capacitance between two lines are used to determine the
crosstalk coefficient. Forward crosstalk is normally much smaller than the
backward crosstalk on microstrip lines except for very long lines ( > 5 feet).
Forward crosstalk does not exist at all on strip lines, since they are made with
a homogeneous medium, so that the inductively and capacitively induced
currents cancel.
The backward crosstalk coefficients for various types of microstrip lines on glass
epoxy boards are shown in Figure 11-18.
11-34
– Z
 E
oe oo

1
+ Z
oe oo
(24)
1