HP 54753A User Manual page 264

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements
TDR Example 6
The graph data in Figure 11-18 will be used to determine the percent of crosstalk
coupling for the circuit of Figure 11-17. From the dimensions of the lines given
in Figure 11-17 (a), K is found to be 0.055 from the graph. This means that if
B
one line (the active line) were driven with a signal, the other line (passive)
would have a coupled signal of 5.5% of the amplitude on the active line, in a
direction opposite to that of the driving signal. Since both 100 Ω lines are active
simultaneously, the reflection observed on the TDR is twice as much, or 11%.
From Figure 11-17, the actual crosstalk can be seen to be about 8%.
In very high speed systems, the exact shape of a line can be important, if
reflections are to be kept to a minimum. The arrangement shown in Figure 11-
19A has been used to investigate the behavior of two different line shapes. For
one line, corners are sharp. This permits the width of the line to be larger at
corners than elsewhere. Figure 11-19B shows that a -7.5% reflection occurs at
point 6 due to the lowered characteristic impedance at the corner. For the other
line, the corners are rounded to produce a constant line width. Figure 11-19C
shows that a constant line impedance exists for the second line. Note that an
inductive reflection, as discussed before, occurs at the end of the line due to
the inductance of the resistor. In conclusion, it is desirable to have smooth,
rounded line edges and constant line widths when designing transmission lines
for high speed systems. Resistor leads should be kept short to minimize
termination inductance.
11-36