IDEAL TRANSMISSION-LINE FUNDAMENTALS - Advanced Signal Integrity for High-Speed Digital Design - page 128

So what can we learn form this? The answer is: symmetry . Whenever a topol-
ogy is considered, the primary area of concern is symmetry. Make certain that
the topology looks symmetrical from the point of view of any driving agent. This
is usually accomplished by ensuring that the lengths, impedances, and loading
are identical for each leg of the topology. The secondary concern is to minimize
the impedance discontinuities at the topology junctions, although this may be
impossible in some designs.

3.5.6 Effect of Rise and Fall Times on Reflections
The rise and fall times of real digital waveforms begin to have a significant
effect on the wave shape when they become less than twice the delay (2 τ d )
of the transmission line. Figures 3-37 and 3-38 show the effect that finite rise
and fall times have on over- and underdriven transmission lines. Notice how
significantly the wave shape changes as the rise time exceeds twice the delay
of the line. When the edge rate exceeds twice the line delay, the reflections are
masked because the amount of time that it takes to transition from a low state to
a high state (or vice versa) exceeds the period of the reflections.

3.5.7 Reflections from Reactive Loads

In real systems, there are rarely cases where the loads are purely resistive. The
input to a CMOS gate, for example, tends to be capacitive. Additionally, bond
wires, vias, lead frames of the chip packages, chip sockets, and daughtercard

t s = 250 ps

v s

25 Ω

50 Ω A

0-2 V

(a)

t r = 10 ps

3

2

t r = 250 ps

1

t r = 500 ps
t r = 750 ps

0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Time, ns

1

−

(b)

Figure 3-37 (a) Overdriven transmission line; (b) example of how increased rise and
fall times mask the reflections.