frames of the chip packages are quite inductive. This makes it necessary to understand how
these reactive elements affect the reflections in a system. In this section we introduce the
effect that capacitors and inductors have on reflections. This knowledge will be used as a
basis in future chapters, where capacitive and inductive parasitic effects are explored in
Reflections from a Capacitive Load.
When a transmission line is terminated in a reactive element such as a capacitor, the
waveforms at the driver and load will have a shape quite different from that of the typical
transmission line response. Essentially, a capacitor is a time-dependent load, which will
initially look like a short circuit when the signal reaches the capacitor and will look like an
open circuit after the capacitor is fully charged. Let's consider the reflection coefficient at
time = TD and at time = t
. At time = TD, which is the time when the signal has propagated
down the line and has reached the capacitive load, the capacitor will not be charged and will
look like a short circuit. As described earlier in the chapter, a short circuit will have a
reflection coefficient of -1. This means that the initial wave of magnitude V will be reflected
off the load with a magnitude of -V, yielding an initial voltage of 0 V. The capacitor will then
begin to charge at a rate dependent on , which is the time constant of an RC circuit, where
C is the termination capacitor and R is the characteristic impedance of the transmission line.
Once the capacitor is fully charged, the reflection coefficient will be 1 since the capacitor will
resemble an open circuit. The voltage at the capacitor beginning at time t = TD is governed
shows a simulation of the response of a line terminated with a capacitive load.
The load capacitance is 10 pF, the line length is 3.5 in. (TD = 500 ps), and the driver and
transmission line impedance are both 50
. Notice the shape of the waveform at the source
(node A). It dips toward 0 at 1 ns, which is 2TD, the time that the reflection from the load
arrives at the source. It dips toward zero because the initial reflection coefficient off the
capacitor is -1, so the voltage reflected back toward the source is initially V
), where V
is the initial voltage launched onto the transmission line. The capacitor then charges to a
steady-state value of 2 V.
Figure 2.22: Transmission line terminated in a capacitive load.
If the line is terminated with a parallel resistor and capacitor, as depicted in
voltage at the capacitor will depend on