2.2. WAVE PROPAGATION
At high frequencies, when the edge rate (rise and fall times) of the digital signal is small
compared to the propagation delay of an electrical signal traveling down the PCB trace, the
signal will be greatly affected by transmission line effects. The electrical signal will travel
down the transmission line in the way that water travels through a long square pipe. This is
known as electrical wave propagation. Just as the waterfront will travel as a wave down the
pipe, an electrical signal will travel as a wave down a transmission line. Additionally, just as
the water will travel the length of the pipe in a finite amount of time, the electrical signal will
travel the length of the transmission line in a finite amount of time. To take this simple
analogy one step further, the voltage on a transmission line can be compared to the height
of the water in the pipe, and the flow of the water can be compared to the current.
Figure 2.2
depicts a common way of representing a transmission line. The top line is the signal path
and the bottom line is the current return path. The voltage V
i
is the initial voltage launched
onto the line at node A, and V
s
and Z
s
form a Thévenin equivalent representation of the
output buffer, usually referred to as the source or the driver.
Figure 2.2: Typical method of portraying a digital signal propagating on a transmission
line.
Summary :
WAVE PROPAGATION At high frequencies, when the edge rate (rise and fall times) of the digital signal is small compared to the propagation delay of an electrical signal traveling down the PCB trace, the signal will be greatly affected by transmission line effects.
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