Digital Signal Processing Reference
In-Depth Information
Return
Load
Copper
plane
Signal
conductor
Dielectri
c
sheet
Microstrip
trace
Return current
Signal current
Generator
Figure 6.1
A transmission line created by a signal trace and its return.
The side view of a trace over a ground plane is shown in Figure 6.2. We can see
that the trace is one plate of a capacitor with the second plate being the return path
(such as a ground plane). The distributed inductance is formed by the current loop
area between the trace and ground plane. The capacitance and inductance values
are determined by the width of the trace and the distance to the ground plane.
As we will see, the inductance and capacitance values determine the trace im-
pedance. This is why an impedance value is really only meaningful when it is quot-
ed with respect to a specific return plane. We will return to this later in this chapter
and somewhat more indirectly in Chapter 7.
6.3
Circuit Model of a Transmission Line
A circuit model for a small segment of trace is shown in Figure 6.3. The trace has
a resistance (
R
) and an inductance (
L
). The capacitance formed between the trace
and ground plane is modeled by capacitor
C
, and the capacitor losses are modeled
Figure 6.2
Distributed parallel plate capacitors and loop inductance formed between the signal
trace and the ground plane return path.
