Digital Signal Processing Reference
In-Depth Information
The actual value of
V
cc
is important in determining the launched voltage.
Sags on
V
cc
caused by many drivers switching all at once (simultaneous
switching noise, described in Chapter 1) result in lower launched voltages.
This can lead to reduced receiver noise margins, and because the amount of
sag depends on the number of lines switching, the reduction in noise margin
becomes data dependent.
•
A higher voltage will be launched when low output impedance output drivers
are used. This may initially increase the received voltage, but care must be
taken because drivers with too low of an impedance will cause unacceptably
high reflection and overshoot voltages.
•
6.6
How Does the Current in the Return Path Behave?
We have seen how current flows down the transmission lines' signal conductor, but
we have not seen how the current flows in the return.
We might think that the signal must first reach the load before current begins to
flow in the return, but in actuality current starts flowing in the return path as soon
as the signal is launched down the signal line.
We can see this by examining the microstrip shown Figure 6.6 and following a
current pulse (
i
) as it travels down the signal line. The current pulse reaches the first
capacitor, causing it to charge. To complete the circuit, the capacitor's displace-
ment current flows in the return path, back to the source. This occurs immediately,
before the signal has reached the load. Displacement current flows again when the
signal current reaches the next capacitor, and the process continues as the pulse
makes its way down the line. By monitoring the return path we observe that return
current continues to flow as each successive capacitor is charged. Because the dis-
tance between each capacitor is so small, a steady, uninterrupted current flows in
the return path. As illustrated by the arrows, this current moves in lockstep with
the signal current, but because it is flowing in the opposite direction it appears to
be a mirror image of it.
6.6.1 How Does Current Flow in Return Paths That Are Not Ground?
In the previous example the return path was a single plane, tied to ground, but the
return could have been a power plane rather than ground. The analysis for that situ-
ation is not presented here, but is similar to the analysis presented next for striplines
when one of the return paths is ground and the other is a voltage plane.
i
V
cc
i
Figure 6.6
Current in the return path is coincident with current
i
fl owing in the signal trace.
