Transmission Lines for Oscilloscope Users, Part 2

Figure 1: A transmission line can be seen as
a series of "buckets" of capacitance charged
to a voltage by the signal as it "walks the line."

In Part 1, we experimented with the signal rise time measurement and saw that it appeared to increase substantially as the length of the interconnecting cable was increased. To understand why, we revisited some basic principles of signal integrity:

1. All interconnects are transmission lines. 
2. Signals are dynamic, and once launched, cannot be prevented from propagating down the transmission line.

Be the Signal

To illustrate the dynamic nature of signals, imagine a very simple, 1 ns long, 50 Ω impedance transmission line. As a 1 V signal is launched into the transmission line and propagates, at each step along the way it asks "What's the impedance of the environment?" at its leading edge. That is the instantaneous impedance, notated as Z. Impedance is always defined as the ratio of a voltage to a current. We know the voltage of this signal (1 V), but how do we find the current at the edge? 

Transmission Lines (Part IV): More Essential Principles

Figure 1: The return current in a transmission line
is as important as the signal current

In our continuing survey of the topic of transmission lines, we'd begun in our last post to cover some essential principles that govern their behavior. We're not quite done with those, so in this post we'll discuss more principles that should be part of the foundation of how you think about interconnects.

Transmission Lines (Part III): Essential Principles

Figure 1: All interconnects are transmission lines with a signal
path and a return path (not ground)

Now that we've covered some of the principles and assumptions that underlie transmission lines in two prior posts, we can now directly address the topic with some essential principles that you need to understand.