How to Connect the Returns to an Unshielded Twisted Pair

Figure 1. Three options for connecting a TDR to a UTP.

During a recent webinar, Dr. Eric Bogatin was asked several questions about how to measure unshielded twisted pairs (UTPs), which are differential pairs with no return path. Here is his answer to one of the questions. For more information, check out the whole webinar on Differential Pairs with No Return Paths.

Q: For the UTP measurement, do you need to connect the SMA connector shield grounds together?

A: Yes. It’s very important to make sure the grounds are connected at the cable connectors.

There are really three ways the connections can be made from a differential TDR to the UTP cable. These three options are shown clockwise from left to right in Figure 1.

Putting Probes in Perspective

Figure 1: Probe, cable, and oscilloscope form a system
that makes or breaks the accuracy of signal acquisitions

Few aspects of using an oscilloscope are as important as the probe: after all, the probe forms both the mechanical and electrical interfaces between the device under test (DUT) and the oscilloscope itself. To feed a signal into an oscilloscope, we're limited to a coaxial connection. Thus, we need a geometry transformer that picks up the signal of interest from the DUT and transfers it to the oscilloscope's coaxial connection.

The Causes of Ground Bounce and How To Avoid It

Figure 1: This cross section of a 100-MHz microstrip
transmission line shows us how a return path should look

We've been discussing the topic of ground bounce on digital I/O lines as well as an effective way to diagnose and analyze it. We've also run through a detailed example of how to measure it using a quiet-low I/O driver as a sense line. Now, let's take a step back and examine the root causes of ground bounce, and also discuss some best design practices for avoiding it altogether.

More on Quiet-Low I/O Drivers and Ground Bounce

Figure 1: To configure an I/O driver as a quiet-low line, its
output is connected directly to Vss on the die

Ground bounce can plague digital I/O lines with bit errors and turn your hair grey trying to uncover the cause in the process. But there is a trick you can use to make the analysis a little easier: using a quiet-low I/O driver as a sense line to reveal the existence, and magnitude, of ground bounce in your system.

About Ground Bounce and How to Measure It

Figure 1: Shown are five I/O drivers
within a package driving signal
lines on a PC board

Designing and/or troubleshooting a system with, say, an MCU driving signals across transmission lines, can be an interesting exercise in patience and diligent sleuthing. Perhaps you're seeing an inordinate amount of bit errors at the receive end of I/O lines but having some difficulty nailing down the source. In many cases, the problem is ground bounce, an issue that can be tough to diagnose and cure. Let's begin an examination of the ground-bounce phenomenon by explaining how it arises and then outlining an approach for finding it.

Inter-Symbol Interference (or Leaky Bits)

Figure 1: Inter-symbol interference, or ISI jitter, is the result
of information from one bit "leaking" to subsequent bits

In reviewing the subject of debugging high-speed serial links, one important aspect of signal integrity we must touch on is inter-symbol interference (ISI). ISI is the phenomenon in which information from one bit "leaks" to some subsequent number of bits.

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.