Whispers about crosstalk

Crosstalk in signal cabling doesn’t come up that often among control system designers because it’s hard to get a handle on. So should you be concerned about this? Editor in Chief Joe Feeley says yes.

 By Joe Feeley, Editor in Chief

 

 

T

he engineers at Belden CDT Electronics define it as unwanted transfer of energy from one signal path coupled to an adjacent or nearby signal path, creating unwanted or erroneous data within a computer link or data system. An easy to understand example of crosstalk is those faint background voices we sometimes hear during a phone conversation. Unless, of course, the FBI is tapping your line.

“Most cable crosstalk problems don’t need detailed analysis,” report electrical engineering consultants William Kimmel, PE, and Daryl Gerke, PE, Kimmel Gerke Associates. "You just need an idea of whether crosstalk might occur and how to handle it.”

Capacitive crosstalk shows up in circuit boards, while inductive crosstalk is mostly a cable issue. Most ground treatments involve a single-point ground, which works well for low-frequency capacitive crosstalk, but is not appropriate for inductive crosstalk, report Kimmel and Gerke.

With high-impedance input circuits, electric field coupling is the key concern, so a single-point ground is adequate, and even preferred. A multiple-point ground will create a potential problem from ground loops.

The multipoint ground also works well for low-frequency magnetic fields, an increasingly important consideration, say the consultants. “If you are dealing with magnetic fields, you need to terminate both ends to provide a return path to cancel the magnetic field. A single-point ground does not provide the return path so cancellation cannot occur.”

So, should you be concerned about this? “The answer is, increasingly, yes,” continue Kimmel and Gerke. “The reason lies in the increasing use of motor drives.”

The use of DC motors or sine wave AC motors is declining as newer motor drives provide more efficiency and far-more-precise speed and position control. This means square waves in the cable, with substantial voltages and currents.

Think of a three-phase drive, where, typically, you switch one phase at a time, so you always have a return path. “But what happens at the transitions—a glitch, and that returns on a fourth path, typically a ground wire or, better yet, a shield grounded at both ends,” warn Kimmel and Gerke.

Looking at the cable from a distance, the currents cancel fully, they say, and the associated magnetic fields cancel almost completely (complete cancellation normally only happens with coaxially spaced conductors). “You can’t get that condition except for special cases—obviously including coax—but you can get closer using a circumferential shield than you can with a simple adjacent ground wire,” the consultants explain. “You’ll get even better cancellation by twisting the wires, then shielding, but that isn’t practical for most power-level applications. So, for magnetic field shielding in motor drives, ground both ends of the shield.”

The shield needs to surround the entire set of power conductors. If there are shields around the individual power conductors, they can be single-point grounded to block electric fields, but this won’t block magnetic fields. “You can block those magnetic fields by grounding that shield at both ends, but you’ll end up carrying a lot more current on the shield than you would like,” they add.

You may be faced with driving a variety of power and signal wires between source and load. The big noise maker is the motor drive, but you might be running other types of signal between the two boxes, including low-level analog, serial data, encoder data, etc.
The consultants argue that the best solution here is separate cables for each kind of signal or power wire. But if you decide to put everything in one cable, you’ll need several sets of shields within the cable.

You might have multiple shields surrounding the conductors. “Now you have the best of both worlds,” say Kimmel and Gerke. “You single-point ground to block electric fields and two-point ground to block magnetic fields. Generally, you ground the inner shield at one end, and the outer shield at both ends. Given a good termination on the outer shield, it will block high frequencies as well as low-frequency magnetic fields. The ground voltage produced by the ground loop is intercepted by the inner shield.”

Measuring crosstalk, according to the Belden engineers, mainly consists of inserting a pulse signal of known strength in a line they refer to as the drive line and measuring the effect of this signal in the adjacent sample line. The result is expressed as % crosstalk.

The good news about cables is that they can usually be changed without modifying the equipment itself—an important bonus when a problem is uncovered in the field.

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