The Perils Of Return Path Gaps

The radio frequency world is full of mysteries, some of which seem to take a lifetime to master. And even then, it seems like there’s always something more to learn, and some new subtlety that can turn a good design on paper into a nightmare of unwanted interference and unexpected consequences in the real world.

As [Ken Wyatt] aptly demonstrates in the video below, where you put gaps in return paths on a PCB is one way to really screw things up. His demo system is simple: a pair of insulated wires running from the center pins on BNC jacks and running along the surface of a piece of copper-clad board to simulate a PCB trace. The end of each wire is connected to the board’s ground plane through a 50 ohm resistor, with one wire running over a narrow slot cut into the board. A harmonics-rich signal is fed into each trace while an H-field EMC probe connected to a spectrum analyzer is run along the length of the trace.

With the trace running over the solid ground plane, the harmonics are plentiful, as expected, but they fall off very quickly away from the trace. But over on the trace with the gapped return trace it’s a far different story. The harmonics are still there, but they’re about 5 dBmV higher in the vicinity of the gap. [Ken] also uses the probe to show just how far from the signal trace the return path extends to get around the gap. And even worse, the gap makes it so that harmonics are detectable on the unpowered trace. He also uses a current probe to show how common-mode current will radiate from a long conductor attached to the backplane, and that it’s about 20 dB higher with the gapped trace.

Hats off to [Ken] for this simple explanation and vivid reminder to watch return paths on clock traces and other high-frequency signals. Need an EMC probe to check your work? A bit of rigid coax and an SDR are all you need

9 thoughts on “The Perils Of Return Path Gaps

  1. One of the things I learned when I’d just started doing pcb layout full-time is provide an uninterrupted ground plane, or if that’s completely impossible (double layer board) at least provide a current return trace that’s exactly beneath the loud trace from end to end, for anything over a couple khz.

  2. One of my first jobs many years ago was as a tuning diode product engineer, and I once made a trip to a company that built mechanical TV tuners trying to convert them to electronic tuning. I met with these two old RF design engineers who really knew their stuff. Their mechanical tuners were enclosed in thick stamped metal casings, with separate “rooms” for the RF amp, the mixer, and the oscillator. One of the old guys described how they could move RF currents around in the walls of the case to get more or less coupling by punching a small slot in the case here or there. It was an art form.

  3. Ran into this problem with an ethernet port on a PCB I CNCed. GND connection passed Kicad’s DRC, but the return path for the TX+/-, RX+/- signals went all over the board. Wired a GND jumper and the ethernet port started “magically” working.

  4. His intentions are commendable, but the explanations are unfortunately rather scant. The issue with such incomplete explanations is that it leaves EMC as “black magic” for many. Much more could have been explained here, and for example, equivalent circuit diagrams could have been used. What a pity.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.