We in the EMC/EMI world often use the saying "Don't put the well next to the outhouse" to describe large EMI sources located next to very sensitive victims.
We in the EMC/EMI world often use the saying "Don't put the well next to the outhouse" to describe large EMI sources (outhouses) located next to very sensitive victims (wells).
Incidentally, this saying probably comes from growing up in Nebraska. I still remember outhouses in my small town. I guess the image stuck.
Here are two of my favorite examples of investigations that involved wells and outhouses.
The first case involved a GPS receiver that was jammed by digital electronics. Although a previous design using similar components had worked fine, the onboard GPS receiver in the new design could not lock on to the satellites.
It was not an easy problem to troubleshoot, since the GPS signal levels are so small in this type of application. The raw GPS signals are in the noise, and sophisticated software techniques are used to recover them. Thus, indirect troubleshooting methods were needed, rather than direct measurements.
Upon examination, I noted the coax cable from the GPS antenna was lying on top of the system processor. I suggested moving it. My client balked (at which point I knew that was not going to be an easy fix), arguing that coax is shielded and doesn't leak.
I countered that no coax is perfect, and that the GPS receiver was looking for nano-Volt signals -- orders of magnitudes below what it would take to jam a nearby digital input.
In any event, when we finally moved the cable, the GPS worked fine. We simply separated the well and the outhouse by a few inches. As a plus, my client no longer believed in the "perfect coax." We've seen similar problems with onboard WiFi and cellular receivers. If you forget everything else, remember this: Radio receivers are extremely sensitive wells.
The second case involved analog sensors that were jammed by a welding arc. A prototype-manufacturing welder used 15 kW of RF to start the arc, which itself could be several inches long. As soon as it started, the electronic controls shut down.
Upon examination, I noted several analog position sensors located within inches of the arc. The sensors, although shielded, were grounded only at one end. This is the preferred technique for 60Hz ground loops, but not for high levels of RF energy.
To make matters worse, the open end of the cable shields were at the sensors.
So we fabricated shield terminations using aluminum foil and hose clamps, and we tried again. Shielding theory said this should work, and it did. But it still amazes me, given the RF power levels. This welder had to be the biggest EMI outhouse I've ever encountered.
The bottom line: Don't put very large EMI sources next to very sensitive EMI victims. Keep the well away from the outhouse.
Daryl Gerke (PE) is a partner at Kimmel Gerke Associates Ltd., an EMI/EMC consulting and training firm. He and his business partner Bill Kimmel (PE) have solved or prevented hundreds of EMI/EMC/ESD problems across a wide range of industries -- computers, industrial controls, medical devices, military systems, vehicles, telecomm, facilities, and more. They have also trained more than 10,000 students in their public and in-house EMC classes. Gerke has a BSEE from the University of Nebraska and resides in Mesa, Ariz.