Differential probes locate emissions

Engineers often employ a magnetic loop when tracking down emissions leaking from seams in equipment enclosures. But you can also use a high-bandwidth (1.8 GHz) differential probe to measure the voltage across a seam. By properly interpreting this measurement, you can estimate the emissions potential of the seam.

You can measure RF leaking through a cabinet's seam by using a differential voltage probe.

When measuring the voltage, consider the following:

• Is the measured voltage coming from a source inside the equipment? To find out, turn off the equipment and make another measurement.
• Is the measured voltage due to voltage across the seam or the probe's common mode response? Short the probe tips together and then to each side of the seam, one side at a time and make test measurements. The voltages should be much lower than the measured voltage across the seam.
• If the measured voltage is really across the seam, determine if that voltage will result in current flow? Without current flow, there would not likely be significant emission. For a metal enclosure whose measurements are a substantial fraction of a wavelength, there will most likely be RF current flow and emission.

To check if there is current flow, put a 75-Ω resistor across the seam. If the impedance across the seam is low, the voltage will not change appreciably. Low impedance allows RF current to flow and indicates a potential problem.

To get the best results from this technique, use a differential probe with an input impedance of at least 200 Ω at the frequency of interest. Many active differential probes have input impedance as low as 20 Ω, which is too low to make a good measurement.

The figure shows how I used a Fischer FCC-BCP-2 probe across an enclosure seam to make a measurement. The probe was connected to an oscilloscope that I set to the 10-mV range. On this range, the presence of any signal indicates a possible compliance problem.

The measurement showed a strong 500-MHz frequency and a smaller component at about 4 GHz. Because the scope's bandwidth was only 1.5 GHz, the 4-GHz component was really much larger than shown and did, in fact, cause an emissions problem. Filling the slot with an EMI gasket practically eliminated the differential voltage, significantly reducing the emissions that escaped through the seam.

While differential voltage measurements are not substitutes for emissions tests, they are effective, and you can easily make them in the development lab to lower development costs and speed equipment to final approval.