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Troubleshooting EMI with a differential probe

20 Nov 2014  | Kenneth Wyatt

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Calculating and adding in the "per metre" factor, this gives us a calculated 79.2 dBuV/m E-field in free space. Because this type measurement will likely be performed over a reflective floor, we might add 6 dB (double the voltage) as a net worst case scenario, due to the reflected wave appearing in phase with the direct wave, for a worst-case E-field of 85.2 dBuV/m. This is as bad as 55 dB over the FCC Class B limit at 10m, depending on the harmonic frequency. Better bond these two connector shells together in the real product!

Example 2
In this next example, I measured the voltage difference between two video connector shells on an original model Apple TV. The result was 35.9 mV between the two. Use Equations 1 and 2, the estimated E-field would be 69.5 dBuV/m, also very high. Connecting long RCA cables to these jacks would fail the emissions test. Figure 7 shows the probe connection and figure 8 shows the resulting measurement.

 Voltage differential

Figure 7: Measuring the voltage differential between the two connector shells of the video RCA connectors of an older Apple TV.

 Differential measurement across a couple RCA jacks

Figure 8: Differential measurement across a couple RCA jacks on the older Apple TV.

A good practice to take when using a differential probe is to perform a "null experiment". That is, make sure that when shorting the two probe tips together, you read zero volts. That way, you'll be sure no extraneous field coupling is affecting the reading you take. The RS-ZD10 probe as a special configuration feature where, after shorting the two probe tips together, any residual voltage can be cancelled out ("zeroed") by pressing the probe button.

I find the differential probe to be a great EMI troubleshooting tool. Because of the 1 MOhm impedance at each probe tip, the probe may be used to measure any two points in a circuit, so long as you're mindful of the maximum voltage limit. Measuring return plane voltages is very useful and will give you an idea how "noisy" the return plane might be. You can also measure between voltage rails or rail to ground. One thing to note, especially when using some of the longer probe tip extensions, is to try to keep the extension part 90-degrees to the plane of the circuit traces, to minimise magnetic field coupling.

1. Glen Dash, EMI: Why Digital Devices Radiate,, 2005.

2. Rohde & Schwarz, RS-ZD10 probe manual.

About the author
Kenneth Wyatt is an independent consultant and specialist in electromagnetic compatibility (EMC) design, test and troubleshooting. In The EMC Blog, he discusses issues, trends, EMC design, and product troubleshooting techniques. One speciality is his use of do-it-yourself probes and use of low-cost test equipment and measurement techniques for evaluating EMC issues on the workbench.

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