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Technical Tidbit - February 2019
A Useful E-Field Near Field Probe for Troubleshooting

Near field E-field probe

Figure 1.
Simple E-Field Probe for Troubleshooting

Abstract: E-field probes can be very useful in EMC settings where measurements are usually made in the far field [1][2]. E-field probes behave quite differently in the near field and when used for troubleshooting circuits, they arre usually in the near field. E-field probes, used in the near field, can be useful for troubleshooting designs by concentrating the field to a small portion of a PCB or system. A useful, easy to build, near field E-field probe is describes and its performance illustrated with data.

Figure 1 shows the completed easy to build E-field probe. In Figure 2, the main components of the probe are shown: a BNC barrel and a carpet tack. The tack is selected to have the nail portion the right size to fit the center pin of the BNC barrel. A carpet tack is better than a thumb tack because the nail section is longer and of the right diameter for the BNC barrel. If the tack is painted like the one in Figure 2, sandpaper or a file should be used to remove the paint from the nail section so it makes good contact with the BNC.

BNC plus carpet tack

Figure 2.
BNC Barrel and Carpet Tack Used to Make the E-Field Probe

Next the end of the BNC barrel is covered with Kapton tape (the stuff that holds Silicon Valley together) which is folded down over the outside of the barrel. The tape should be wide enough to completely cover the end of the BNC barrel and some extra. The tack is inserted through the tape into the BNC. A this point, the head of the tack is resting on the tape, insulated from the BNC barrel. Then a second piece of Kapton tape is applied over the track to insulate it. This piece of tape should be at 90 degrees to the first piece on the outside of the BNC barrel. Then a third piece of tape should be added around the circumference of the BNC to hold the other two pieces down. The probe should look like the one in Figure 3 at this point.

Tack inserted into BNC

Figure 3. Probe Covered in Kapton Tape

A final touch of heat shrink tubing can be added as shown in Figure 4 for a nice look and to insure the layers of tape are held down. The tubing also serves to make a grip for holding the probe securely.

Finished E-Field probe

Figure 4. Heat Shrink Tubing Used to Finish the Probe

Figure 5 shows how the probe is used. It is generally held at right angles to a PCB and fed from a high voltage pulse generator to inject capacitively coupled current locally into the PCB at one spot.

Use of probe on a PCB

Figure 5.
Using the E-Field Probe to Scan a PCB

Figure 6 shows a Fischer Custom Communications "EFT" high voltage pulser with external attenuators feeding the E-field probe. The pulse shape has a fast rise, slow fall, and is designed not to burn out most 2 Watt attenuators. For the measurements below, a single 6 dB attenuator was used to keep the scope plot on the screen.

Using Fischer Custome Communications pulser to drive probe

Figure 6. Using a Fischer Custom Communications "EFT" High Voltrage Pulser and Attenuators to Drive the E-Field Probe

With the pulser running, the setup shown in Figure 7 was used to measure the injected current from one probe to a second probe with a 50 Ohm termination at the scope input. The result gives an idea what might be injected into a PCB. The result is shown in Figure 8. The waveshape will vary considerably with the length of the cables and nearby objects as the outside of the coax shields are in the current path. Injected current into the receiving probe is matched with a current flowing on the cable shield in the opposite direction (traveling out of the cable shield and along the outside of the coax shield away from the probe. Injected E-fields effects on a PCB are usually less contained in area than H-fields that form local current loops in a ground plane or signals injected into signal traces. H-field injection is not particularly sensitive to surrounding objects like E-field injection is.[3]

Figure 8 shows a peak voltage injected into the second probe of about 8 Volts, a significant amount. The pulse width is about 5 ns, the risetime of the pulser. There is a much smaller negative pulse corresponding to the longer fall time of the pulser, about 25-30 ns.

probe to probe coupling

Figure 7. Coupling Two E-Field Probes to Measure Injected Voltage Into a 50 Ohm Load

injected current waveform

Figure 8.
Injected Voltage Plot into 50 Ohms With 6 dB Pad on Scope Input
Vertical = 2 Volts/div, Horizontal = 10 ns/div

I have seen PCBs where a portion of the surface produced a response from E-field injection that correlated with an ESD issue and other parts of the same board responded to H-field injection, also producing a response that correlated with the ESD issue.

Summary: E-field injection is useful in debugging of electronic equipment. This article presented a simple probe for accomplishing this. When coupled with the small battery powered pulser from Fischer Custom Communications, it makes a handy troubleshooting tool.

Links in this article:
  1. https://incompliancemag.com/article/near-and-far-fields-simplified-for-emiemc/
  2. https://m.eet.com/media/1140931/19213-150828.pdf
  3. Noise Injection for Design Analysis and Debugging
Equipment Used in This Article
  1. Fischer Custom Communications "EFT pulser"
    Contact: Alan Fischer
    Email: (sales@fischercc.com
    Web: http://fischercc.com)
    Tel: 310-303-3300
    EFT Waveshaping Module
    P/N: 170808-1033-4
    Power Supply Module (for pulser)
    P/N: 170808-1033-1
  2. Tektronix MDO4104B-6 Oscilloscope/Spectrum Analyzer
    Contact: Wilson Lee
    Email: wilson.lee@tektronix.com
    Web: http://tek.com
    Tel: 503-627-5830
  3. Mini-Circuits, HAT-6, 50 Ohm, 6 dB attenuator

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Copyright 2019 Douglas C. Smith