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Douglas C. Smith

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Technical Tidbit - July 2004
Induced Voltages via Electric and Magnetic Fields - ESD Immunity

radiated ESD test setup

Figure 1.
Magnetic Loop Used to Pickup EMI from an ESD Event

ESD can have significant effectson electronic system operation. The effects of electric and magnetic fieldcoupling is explored using E and H field probes. Conclusions are drawn onresulting system effects and the nature of the fields.

Discussion: To gain insight into the nature of the fields radiatedfrom ESD events, measurements were made using magnetic field and electricfield probes. The source of ESD events used was the generator described onthis website in a Technical Tidbit, June 2001, A StaticField Powered EMI Source.The source is composed of two lengths of copper tape with a small gap in-between.Sparks are induced by bringing a static charge close to the copper foil.In this case, the spark gap is a fraction of a millimeter resulting in fastdi/dt with risetimes of current on the order of a couple hundred picoseconds.

Figure 1 shows the basic test setup. The AM radio in the picture is usedto indicate that the generator is actually generating sparks as evidencedby "pops" in the radio as the charged styrofoam is moved over thecopper tape. A standard 6 cm 7405 shielded loopwas used to pick up the radiated magnetic fields of the ESD event by responding(mostly) to magnetic fields (there is some electric field response, see Signal and Noise Measurement Techniques UsingMagnetic Field Probes (~600K pdf file)on this site for more information). Voltages picked up by the loop due tomagnetic fields would be comparable to that picked up by loop structuresof similar dimensions, cables for instance, in a system. The loop was connectedusing high quality RG-142B/U coaxial cable to a 50 Ohm termination in thescope. Ferrite cores were used to help isolate the shield of the cable frombecoming part of the antenna structure.
Figure 2 shows the loop output for this test setup. The importantparameter to note is the peak voltage of over 3 volts. Such interferenceis a real problem when looking for glitches with oscilloscope probes. Loopsare always formed when making voltage measurements and interference likethis easily gets into the measurement. In addition, it is easy to see howradiated fields from ESD can effect systems.

 loop output

Figure 2. Loop Output from Test Setup of Figure 1

Figure 3 shows a similar measurement setup for electric fields. The probe is part of the same 7405 probe set containing the loop probe in Figure 1. The probe is essentially an insulated metal ball connected to the center conductor of the coax. The shield of the cable extends up to the ball but is not connected.The overall antenna structure has two parts: first, the ball and second,the shield as far back as the ferrite cores, an asymmetrical dipole of sorts.The RG142B/U cable connects to a 50 Ohm termination in the scope,

  E-field setup, right

Figure 3. E-field Test Setup - Right Side

Figure 4 shows the measurement result using the ball probe. Note the amplitudeof about 2 volts, a significant amount of EMI. The ringing frequency is likely dueto a combination of the resonant frequency of the copper tape structure at about 500 MHz and the ball and cable shield antenna.

scope plot for E-field right side

Figure 4. "Ball Probe" Response to an ESD Event
(probe on right side of event)

For the same movement of the charged styrofoam, but with the ball probe nearthe left hand side of the copper tape antenna as shown in Figure 5, the oppositefield is picked up by the loop. The result is shown in Figure 6. The amplitudeof the response is about the same, a peak of 2 Volts, but the polarity isreversed. For a fixed position of the ball, an opposite polarity waveformcan also be made by moving the styrofoam in the opposite direction.

E-field setup, left

Figure 5. E-field Test Setup - Left Side

scope plot for E-field left side

Figure 6. "Ball Probe" Response to an ESD Event
(probe on left side of event)

A question: The polarity of the E field probe output inverted when the probewas moved from the right side of the spark gap in the copper foil to the leftside. What happens to the output of the H field probe if it is moved thesame way? A hint can be found in a pair of Technical Tibits on this sitethat cover a completely different topic:
Summary:ESD can have strong effects on systems including by radiation. Data presentedshows that even relatively low voltage events from small arcs can producesignificant voltages from radiated EMI in nearby structures.  Also notableis that the opposite polarity of field can be generated from the same dischargeas seen from a different perspective in space.

Additional information on this site regarding ESD effects on systems includes:
Thanks to Agilent Technologies for supplyingthe scope for this experiment. The model used for this article was an Agilent Technologies 54845a, an 8 GSa/secunit that is now replaced by a much faster scope, the 54853a.

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Copyright © 2004 Douglas C. Smith