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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 effects on electronic system operation. The effects of electric and magnetic field coupling is explored using E and H field probes. Conclusions are drawn on resulting system effects and the nature of the fields.

Discussion: To gain insight into the nature of the fields radiated from ESD events, measurements were made using magnetic field and electric field probes. The source of ESD events used was the generator described on this website in a Technical Tidbit, June 2001, A Static Field 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 fast di/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 used to indicate that the generator is actually generating sparks as evidenced by "pops" in the radio as the charged styrofoam is moved over the copper tape. A standard 6 cm 7405 shielded loop was 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 Using Magnetic Field Probes (~600K pdf file) on this site for more information). Voltages picked up by the loop due to magnetic fields would be comparable to that picked up by loop structures of similar dimensions, cables for instance, in a system. The loop was connected using high quality RG-142B/U coaxial cable to a 50 Ohm termination in the scope. Ferrite cores were used to help isolate the shield of the cable from becoming part of the antenna structure.
Figure 2 shows the loop output for this test setup. The important parameter to note is the peak voltage of over 3 volts. Such interference is a real problem when looking for glitches with oscilloscope probes. Loops are always formed when making voltage measurements and interference like this easily gets into the measurement. In addition, it is easy to see how radiated 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 amplitude of about 2 volts, a significant amount of EMI. The ringing frequency is likely due to 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 near the left hand side of the copper tape antenna as shown in Figure 5, the opposite field is picked up by the loop. The result is shown in Figure 6. The amplitude of the response is about the same, a peak of 2 Volts, but the polarity is reversed. For a fixed position of the ball, an opposite polarity waveform can 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 probe was moved from the right side of the spark gap in the copper foil to the left side. What happens to the output of the H field probe if it is moved the same way? A hint can be found in a pair of Technical Tibits on this site that cover a completely different topic:
Summary: ESD can have strong effects on systems including by radiation. Data presented shows that even relatively low voltage events from small arcs can produce significant voltages from radiated EMI in nearby structures.  Also notable is that the opposite polarity of field can be generated from the same discharge as seen from a different perspective in space.

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

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Copyright © 2004 Douglas C. Smith