IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 43, NO. 2, MAY 2001 187 Measurement of Magnetic Fields Radiated from ESD Using Field Sensors Graziano Cerri, Member, IEEE, Francesco Coacci, Luca Fenucci, and Valter Mariani Primiani, Member, IEEE Abstract—The goal of this paper is to show that commercial sensors, whose frequency response is not specifically designed, can be effectively used to measure very fast transient fields applying a proper reconstructing procedure based on the knowledge of the sensor transfer function. To do this, it is necessary to characterize a structure supporting a transverse electromagnetic (TEM) field, that will be used to set up a calibration procedure for elementary magnetic field sensors. The approach is completely analytical and allows us to know rigorously the field inside the structure. From the knowledge of this field, the transfer function of the sensor, in ampli- tude and phase, is evaluated up to 2 GHz. The complete character- ization of the sensor allows us to reconstruct the sensed field from its output voltage waveform. The calibration procedure is carried out in time domain and therefore the fast Fourier transform (FFT) algorithm is used to achieve the sensor transfer function, as well as an inverse FFT (IFFT) is necessary to retrieve the transient im- pinging field. An experimental validation of the procedure shows the consistency of the approach. Index Terms— ESD, field probes, magnetic field sensors, trans- verse electromagnetic horn cell. I. INTRODUCTION E LECTROMAGNETIC transient fields radiated by electro- static discharge (ESD) currents or during electrical fast transient (EFT) tests are strong pulsed sources of interfering sig- nals for electronic equipment, and therefore it is necessary to measure the intensity of the disturbance to adopt the most suit- able actions to avoid electromagnetic interference (EMI) prob- lems. Also the electromagnetic characterization of the source from a theoretical point of view requires an experimental validation of the model by means of field measurements; consequently the complete understanding of the ESD physical mechanism could lead to improve ESD standards, including radiated field speci- fications [1]. Unfortunately, measurement of fast transient fields is not an easy task because it requires sensors having flatness of the am- plitude response over a wide frequency range, a minimum phase distortion, a proper sensibility and reduced dimensions for min- imum field perturbation for point measurements [2]–[4]. The realization of such sensors requires an accurate design and a considerable technological effort. An alternative solution is the use of the transverse electro- magnetic (TEM) horn antennas for a direct measurement of the Manuscript received October 13, 1999; revised November 16, 2000. The authors are with the University of Ancona, Department of Elettronica ed Automatica, 60131 Ancona, Italy. Publisher Item Identifier S 0018-9375(01)04065-0. ESD electric radiated field [5], [6], but these antennas have gen- erally large dimensions and therefore they are not suitable for point measurements, in particular when most of measurements have to be carried out very close to the source where fields vary rapidly as a function of position. Moreover, these antennas do not give the magnetic field that dominates in proximity of a closed current loop, like the one produced by a standard ESD gun, and they also disturb the field being measured. Some authors proposed to characterize an EMP sensor by its time domain sensor factor [7]. This implies a purely derivative behavior of the sensor leading to a bandwidth limitation for a prescribed dimension fixed by the required sensitivity. Other authors overcome this difficulty evaluating the sensor discrete transfer function [8], and obtaining the model from a system identification approach applied to the time domain sensor response. In this case, the problem is the choice of the calibrating struc- ture, that must provide a uniform and known electromagnetic field over a wide frequency range. For example, in [8], a TEM cell was used, but its disadvantage is the low upper frequency limit and therefore the calibrated sensor cannot be applied to sense fast transient ESD field. This trouble can be overcome by using a wire-TEM or giga-TEM cell that assures a much wider bandwidth, but it requires a good matched termination all over the work bandwidth, which makes the structure expensive. In this paper, we propose a structure and a calibration pro- cedure that represent a compromise between cost and effective- ness: a TEM horn cell is adopted to calibrate a sensor according to a time domain technique. This approach was chosen because in our laboratory a “transfer standard” or a “primary standard” are not available, and so we decided to adopt a calibration method based on the calculated field strength [9, Sec. 4.1]. The TEM horn antenna has been used for many years, be- cause its broad bandwidth and its non dispersive behavior are suitable to radiate and receive pulsed signals preserving their time domain waveform. Therefore, this structure was studied in the past [10], [11], but also, recently, it received much attention in order to develop more accurate models [12], [13]. In this paper, the TEM horn structure is designed to generate a known and uniform transverse field for calibration of sensors over a wide frequency range: the transverse field distribution is achieved by a rigorous analytical solution of the Laplace’s equation. With respect to the conical transmission line [9, Sect. B.8] our TEM horn cell is not terminated and so the problem of designing 0018–9375/01$10.00 © 2001 IEEE