Efficient magnetic field measurements

Measuring magnetic fields of equipment under test at low frequencies which is received by loop antennas using an EMI receiver with small bandwidths takes much time and can even reach a week for a standard measurement. This waste in time could be avoided by applying time domain measurements. Commercial time-domain EMI (TDEMI) measurement equipment are sampling at the intermediate frequency, and are still relatively expensive. Base-band sampling using low-cost digitizers, or an oscilloscope, is proposed in this paper. The measured data is converted to the typical results in frequency domain via fast Fourier transform, while the necessary large dynamic range is achieved via oversampling. This methodology decreases the cost of the test equipment, but most of all reduces the measurement time drastically compared to the classical frequency domain measurement approach. The setup is described. Measurements with a conventional receiver parallel to the digitizer using a signal generator powering a radiating loop have been performed. Also measurements on an actual interfering source with pulsed magnetic fields are performed. It is shown that the measurement results are the same, while the measurement time has been reduced drastically.

I. Introduction

Radiated emission measurements of magnetic fields (NRE01, RE101) according to military standards are performed between 30 Hz and 100 kHz [1], [2] with bandwidths starting at 10 Hz. As the step size is thus 5 Hz, and the measuring time at least some hundreds of milliseconds per step, the measuring time is very long. The test is also requiring a large measuring time because the field has to be measured at many positions around the Equipment Under Test (EUT). Although proposals have been made to make the measurement methodology smarter, these have not been accepted, mainly because the custodian of the standard [2] is aerospace and in that application area only small objects are measured. However, when testing large EUT, the test time can be one week, resulting in very high costs. A shortened test methodology is also not always possible because observers are attending the tests, and it is not in their (commercial) interest to decrease the test time. Time Domain ElectroMagnetic Interference (TDEMI) analysers are very popular, and can be used to decrease test time drastically. These TDEMI analysers are using an intermediate frequency (IF). And they are expensive. In this paper a low-cost digitizer, like a Picoscope, for the NRE01 [1] or RE101 [2] test procedure is presented. Experiments have been performed with the standardised calibration/verification setup, as mentioned in the standards, with a conventional ElectroMagnetic Interference (EMI) analyser connected in parallel. Also actual magnetic field emissions have been measured, showing that the test time and the cost of test equipment can be decreased drastically.