Time-Domain EMI Measurements using a Low Cost Digitizer to Optimize the Total Measurement Time for a Test Receiver

Abstract

By analyzing electromagnetic interference (EMI) based on its spectral components important time-domain information is lost. Conventional super-heterodyne frequency band stepping EMI test receivers have to use long dwell times for every frequency which makes the total measurement time too long. Often dwell times are set too short, causing time-varying interference, or cyclo-stationary interference, to be detected improperly. The concept of time-varying EMI is also not incorporated in standards, which are only based on frequency domain limits. To catch these time-varying interferences the receiver has to measure for at least one repetition period. Measuring many spectral components for at least the repetition of a cyclo-stationary signal causes detrimental measurement times. Time-domain electromagnetic interference (EMI) analyzers have been proposed to reduce these long measurement times, but remain expensive. To reduce costs the utilization of time-domain EMI measurements using a low cost digitizer is examined. A PicosScope in conjunction with Digital Signal Processing (DSP) is used to create the possibility to estimate the total measurement time of a conventional EMI receiver based on the dwell times. This can be used to optimize the total measurement time needed for the test receiver, while still complying to standards, resulting in reduced measurement times otherwise needed in expensive test labs. A short-time Fast-Fourier transform (STFFT) is used to examine the interfering source in both frequency and time simultaneously. It was also shown that the conventional EMI receiver perceives certain time varying signals as continuous waves due to the spectral nature of this receiver.