Hidden aspects in CISPR 16-1-1 full compliant fast fourier transform EMI receivers

With the introduction of Fast Fourier Transform (FFT) EMI receivers, the time required to perform electromagnetic interference (EMI) measurements has been drastically reduced. In theory, these instruments allow measuring the whole band at once. However, aspects related to (i) the level and band of input interference, (ii) prescriptions in EMI normative (i.e., CISPR 16-1-1), and (iii) achievable performance with today's state-of-the-art A/D converters, undermine the actual benefit of FFT receivers and pose limitations for their indiscriminate use for CISPR 16-1-1 full compliant measurements. In this article, we treat in detail the dynamic range in FFT receivers, discussing possible measurement errors in case of insufficient dynamic range. Through a Lab test we show that CISPR 16-1-1 full compliance requirements challenge dynamics in EMI measurements. We also show by means of simple examples that, in contrast to claims of some manufacturers, there are no A/D approaches that can improve the dynamic range of FFT receivers, other than the use of more advanced converters (higher effective number of bits). Instead, we advise the use of preselection as the only mean to achieve sufficient dynamics. Finally, we discuss the measurement error that FFT receivers may introduce during the processing of the digitalised signal if the related windowing is not accurately designed.