A Noise Spectrum Measurement System Using the Fast Fourier Transform

A portable multichannel battery-operated measurement system was developed to measure the rms magnetic-field noise spectrum in the frequency range from 100 Hz to 375 kHz. During each measurement, the entire spectrum is measured simultaneously through the use of time-domain recordings which are later analyzed by Fast Fourier Transform (FFT) processing. Dynamic ranges of 60 dB in a 125-Hz bandwidth are obtained for spectra covering the range from 100 Hz to 100 kHz. The method also allows a three-dimensional display of the way spectrum occupancy changes with time. Some advantages of the system are portability, rms measurement, 100 percent probability of signal intercept, data gathering times measured in seconds or milliseconds, simultaneous coverage of several decades of frequency, no "picket fence" amplitude uncertainty with the associated potential 3-dB error, and increased frequency measurement resolution. Some examples of electromagnetic noise (measured in and above coal mines) are given, primarily to show the capability of the measuring system.