Noise In Time Domain EM Systems

This paper considers the signal-to-noise performance of a time-domain electromagnetic (TEM)<br>system that we developed with funding from NAVEODTECH (Indian Head, MD). TEM systems<br>require wide bandwidth and large dynamic range. Wide bandwidth is not conducive to low noise levels.<br>However, large dynamic range requires low noise levels. Stacking of successive repetitions of the signal<br>is a commonly, if not ubiquitously, applied method for effectively narrowing the bandwidth and for<br>lowering the effective noise levels. The method of signal repetition and the corresponding stacking<br>algorithm can have a significant influence on the performance of these systems.<br>We define noise sources for this system as environmental noise, system generated magnetic field<br>radiation noise, system electronic noise, motion noise, transmitter noise, and mechanical and physical<br>noise.<br>The TEM system we have developed includes a novel transmitter and a versatile computer<br>interface that provides flexibility in controlling the period, repetition rate, and successive polarities of<br>transmitted signals. This unprecedented control of the transmitter output has provided us with the<br>opportunity to test transmitter waveforms and stacking algorithms.<br>In this paper, we compare relative contributions and importance of each noise source and we<br>further compare the effects of using different transmitted waveforms and corresponding stacking<br>algorithms. Careful consideration of noise has helped us build a prototype system with superior<br>sensitivity.