Reduction of noise in near-field measurements

Near-field measurements are sometimes corrupted by ambient noise from either far- or near-field sources. Far-field noise can be removed by performing measurements in a shielded room, but this option is not always available. An adaptive noise cancellation technique is proposed for cancelling ambient electromagnetic interference from measurements taken in a noisy environment. A time-domain, non-recursive, block-processing algorithm was selected to generate the adaptive linear filter. Measurements in the near field of a clocked-logic electronic device in a noisy environment are compared with measurements of the same device in a noise-free environment to demonstrate that the algorithm properly removes ambient electromagnetic noise from the recorded signals. Electromagnetic noise signals in the frequency range of 10 to 700 MHz are reduced in magnitude by 20 dB or more, often to the level of thermal noise.