Whitepaper
Pre Compliance Emissions Testing: Accounting for Ambient Noise
This technical note from Com-Power Corporation addresses one of the central challenges of pre-compliance radiated emissions testing outside an anechoic chamber: the presence of ambient RF signals that can obscure or mask emissions from the equipment under test. Three practical techniques for improving signal-to-noise ratio are presented and illustrated with spectrum analyzer screenshots. The first involves narrowing the resolution bandwidth (RBW) of the spectrum analyzer, which separates closely spaced EUT emissions and ambient signals that appear merged at wider bandwidths, though at the cost of longer sweep times. The second technique is rotating the test antenna polarization; since ambient noise sources typically have a fixed polarization, rotating the antenna 90 degrees relative to the ambient source can substantially suppress it while the EUT's own emissions remain visible. The third approach is reducing the measurement distance: because EUT emissions diminish with distance while ambient signals from far-off sources remain roughly constant, moving the antenna closer to the EUT significantly improves the EUT signal above the noise floor, with the document demonstrating improvements from 3.5 dB at 10 meters to over 20 dB at 1 meter.